HIPPARCHOSHIPPARCHOS The Hellenic Astronomical Society Newsletter Volume 2, Issue 11

ISSN: 1790-9252 Contents HIPPARCHOSHIPPARCHOS Volume 2, Issue 11 • June 2014

ISSN: 1790-9252 Hipparchos is the official newsletter of the Hellenic Astronomical Society. Message from the President ...... 3 It publishes review papers, news and comments on topics of interest to as- Hel.A.S. Events tronomers, including matters concern- ing members of the Hellenic Astro- 11th Hellenic Astronomical Conference ...... 4 nomical Society. 1st Summer School of the Hellenic Astronomical Society ...... 6

REVIEWS Editorial board • Christos Efthymiopoulos Formation: a Persistent Mystery by Konstantinos Tassis ...... 7 (RCAAM, Academy of Athens) • Kleomenis Tsiganis Orbital Evolution in Extra-solar systems (Univ. Thessaloniki) by George Voyatzis ...... 13

• Alceste Bonanos Superluminous Supernovae (IAASARS, National Observatory by Emmanouil Chatzopoulos ...... 18 of Athens)

Contact person Research Projects in Astronomy in Greece Funded by the GSRT Christos Efthymiopoulos “Excellence I and II” (Aristeia) Actions Academy of Athens Aristeia I: Research Center for Astronomy & Applied Mathematics Revealed by their own Dust: Identifying the Missing Links Soranou Efessiou 4 in Massive Star Evolution - MissingLinks ...... 24 Athens, GR-11527, Greece The origin of astrophysical magnetic fields - CosmicBattery ...... 25 Tel: +30-210-6597513 Fax: +30-210-6597602 A Step in the Dark: The Dense Molecular Gas in - DeMoGas ...... 26 E-mail: [email protected] Unveiling the Physics of Supermassive Black Holes and Relativistic Jets With Optical Polarization Observations of Blazars - ...... 27 Editorial Advisors RoboPol • V. Charmandaris Aristeia II: (Univ. Crete & NOA) The Manchester-Athens Wide-Field (Narrow-Band) Camera: • D. Hatzidimitriou (Univ. Athens) A Deep Sky-Survey of the Extensive Line Emission Regions • K. Kokkotas at High Galactic Latitudes - MAWFC ...... 28 (Univ. Tübingen - Univ. Thessaloniki) The Quest for Relativistic Signals in the X-Ray Light Curves • M. Plionis (Univ. Thessaloniki) of Active Galactic Nuclei - AGNQUEST ...... 29 • L. Vlahos (Univ. Thessaloniki) Solar Small-scale Events and their Role in the Heating of the Solar Atmosphere - SOLAR ...... 30 Printed by ZITI Publications • www.ziti.gr

Cover Image: Schematic representation of the effect of solar eruptions on the terrestrial magnetosphere and geospace (Image courtesy: NASA). See page 6 for the First Hel.A.S. Summer School on "Physical Proc- esses and Data Analysis in Heliophysics", to be held in Athens, September 1 to 5, 2014. Message from the President

3 June 2014

Dear friends,

t is with pleasure that I am writing this The economic crisis of the last four liophysics. I believe this is quite appro- Imessage. My term as President of the has affected everyone, but our So- priate, since the study of Solar Physics is Hellenic Astronomical Society comes to ciety, again thanks to its members, has quite strong in Greece. an end in ten days and I am leaving with not done badly. A significant reserve has very fond memories. been established and the Society is able I would like to thank our members for to guarantee to all Greek Astronomers entrusting me with the presidency of I believe that our Society is on a “curve free publication of their articles in As- our Society in the past four years and of growth” and there is no sign that this tronomy & Astrophysics, despite the I wish success to the new Council that will change in the future. Thanks main- change in the government’s policy. Pay- will be elected on June 13. The depart- ing Council is here to help in any way ly to our young colleagues, the quality ment of page charges would be prohibi- possible. and quantity of astronomical research tive to our members. in Greece is constantly improving. Our young members have had their fair share Last, but not least, I want to mention the of the EXCELLENCE (ARISTEIA) grants establishment by our Society of a Sum- of the General Secretariat for Research mer School every even . The first and Technology. Also, our members con- such School will take place this com- Nick Kylafis tinue to be successful with internation- ing September and it will be on Physi- President of Hel.A.S. al grants. cal Processes and Data Analysis in He-

HIPPARCHOS | Volume 2, Issue 11 3 11th Hellenic Astronomical Conference

Under the Auspices of H.E. the President of the Hellenic Republic Dr. Karolos Papoulias Hel.A.S. EVENTS Hel.A.S.

he Hellenic Astronomical Confer- edge the support of the following spon- dition of “Best Ph.D Award” Talk (E. T ence, organized by the Hellenic sors: Academy of Athens, National Ob- Papastergis: Statistical analysis of ALFAL- Astronomical Society (Hel.A.S.), is the servatory of Athens, Biomedical Re- FA galaxies: insights in formation & major scientific event of the greek as- search Foundation of the Academy of near-field cosmology). tronomical community. The Conference, Athens. Finally, there was a talk “in memo- which takes place every two years in a The official opening of the confer- ry of John Hadjidemetriou” delivered by different part of Greece, brings together ence took place at the central building Hel.A.S. Honorary President G. Conto- scientists with research interests in of the Academy of Athens. There was a poulos. Astronomy, Astrophysics, and Space public outreach talk (I. Contopou- The conference hosted four scientif- Physics. los: “Magnetic fields and Black Holes in the ic sessions, namely: The 11th Conference of Hel.A.S. was Universe”). This was preceded by short – Session 1: «Sun, Planets and Inter- held in Athens, from 8 to 12 September addresses by all former presidents of planetary Medium» 2013. The conference was co-organised Hel.A.S. (G. Contopoulos, J. Seiradakis, – Session 2: «Extragalactic Astrono- by the Research Center for Astronomy P. Laskarides, and K. Tsiganos), on the my and Astrophysics» and Applied Mathematics of the Academy occasion of the 20th anniversary of – Session 3: «Cosmology and Relativ- of Athens. The conference took place at Hel.A.S. istic Astrophysics» the main auditorium of the Biomedical The conference reached a record – Session 4: «, Our Galaxy and Research Foundation of the Academy number of 178 registered participants. the Local Group» of Athens. The welcome reception was There were five plenary talks: hosted in the historical central building – A. Vourlidas: Hurricane Season in the There were in total 78 talks (23 in sec- of the Academy of Athens, while the Inner Heliosphere: Observations of Cor- tion 1, 29 in section 2, 14 in section 3, official conference dinner was hosted onal Ejections during Solar Maxi- and 18 in section 4) and 68 posters (33 in the gardens of the historical central mum in section 1, 11 in section 2, 4 in section 3, and 20 in section 4) presented. The building of the National Observatory – R-P. Kudritzki: Supergiant Stars as Ex- oral and poster presentations were made of Athens, at the Nymphes hill (Lofos tragalactic Probes of Cosmic Abundanc- public (see http://www.helas.gr/conf/2013/ Nymfon), very close to the Acropolis. es and Distances presentations.php and http://www.helas.gr/ Scientific Organizing Committee – T. Courvoisier: Building up a European conf/2013/posters_pres.php respectively). (SOC): Astronomical Community The abstracts of all presentations were N. Kylafis (chair), A. Anastasiadis, A. Bo- – F. Combes: Molecular gas in galaxies linked to the ADS database. nanos, C. Efthymiopoulos, I. Papadakis, K. across the Hubble time Tsiganis and N. Vlahakis. – M. Plionis: Recent developments in Cos- We were very happy for the quality and Local Organizing Committee mology high level of research results present- (LOC): The conference invited a young astron- ed during the 11th astronomical con- C. Efthymiopoulos (Chair), C. Gontikakis, omer to give a “Highlight Talk”. This was ference. This only generates confidence M. Harsoula, N. Delis, L. Tsigaridi, M. Kat- delivered by A. Fragos: The Origin of Black and optimism for the future of Greek sanikas, G. Aggelopoulou and M. Zoulias. Hole Spin in Galactic Low-mass X-ray bina- astronomy. We would like to gratefully acknowl- ries. Also, the Hel.A.S. continued the tra-

More details about the conference can be found at http://www.helas.gr/conf/2013/

HIPPARCHOS | Volume 2, Issue 11 4 11th Hellenic Astronomical Conference picture

HIPPARCHOS | Volume 2, Issue 11 5 The 1st Summer School of the Hellenic Astronomical Society

uring the General Assembly of the as the opportunity for them to meet of Athens. The central theme is: Hel.A.S. in September 2013, the and interact with more experienced re- Hel.A.S. EVENTS Hel.A.S. D “Physical Processes Hel.A.S. council proposed to organize searchers. and Data Analysis in Heliophysics” a summer school on even years, when The 1st Hel.A.S. Summer School will there are no Hel.A.S. Conferences. The take place in September 2014, in collab- The School will be held in Athens, from purpose of the schools will be to of- oration with the National Observato- the 1st until the 5th of September, 2014. fer training to the younger members of ry of Athens (NOA) and the Research Hel.A.S., such as graduate students and Center for Astronomy and Applied More information can be found at: young postdoctoral researchers, as well Mathematics (RCAAM) of the Academy http://www.helas.gr/school/2014

Organizing Committees Registration Venue Scientific Organizing Committee The School participation is limited to The 1st Summer School of Hel.A.S. Chair: A. Anastasiadis (NOA) 30-35 persons. A registration fee of 60 € will take place at the “Kostis Pala- Members: I. A. Daglis (UoA), M. Georgou- has been set to cover writing materi- mas” Building of the University of Ath- lis (RCAAM), S. Patsourakos (UoI), al and coffee break costs. The registra- ens (Akadimias 48 & Sina Street, en- K. Tsinganos (NOA-UoA), G. Tsiropoula tion fee will be paid in cash upon arrival. trance from Akadimias). (NOA) and L. Vlahos (AUTH) Participants are encouraged to complete the Registration Form available from the Local Organizing Committee School website (before July 15, 2014). Chair: M. Georgoulis (RCAAM) http://www.helas.gr/school/2014/ Members: C. Gontikakis (RCAAM) and register.php G. Tsiropoula (NOA)

HIPPARCHOS | Volume 2, Issue 11 6 Star Formation: a Persistent Mystery by Konstantinos Tassis Department of Physics, University of Crete REVIEWS formation in different environments (the Under certain conditions, self-gravity Abstract rate at which stars form from a specif- overpowers all opposing agents. If this oc- ic amount of gas) and the stellar multi- curs, the formation of one or more stars Despite its central importance in de- plicity. All these quantities are critical in- is inevitable. In this review, we will focus veloping a predictive theory of star for- puts for simulations of both galaxy for- on the initiation of star formation: the mation, the initiation of star formation mation and evolution, as well as plane- processes through which gravity manag- has been one of the most persistent tary formation – but, lacking a star for- es to dominate and force the formation problems in modern astrophysics. The mation theory, they are supplied by sim- of a protostar. Although the open ques- main open questions revolve around plified empirical recipes. If we are there- tions on what happens from that point the interpretation of the low efficiency fore to be able to claim true under- on are enough to fill books rather than of star formation: why are molecular standing of how galaxies evolve in the review articles, here we will only tackle clouds supported against gravity? and cosmos, and of how planets, the hosts of the onset of star formation – partly for what drives small fragments of these life, are formed, we have to first under- the sake of brevity, but also because these clouds to form, lose their support, and stand how stars form. first stages provide the initial conditions collapse to form protostars? Here, the The raw material out of which stars for everything that follows. two main contestant theories (mag- are formed exists in the space between In this review, we will first discuss netic support and fragmentation, and stars, which is not empty. Rather, it con- the properties of molecular clouds as turbulent support and fragmentation) sists of gas (mostly hydrogen, with about they are known to us by different types are reviewed, together with promising 25% by mass helium and traces of heavi- of observations, and we will then briefly observational tests that may help us er elements), dust grains, cosmic rays, review the main theoretical ideas on the distinguish between them. and it is permeated by magnetic fields onset of gravitational collapse in molec- and photons. These ingredients are col- ular clouds, emphasizing the still - open lectively referred to as the interstellar questions. Despite its central impor- medium (ISM). The ISM is not uniform: it tance, the initiation of star formation has Introduction exhibits a wide range of densities. ISM been one of the most persistent prob- Stars have monopolized the interest of condensations are known as interstel- lems in modern astrophysics. astronomers for centuries while more lar clouds. Newborn stars are observed recently the focus has been shifted to in the densest, coldest of these clouds. galaxies as the fundamental unit mak- In these stellar nurseries the bulk of hy- Observations of Molecular ing up the cosmic web. But at the foun- drogen gas is in molecular form – hence Clouds dation of modern astrophysics lies the the name molecular clouds. process of the formation of stable, hy- Star formation is driven by the self- 1. Mass Distribution drogen-burning stars, the building blocks gravity of the gas and its interplay with and Kinematics of galaxies, from rarified gas. Star forma- other forces associated with thermal tion is a complex physical process, which pressure, turbulence, and magnetic fields Molecular clouds consist mostly of mo- has operated over most of cosmic time (since the molecular gas is weakly ion- lecular hydrogen (H2). However, H 2 in and is not yet fully understood. While ized by cosmic-ray interactions). In the molecular clouds cannot be directly ob- the study of star formation is a funda- interstellar medium, the energy den- served: the two nuclei in the H2 mole- mental and exciting topic on its own, the sity associated on average with turbu- cule are identical, so the molecule does development of a predictive theory of lence, magnetic fields, thermal pressure, not have any permanent dipole mo- star formation is essential for many oth- cosmic rays, and the interstellar photon ment nor any dipolar rotational transi- er fields, like galaxy formation and evo- field is in rough equipartition, around 1 tions. Rather, the lowest energy transi- lution and cosmology. eV per cc each (Draine 2011). In devel- tions are purely quadrupole rotational A successful theory of star forma- oping a theory of star formation, there- ones, which are not excited in gas with tion should be able to provide, from first fore, none of these ingredients can be ig- lower than about 100K – principles, the stellar initial mass func- nored; instead, all have to be treated as much higher than typical tion (what fraction of stars are born at potentially important players. This makes of molecular clouds (~10K). Thus, the each mass interval) and its dependence star formation an inherently complex bulk of the gas in molecular clouds does on local conditions, the efficiency of star and difficult to study process. not emit radiation.

HIPPARCHOS | Volume 2, Issue 11 7 Then, how does one go about ob- serving molecular clouds? By relying on

constituents other than H2: interstellar dust, and tracer molecules. Interstellar dust comprises about 1% by mass of molecular clouds. It absorbs light of background stars, and as a result molecular clouds are seen in the opti- cal as dark patches on the sky obscur- ing starlight (Figure 1a). The absorbed starlight heats the dust, and it is thus re- emitted in submillimeter wavelengths as thermal radiation (Figure 1b). The mor- phology traced by submillimeter emis- sion, as expected, correlates very well with extinction maps that rely on mea- suring the amount of starlight absorp- tion to quantify the amount of interven- ing dust (Figure 1c). The morphology of molecular clouds revealed in this way is complex. The gas within a molecular cloud is distributed in a very non-uniform manner, with varia- tions between low- and high-density re- gions. On large scales, the clouds appear filamentary. A good example is shown in Figure 2, which depicts a dust thermal emission map of the Polaris Flare cloud Figure 1: A region in the Taurus molecular cloud as seen in optical emission (a), far infrared (b), as revealed by ESAís Herschel Space through an extinction map (c), and in CO emission (d). Observatory. The spider-webólike struc- Credit: (a),(b): ESO/APEX (MPIfR/ESO/OSO)/A. Hacar et al./Digitized Sky Survey 2. Acknowl- ture is striking. edgment: Davide De Martin. (c),(d): J. Pineda. Protostars appear inside small- er-scale, more compact overdensities, known as molecular cloud cores. Cores about the mass and morphology of the appear detached from the parent cloud- frequently reside along filaments. The gas traced by the dust, the presence of sís turbulent field (Goodman et al. 1998) mass distribution of cores is similar in tracer molecules allows us to study line and are observed to be collapsing. Even shape with the mass distribution (ini- emission and thus open a window into then, though, core lifetimes are observed tial mass function) of stars (Könyves et molecular cloud kinematics and dynam- to be longer than a free-fall time (Enoch al. 2010). Thus they are the fundamental ics. This is achieved through the Doppler et al. 2008, Evans et al. 2009): even when unit of the star formation process, and effect and the velocity information that gravity has won and the formation of a understanding the way that cores are we can extract from it. Such studies have protostar is on its way, the forces op- formed is central in understanding star revealed two crucial pieces of informa- posing it still play an important role in formation. tion regarding the initial stages of star the process of star formation.

H2 is not the only molecule present formation. Molecular spectral lines are thus an in molecular clouds. Chemical processes First, molecular clouds are not col- invaluable source of information in mo- in the interstellar medium combine hy- lapsing as a whole. Star formation is a lecular clouds. However, the interpreta- drogen and higher mass atoms, includ- battle between gravity and the forces tion of such observations is complicated ing C, N, O, and heavier elements, into that oppose it, and, while gravity has to by the fact that molecular tracer species a variety of molecular species, including eventually win if we want a star to form, are not a fair tracer of the H2 distribu- complex and organic compounds. A par- this battle is not won at the molecular cloud tion, because of the complex and nonlin- ticularly abundant and commonly ob- scale. Rather, most of the mass of a mo- ear effect of interstellar chemistry. Mo- served molecule that is very frequently lecular cloud is supported against gravity, lecular abundances are determined not used to trace molecular gas is CO (Fig- and the efficiency of the star formation only by the local gas density but also by

ure 1d). In contrast to H2, CO and other process is low. Stars are only formed the dynamical evolution of a particular non-homonuclear tracer molecules used within molecular cloud cores, which molecular cloud region. As a result, all in molecular cloud observations have represent a small fraction of the mass information that is obtained by molecu- rotational transitions with low excita- (a few percent) of the entire cloud (e.g., lar cloud observations is viewed through tion temperatures and thus emit even Johnston et al. 2004). a “chemical lens”, rendering the study of at the low temperatures and densities Second, molecular cloud linewidths interstellar chemistry a critical ingredi- characteristic of molecular clouds. are supersonic, and clouds on large ent in developing a theory of star for- While dust extinction and continu- scales exhibit random, turbulent mo- mation. Figure 3 (from Bering & Tafalla um emission can only offer information tions. In contrast, molecular cloud cores 2007) shows a characteristic example of

HIPPARCHOS | Volume 2, Issue 11 8 Figure 2: Filamentary structure in the Polaris Flare molecular cloud. Figure 3: Core B68 in optical (upper left), dust continuum emission 18 + Credit: ESA and the SPIRE & PACS consortia, Ph. André (CEA Saclay) (upper right), and viewed through the chemical lens (C O and N2H for the "Gould’s Belt survey" Key Programme Consortium. molecular transitions, lower left and right respectively). Credit: Bergin & Tafalla 2007. a core seen through the “chemical lens”. gest axis (perpendicular to the magnet- their longest axis (perpendicular to the In the lower two panels, core B68 (seen ic field). The transmitted light (optical) is magnetic field). Thus linear polarization in the upper panels in the optical and thus partially polarized in the direction studies of molecular clouds trace the dust continuum emission) is mapped of the magnetic field. In emission (far in- plane-of-the-sky component of the mag- 18 + through C O and N2H molecular line frared), on the other hand, grains act like netic field, both in emission and absorp- emission. CO is anticorrelated with the tiny dipolar antennae, emitting light that tion, with the polarization direction of dust continuum peak as it mainly traces is partially polarized in the direction of emitted and transmitted light being per- the lower density parts of a core, while pendicular to each other. + N2H is only excited at higher densities Another way to study the plane-of- and traces the high-density parts. the-sky magnetic field relies on the study

E-vector of polarization of molecular line emis- 2. Magnetic Fields sion (Goldreich-Kylafis effect, Goldreich B-field & Kylafis 1981, 1982). The component of The presence of dust and molecules the magnetic field that lies along the line other than H2 in molecular clouds en- of sight can be studied through the Zee- able us to also study the magnetic field man effect. In the latter case, it is the in these structures. strength of the magnetic field along that Dust grains can reveal information specific direction that is measured. about the magnetic field through the Linear polarization maps have re- alignment of their angular momentum vealed that a large-scale ordered mag- vector with magnetic field lines. Nee- E-vector netic field component exists in molec- dle-like grains acquire angular momen- ular clouds, indicating that the turbulent tum through thermal collisions or non- flows are not sufficient to completely thermal processes (such as interaction tangle up the field and eliminate all of its E-vector with photons), in rough rotational ener- ordered structure. An example is shown gy equipartition along their principal ax- optical in Figure 5, which depicts a linear po- es. The angular momentum is then great- larization map of the transmitted light est in the direction of the shortest axis. from background stars for the Pipe neb-

The presence of the magnetic field sub- far infrared ula (Alves et al. 2008). sequently leads to an alignment of the Detailed maps of magnetic fields sur- angular momentum (and thus, the short- Figure 4: Linear polarization in molecu- rounding molecular cloud cores have est axis) along magnetic field lines (Fig- lar clouds. Emitted light is polarized along revealed the so-called hourglass-shape ure 4). Grains in such a configuration act the longest axis of the dust grains (perpen- morphology, caused when an ordered like a polaroid with respect to light pass- dicular to the magnetic field). Transmitted large-scale field is “pinched” toward the ing through the cloud: they preferentially radiation from background stars is polar- center by the dynamical contraction of absorb the polarization component that ized perpendicular to the long axis of dust a collapsing core (Figure 6, Girart et al. grains (parallel to the magnetic field). has the E-vector parallel to their lon- 2006).

HIPPARCHOS | Volume 2, Issue 11 9 Figure 5: Magnetic field in the Pipe molecular cloud as traced by opto- Figure 6: Hourglass morphology of the magnetic field in NGC1333, polarimetry of background stars. observed through studies of polarization of dust continuum emission. Credit: Alves et al. 2008. Credit: Girart et al. 2006.

How do cores form? ever have between a thousand the effective pressure associated with Theoretical ideas and a million solar masses – much high- random motions of turbulent eddies, er than the Jeans mass. So whatever sup- and magnetic fields. As we have seen, In the previous section we reviewed the ports them, it is not thermal pressure. the presence of both turbulent motions evidence that point toward a low effi- Another in principle plausible mech- and magnetic fields has been observa- ciency for the star formation process: anism for support against gravity is ro- tionally confirmed in molecular clouds. most mass of the cloud is supported tation. Interstellar clouds carry angular Determining which of the two mecha- against gravity, as clouds are not collaps- momentum at the very least because of nisms dominates in different cloud types ing as a whole; only a small percentage of their participation to the Galactic rota- is still a matter of active research and the cloud mass resides in cores where tion. In fact, that specific angular momen- heated debate in the field. The answer stars will form; and the lifetimes of cores tum by itself is so large that it exceeds to the second question – how do clouds are longer that the free-fall timescale. by at least two orders of magnitude the fragment to form cores – also depends The question then that naturally arises specific angular momentum associat- on whether it is turbulence or magnetic is: why is the efficiency low, and what is ed with single and binary stars – so a fields that dominate on large scales. the mechanism that regulates the star centrifugal barrier would certainly form Let us first consider the case where formation process? long before contraction would bring turbulence is dynamically more impor- At the heart of this problem lie the the density up to protostellar densities. tant that the magnetic field, and the mo- mechanisms by which molecular clouds A mechanism is therefore needed to lecular cloud as a whole is support- (a) are supported against gravity as a transfer angular momentum from a col- ed predominantly by turbulence (Mac whole and (b) fragment to form cores lapsing core to the surrounding medium Low & Klessen, 2004 Elmegreen & Sca- which will then go on to collapse. if stars are to form in the first place (this lo 2004). The velocities correspond- Regarding the first question, there is requirement is known as the angular mo- ing to the supersonic linewidths in such one contestant that certainly does not mentum problem of star formation). It is clouds are thought to be super-Alfven- enter the competition for cloud support: now generally accepted that this mech- ic (higher than the Alfven speed, which thermal pressure. The reason for this anism is magnetic breaking (Mouschovias is the characteristic speed of propaga- lack of effectiveness of thermal pressure & Paleologou 1979): magnetic field lines tion of a certain type of magnetohydro- in battling gravity is the very low tem- connect dense with more diffuse, larg- dynamic waves). As a result, the magnet- peratures of molecular clouds. Thermal er-scale material, and transfer angular ic field does not have time to react to pressure can only support structures of momentum away from the dense, con- disturbances caused to it by the turbu- up to a certain mass, known as the Jeans tracting part. It turns out that magnetic lent velocity field of the gas. The magnet- mass. For typical molecular cloud tem- breaking is so fast and efficient that ro- ic field is then relatively weak and mag- peratures, the Jeans mass is about 5M¤. tation does not contribute substantially netic field lines can be dragged around This means that any structure more to the support of a molecular cloud as a by the random turbulent motions, so massive than this limit cannot be ther- whole against gravity. they are expected to appear rather tan- mally supported and, in absence of other What is then the main agent that op- gled. In this case, overdensities are cre- means of support, will collapse under its poses gravity on molecular cloud scales? ated by compression in regions where own self-gravity. Molecular clouds how- Two mechanisms have been proposed: turbulent flows converge. Some of these

HIPPARCHOS | Volume 2, Issue 11 10 overdensities have enough mass to ex- ceed the Jeans limit, and these go on to collapse and form stars. Others are be- low the Jeans limit, and as a result even- tually re-expand – these are known as “transient clumps”. Next, let us consider the case where the magnetic field is dynamically more important than turbulence (Mouscho- vias & Ciolek 1999, Mouschovias, Tas- sis, & Kunz, 2006). In this case, it is im- portant to first take a close look at the Figure 7: Microphysics of magnetic molecular clouds. Magnetic forces are felt by charged par- microphysics of how the magnetic field ticles and communicated to the bulk of the gas through collisions. Because the degree of ion- ization is low, a drift can develop between field lines and charged particles on the one hand, and “communicates” with the gas. the neutral fluid on the other. As a result, neutrals can diffuse through field lines towards cen- The magnetic field can of course on- ters of gravity (ambipolar diffusion). ly directly “talk” to the charged particles in a molecular cloud: ionized atoms and molecules, free electrons, and charged the effectiveness of the magnetic field in to be subcritical, then magnetic forc- dust grains. The magnetic field is well opposing gravity is the ratio between es alone are adequate to support them coupled to the charged components of mass and magnetic flux (Mouschovias & against their self-gravity, and turbulent the cloud and the magnetic field lines Spitzer, 1976). If the mass-to-flux ratio in effective pressure is not necessary, even follow the motion of the charged par- a region is below a critical value (the re- if it is present. Measuring this important ticles. The magnetic force is communi- gion is magnetically subcritical), the mag- quantity requires separate knowledge cated to the bulk of the molecular cloud netic field wins, and the region is sup- of the mass and magnetic field strength mass, which is in the form of neutral H2 ported against gravity. If the mass to flux of a particular region. Important obsta- molecules, through collisions between ratio however exceeds the critical value cles in obtaining reliable measurements charged particles and neutrals. If the de- (the region is magnetically supercritical) of the mass-to-flux ratio are geometri- gree of ionization in a cloud is high, then then gravity wins, and the region collaps- cal projection effects, difficulty to mea- these collisions are frequent, the mag- es due to its self-gravity. Thus, in this sce- sure magnetic field strengths in low-den- netic field and the neutrals are well-cou- nario, clouds (which are not collapsing) sity regions, and the effect of chemistry pled, and the magnetic field lines and the are magnetically subcritical as a whole, – since magnetic fields can only be as- neutral fluid move together. This state is while cores are magnetically supercriti- sessed through molecular line obser- known as “magnetic flux freezing.” cal. The question in this case is how one vations (Crutcher, Hakobian & Troland Molecular clouds, however, do not can form supercritical fragments within 2009, Mouschovias & Tassis 2009). have a high degree of ionization. They a subcritical parent cloud. The answer is Super-Alfvenic or sub-Alfvenic tur- are self-shielded against ionizing radi- provided by ambipolar diffusion: because bulence? The Alfven speed in a cloud, as ation from massive stars (outer layers of the imperfect coupling between the compared to the characteristic speed of the cloud attenuate the high-energy magnetic field and the neutral gas, neu- of random motions (determined from photons and protect the inner layers), tral molecules can diffuse between field the supersonic linewidths of molecular so they can only rely on the more pen- lines and increase the mass-to-flux ra- lines) characterizes the relative strength etrating cosmic rays (relativistic charged tio around centers of gravity, until it ex- between magnetic field and turbulence, nuclei) to provide some ionization. As ceeds the critical value, at which point a and could in principle by itself be indic- a result, the coupling between magnet- supercritical fragment is formed. Cores ative of which of the two scenarios dis- ic field lines and neutral particles is im- that are dense enough to be observ- cussed above is more likely to occur in perfect, and a drift can develop between able in magnetically supported clouds specific clouds. However, knowledge of magnetic field lines and the neutral gas are generally already magnetically super- the Alfven speed requires knowledge of towards centers of gravity. This process critical, and they are in the process of the magnetic field and the density of a is known as ambipolar diffusion, and it al- collapsing dynamically until a hydrostatic region, quantities which are affected by lows the slow increase of mass associat- protostar is formed at their center. all the observational uncertainties dis- ed with a region of fixed magnetic flux Distinguishing between these two cussed for the mass-to-flux ratio above. (Figure 7). paths to core formation is presently the Thus, indirect methods have been devel- Let us now go back to thinking about focus of great theoretical and observa- oped (Hildebrand et al. 2009, Houde et a molecular cloud that is globally sup- tional efforts by researchers of the early al. 2009, Heyer et al. 2008) ported by the magnetic field, which phases of star formation. A few promis- Ordered or tangled magnet- dominates over turbulence. The veloci- ing approaches in this direction are dis- ic field? The degree of entanglement ties associated with turbulent flows in cussed below. of magnetic field lines is another mea- this case are sub-Alfvenic or Alfvenic, Magnetically supercritical or sub- sure of the relative importance between and the magnetic field lines cannot be critical clouds? The mass-to-magnetic- magnetic field and turbulence in a mo- dragged around effectively by turbulent flux ratio quantifies the relative impor- lecular cloud. This information can be flows, so the magnetic field appears rath- tance between magnetic field and gravity. accessed through polarimetric mapping er ordered. The quantity that determines If molecular clouds as a whole are found of the plane-of-the-sky component of

HIPPARCHOS | Volume 2, Issue 11 11 the magnetic field (Dotson et al. 2010, amount of time it has spent at a specific cal modeling and optopolarimetric map- Clemens et al. 2012). However, once the density. Magnetic forces keep mediating ping of molecular clouds in transmitted first stars are formed within the cloud, the rate of collapse of cores even after light from background stars, using the their energy output can significantly al- a supercritical fragment is formed, while RoboPol optopolarimeter, which is an ter the magnetic field geometry of the once turbulence decays, and in the ab- instrument specifically designed and built parent cloud. For this reason, mapping of sence of a significant magnetic field, col- for the 1.3m telescope at the Skinakas quiescent clouds that do not host any lapse proceeds on a free-fall timescale. Observatory, and represents a collab- protostars yet are the most promising In this way, even cores of similar total orative effort between FORTH and the targets for such studies. lifetimes will exhibit different molecular University of Crete, Caltech, the Max- Orientation of magnetic fields abundances if they have reached a cer- Planck Institute for Radioastronomy in in clouds and cores. Because magnet- tain evolutionary stage through the tur- Bonn, the Nicolaus Copernicus Univer- ic forces only act perpendicular to the bulent or magnetic paths. However, oth- sity in Poland, and the Intra-University magnetic field, clouds and cores in re- er parameters, such as temperature, el- Center for Astronomy and Astrophysics gions with dynamically important mag- emental abundances, and the cosmic ray in Pune, India. netic fields will tend to be more com- ionization rate can significantly affect the pressed in the direction parallel to the cloud chemistry and the resulting abun- field where there is no magnetic sup- dances (Tassis, Willacy, Yorke & Turner Acknowledgements port. As a result, the mean magnetic field 2012a,b, Tassis, Hezareh & Willacy 2012). I gratefully acknowledge comments by will be preferentially oriented parallel to As a result, chemodynamical studies of Gina Panopoulou and Aris Tritsis that the shortest axis of the molecular cloud molecular clouds are complex and time- improved this review; Felipe Alves, Ted or core. If magnetic fields are dynamical- consuming. Bergin, Miquel Girart , and Jorge Pineda ly unimportant compared to turbulence, Although all tests discussed above for their kind permission to use their il- then magnetic fields are dragged around are being actively pursued, the results lustrative figures in this review; and the by turbulence and the mean magnetic tend to be no more than a factor of two editors of Hipparchos for their interest field has a random orientation with re- away from agreeing with either scenario. in Star Formation physics. This work is spect to the molecular cloud or core The key therefore to resolving this con- supported by FP7 through Marie Cu- principal axes. However, projection ef- troversy is entering the era of precision rie Career Integration Grant PCIG- fects can make observations of this ef- interstellar medium observations. The tim- GA-2011-293531 “SFOnset” and the fect difficult to interpret (Tassis 2007, ing to be studying star formation physics EU FP7 Grant PIRSES-GA-2012-31578 Tassis et al. 2009, Chapman et al. 2013). is particularly good, because of the up- “EuroCal”. This work was partially sup- Molecular abundances. The abun- coming torrential flood of data through ported by the “RoboPol” project, which dances of specific molecular species de- current and upcoming instruments and is implemented under the “ARISTEIA” pend on the dynamical evolution of a missions, including Herschel, ALMA, SO- Action of the “OPERATIONAL PRO- cloud. The rate of a chemical process de- FIA, SKA, and the JWST. GRAMME EDUCATION AND LIFE- pends on the local density, and therefore The astrophysics group at the Uni- LONG LEARNING” and is co-funded the resulting abundances of any specific versity of Crete is pioneering such stud- by the European Social Fund (ESF) and molecule in some region depend on the ies in two distinct fronts: chemodynami- Greek National Resources.

References

Alves, F.O., Franco, G.A.P., & Girart, J.M.2008, J.K., et al. 2009, ApJS, 181, 321 views of Modern Physics, 76, 125 A&A, 486, L13 Girart, J.M., Rao, R., & Marrone, D.P. 2006, Mouschovias, T. Ch., & Ciolek, G. E. 1999, André, P., Men’shchikov, A., Bontemps, S., et Science, 313, 812 NATO ASIC Proc. 540: The Origin of al. 2010, A&A, 518, L102 Goldreich, P., & Kylafis, N.D. 1981, ApJL, 243, Stars and Planetary Systems, 305 Bergin, E.A., & Tafalla, M. 2007, ARA&A, 45, L75 Mouschovias, T. Ch., & Paleologou, E. V. 1979, 339 Goldreich, P., & Kylafis, N.D. 1982, ApJ, 253, ApJ, 230, 204 Chapman, N. L., Davidson, J. A., Goldsmith, P. 606 Mouschovias, T. Ch., & Spitzer, L., Jr. 1976, F., et al. 2013, ApJ, 770, 151 Goodman, A. A., Barranco, J. A., Wilner, D.J., ApJ, 210, 326 Clemens, D. P., Pinnick, A. F., Pavel, M. D., & & Heyer, M.H. 1998, ApJ, 504, 223 Mouschovias, T. Ch., Tassis, K., & Kunz, M. W. Taylor, B. W. 2012, ApJS, 200, 19 Heyer, M., Gong, H., Ostriker, E., & Brunt, C. 2006, ApJ, 646, 1043 Crutcher, R.M., Hakobian, N., & Troland, T. 2008, ApJ, 680, 420 Mouschovias, T. Ch., & Tassis, K. 2009, MN- H. 2009, ApJ, 692, 844 Hildebrand, R. H., Kirby, L., Dotson, J. L., RAS, 400, L15 Dotson, J. L., Vaillancourt, J. E., Kirby, L., et al. Houde, M., & Vaillancourt, J. E. 2009, Tassis, K. 2007, MNRAS, 379, L50 2010, ApJS, 186, 406 ApJ, 696, 567 Tassis, K., Dowell, C. D., Hildebrand, R. H., Draine, B. “Physics of the Interstellar and Houde, M., Vaillancourt, J. E., Hildebrand, R. Kirby, L., & Vaillancourt, J. E. 2009, MN- Intergalactic Medium”, 2011, Prince- H., Chitsazzadeh, S., & Kirby, L. 2009, RAS, 399, 1681 ton University Press, Princeton ApJ, 706, 1504 Tassis, K., Willacy, K., Yorke, H. W., & Turner, Elmegreen, B. G., & Scalo, J. 2004, ARA&A, Johnstone, D., Di Francesco, J., & Kirk, H. N. J. 2012, ApJ, 753, 29 42, 211 2004, ApJL, 611, L45 Tassis, K., Willacy, K., Yorke, H. W., & Turner, Enoch, M.L., Evans, N. J., II, Sargent, A.I., et al. Könyves, V., André, P., Men’shchikov, A., et al. N. J. 2012, ApJ, 754, 6 2008, ApJ, 684, 1240 2010, A&A, 518, L106 Tassis, K., Hezareh, T., & Willacy, K. 2012, Evans, N.J., II, Dunham, M. M., Jørgensen, Mac Low, M. M., & Klessen, R. S. 2004, Re- ApJ, 760, 57

HIPPARCHOS | Volume 2, Issue 11 12 Orbital Evolution in Extra-solar systems by George Voyatzis Section of Astronomy, Astrophysics and Mechanics, Department of Physics, Aristotle University of Thessaloniki, Greece REVIEWS

Abstract Nowadays, extra-solar systems are a hot research topic including various as- pects, as observation, formation, com- position etc. In this paper, we present the main orbital and dynamical fea- tures of these systems and discuss the evolution of planetary orbits in multi- planet systems. Particularly, we consid- er the dynamics of a two-planet sys- tem modeled by the general three body problem. Complicated orbits and chaos due to the mutual planetary interac- tions occur. However, regions of regular orbits in phase space can host plane- tary systems with long-term stability.

1. Introduction Figure 1: The blue histogram shows how many have been discovered until the indi- cated year. The abrupt increase of the number in 2014 (although this number refers to March, After the detection of the first three 6th) is due to the addition of a set of 702 planets discovered by the KST. The red histogram planets orbiting the PSR B1257+12 shows how the 1778 planets are arranged in the 1099 planetary systems. in 1992 and the first planet orbiting the main-sequence star 51 Pegasi in 1996, a significant growth of detected exoplan- sisting of two planets. The most “popu- et Imager (see figure 2). The region of 3 ets took place, which currently seems to lated” system is Kepler-90 (around the observations has a radius of about 10 be rather exponential. One of the most star KOI-351), which seems to consist of l.y. and it is estimated that our galaxy 2 reliable catalogs of exoplanets is given by seven planets . contains 400 billion planets. The closest the Extrasolar Planets Encyclopedia (EPE) The first detection method used was to our Solar system is the Ep- [1]. Very recently, on 6th March 2014, a the method of or Doppler silon Eridani b in a distance of 10 l.y. set of 702 new planet candidates, ob- method and most of the known exoplan- Most exoplanets are classified as hot served by the Kepler Space Telescope ets have been discovered by this meth- Jupiters, namely gas giant planets that (KST), were added in this catalog, which od, which is still widely used. The tran- are very close to their host star, e.g. HD now includes 1778 planets arranged in sit method has also been proved efficient, 102956 b which has mass equal to Jupi- 1099 planetary systems1. In the blue his- mainly after its application by the KST. togram of figure 1, we present how the Gravitational microlencing, reflection/ number of discovered exoplanets in- emission methods, polarimetry e.t.c. can creases year by year and in the red one also be used as detection methods [2]. how the planets are distributed in the Direct imaging has given few yet spectac- planetary systems. A number of 637 sys- ular results, firstly, with the discovery of tems contains only one planet, while the the planet Formalhaut b by the Hubble rest 1141 planets are arranged in 462 telescope and, recently with the discov- multi-planet systems most of them con- ery of Beta Pictoris b by the Gemini Plan- Figure 2: The first light image of an ex- oplanet (Beta Pictoris b) by the Gemini Telescope (announced in 7 January 2014 1. This number changes day by day, including 2. In 2013 it was announced that the system by NASA JPL, http://planetquest.jpl.nasa. new discoveries or excluding planets for which a HD10180 has 10 planets, but today only six gov/news/144) confirmation study had failed. planets have been confirmed

HIPPARCHOS | Volume 2, Issue 11 13 ter’s mass (1M ), semimajor axis 0.08AU J (years) and period 6.5 days. Very massive planets 0369 12 15 1 HD 7449 b,c have been found, either close to their semi-major axis - eccentricity orbital period - eccentricity host star (e.g. Kepler-39 b with mass mb=1.1 MJ mc=2.0 MJ 0.8 20MJ, semimajor axis 0.15 AU and period 21 days), or very far from their host star b

y 0.6 ) (e.g. HIP 78530 b has mass 24M , semim- it

J c i U r t A (

ajor axis 710AU and period 12Ky). As de- Y

tection techniques are improved, small- ecce n 0.4 er planets are detected. Many of them

c have masses equal to 10-20 times the 0.2 mass of the Earth, are possibly gaseous and are called Mini-Neptunes. Neverthe- 0 X (AU) less, planets with mass of the order of 02468 10 semimajor axis (AU) the Earth’s and of similar radius can be detected by the KST. They are possibly Figure 3: The distribution of exoplanets Figure 4: The orbits of the giant planets terrestrial (rocky) planets and become in the planes a-e (blue dots) and T-e (red around the star HD 7449. The eccentrici- very interesting for detailed study, when crosses). Most of the planets have small ties of the inner and the outer planet are they are located in a habitable zone, e.g. semi-major axis (or period) and small ec- 0.82 and 0.53, respectively. The semimajor the Gliese 667C c [3]. centricity. There are few more planets, axes are 2.3AU and 4.96AU, respectively. which are located outside the axis’ limits Studying the general structure of the of the plot and they are not shown. known extrasolar systems we can cer- tainly argue that our Solar system is an exception. Thus, the proposed mech- extreme planet is HD 20782 b with planet systems, planets are almost co- anisms for formation and evolution of a=1.38AU and e=0.97. High eccentric planar, i.e. the mutual inclination ∆I = I1– planetary systems should be revised and orbits are also observed in multi-plan- I2 between two planets is close to ze- generalized. In the following, we focus on et systems. E.g. in the two-planet sys- ro. However, cases of high mutual incli- the orbital characteristics of multi-plan- tem around the star HD 7449 the inner nation between planets have also been et systems, the conditions required for and the outer planets have eccentricity observed, e.g. the planets c and d around their long-term stability and the role of 0.82 and 0.53, respectively (see figure 4). Upsilon Andromeda seem to have mutual planetary resonance. The formation of such highly eccentric inclination larger than 30o. planetary systems is not sufficiently sup- Mean motion resonances (MMR) in ported by the current theories. Howev- multi-planet systems appear frequent- 2. Orbital and dynamical er, long-term evolution stability is possi- ly. Two planets (1 and 2) are in a MMR, features ble and can be proved as we explain in when the ratio ρ = T1/T2 of their orbital Considering a system consisting of a star the last section. period is close to a ratio r = p/q of two and a planet (assumed as point mass- The inclination Io of the orbital plane (small) integers. Computing the ratio ρ es) we expect Keplerian elliptic plane- with respect to the observer is defined of all planet pairs appearing in the multi- tary orbits around the star with peri- as the angle between the normal to the planet systems of the EPE list we obtain od T, semi-major axis a, eccentricity e planet’s orbital plane and the line of the a significant number of resonant pairs. and inclination I. From the observations observer to the star. Most observation- Certainly this number depends on the and after applying particular fitting meth- al methods, and mainly the transiting divergence δ = ( ρ – r) / r that we set as a ods (see e.g. [2]) these orbital param- method, can observe planets only with threshold. For divergence less than 5%, o eters can be estimated approximately inclinations Io≈90 . For radial-velocity 2% or 1% we find respectively 419, 174 for each observed planet. In figure 3, we observations the inclination is very im- or 83 resonant planetary pairs. Their dis- plot the distributions a-e and T-e of 623 portant for the computation of the plan- tribution to each value r is shown in fig- planets for which we have an estimation etary mass because the particular mea- ure 5. of the orbital parameters in the list of surements provide us with an estimation EPE. The fact that the majority of plan- of the quantity m×sin Io. 3. The three body model The inclination I defined between the ets appears with small semi-major axes and orbital evolution and periods is possibly caused by an ob- normal of the orbital plane and the stel- servational selection bias, since large and lar rotational axis is the most important A two-planet system can be modeled by close to the star planets are more eas- from a dynamical point of view. In this a system of three point masses m0, m1 ily detectable. Most of these close-to- sense, particular studies indicate that and m2 representing the star, the inner star planets have also small orbital ec- most planetary systems are inclined [5]. and the outer planet, respectively. Thus o centricities possibly caused by tidal cir- Cases where I≈180 , i.e. planets moving m0>>m1,2 and initially it holds a1

HIPPARCHOS | Volume 2, Issue 11 14 or the “exact resonance” is given by the 90 stable stationary solutions i.e. the mini- ma of the averaged Hamiltonian where s

t 80 "± 5%" σi=const. In resonant evolution, the

ane 70 l "± 2%" semimajor axes are not invariant (as in p

f 60 "± 1%" the secular evolution), but we can derive o

s the constraint

r 50 i 2 2 pa 40 β1a1+ β1a2 = const., f o

r 30

e where βI are almost constants and de- b 20 pend on the masses and the resonance. m u 10 The conservation of the angular mo- N mentum can be used for reducing fur- 0 ther the degrees of freedom by two. This 4/3 3/2 5/3 2/1 7/3 5/2 3/1 7/2 4/1 5/1 6/1 7/1 is achieved by choosing a suitable rotat- resonance ing frame Oxyz, where the star and one Figure 5: The distribution of resonant planetary pairs at each resonance p/q. Different color of the planets (say planet 1) are always

bars correspond to different threshold values of the difference of the ratio T1/T2 from the ratio located on the plane Oxz and Oz ax- p/q. For each pair we have set as T1 the greater orbital period. is is chosen to coincide with the vec- tor of angular momentum. In this rotat- ing frame, the planet 2 is given by the tion the inner planet may become outer 2 Gm m Gm m components (x, y, z) = (x2, y2, z2), but on- H = − 0 k − 1 2 R (a , e , I , ω , Ω , λ ) ∑ i i i i i i ly the component χ = x1 is required for and vice versa. k =1 2a a k 2 the determination of the planet 1. Thus Let Xi be the position vector of the three bodies in an inertial frame. Fol- the system is described by four degrees where ωi is the argument of periastron, lowing Poincaré, we can consider for of freedom and Hamiltonian Ωi is the longitude of ascending node the planets the astrocentric position and λ is the mean longitude (index i =1,2 i H = H (χ, x, y, z, pχ, px, py, pz) vectors ri =Xi – X0 and the barycentric refers to the particular planet) [7]. From momenta pi = m i (dXi / dt) and write the the above Hamiltonian simplified mod- where pχ, px, py and pz are the conju- Hamiltonian of the system in the form els can be constructed by the method of gate momenta [9]. In this formalism, we H = H 0 + H 1 , where averaging. can obtain directly the well known cir- 2 ⎛ p2 µ β ⎞ By averaging that fast motion on the cular restricted three body problem, if H = k − k k 0 ∑⎜ ⎟ ellipse, given by the angles λi, we obtain we set m2 = 0 , χ = const. and n = const., k =1 2β | r | ⎝ k k ⎠ the secular model. Up to second or- where n is the angular velocity of the ro- der in masses we obtain that the phase tating frame. Gm m p p H = − 1 2 + 1 2 space structure depends on the plane- Hamiltonian, H, written for the ro- 1 | r − r | m 1 2 0 tary mass ratio m1/m2. Also, it turns out tating frame is convenient, when we that the semimajor axes remain almost want to study the dynamics through

m0 mk constant and in a coplanar system the the periodic orbits of the system. The µk = G (m0 + mk ), βk = . eccentricities e1 and e2 oscillate slowly families of periodic orbits of the re- m0 + mk with opposite phases. Actually, in a 3D stricted three body problem, general- system the conservation of angular mo- ly, are continued to the general three The term H­0 describes the evolution of the planets in the framework of the two mentum is expressed as body problem and can be computed in body problem (unperturbed star-planet a systematic way [10][9]. Periodic or- 2 2 system). The second term includes the α1 1− e1 sinI 1 + α2 1−e 2 sinI2 ≈ const. bits are associated with the stationary planetary interactions and is a pertur- solutions of the approximate averaged bation term for the integrable part H0. where αι are almost constants, which de- Hamiltonian mentioned above. Families Subsequently, we have a nonintegrable pend on the masses and semimajor axes of periodic orbits are either “circular” model of 6 degrees of freedom and the of the planets. Another feature of secu- or “elliptic”. Along a circular family the corresponding canonical equations of lar dynamics is also the libration of the ratio ρ = T2/T1 of planetary periods var- motion are integrated numerically. In difference ∆ϖ=ϖ2-ϖ1 of the longitude of ies. Families of elliptic orbits bifurcate this system we have the conservation pericenter of the two planets [8]. from the circular family, when ρ = p/q of energy and the three components of When the system is close to a MMR, (i.e. at resonances). Along these bifur- the angular momentum vector [6]. the averaging should exclude slow “an- cating families ρ remains almost con- A second formalism for the planetary gle combinations” [7][8]. For a reso- stant. Thus, all elliptic periodic orbits three-body model results if we express nance r = p/q we can define the resonant are resonant and, actually, present the 3 the Hamiltonian in orbital elements (slow) angles σi = q λ1 – p λ2 + (p – q) ϖi. If “exact” dynamical resonance [11]. Lin- σ1 or σ2 librates we argue that the plan- ear stability analysis can be performed 3. More precisely we use canonical Delau- etary system involves inside the reso- classifying the periodic orbits as stable nay-like variables nance. The center of a resonant domain or unstable.

HIPPARCHOS | Volume 2, Issue 11 15 4. Orbital Evolution creases its eccentricity to values almost dicate chaotic motion while dark ones of HD 82943b,c up to 1 (collision with the star). (blue-green) corresponds to regular In order to understand the under- evolution. In this section, we present an example lying dynamics of the above evolution, In panels (a) and (b) of figure 7, we of the dynamical analysis of the orbit- we depict the qualitative type of evolu- consider all possible eccentricities of al evolution of the extra-solar system tion of all orbits in particular domains of the two planets (keeping the rest orbit- around the star HD 82943 (with mass the phase space by constructing dynam- al parameters as it is defined in the two 1.18 MSun). Three planets have been dis- ical maps of stability, namely we consid- configurations A and B, respectively). covered for this star, the planets b,c and er plane grids of initial conditions and We obtain that in configuration A (pan- d with period 442, 219 and 1078 days, for each grid point we evolve the orbits el a) there is a strip of regular orbits. respectively. The first two planets have and classify them as regular or chaotic The backbone of this region is the sta- masses ∼4.8 MJ and the third one is quite by computing a chaoticity index e.g. the ble 2:1 resonant family of periodic or- smaller with mass 0.29 MJ. So, we can ne- DFLI in our case [12]. In the maps pre- bits for the particular planetary mass- glect the interaction of the third plan- sented below, light (yellow) colors in- es (the gray characteristic curve). If we et to the other two heavy planets and study the evolution in the framework of the three body model with inner plan-

et (P1) the planet c and outer planet (P2) HD 82943 b,c

the planet b. The orbital parameters are g e y t

d =0+ (Ky)

0.4 =1.5+ (Ky)

c i t t i P1 0 the following r t =

ϖ 0.2 Δ cce n

2

e P , P2 ο o ϖ1=ϖ2=0 A P2 m (MJ) a (AU) e ϖ ( ) 0 n

o 90 P1 i t

a 45 P1 r ϖ P1 4.78 0.75 0.425 133 u 0 Δ g i

f -45 n

o -90

P2 4.80 1.20 0.203 107 C 0 0.4 0.8 1.2 1.6 time (Ky) g

e 1 No estimation is given for the posi- d y

t 26 t =1.5+ (Ky) 6 0.8 − =0+ (Ky) c i = t

i P1 2 ϖ r - tion of the planets in their elliptic or- t 0.6 P1 Δ n =

ϖ 0.4 cc e Δ P2 0.2 e

bits. The two planets are coplanar with , 0

B P1

o 180 I =19 . Also the system is resonant with n

0 o i

t 90 a r T /T ≈2.0. ϖ

u 0 2 1 Δ g i

f -90 For the planetary ratio m2/m1≈1 and n o -180 C 0 0.4 0.8 1.2 1.6 P2 for the region of the given eccentricity time (Ky) P2 values, we find that there exists a fami- ly of periodic orbits, which is symmetric Figure 6: The evolution of planets c (P1) and b (P2) of the system HD 82943 for the two differ- and corresponds to a planetary configu- o ent initial configurations A and B (see the text). In the left panels, the time evolution of the ec- ration with aligned planets, i.e. ∆ϖ = 0 , centricities and the planetary apsidal difference is presented. In the middle, we show the initial and when the inner planet is at perias- configuration of the system and the orbit in its first moments of evolution. In the right, the plan- tron the outer planet could be found etary orbits at about 1.5Ky are shown. The destabilization in the configuration B is obvious. o at apoastron, i.e. σ1 = 0 . Thus, we will consider for our analysis two plane- tary initial configurations: configuration A with ∆ϖ = 0 o and configuration B with ∆ϖ = –26o (the value given in the list).

We consider that P1 is initially located at 4 periastron and P2 at apoastron . In Figure 6, we present the evolution of the eccentricities and the apsidal dif- ference ∆ϖ for configurations A and B. When the planets are aligned (configura- tion A) the system evolves regularly, ∆ϖ librates indicating the resonant evolution and the eccentricities show small (anti- phase) oscillations. However, if we con- sider the configuration B, we see that ∆ϖ rotates. Although, the evolution ini- tially seems regular, after 1.2 Ky the sys- tem is destabilized. Small planetary en- Figure 7: Dynamical maps of stability for the system HD 82943. a) map in the plane of initial counters occur and the outer planet in- eccentricities for the configuration A, b) map in the plane of initial eccentricities for the con- figuration B, c) map in the plane of initial resonant angles (σ1, ∆ϖ) at (e1,e2)=(0.425, 0.16). Yel- low regions indicate chaos while dark colors correspond to regular orbits. The star shows the 4 This is the better configuration with re- starting point of the system. spect to stability

HIPPARCHOS | Volume 2, Issue 11 16 project the planetary evolution in the In panel (c) of figure 7, we consider When two planets have large enough plane of eccentricities, we obtain os- the eccentricity values (0.42, 0.16), men- eccentricities, then planetary close en- cillations centered at the eccentrici- tioned above, and construct a dynami- counters (and subsequently destabiliza- ty values of a periodic orbit at about cal map for a grid of initial conditions tion) are, generally, unavoidable unless

(e1,e2) = (0.42, 0.16). When the plan- with all possible planetary alignments the system is in resonance. Resonanc- ets are not initially set to be aligned ∆ϖ and initial angle positions (present- es can offer a phase protection, i.e. al- o (∆ϖ ≠ 0 ), the regions of regular orbits ed by the resonant angle σ1). The central though orbits are close (or intersect), in the dynamical stability maps shrink regular region is associated with the pe- the planets cannot be found close to and show a shift from the characteristic riodic orbit, located in this map at (0,0). each other. It has been shown that sta- curve of periodic orbits. Subsequently, Again, we see that the configuration B ble resonant periodic orbits can exist for ∆ϖ = –26o the system seems to be locates the system in the chaotic region. for very large eccentricities and there- located in the chaotic region (but close We may conclude from the above anal- fore, regions of stability can be located. to the regular one). Thus, the system ysis that the orbital parameters given in However, as eccentricities increase, the destabilizes and shows diffusion in the the EPE list should be revised. Namely, stability domain around the central peri- wide chaotic sea of the phase space, stability is guaranteed for |∆ϖ|<26o. Al- odic orbit shrinks. Thus, a real planetary where the evolution is strongly irreg- so, if we assume slightly smaller plane- system with large eccentricities may be ular. In these dynamical regions, the tary masses, then the regular region is found only very close to a stable res- system suffers from close encounters, expanded and the system can be locat- onant periodic orbit. This, for example, which possibly lead one of the planets ed in the regular region. Possible plane- should be the case for the system HD to a collision with a star or to escape tary mutual inclination may also be a sta- 7449 presented in figure 4. or to a planet-planet collision. bilization factor.

References

[1] F. Roques and J. Schneider, The Extra- [6] Beauge C., S. Ferraz-Mello, T. Michtchen- etary dynamics - periodic orbits and solar Planets Encyclopedia, http://ex- ko, in Extrasolar Planets, edited by R. dynamical stability. MNRAS, 395, 2147, oplanet.eu Dvorak, Wiley-Vch, 2008 2009. [2] Cassen P., Guillot T. and Quirrenbach A., [7] Murray C.D. and Dermott S.F., Solar [11] Hadjidemetriou J., Symmetric and Extrasolar Planets, Springer, 2006. system dynamics, Cambridge Univer- asymmetric librations in extrasolar [3] Planetary Habitability Laboratory, Uni- sity Press, 1999 planetary systems: a global view. CM- versity of Puerto Rico, http://phl.upr. [8] T. Michtchenko, S. Ferraz-Mello, Beauge DA, 95, 225, 2006. edu/hec. C., in Extrasolar Planets, edited by R. [12] Voyatzis, G.: Chaos, order, and period- [4] Sun Y.S, Ferraz-Mello S., Zhou J.L., Exo- Dvorak, Wiley-Vch, 2008 ic orbits in 3:1 resonant planetary dy- planets: Detection, Formation and Dy- [9] Antoniadou, K.I., Voyatzis, G, 2/1 reso- namics. Astrophys. J. 675, 802, 2008. namics, Cambridge University Press, nant periodic orbits in three dimen- 2008 sional planetary systems, CMDA, 115, [5] Atkinson N, Most Exoplanetary Solar 161, 2013. Systems Have Inclined Orbits, http:// [10] G. Voyatzis, T. Kotoulas, and J. D. Hadji- www.universetoday.com/82601/ demetriou, On the 2/1 resonant plan-

HIPPARCHOS | Volume 2, Issue 11 17 Superluminous Supernovae by Dr. Emmanouil Chatzopoulos Enrico Fermi Postdoctoral Fellow Department of Astronomy & Astrophysics, FLASH Center for Computational Science, University of Chicago REVIEWS

The discovery of a new class ceptional , the unprecedented of Supernovae light curve shape and the unique spectra of this event did not resemble any of the Supernovae are some of the most spec- characteristics of SN explosions discov- tacular phenomena in the Universe, ered to date. marking the explosive deaths of mas- The following year, TSS discovered

sive stars (M* ≥ 8 M¤; Type II and Type yet another SLSN with a very broad Ib/c SNe and subtypes), or the thermo- light curve shape, SN 2006 gy [5,6,7]. SN nuclear explosions of white dwarfs that 2006 gy has been one of the most well exceed the Chandrasekhar mass (~1.4 observed SLSN with photometric ob- M¤; Type Ia SNe). The exceptional lumi- servations spanning from the ultraviolet nosity produced by Type Ia events, typi- (UV) to the infrared (IR) and high qual- cally reaching peak absolute visual mag- ity contemporaneous spectra for hun-

nitudes of MV~-19, allow us to detect dreds of days after the explosion, un- them at great, cosmological distances til the transient became dimmer than yielding information about the very na- detection limits. SN 2006gy possessed ture of the Universe. Typical Type II SN Figure 1: Discovery image of SN 2008am optical spectra dominated by narrow events are not nearly as luminous as showing its position in its host galaxy emission lines of hydrogen reminiscent Type Ia events but are important in that [44]. to those of Type IIn SN explosions (“n” they provide information about the evo- for “narrow” indicating the presence of lution and mass-loss history of massive narrow lines). The origin of these lines stars as well as heavy nucleosynthesis looking for new transients by compar- is believed to be a shock propagating in and the chemical enrichment of the in- ing images taken in previous scans (dy- the circumstellar medium (CSM) around terstellar medium. namically comparative photometry). The the explosion, produced by the interac- Some of the main early most productive of these telescopes, tion of the SN ejecta with that medi- search projects, such as the Supernova ROTSE-IIIb that is installed at the Mc- um. The LC of SN 2006gy took about Cosmology Project [1] and the High-Z Donald Observatory in West Texas led 70 days to rise to peak luminosity at the Supernova Search Team [2] were focus- to the prompt discovery of a large num- rest frame of the event indicating a large ing on the discovery of Type Ia events ber of new transients. Targets of inter- diffusion mass associated with it. The due to their importance as cosmological ests discovered by ROTSE are then combination of high peak luminosity and standard candles. The basic character- passed to the observing queue of the slow LC evolution led to the theory that istic of these surveys is that they were Hobby-Eberly Telescope where spec- SN 2006gy was powered by either the targeted searches; the telescopes used tra are obtained by the Low Resolution result of the explosion of an extreme-

had relatively small fields of view and fo- Spectrograph (HET-LRS) instrument al- ly massive star (M* ≥ 100 M¤) or the in- cused on large bright spiral galaxies in lowing for the final classification of the teraction of massive SN ejecta with an the and Coma clusters. Therefore transients. also massive CSM shell, thought to have these searches were biased in the sense In 2005 the TSS project discovered been ejected by the progenitor star in that they ignored potentially interesting the first of what was going to be a new, the years prior to explosion [6]. The TSS transient events occurring in other, low- unique and rare class of supernovae ex- has discovered a dozen more SLSN up er luminosity environments. plosions, the Superluminous Superno- to date (Figure 1) including some unclas- The first untargeted, unbiased super- vae (SLSN) events. At the redshift of z = sified transient phenomena. search, the 0.2832, SN 2005ap [4] reached peak un- The TSS project was later comple- Project (TSS; later RSVP: ROTSE Super- filtered in excess of mented by more efficient and high re- nova Verification Project, [3]) uses four -22 within 1-3 weeks after explosion fol- sponse transient search projects such as fully automated, unfiltered, large field lowed by a rapid decline. Consequent- the Palomar Transient Factory [8] and of view telescopes, the ROTSE-III tele- ly, the corresponding peak luminosity for Pan-STARRS that, altogether account scopes (ROTSE: Robotic Optical Tran- this event was greater than 1044 erg/s, to the discovery of more than 30 SLSN sient Search Experiment) that scan large or comparable to the total luminosity events up to date [9]. This sample of SL- portions of the sky in a nightly basis of the whole Milky Way galaxy! The ex- SN has a revealed a great degree of di-

HIPPARCHOS | Volume 2, Issue 11 18 versity as the main striking characteris- tic of this class of events: a variety of LC shapes, peak , decline rates as well as spectroscopic characteristics that raise intriguing questions about the nature of these events, the implications for massive star evolution as well as their potential role in the chemical en- richment of the Universe.

The observed properties of SLSNe

The LCs of SLSNe exhibit differences in terms of peak luminosity, rise time to maximum light, general LC shape and late time decline rate with some being consistent with the radioactive decay rates of 56Ni and 56Co, in a similar man- ner with Type Ia and some Type II events, while many are not. Therefore, classifi- cation of SLSN based on the LC alone Figure 2: LCs of different types of SNe as compared to SLSNe [9]. is incomplete and it does not capture the physics since in some cases the LC shows a “linear” decline in logarithmic (in luminosity) plots similar to that of Type IIL SNe (SN 2006gy) and in other cases the LC are characteristic of Type II and Ib/c events. Figure 2 shows a collection of SLSN LCs. The observed differences in peak luminosity as well as total energy radi- ated (1051–1053 erg) hint that the pow- er source is not standard but comes in varying degrees of intensity. The differ- ences in the timescales to rise to maxi- mum luminosity on the other hand are indicative of different diffusion masses associated with the SLSN ejecta. More specifically, the SN ejecta (or diffusion) mass can be estimated by the following formula [10, 11, 12]:

3β c M = v t2 ej 10κ ej d where β = 13.8 is a constant coming from integrating characteristic SN ejec- Figure 3: Spectra of SLSN-II [9]. ta density profiles, c is the speed of light, κ is the optical opacity of the SN ejec- ta (0.38 for hydrogen rich, 0.2 for helium rich and 0.1 for H/He poor material), vej in some cases (SN 2005ap) while they generally classified in two categories: hy- is the typical ejecta velocity and can be can exceed 100 days in other cases (CSS drogen-rich spectra that show emission measured by spectroscopic features and 100217; [13]). Such a variety of SN ejec- Balmer lines of H and hydrogen-poor td is the diffusion time and is character- ta mass could translate to either a vari- spectra that lack H and, in some cas- istic of the rise time to maximum light. ety of progenitor masses or even a va- es, He as well. Based on this observable

The dependence of Mej to the sec- riety of circumstellar masses around the alone the SLSNe are categorized in SL- ond power of td is the reason why there progenitor star as we will discuss in the SN-II and SLSN-I in accordance with the is a variety of diffusion masses charac- next section. categorization of normal luminosity SNe teristic to SLSN since their rise times The spectra of SLSNe also show dif- [9]. Some SLSN-I like SN 2007bi [14] to maximum can be as low as <10 days ferences amongst them but they can be show Fe, Co and Ni lines at the very late

HIPPARCHOS | Volume 2, Issue 11 19 during the late stages of their evolution that are characterized by high tempera- 9 tures (Tcore > 10 K) but relatively low 4 5 3 densities (ρc > 10 – 10 g/cm ). At this thermodynamical regime rapid produc- tion of electron-positron pairs is favored in the core. Under such conditions, the rate of the electron-positron pair pro- duction can even exceed the rate of gamma-ray generation by nuclear fu- sion and a large fraction of gamma-rays can be absorbed by the newly generated pairs before those annihilate. This pro- cess softens the equation of state and the adiabatic index falls below 4/3 such that radiation pressure support is re- moved and the C/O core begins to col- lapse under the influence of gravity. The collapse leads to higher core tempera- tures and densities and due to the high (steep power law) sensitivity of pair- production and nuclear reaction rates to the temperature the overall process becomes a runaway effect leading to the Figure 4: Spectra of SLSN-I [9]. continuous contraction of the core up to the point where the available carbon times that might be indicative to the ra- ed that SLSN-I are the most rare SLSN and oxygen fuel is ignited and explosive- dioactive decays of 56Ni and 56Co and events with a rate of ~32 events / Gpc3 ly burns releasing > 1051 ergs of ener- are referred to as SLSN-R where “R” /year while SLSN-II have a rate of ~151 gy and leading to the formation of sev- stands for “radioactive”. This subclass is events / Gpc3 /year assuming a Hub- eral solar masses of 56Ni in some cas- of particular interest as it has raised the ble constant of 71 km/s/Mpc [17]. This es. The decay of this very large amount question of whether those events are amounts to approximately just 1 SLSN of radioactive 56Ni, that can be 10-100 related to the long theorized Pair Insta- event for every 400 to 1300 core-col- times more than that produced in typi- bility Supernova explosions (PISNe) that lapse SNe (Type II or Ib/c). cal CCSNe and Type Ia SNe, can provide represent the deaths of extremely mas- the input heating to power the PISN LC sive stars (>120 M¤) producing sever- How are SLSNe powered? to superluminous levels. al solar masses of 56Ni powering their From the available set of SLSNe LCs to superluminous levels. We return The unique diversity of the observed only a very limited number of events to this topic in the next section. Figures properties of SLSNe has triggered in- have been discussed as PISNe candi- 3 and 4 show some examples of SLSN-II tense theoretical work with the purpose dates based on the properties of their and SLSN-I spectra accordingly. The lack to unveil what powers these events. The LCs and late time spectra. Some nota- of H-features in SLSN-I events can be in- debate today rests on the two main hy- ble examples are SN 2007bi [14] and dicative of the lack of H overall in their potheses that either the SLSN diversity SN 2010hy [24, 25]. The most widely ejecta meaning that the progenitors of is due to a unique power input mecha- debated PISN candidate is SN 2007bi. these events lost their H/He envelopes nism that involves many parameters or Some authors [14,26] have present- maybe in a similar manner than the pro- to a different power input mechanisms. ed radiation hydrodynamics model- genitors of SN Type Ib/c did. Up to date, three main mechanisms have ing of the LC of SN 2007bi and con- SLSNe, especially SLSN-I are also been proposed as the power inputs of cluded that a PISN resulting from a He- unique in that they are usually discov- SLSNe: Pair-Instability Supernovae, mas- rich core of about ~110 M¤ can ade- ered in low luminosity, sub-solar metal- sive SN ejecta – circumstellar matter quately reproduce the observed pseu- licity dwarf host galaxies [15]. In some (CSM) interaction and spin- do-bolometric LC. A caveat to this re- cases for SLSN-I SNe the hosts are not down. sult is the lack of a large number of da- even detected [16]. Figure 5 shows the Pair-Instability Supernova explo- ta points during the rising part of the metallicity of some known SLSN hosts. sions have been theorized since the late PISN LC that does not allow for an ac- Finally, the most apparent character- 1960s [18, 19, 20, 21, 22, 23] and result curate estimate of the explosion date istic of SLSNe is that they are a partic- from the collapse of the carbon/oxygen and, therefore, the real rise-time of the ularly rare phenomenon, with an aver- cores of very massive stars (MZAMS > LC that is related to the ejecta mass age discovery of one SLSN a year during 120 M¤) that encounter the electron- deduced. Another concern is raised by the first years of TSS that increased to positron pair instability. More specifi- recent results from non-local thermo- a few with the introduction of PTF and cally, very massive stars naturally devel- dynamical equilibrium (n-LTE) spectro- PanSTARRS. More formally, it is estimat- op massive C/O cores (MCO > 60 M¤) scopic modeling of PISNe [27] that find

HIPPARCHOS | Volume 2, Issue 11 20 shock that propagates into the CSM and a reverse shock that propagates back into the SN ejecta (see Figure 6). This double shock structure will then evolve inwards in mass coordinates depositing shock kinetic energy into the SN ejec- ta that can be efficiently converted in- to luminosity powering the LC of a SL- SN. The efficiency as well as magnitude of the luminosity that will be deposited by the shocks in the interaction region depends strongly on the original combi- nation of the SN ejecta mass, the CSM mass, their density profiles as well as the SN energy and the radial separation be- tween the CSM shell and the progenitor star. Efficiency is generally maximized for comparable SN ejecta and CSM masses [37] and the LCs SLSN events such as SN 2006gy, SN 2008am [44], SN 2010gx [45] and many others have been suc- cessfully modeled with this mechanism [37, 38, 39]. The CSM interaction is actually ob- Figure 5: Metallicities of some known SLSN hosts [15]. served at least in the case of SLSN-II events in a matter similar to that for that the spectral properties and color that of violent, massive SN ejecta – CSM normal luminosity Type IIn SNe. These evolution of PISNe do not have any re- interaction [32, 33, 34, 35, 36, 37, 38, 39]. events often possess spectra that are semblance with that observed for SN It is known that massive stars lose mass dominated by Balmer intermediate 2007bi. More specifically, PISN are ex- throughout the evolution either via width (~1000-5000 km/s) emission lines pected to be intrinsically red events steady state winds or via episodic mass- of Hydrogen. These are Case B recom- while the color evolution of SN 2007bi, loss events. The exact properties of bination lines that are produced by the especially at early times is particularly mass-loss in massive stars during their CSM shock ionizing the material which blue in color. Yet another strong coun- very late stages of evolution leading to then quickly cools and recombines. It ter-argument against the PISN hypoth- supernova are not well known but a va- is unclear whether the measured width esis for the case of SN 2007bi is the riety of mechanisms have been proposed of these emission lines is characteris- measured high metallicity of the host including mass-loss due to gravity waves tic of bulk kinematic motion, however, galaxy (Z > 0.4 Z¤). At such high me- originating from vigorous convective since single and multiple electron scat- tallicity a massive star would experi- shell burning [40,41] or Luminous Blue tering effects can broaden these line ence extreme radiative driven mass- Variable (LBV) – type mass loss from profiles. In a similar manner one would loss during the main-sequence that stars with luminosities close to their Ed- expect intermediate with CSM lines in would drive the final mass of the star dington limit such as η – Carina [6] or the spectra of H-poor events charac- below the limits required for PISN to even a “milder” form of pair-instability teristic of other elements, such as car- be encountered. A relevant parameter events (Pulsational PISNe) occurring at bon and oxygen. Such lines might have study has shown that the upper metal- a narrow CO core mass range (40 M¤ been observed in the first and second licity limit for PISNe to be encountered < MCO < 60 M¤) that does not lead to spectrum of SN 2007bi but also in oth- based on models of star formation and the total disruption of the star but rath- er cases. The possibility that SN 2007bi the known initial mass function (IMF) er to multiple ejections of the outer, less and other SLSN-I events are powered is Z ~ 0.3 Z¤ and even in that case the gravitationally bound shells [31,34]. Such by H-poor CSM interaction or by a progenitor would have to have a ZAMS violent mass-loss eruptions have been combination of H-poor interaction and 56 mass in excess of 300 M¤ to become observed around massive evolved stars radioactive decay of Ni is therefore a PISN [28]. It’s worth noting that the (with η – Carina being the best exam- worth considering [31]. LC models of most massive stars ever detected have ple) or even recorded in the weeks pri- SN ejecta – CSM interaction seem to masses of that order [29]. or to the actual SN explosion as in the well reproduce the majority, if not all, As we will discuss below, other al- case of SN 2009ip [42,43]. observed SLSN LCs albeit the natural- ternatives have been proposed to ex- The pre-SN mass loss naturally sets ly large number of parameters associat- plain the nature of SN 2007bi including the initial conditions of the environment ed with this phenomenon [39]. Includ- magnetar spin-down power [30] and H- within which the SN will occur. The SN ing the uncertainty of CSM composi- poor CSM interaction [31]. ejecta will collide with the surrounding tion (H-rich versus H-poor) and con- The second power input mechanism CSM formed by mass-loss leading to the sidering the complexity of CSM envi- proposed for the majority of SLSNe is formation of a strong radiative forward ronments around some massive stars

HIPPARCHOS | Volume 2, Issue 11 21 like η – Carina models of CSM interac- tion can naturally explain the diversity of SLSN characteristics. Finally, the third proposed SLSN LC power input mechanism proposed is that of energy released by the spin- down of a newborn magnetar follow- ing a core-collapse SN explosion [46, 47]. Newly born highly magnetized (B ~ 1014 G) millisecond rap- idly spin-down converting their magne- Figure 6: Illustration of SN ejecta - CSM in- to-rotational energy into radiation that teraction [33]. can then heat the expanding SN ejecta. Although some n-LTE models of mag- netar spin-down seem to accurately re- produce the spectra of SN 2007bi and Future prospects the luminosities of many SLSNe (such The discovery of SLSN challenges our (Population III) stars are expected to as SN 2008es) concerns have been knowledge about massive stellar death be significantly more massive than con- raised about whether the non-thermal and pre-SN mass-loss and opens new temporary stars therefore dying as en- magnetar radiation sufficiently ther- ways to probe energy generation by ergetic SN explosions [49, 50, 51]. The malizes in the ejecta [48]. The magne- supernovae. The modern high cadence extreme luminosities and long durations tar spin-down model is still under de- rapid response transient search projects of these events can allow them to be bate but it has gained growing support led by two of the worldís leading astro- observed by the next generation space amongst the community over the last physics research institutions, the PTF telescopes such as JWST and WFIRST at year. Rigorous and self-consistent radi- at Caltech and PanSTARRS at Harvard redshift above 20-30 [52, 53, 54, 55, 56, ation hydrodynamics and n-LTE spec- University have increased the number of 57, 58]. Studying the properties of these tral evolution modeling is required to the discoveries of these events and fur- Population III explosions can thus help provide more insight to the question of ther confirmed their striking diversity. us investigate the chemical enrichment what mechanism best describes the ob- In addition to all that, SLSNe in the of the primordial interstellar medium, servables and this is an ongoing endeav- form of PISNe or CSM interaction pow- which sets the stage for the future gen- or and a subject of debate amongst su- ered SNe can also be an important erations of stars to be born. pernova astrophysicists. probe of the early Universe. The first

References

[1] Perlmutter (2003), Physics Today, 56, [22] Ober et al. (1983) A&A, 119, 61 [39] Chatzopoulos et al. (2013) ApJ, 773, 040000 [23] Heger & Woosley (2002) ApJ, 567, 76 [2] Schmidt et al. (1998) Apj, 507, 46 532 [40] Quataert & Shiode (2012) MNRAS, [3] Quimby et al. (2005) Bulletin of the [24] Vinko et al. (2010) CBET, 2476, 1 423, L92 American Astronomical Society, 37, [25] Kodros et al. (2010) CBET, 2461, 1 [41] Shiode & Quataert (2014) ApJ, 780, 171.02 [26] Kasen et al. (2011) ApJ, 734, 102 96 [4] Quimby et al. (2007) ApjL, 668, L99 [27] Dessart et al. (2013) MNRAS, 428, [42] Smith et al. (2014) MNRAS, 438, 1191 [5] Ofek et al. (2007) ApjL, 659, L13 3227 [43] Margutti et al. (2014) ApJ, 780, 21 [6] Smith et al. (2008) ApJ, 686, 485 [28] Langer et al. (2007) A&A 475, L19 [44] Chatzopoulos et al. (2011) ApJ, 729, [7] Smith et al. (2010) Apj, 709, 856 [29] Crowther et al. (2010) MNRAS, 408, 143 [8] Law et al. (2009) PASP, 121, 1395 731 [45] Chen et al. (2013) ApJL, 763, L28 [9] Gal-Yam (2012) Science, 337, 927 [30] Dessart et al. (2012) MNRAS, 426, [46] Woosley (2010) ApJL, 719, L204 [10] Arnett (1979) ApjL, 230, L37 L76 [47] Kasen & Bildsten (2010) ApJ, 717, 245 [11] Arnett (1980) ApJ, 237, 541 [31] Chatzopoulos & Wheeler (2012) ApJ, [48] Bucciantini et al. (2005) A&A, 443, 760, 154 [12] Arnett (1982) ApJ, 253, 785 519 [32] Chevalier (1982) ApJ, 258, 790 [13] Drake et al. (2011) ApJ, 735, 106 [49] Greif et al. (2011) ApJ, 737, 75 [33] Chevalier & Fransson (1994) ApJ 420, [50] Stacy et al. (2010) MNRAS, 403, 45 [14] Gal-Yam et al. (2009) Nature, 462, 268 624 [51] Stacy et al. (2013) MNRAS, 431, 1470 [34] Woosley et al. (2007) Nature, 450, [15] Stoll et al. (2011) Apj, 730, 34 390 [52] Scannapieco et al. (2005) ApJ, 633, 1031 [16] McCrum et al. (2014) arXiv: 1402.1631 [35] Smith & McCray(2007) ApJL, 671, L17 [53] Pan et al. (2012) MNRAS 422, 2701 [17] Quimby et al. (2013) MNRAS, 431, [36] Chevalier & Irwin (2011) ApJL, 729, 912 L6 [54] Hummel et al. (2012) ApJ, 755, 72 [18] Barkat et al. (1967) PRL, 18, 379 [37] Ginzburg & Balberg (2012) ApJ, 757, [55] Whalen et al. (2013a) ApJ, 777, 110 [19] Rakavy & Shaviv (1967) ApJ, 148, 803 178 [56] Whalen et al. (2013b) ApJ, 768, 195 [20] Rakavy et al. (1967) ApJ, 150, 131 [38] Moriya et al. (2013) MNRAS, 428, [57] Whalen et al. (2013c) ApJL, 762, L6 [21] Rakavy & Shaviv (1968) Ap&SS, 1, 429 1020 [58] Whalen et al. (2014d) ApJ, 781, 106

HIPPARCHOS | Volume 2, Issue 11 22 Research Projects in Astronomy in Greece Funded by the GSRT “Excellence I and II” (Aristeia) Action RESEARCH PROJECTS

n two successive calls, with deadlines with the financial means to reach a state These are: A. Bonanos (IAASARS, Na- I expiring September 2011 and June of so-called “autonomy”. This is defined tional Observatory of Athens), I. Co- 2012 respectively, the Greek General by the GSRT as the possibility to develop ntopoulos (RCAAM/Academy of Ath- Secretariat for Research and Technolo- a principal investigator’s own research ens), V. Pavlidou (Department of Phys- gy (GRST) invited researchers in univer- group, in order to pursue and further ics, University of Crete), M. Xilouris sities and research institutes to submit expand the PI’s scientific vision by carry- (IAASARS, National Observatory of proposals in the framework of the pro- ing research hosted in a Greek university Athens), for Aristeia I, and P. Boumis gram “Excellence” (Aristeia). Aristeia is or research institute. The Hellenic As- (IAASARS, National Observatory of Ath- an action taking place within the frame- tronomical Society has gladly observed ens), I. Papadakis (Department of Phys- work of the Ministry of Education Oper- the vivid participation of the Greek as- ics, University of Crete), and G. Tsiro- ational Program for Education and Life- tronomical community in the Aristeia poula (IAASARS, National Observatory long Learning. It is a highly competitive calls. We would like to congratulate all of Athens) for Aristeia II. program aiming to provide researchers, those who took part in the process, and The following is a brief account of who have demonstrably met standards in particular our seven members who the astronomy research projects funded of excellence in their personal research, have won an Aristeia grant. under Aristeia.

Hellenic Republic Ministry of Education and Religious Affairs

HIPPARCHOS | Volume 2, Issue 11 23 Revealed by their own Dust: Identifying the Missing Links in Massive Star Evolution - MissingLinks Principal Investigator: A. Bonanos Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing, National Observatory of Athens RESEARCH PROJECTS

e have undertaken a systemat- What is new about our approach is from LBVs mostly episodic, or can it be W ic study of rare, luminous dusty that we propose a systematic study of also in the form of a continuous wind? massive stars in nearby (D<10 Mpc) these very interesting objects, “missing What is the frequency of major mass galaxies, which correspond to progen- links in massive star evolution”. We take ejection events, similar to eta Car? Does itors of future core-collapse superno- advantage of the abundant mid-IR imag- every massive star have one such event vae and possibly transients. Luminous ing data of nearby galaxies existing in the in its lifetime, one in a hundred stars, blue variables (LBVs), supergiant B[e] Spitzer archive to identify the luminous or are such events even less common? Aristeia I (sgB[e]), some Wolf-Rayet stars and red mid-IR population of stars in the star- Does that frequency depend on the me- supergiants (RSGs) belong to this class forming spiral and dwarf irregular gal- tallicity? Do massive stars have a major of dusty objects, their rarity implying a axies of the Local Group and in more mass ejection episode before they ex- very short but perhaps critical stage in distant galaxies, such as M83. Follow-up plode as supernovae? What is the evolu- the evolution of massive stars. The role spectroscopy has so far revealed sever- tionary status of sgB[e] stars? The pro- of mass loss from massive stars, espe- al candidate luminous blue variables (LB- posed research is an exploratory study cially episodic mass loss in evolved mas- Vs), red supergiants (RSGs) and yellow of a new regime, and the investigators sive stars, is one of the outstanding open supergiants (e.g. Britavskiy et al. 2014, fully "expect to find the unexpected" questions facing theory, A&A, 562, 75). and in fact they already discuss some of and the proposed research has the po- The proposed research program will their "unexpected" findings for luminous tential to dramatically transform this field address a number of important scientif- dusty stars. by providing crucial data on the proper- ic questions, such as: Can the presence ties and statistics of systems most ob- of significant mid-IR excess be used as a scured by their own mass loss. hallmark of being a LBV? Is the mass loss

A.Z. Bonanos, N. Britavskiy, S.J. Williams, M. Kourniotis, P. Boumis (National Observatory of Athens, Greece), A. Mehner (ESO, Chile), J.L. Prieto (Princeton, USA) (including postdocs, students & collaborators).

Figure: Left: Spitzer/IRAC image of M83, located at 4.5 Mpc (3.6 μm in blue, 4.5 μm in green and 8 μm in red; NASA/JPL-Caltech). We have ana- lyzed these archival mid-infrared data along with near-infrared follow-up images to select dusty obscured candidates (Williams et al., in prep.). Right: [3.6]-[4.5] color magnitude diagram for WLM, one of the dwarf irregular galaxies under investigation. All targets we have followed up with spec- troscopy are labeled. We have discovered 5 red supergiants (RSGs), 3 emission line objects (candidate LBVs and supergiant B[e] stars) and a quasar at z=0.62 (Britavskiy et al., in prep.).

HIPPARCHOS | Volume 2, Issue 11 24 The origin of astrophysical magnetic fields - CosmicBattery Principal Investigator: I. Contopoulos Research Center for Astronomy and Applied Mathematics, Academy of Athens RESEARCH PROJECTS agnetic fields are an important el- M ement of the Universe, their ori- gin, however, remains one of the most fundamental unsolved questions of mod- ern astrophysics. A few years back, we proposed the mechanism of the Cosmic Battery for the generation of astrophysi-

cal magnetic fields in the vicinity of the Aristeia I accretion disk around black holes and active galactic nuclei. Recent observations of galactic X-ray binaries and extragalactic radio sources attest to the important role of this mechanism in astrophysics. Our Proposal aims to give a final answer to the question on whether the Cosmic Battery can indeed account for the origin of magnetic fields in the Universe. The astrophysical problem that we are investigating combines General Relativity, non-ideal Magneto-Hydrody- namics, and Radiation Transfer. Our core research team consists of the Principal Figure: The accretion disk of hot plasma swirling around a supermassive black hole generates Investigator, a main member, one post- powerful magnetic fields. The disk’s rotation twists the field into a funnel shape. These field lines doctoral researcher, and two graduate constrict and direct the outflow of high-speed plasma from the black hole’s vicinity. The result is a narrow, tapered, extragalactic jet. Credit: NASA, ESA, and A. Feild (STScI) students. Our effort encountered great bureaucratic hurdles which resulted in the non-implementation of the numeri- through the electromagnetic spin- Our results to date have been presented cal simulation part of the Proposal. For- down of the maximally rotating black in the form of scientific papers in the As- tunately, its theoretical part is proceed- hole that forms during the collapse of trophysical Journal, a chapter in a special ing normally, with very important results its supermassive stellar progenitor, volume (Springer), seminars at research up to now: iii) We proposed a GRB sub-category Institutes in Greece (Athens, Thessalo- i) We obtained the general structure of (tentatively named «100sec GRB») niki, Crete) and abroad (Cornell, Princ- Kerr black hole magnetospheres and which may function as «Standard eton, Purdue, Southampton), presenta- emphasized the physical significance Candles» in Cosmology, tions at international conferences (Ma- of the inner and outer light cylinders drid, Granada, Les Houches, Dallas, Pots- iv) We calculated numerically the ra- in the electromagnetic extraction of dam, Athens, etc.), and a special Work- diation field of the accretion disk the black hole rotational energy, shop (Academy of Athens, March 2013). around a rotating Kerr black hole Completion of the Program is expected ii) We proposed a new mechanism for and the resulting magnetic field gen- by the end of September 2015. the generation of high energy radi- erated through the mechanism of ation in Gamma Ray Bursts (GRB) the Cosmic Battery.

HIPPARCHOS | Volume 2, Issue 11 25 Unveiling the Physics of Supermassive Black Holes and Relativistic Jets with Optical Polarization Observations of Blazars - RoboPol Principal Investigator: V. Pavlidou Department of Physics, University of Crete RESEARCH PROJECTS

lazars are the most active galaxies B known. They are powered by rela- tivistic jets of matter speeding towards us almost head-on at the speed of light, radiating exclusively through extreme, non-thermal particle interactions, ener- gized by accretion onto supermassive Aristeia I black holes. Despite intensive observa- tional and theoretical efforts over the last four decades, the details of blazar astrophysics remain elusive. The launch of NASA’s Fermi Gamma-ray Space Tele- scope in 2008 has provided an unprec- edented opportunity for the systemat- ic study of blazar jets and has prompt- ed large-scale blazar monitoring efforts across wavelengths. In such a multi-wave- length campaign, a novel effect was dis- covered: fast changes in the optical po- larization during gamma-ray flares. Such events probe the magnetic field struc- ture in the jet and the evolution of dis- turbances responsible for blazar flares. Figure: RoboPol Comissioning at the Skinakas 1.3m telescope. Their systematic study can answer long- standing questions in our theoretical un- derstanding of jets; however, until recent- the 1.3m telescope of the University of ma-ray—loud blazars differ significantly ly, optical polarimetry programs were Crete’s Skinakas Observatory, and it was from those of gamma-ray—quiet blazars not adequate to find and follow similar successfully commissioned in May 2013, (gamma-ray—loud blazars are more po- events with the efficiency and time-res- while its first, very successful observing larized); and it detected 13 new polar- olution needed. season, was concluded in November of ization angle rotation events (compared The RoboPol program, a collabora- the same year. Since April 2014. RoboPol to 17 such events ever observed with tion between the University of Crete, has been observing blazars as part of its all other optopolarimetric efforts com- Caltech, the Max-Planck Institute for second observing season, monitoring bined). Additional science that was per- Radioastronomy, the Inter-University over 100 blazars with a dynamical ob- formed wit the RoboPol instrument in- Centre for Astronomy and Astrophys- serving schedule, using a large amount cluded the detection of polarization ics in India, and the Nicolaus Copernicus of telescope time: 4 nights a week, on in one gamma-ray burst afterglow; the university in Poland, was envisioned as average. complete mapping of the magnetic field a focused, legacy project aimed to settle During the 2013 season, RoboPol of a translucent interstellar cloud (the this question by providing blazar opto- performed the largest single- sur- Polaris Flare) that was recently mapped polarimetric data of unprecedented vol- vey of optopolarimetric properties of by the Herschel Space Observatory; and ume and quality. To this end, the RoboPol gamma-ray—loud blazars; it established the unexpected discovery of high degree instrument was specifically designed for that the polarization properties of gam- of polarization from Be/X-ray binaries.

HIPPARCHOS | Volume 2, Issue 11 26 A Step in the Dark: The Dense Molecular Gas in Galaxies - DeMoGas Principal Investigator: M. Xilouris Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing, National Observatory of Athens RESEARCH PROJECTS

arious types of galaxies are spread X-rays through radio wavelengths have we can only observe the low transitions V out throughout the Universe. The produced a consensus picture of local of this molecule from ground-based fa- basic components of galaxies are stars, LIRGs, showing that they are mergers cilities. We have done so by using the JC- gas and dust. During their evolution these between gas-rich galaxies, where the in- MT telescope in Hawaii and the IRAM- three components are interacting with teraction triggers some combination of 30m telescope in Spain. For the high- each other. There are regions in the gal- dust-enshrouded star-formation and Ac- er transition states we made use of the axies, the so-called molecular clouds, with tive Galactic Nuclei (AGN) activity. Herschel Space Observatory of the Eu- large quantities of dust and gas (mostly Left: Composite image of NGC6240 ropean Space Agency. This effort provid- Aristeia I atomic and molecular hydrogen) providing (HST/ACS B-band in blue, HST-ACS I- ed us with a unique database for explor- ideal conditions for the creation of new band in green and Spitzer/IRAC 8 μm ing the properties of the gas in dense stars. Stars are forming, live their lives and in red. Right: The Spectral Line Energy molecular clouds. It is worth mentioning die. During the last stages of their lives Distribution (SLED) of the CO mole- that studies so far have mostly been lim- they «donate» their materials (gas and cule decomposed in two dense compo- ited to the first two transitions of CO metals) to the interstellar space (via stel- nents (A) and (B) (red and blue dotted (J = 0-1 and 1-2). With our database we lar winds or violent explosion events) and lines) and a lower density component can, for the first time, study transitions as a result, conditions for birth of new (C) (pink) which accounts for the low- up to J = 13-12. This provides us with stars are set. This cycle between star for- J CO line emission (Papadopoulos, et al. the tools to probe deep into the mo- mation and enrichment of the interstel- 2014, ApJ, in press). lecular clouds and determine the condi- lar medium is keeping the galaxies as live In our study we analyzed observa- tions in the densest environments of the entities that evolve with time. tions of this specific group of galaxies interstellar medium in galaxies. There is a special group of galax- obtained through a long-term campaign The «DeMoGas» group is com- ies that emit strongly at infrared wave- during the past ~10 years. These obser- posed of the following members: Man- lengths. These galaxies are called Lu- vations (at far-infrared and sub-millime- olis Xilouris (PI; NOA), Ioanna Leonidaki minous Infrared Galaxies (LIRGs) and ter wavelengths) mainly reflect the pres- (NOA), Padelis Papadopoulos (Cardiff Uni- have infrared luminosities (LIR) higher ence of the molecule of carbon monox- versity), Paul van der Werf (Leiden Obser- than 1011 L⊙ (they are called ULIRGS ide (CO) through its various transition vatory), Thomas Greve (University College –Ultra Luminous Infrared Galaxies– states but also other important mole- London), Zhi-Yu Zhang (Royal Observatory, 12 13 when LIR=10 L⊙). Over the last de- cules such as CO and HCN. Due to Edinburg), Panos Boumis (NOA), Alceste cade comprehensive observations from limitations due to Earth’s atmosphere Bonanos (NOA).

Figure: Left: Composite image of NGC6240 (HST/ACS B-band in blue, HST-ACS I-band in green and Spitzer/IRAC 8 μm in red. Right: The Spectral Line Energy Distribution (SLED) of the CO molecule decomposed in two dense components (A) and (B) (red and blue dotted lines) and a lower density component (C) (pink) which accounts for the low-J CO line emission (Papadopoulos et al. 2014, ApJ, in press).

HIPPARCHOS | Volume 2, Issue 11 27 The Manchester-Athens Wide-Field (narrow-band) Camera: A Deep Sky-Survey of the Extensive Line Emission Regions at High Galactic Latitudes - MAWFC Principal Investigator: P. Boumis Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing, National Observatory of Athens RESEARCH PROJECTS

he sky at high galactic latitudes is tested completely in Greece and will totype (Fig.1 left), it is an enhanced ver- T host to a wide range of extensive conduct a large-area sky survey that will sion of the one used very successfully phenomena that emit faintly in optical provide maps at less than 1 arcmin res- several years ago (MWFC; Johnson et lines over a range of excitations. These olution, in order to investigate the very al. 1978, Boumis et al. 2001, Dickinson features remain largely unexplored for extensive, but faint, line emission regions 2002). Figig.1 (right) illustrates success- the overwhelming majority of the ob- over the whole sky. We will make deep ful use of the prototype device where Aristeia II serving programmes of the World's larg- observations of the northern sky in the the faint but extremely extensive Eri- est telescopes, which have been concen- optical emission lines of Hα, [OIII] and danus nebulosity has been imaged in the trated on achieving high angular reso- Hβ, from astronomical sites. The suc- light of Hα (Boumis et al. 2001). lution over small fields. First of all, the cessful outcome will have a significant foreground, very diffuse, line emission impact on topical astronomical areas of Team Members: from the galactic plane needs accurate research e.g. subtracting the foreground Prof. John Meaburn (Jodrell Bank Centre evaluation down to a resolution of 1 arc- for the cosmic microwave background; for Astrophysics, UK) min to improve the interpretation of the estimating the electron temperature of Dr. Alexandros Chiotellis (postdoc, Cosmic Microwave Background (CMB). the warm ionized gas by comparison IAASARS, NOA) Then there is the 100 degree long non- with radio data; investigating the giant, Dr. Joan Font (postdoc, IAASARS, NOA) thermal radio spur apparantly project- high latitude, radio filaments from the Mr. Vassilis Loizos (IT specialist, ing from the Galactic centre. The ques- Galactic center or very close objects IAASARS, NOA) tion still remains as to whether or not in the Galactic plane of extreme angu- Dr. Clive Dickinson (Jodrell Bank Cen- this is a nearby or lar extent; detecting the northern end tre for Astrophysics, UK) the more dramatic ejection of relativ- of the LMC/SMC HI stream; investigat- Dr. Alceste Bonanos (Associate Re- istic particles from the Galactic nucleus ing the Fermi/WMAP haze/bubbles. searcher, IAASARS, NOA) into the Galactic halo. Remarkably, no Note that this new camera is a 10 Dr. Manolis Xilouris (Senior Researcher, optical identification has yet occurred. In times more sensitive upgrade of a pro- IAASARS, NOA) addition, the northern end of the huge HI Magellanic stream, which is certainly partially ionized, is yet to be explored at optical wavelengths although it is ex- pected to emit Hα and [OIII] lines. The complexity of the nearest HII 30 degree diameter, `bubble' in Eridanus also needs evaluating with far deeper emission line observations to distinguish between its radiatively ionized and more filamenta- ry, collisionally ionized components. It is the extremely large angular sizes of these phenomena that inhibits their ob- servation. The proposed project is to design and construct a state-of-the-art, wide- Figure 1: Left: Photo of the prototype Manchester Wide-Field Camera, being tested in the lab field (~30 degree diameter), narrow- (circa 2001). The original design was large and bulky. band, optical filter camera - The `Man- The Apogee Ap7-p CCD was not entirely adequate (pixels too large, array too small, dark cur- rent too high and filters only 50 mm diam.). chester-Athens Wide Field Camera' Right: Deep, mosaic Hα image of the high Galactic latitude Eridanus shells, made with this early (MAWFC). The standalone camera will version of the camera. Note the large field-of-view of the image (coordinates are in degrees; be the first scientific instrument for as- Boumis et al. 2001). tronomy that will be constructed and

HIPPARCHOS | Volume 2, Issue 11 28 The Quest for Relativistic Signals in the X-Ray Light Curves of Active Galactic Nuclei - AGNQUEST Principal Investigator: I. Papadakis Department of Physics, University of Crete RESEARCH PROJECTS

ctive Galactic Nuclei (AGN) are the results from the first studies with istic effects, like Doppler boost for ex- A the most powerful persistent XMM-Newton were published. In fact, ample. As a result, we expect significant sources in the Universe. It is currently the fraction of AGN which show this periodic or “quasi-periodic oscillations” thought that energy in these objects is line in their spectra is currently rather (QPOs) to appear in the power spec- liberated by the accretion of matter on- unknown. Furthermore, “warm absorb- tral density functions (PSDs) of AGN to a super-massive black hole (BH) at ers” and/or partial-covering of the cen- light curves in the 5-6.3 keV band light their center. The AGN emission is highly tral source from very “thick” absorbers, curves, i.e. energy bands where the re- Aristeia II variable at all wavelengths, with the X- can introduce significant complexities in- flection components constitute a signifi- ray flux showing the fastest, and largest to the iron Κα band, which prevents the cant percentage of the observed flux. amplitude variability in any of the wave- accurate determination of their intrinsic «AGN-QUEST» is a research proj- length ranges. This fact implies that X- width and shape. ect which was recently awarded an «Ex- rays originate from a small region very However, recent detections of X-ray cellence II» («Aristeia II») grant for the close to the central source, and could “negative” time lags in a few AGN sup- GSRT, to study the flux variability of the be as small as ~ 10 gravitational radii, Rg. port the view of X-ray reverberation iron line in a carefully selected sample of If the X-ray source illuminates optically across the inner accretion disc in these ~ 15 AGN with known Black Hole mass. thick cold matter, we expect to observe objects. For example, the radius of the The main objective is to construct light a Compton reflection spectral compo- X-ray reprocessing region in MCG -6- curves in the 5-6.3 keV band, using the nent containing a series of fluorescent 30-15 may be less than ~ 5Rg, i.e. smaller vast amount of data that exist in the ar- lines. The most prominent line is the Κα than the radius of ISCO around a non- chive of current and previous X-ray mis- line emitted by neutral iron at 6.4 keV. If rotating BH. On the other hand, glob- sions (i.e. ASCA, XMM-Newton and Su- Compton reflection originates in the in- al simulations of thin discs where mag- zaku), and to use Fourier techniques to ner accretion disc it will be affected by netorotational instabilities operate, in- search for periodic/quasi-periodic signals the black hole’s strong gravitational field dicate that they should be strongly in- at frequencies which are representative and the line profile will be “relativistical- homogeneous. When combined togeth- of the Keplerian orbital time scale at the ly” skewed and asymmetric, with a red er, these results imply that the line emis- ISCO radius for a Schwarzschild and a wing extending towards low X-ray en- sion in AGN may originate from dense Kerr black hole. Hopefully, the project ergies due to gravitational redshift and accretion disc filaments, which rotate results will help us better understand transverse Doppler redshift. around the central source with relativ- the picture of the innermost gravitation- By the end of the 1990s, it was be- istic velocities. If this is the case, the re- al radii around BHs, which is currently lieved that the relativistically broadened processed emission from these filaments one of the most active research fields in iron line was a common feature in AGN. (i.e. the broad iron line) will be period- High Energy Astrophysics. The situation became less clear after ically modulated due to various relativ-

HIPPARCHOS | Volume 2, Issue 11 29 Solar small-scale events and their role in the heating of the solar atmosphere - SOLAR Principal Investigator: G. Tsiropoula Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing, National Observatory of Athens RESEARCH PROJECTS

olar small-scale structures are of tion with the magnetic field, of their for- several ground-based and space in- S pivotal importance because they in- mation mechanism(s), and of their role struments that observe in a range of termittently connect the in coronal heating. These goals will be temperatures, and chromosphere to the corona achieved by using high spatial and tem- – The determination of their physi- through their magnetic fields. Extensive poral resolution multi-wavelength and cal parameters (3-D morphology, past observations aimed at gaining clues magnetic field observations of the Sun temperature, density, velocity) and on their true nature, and many attempts obtained by space and ground-based in- changes of these parameters during

Aristeia II were made to provide theoretical mod- struments and with advanced 3D MHD their lifetime, els for processes responsible for their numerical simulations, inversion tech- – The comprehension of their interre- generation and sustain. Despite these niques for the line profiles and magnet- lationship, developments, the state of our present ic field extrapolation codes. These stud- – The derivation of a relationship be- understanding of these structures, as ies are crucial to reveal how the Sun’s tween their morphology and dynam- well as their interrelationship and the magnetic field drives the dynamics of its ics and the three-dimensional mag- role they play in the heating and mass atmosphere and how this atmosphere netic field structure, balance of the solar chromosphere and is heated. corona, remains far from satisfactory. – The determination of their driv- The aim of the funded research un- More specifically, the scientific objec- er mechanism(s) through 3D MHD der the ARISTEIA II Action is the der- tives of the funded project are the fol- numerical simulations and forward ivation of accurate physical parame- lowing: modeling, ters of small-scale events, the compre- – The study of the short term dynam- – The study of their role in the energy hension of their dynamical behavior, of ics of solar small-scale events ob- and mass balance of the solar atmo- their interrelationship, of their associa- served in different wavelengths by sphere.

Visit our website http://www.helas.gr The above web server contains information, both in greek and english, about the Hellenic Astronomical Society (Hel.A.S.), the major organization of profes- sional astronomers in Greece. The Society was estab- lished in 1993, it has more than 250 members, and it follows the usual structure of most modern scientif- ic societies. The web pages provide information and pointers to astronomy related material, useful to both professional and amateur astronomers in Greece. It contains a directory of all members of the Society, as well as an archive of all material published by the Society, including electronic newsletters, past issues of “Hipparchos”, and proceedings of Conferences of Hel.A.S. The server is currently hosted by the Univer- sity of Thessaloniki.

HIPPARCHOS | Volume 2, Issue 11 30

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