e d surrounding remnants pulsars with isgreater thana number of nants o the o theCrab. along-standingbeen puzzlewith has This thesurrounding medium. detected with theinteraction but we have not we theinner see pulsarbubble, the Crab Nebula, In thesurrounding medium. with thestarinteracts of outer part st and par fields magnetic bubble arelativistic of implies thepresence of synchrotron The radiation thenebula. energetic requirements of to wasshown account for the period, its33msec with pulsar, thecentral spin-down The of in1968. neutronmagnetized star, rotating arapidly pulsar, acentral discovery the until of known wasnot theparticles but thesource of inthe1950’s, particles tic the nebula wasidentifiedassynchrotron from radiation relativis- from radiation amorphous theinterior The of 1). (SN 1054)(Fig. 1054 the Crab Nebula thesupernova of with anditsassociation such stars hasbeen Rosetta The stone for theend states of sion. collapses inatremendous are andtheouter ejected explo- parts T USA Virginia, University of Astronomy, of Department Chevalier, Roger A. Collapse Supernovae The ofCore- aftermath eur xp o e l z nayug(g 10 r uenv enn,thepulsaris In ayoung supernova remnant, (age ~1000yr) F lar e oph e e rmn er,theCrab pulsarandtheolder Velaor manypulsarwere years, show that they end in spectacular eventsshow endinwhich thecore inspectacular thatthey stars considerably more massive thanthesun he lifecycles of c .Acmiaino atr a loe hsavne but factors hasallowed thisadvance, Acombination of n. nl ted to affect the inner part of the exploded star, while the while theexploded star, ted of to theinner affect part y pulsar jecta intojecta ashell. t f y ic sics news uenve htstainhsrcnl hne;the situation That hasrecently changed; supernovae. les, w s thatc hic h ar MAR ould b e responsiblee for sweeping the surrounding CH / APRIL e c Article available at lear 2005 l y link ed toed theexplosive rem- http://www.europhysicsnews.org or 25 M around Above somemass, initial for curve. aplateau inthelight implies thathydrogen andthePstands ispresent inthespectrum where theII Typesion identified canbe with IIPsupernovae, explo- of type This isfromlight theheated hydrogen envelope. s thesestars to leads aluminous explosion The of lope intact. li details andreferences found canbe inChevalier(2005). Full massive stars. links andtheimplicationsfor theend states of these isto thispaper describe aimof The collapse remnants. core to thedifferent kindsof ultimately, and, supernova types ences inpresupernova linked masslosscanbe to thedifferent differ- These increasing stellar mass(andluminosity). initial with massive stars show thatmasslossrates increase studies of tionary stellar masslossandevolu- of Observations thestar. center of masslossandthecore development atthe depends on therate of thefinaloutcome Above thatmass, thesun. is themassof Star S lig inwhich there curve) isamoreIIL (linearlight drop rapid from the t e believed to occur in stars with an initial mass>8M believed toaninitial occur instarswith Core collapse is core collapse supernovae. of types the various massive stars and thefinalevolution of in ourunderstanding of observed. thesurrounding mediumisalso with theinteraction many cases, In generatedic fields by thecentralpulsarasinCrab Nebula. andmagnet- particles relativistic by apulsar-generated nebula of a young surrounded pulsarinasupernova remnant istypically have observations These that shown XMM-Newton). Chandra, excellentwith vatories (ASCA, resolution spatial andsensitivity X-ray space obser- thelaunch hasbeen of important particularly up v v upernova types ol es asc ht maxim ttesm iea hs icvre,there have advances been At asthesediscoveries, thesametime e s j u ime o r t ᭪ nova over anextended (100days) time which the during ust ab io , oto thehydrogen envelope thestellar islostduring most of n, o l xeddrdsas ihms ftheir hydrogen enve- mostof with extended red stars, ol, f explosion. These objects can be identified with Type identified with canbe objects These explosion. f b http://dx.doi.org/10.1051/epn:2005206 o u um thanintheIIPcase. v t there left to isenough give anextended envelope at e thelimitf o r core collapse are expected to end their A oeahge as around bove mass, ahigher ᭣ image oftheC N bubble out by thepulsar swepthas been exploded starthat from the emission isgas filamentary reddish, Large Telescope). Observatory,Very S power (European by thepulsar b that haveparticles relativistic radiation from synchrotron emission is smooth bluish, outhern een acc ebula. F ig . 1: . ᭪ T T where M , Optical eler FEATURES he he a t r ed ab 59 ᭪ features FEATURES

As described above, the supernova types IIP, IIL, and Ib/c have increasing amounts of mass loss, which has implications for the composition distribution of the freely expanding ejecta. In a Type IIP supernova, most of the explosion energy ends up in the H envelope, whereas in the other types, the energy is in heavy ele- ment rich material. If the explosion were strictly radial and spherically symmetric, the explosion would have a set of compo- sition layers, like an onion, with the heaviest elements at the center. Actually, composition discontinuities can be accompanied by strong density gradients; when the denser, interior material is decelerated by the outer lower density material in the explosion, the interface is subject to Rayleigh-Taylor instabilities. The result is that clumps of heavy elements go out to high velocities, while ᭡ Fig. 2: A one-dimensional model for a young supernova bubbles of light elements move to low velocities. The effects of remnant (age 500 yr); the density is in gm cm-3 and the this redistribution of elements can be seen in the late spectra of radius in cm. Starting from radius = 0, there is a freely supernovae, when the ejecta have become optically thin. expanding pulsar wind, a region of shocked pulsar wind The deceleration of an inner layer is accompanied by a reverse (pulsar bubble), a swept up shell of ejecta bounded by a shock wave, a shock front that eventually moves back toward the shock front at Rp, a region of cold, freely expanding ejecta, center of the explosion. Material brought back to the center in and a region of hot gas (SSDW) bounded by a reverse shock this way can be important for fallback onto the central compact at R and a forward shock at R (from Blondin, Chevalier & 2 1 object. Some fallback is expected for gas that is marginally Frierson 2001). bound to the compact object. In high mass stars, including most single stars that are expected to end as Type IIL or Ib/c super- 35 M᭪, the hydrogen envelope is completely lost during the stel- novae, the fallback is expected to be strong, leading to the lar evolution. These stars are observed as relatively compact, hot formation of black holes. In other cases, the accreted mass is stars. The explosions of these stars can be identified with Type Ib <0.1 M᭪. The nature of the fallback can depend on the type of or Ic supernovae, which do not have hydrogen in their spectra. supernova. Even if the neutron star is born with slow rotation, the The rate of star formation is dominated by the lower mass stars, inward mixing of material can bring higher angular momentum so that the expected rate of Type IIP supernovae is about 10 material into the vicinity of the compact object. times the rate of Type IIL and 5 times the rate of Type Ib/c. Finally, different types of circumstellar medium, built up by Observational data are not yet sufficiently detailed to determine stellar mass loss, are expected in the different types of supernovae. the relative rates. However, the above results are for single mas- The speed of a stellar wind is related to the escape velocity from sive stars. A significant fraction of massive stars are in binary the star, so that extended red supergiant stars, which are the systems and mass transfer and loss due to binary effects could expected supernova progenitors of stars with hydrogen envelopes, -1 influence the relative rates of various core collapse supernovae. have relatively slow winds (vw=10-20 km s ). These stars thus The importance and detailed effects of binary evolution remain have a relatively high surrounding wind density, Úw, because in a 2 one of the most uncertain aspects of stellar evolution. steady spherical flow ρw=(dM/dt)/(4πr vw), where dM/dt is the mass loss rate and r is the radius from the center. The value of Supernova properties dM/dt depends primarily on the luminosity of the star, which is The central core collapse in a massive star triggers an explosion of higher for the higher initial mass stars. Thus, for single stars, the the rest of star, by a mechanism that remains poorly understood. Type IIL supernovae are expected to have the densest circumstel- A mechanism involving transfer of energy from neutrinos has lar winds, while the Type IIP are at lower density. Binary been favored, but has not yet provided a convincing model for an interaction can also lead to dense winds in Type IIL supernovae. explosion; this has recently led to the exploration of other possi- The expansion of the wind is ultimately limited by the pressure ble mechanisms, involving rapid rotation and magnetic fields. In of the surrounding medium. The high ram pressure in the denser the immediate aftermath of the explosion, the behavior of the lay- winds means that they can expand out 5 parsecs (1 parsec = 3.26 ers near the collapsed core depend on the explosion mechanism, light-years = 3.09 x 1018 cm) or more from the progenitor star. but, as one moves out in the star, the shock acceleration of the stel- Type Ib/c supernovae are thought to have more compact progen- -1 lar layers is primarily determined by the explosion energy. The itors with higher velocity winds, vw=1000 km s . Their mass loss eruption of the explosion shock from stellar surface gives a burst rates are comparable to those expected around Type IIL super- of hard, typically ionizing, radiation. This emission has not yet novae, but the high wind velocity leads to a relatively small been directly observed from a supernova, although the effects of surrounding density. such emission can be seen in the nearby supernova SN 1987A. These expectations have been borne out by observations of The time for the shock front to traverse the star is < 1 day, and extragalactic supernovae at ages up to a few years. The interaction is followed by a period in which the internal energy of the explo- with the surrounding wind can be observed at radio and X-ray sion is converted to kinetic energy. After about a week, the wavelengths, as well as optical in the case of a dense wind. The expansion tends towards a homologous, free expansion. The den- interaction with the wind gives rise to hot shocked gas and rela- sity profile that is set up by the expansion is fairly flat in the central tivistic particles that are probably produced by the shock wave region and has a steep power law profile in the outer parts where acceleration mechanism. The relativistic electrons radiate syn- the shock front has accelerated through the decreasing density lay- chrotron emission in the magnetic field of the shocked region. The ers of the star. The density of gas at a given velocity drops as t-3 early absorption of radio synchrotron emission and the synchro- due to the 3-dimensional expansion. The typical energy of a core tron luminosity provide indicators of the circumstellar density. collapse supernova is 1051 ergs (1022 H bombs). The thermal emission from the hot gas at X-ray wavelengths

60 europhysics news MARCH/APRIL 2005 e incr usrpwrotu n steaeo thenebula. pulsar power output andtistheage of where dE/dtisthe thenebulato (dE/dt)t, of internal energies estimated anebula canbe by and comparing thekinetic status of spun d no e the do pulsarnebulae notreach thebreakout phase, If spinrate. high building bubble arelativistic atsuch do notbegin a ing thatthey imply- to appear nebulae have undergone expansion, thisrapid t a thebubbleerates because issweepingthatexpanding upgas h uenv,~10 the supernova, pulsar required to do of thisiscomparable to energy the kinetic t fluid therelativistic canbreak thevolume out andfill out profile, the ag thefilaments islessthan cated by thattheexpansion thefact age of filaments intheCrab isdirectly indi- acceleration The of 1). (Fig. intheCrab Nebula observed structure thefilamentary of ly origin fluid gives which isthelike- to theRayleigh-Taylor rise instability, be described by apower-law described be inradius, thesupernova can thepulsarpower input issteady profile andtheinner density of If 2). inFig. thesupernova (cold ejecta ta of approximationinitial to thepulsarnebula. provides an index adiabatic 4/3, with relativistic fluid, bubble of nebula asa thepulsarwind of Adescription chrotron emission. filaments inthesyn- of position theobserved compatible with termination shock was thewind of was found thattheposition where it wasfirst model applied to theCrab Nebula, of type This 2). andconvertswind power thewind to(Fig. internal energy must passthrough ashock wavethe wind thatdecelerates the thebubble, to theslow compatible be outside gas of moving with In order moves out from velocity. relativistic thepulsarathighly that star losesrotational wind through amagnetized energy spinningneutron The thesupernova. relativisticfluid inside of of asupernova generates abubble apulsarinside of presenceThe of Pulsar windnebulae supernovae are roughly inaccord expectations. with of cumstellar densities inferred around types thevarious cir- The supernovae routine. extragalactic nearby detection of ( X-ray observatories new The thecircumstellar density. provides another estimate of eur hydrogen atabout moving shows thepresence ejecta of of shell theemission from the theCrab Nebula, In thecaseof supernova. the nant gives on information thenature valuable diagnostic of T Young supernova remnants withpulsars the int a oainlpro ftepla,aot9 sc No observed msec. 9 about thepulsar, of ial rotational period o theshe xpand int wa epeec fapla idnbl nieo asupernova rem- nebula inside of apulsarwind he presence of ᭤ (Observatory/Smithsonian Astrophysical(Observatory/Smithsonian Observatory) the r The radius of the interaction withthecircumstellar medium. lef its central isthe blueregion to pulsar(135msec period) the I The pulsar wind nebula expands into pulsarwind The thefreely expanding ejec- r ihri nerypaeo steady power input (assuming y are either phase inanearly of oph f F y fr eases asap t ofthec v ig the pulsarmaintainsitsp ol ftheparent supernova SN1054. e of mati bu asc.(NASA/Chandra X-ray emnant 7parsecs. isabout e . o o to ftepla antcfil) or thepulsars have field), thepulsarmagnetic ution of r y 3: w m it. The fact that a light fluid isaccelerating thatalight adenser fact The m it. a sics news c l n fr X-r t l o h tel rpigpr fthesupernova density o thesteeply dropping of part io ne.The brighter emissionisfrom gasheated by enter. a f circumstellar interaction. The energy from energy The the circumstellar interaction. f n is self-similar and the radius of the wind bubble thewind of andtheradius n isself-similar o y imageofG292.0+1.8. o m their initial rotation rates. The evolutionary The rotationm their initial rates. w rlwi ie R~t er law intime, 51 MAR rs which places anupper limiton theini- ergs, hnr,XMM-Newton Chandra, CH / APRIL o 2005 w e r andtheb (6-m) The pulsarnebulawith ρ ~ r / (5-m) -m . (with mabout1), (with T ) have made the ub he r b le isable to a dius accel- r r thepulsars are inferred to inthe be of periods initial The types. va pulsar in themasslossd o stellar evolution thecore because depend properties on themass for starmodel the inasingle would This surprising be radiation. dipole spin-down by magnetic deduced from theassumption of fields for thepulsarmagnetic sameistrue The supernova type. estimatedThe age is3200years. supernova. asexpectedfor thiskindof and heavy element rich, ejecta moving atabout2000kms moving ejecta supernova of observations Optical aTypegesting IILexplosion. l h bet icse oti on aeae fabout 10 to discussed theobjects thispointhaveAll ages of SN 1987Aandc several M andtheemittingmass, 7pc, theemittingregion, of radius The 3). asapulsarnebula aswell (Fig. strong circumstellar interaction, searches for emission surrounding theCrab Nebula are needed. adefinitive casehasnotyet made anddeeper been plausible, thisscenario is Although expands andbecomes unobservable. would have wind supergiant to led thatadiabatically hotgas thered with interaction Early low regionaround density thestar. thatcancreate fastwind a thestarhasastrong, a massive star; evolution for phase of andlongest, istheinitial, This progenitor. themassive star evolution of from themainsequence phase of attributedtobe expansion into bubble thelow left wind density thiscan In aType IIPmodel, extensive multiwavelength efforts. despite interaction around the Crab Nebula observed, hasbeen No circumstellar acceleration bubble. bywith thepulsarwind are shell thegas consistent andacceleration radius observed of The as expected for expansion into thesupernova ejecta. theCrab Nebula to isobserved free inessentially be expansion, of 2000 kms htacmatojc omdi h xlso.However, intheexplosion. that acompactformed object showing Neutrinos were detected from thesupernova in1987, y a we have opportunity best The to study theaftermath of e ang ounger explosion supernova isthenearby SN1987AintheLMC. f the progenitor. However, if close binary evolution plays close binary arole if However, theprogenitor. f lation isnotsurprising. T h perneo G292.0+1.8isdifferent inthatitshows appearanceof The he e 10-100mse s t r e is o est ᭪ - 1 r ossetwt es urudn id sug- are consistent adense surrounding wind, with , , as expected in a Type IIP supernova. The outer The edge asexpected inaType IIPsupernova. , noug imat hi gs nta usrpros andsuperno- pulsarperiods, initial e their ages, h inf e c, onclusions t e r but the periods arebut notcorrelated theperiods the with mining thes o rmation on 9youngrmation thatcontain nebulae -1 up show thatitishydrogen-poor e r oatp,telc fcor- thelack of nova type, FEATURES 3 years. 61 features FEATURES

Lithuanian Physical Society Zenonas Rudzikas, Rasa

he history of physics teaching and physics research in Lithua- T nia goes back to the sixteenth century, when in 1579 the University was founded in Vilnius, the oldest university in Eastern Europe. Physics was always taught there together with the other natural sciences. The newest achievements in physics were pre- sented to the students with some delay, mainly caused by low speed of travel of the newest scientific literature to this then remote part of Europe. ᭡ Fig. 4: Optical image of SN 1987A in the Large Magellanic In 1773 the Rector of Vilnius University, M.Poczobut-Odlan- Cloud, a companion galaxy to our Milky Way Galaxy (taken 28 icki, presented a draft for an Academy of Sciences in Vilnius; November, 2003).The ring, believed to be a circular ring with a however, perhaps due to the complicated political situation in that radius of 0.2 pc observed at an angle, was created by region at that time, the initiative did not become a reality. presupernova mass loss. The bright spots on the ring are In the 19th century physics was taught in the old Vilnius Uni- where the supernova shock front is driving shock waves into versity in a particularly modern way, because at that time there the dense ring material. The asymmetric central region is existed especially good contacts of local professors with their col- supernova debris heated by radioactive decays. leagues in France, England and other European countries. It also (NASA/Hubble/STScI) became common to buy modern equipment from these and other countries. The Vilnius University observatory also had strong multiwavelength observations since that time have failed to give links with a number of observatories in Western Europe. any evidence for the compact object. The optical emission from The recent roots of the Lithuanian Physical Society (LPS) the region of the explosion can be attributed to power input by originated in the Lithuanian Scientific Society (LSS) founded in radioactive decays of 44Ti (Fig. 4). Current limits on the lumi- 1907, when Lithuania was still the so-called North-West Region of nosity of any compact object are orders of magnitude smaller that the Russian tsarist empire. The core of the LSS consisted of edu- the power output of the Crab pulsar, and are lower than expecta- cated Lithuanians of various specialities, physicists and engineers tions for accretion onto a quiet neutron star. Accretion onto a included, who had the desire to collect and preserve the Lithuan- black hole, with low radiative efficiency, is one possibility, but a ian ethnographical cultural artefacts, to contribute to the quiet neutron star with little mass around it cannot be ruled out. education of the people, and to build up the national self-esteem. Fortunately, the emitting supernova ejecta are becoming more Much attention was paid to collecting data on geology, botany, extended and more optically thin with time, so it is just a matter zoology and other branches of science, to writing and publishing of waiting until the mystery at the center reveals itself. textbooks and popular scientific literature. The increasing power of space and ground-based observatories The LSS had its own library, its collection of manuscripts, an is giving us the possibility of following through the supernova ethnographic archive, and museum. It also edited the journal explosion to the effects of a central compact object and circum- “ ” (“Lithuanian Nation”) (1907-1936). stellar interaction around massive stars. These phenomena tell Special attention was paid to writing and publishing textbooks. us about fundamental astrophysical issues: the ejection of heavy During 1915-1920 about 100 textbooks for Lithuanian schools elements and energy into the interstellar medium and the early were written and published. When the Vilnius region was occu- evolution of neutron stars. The number of well-observed objects pied by Poland in 1920, the LSS became very active in opening the is still small; as it increases, we will have a clearer view of the new university in Kaunas, then the temporal capital of Lithuania. diverse paths of massive star death. In 1928 the Society of the Students of Physics and Mathemat- ics was founded at Kaunas University, and in 1931 the Lithuanian About the author Society of Natural Scientists. The latter was active up to 1940. It Roger Chevalier is W.H. Vanderbilt Professor of Astronomy at had about 200 members representing the physical, mathematical, the University of Virginia. He is a member of the National chemical, biological, geological and medical sciences. Academy of Sciences and was awarded the 1996 Dannie Heineman Prize for Astrophysics for his research on supernovae.

References Blondin, J.M., Chevalier, R.A., Frierson, D.M., 2001, “Pulsar Wind Nebu- lae in Evolved Supernova Remnants,”Astrophys. J., 563, 806 ᭣ Fig. 1: Chevalier, R. A. 2005, “The Young Remnants of Core Collapse Superno- Emblem va Remnants and Their Supernovae,”Astrophys. J., 619, 839 of LPS

62 europhysics news MARCH/APRIL 2005