Astronomical News

Report on the Workshop 400 Years of Stellar Rotation

held at Natal, Brazil, 21–26 November 2013

José Renan de Medeiros1 Claudio Melo2 Luca Pasquini2

1 Departamento de Fisica, Universidad Federal do Rio Grande do Norte, Natal, Brazil 2 ESO

The workshop marked the 400th anni- versary of the observation of sunspots by Galileo and his deduction of the rotation of the Sun. The topics covered extensively both the theoretical and observational aspects of stellar rotation for stars of all types, from pre-main sequence to evolved stages, and includ- ing binary stars and stars hosting plan- ets. A summary of the selected themes is presented.

Figure 1. The participants at the workshop "400 rational mind in many fields, a real man Motivation years of Stellar Rotation" by the beach at Tabatinga. of the rinascimento, dominating philoso- phy, music and painting. Paolo Molaro Stellar rotation is both a very old topic, tributed talks and almost 50 posters. The also presented the fascinating story of going back to the time of Galileo, and topics were organised into nine sessions, a mysterious portrait that might be of a very topical one, at the forefront of including: pre-main sequence and low- Galileo himself (Molaro, 2012). ­current research in stellar physics. As mass stars; massive stars; evolved stars; described on the webpage of the confer- modelling evolution with rotation; rotation, To summarise the results of such a ence1, Galileo Galilei reported in 1613 magnetic fields and binaries. A session broad-ranging conference and select sci- in his book the “l’Istoria e dimostrazioni on the Sun was followed by: rotation and entific highlights is a difficult task, which intorno alle macchie solari e loro accidenti” chemical abundances; rotation, activity may lead to unfair omissions. Accepting evidence for sunspots and the inter­ and planets; and the conference ended this risk, we present a short summary pretation of their motion as due to Solar with sessions devoted to the legacy of of some, subjectively selected, scientific rotation. A few centuries separated this space missions and to our understanding highlights covering many aspects of first reported evidence of the rotation of of stellar rotation. observations and theory, including rota- the Sun from the first studies at the begin- tional periods, magnetic fields, activity ning of the 20th century, by Eddington, The conference also featured two and abundances, dynamo theory and its Milne, von Zeipel and others, addressing ­enlightening talks about Galileo and his relationship with stellar structure and dif- the impact of rotation on stars from a cultural environment. In his opening ferential rotation. theoretical point of view. Thus, 400 years review “­Galileo, the man and the scien- after the discovery of Galileo, the subject tist”, Alberto Righini summarised the The many different talks presented recent of the rotation of the Sun and of other entire scientific life of Galileo with empha- results both from theoretical and obser- stars is still a very active area of research, sis on his discovery of sunspots and vational standpoints. Experiments like as testified by the 144 scientists from ­rotation. He was of the opinion that Galileo SOHO have provided a detailed view of 21 countries who attended the confer- Galilei was close to a sort of Florentine the internal and surface rotation of the ence held in the beautiful city of Natal, Protestant movement and this motivated Sun. The precise photometry obtained Brazil. The conference was co-sponsored the strong reaction of the Catholic with the Kepler and CoRoT satellites by ESO, the University Federal of Rio Church. He has described this thesis in has made rotation periods for thousands Grande do Norte and the International full in his book on Galileo (Righini, 2008). of stars available. In addition, projected Institute of Physics of Natal. In his talk “Galileo, the artist and the rotational velocities (Vsini) are provided ­scientist”, given on the final day of the for thousands of stars, thanks to ground- conference (not at the conference venue, based high-resolution spectroscopy. Conference themes but in the exotic surroundings of Tabatinga These data offer the unique possibility of Beach), Paolo Molaro emphasised that detailed studies of the behaviour of the The conference occupied five full days, Galileo was not only the father of modern rotation of the Sun in time, as well as the with more than 60 invited reviews, con- science, but also an innovative and inspi- evolutionary behaviour of stellar rotation

The Messenger 155 – March 2014 55 Astronomical News Renan de Medeiros J. et al., Report on the Workshop “400 Years of Stellar Rotation”

all along the Hertzsprung–Russell (HR) tion, to the interferometric measurements As usual more information does not nec- diagram. Techniques like interferometry of flattening (presented by Antoine essarily mean more answers, but rather allow the shape of fast-rotating stars to ­Merand and John Monnier) and depar- more questions! For many of the stars be explored directly and spectropolarim- tures from the Von Zeipel darkening law. studied with other techniques, the physi- etry provides information about the ex­­ A positive conclusion is that evolutionary cal characteristics can also be derived ternal magnetic field and, together with models of massive stars incorporating by using asteroseismology. In contrast to measures of rotation, gives indications stellar rotation seem to be able to repro- main sequence stars, these data show on how these two properties interact. duce many of the observations. Models that in evolved stars, the nucleus rotates Asteroseismology also provides invalua- are therefore an essential tool to predict about 13 times faster than the external ble information on the internal rotation the production of key elements produced atmosphere. This finding indicates that of stars and poses new challenges to by massive, very metal-poor stars, that the cores rotate slower than expected if stellar modellers. are barely observable at present. The the star were a solid body, but faster impact of satellite missions has also dra- than if a shellular rotation model (i.e., one matically changed what we know about where the rotation in the interior of the Theory and models white dwarfs, revealing large discrepan- star mainly depends on the radius, so cies between observed rotation periods that each incremental shell rotates with From a theoretical point of view, rotation and spectroscopically determined Vsini the same angular velocity, but different does appear to be an essential ingredient values. In an equally energetic talk, Andre from the nearby shells) is assumed. The for understanding the evolution of single Maeder walked us through a number ­presence of a certain amount of coupling and close binary stars. Rotation has an of the present puzzles (such as the abun- between the core and the surface is impact not only in the area of stellar dance of nitrogen at low metallicity and deduced. This coupling is of course fun- physics, but also on the evolutionary sce- the s-elements), where rotation of massive damental to understanding the internal narios and lifetimes of different types of stars could play a role, concluding that ­mixing of the chemical elements, and stars, leading to the modification of stellar the early Universe was controlled by stel- may therefore help to explain some of the populations, and thus the photometric lar rotation. peculiar abundances observed in giants. evolution of galaxies. It may change the What, however, is the missing angular yields of some elements, especially at momentum transport process: internal very low metallicity, providing new insights Observations gravity waves or a magnetic field? into the early chemical evolution of gal­ axies. Rotation has an impact on the Currently there are a striking number The same kind of data allows the study nature of progenitors of core-collapse of stars for which the ages and stellar of gyrochronology, i.e., how stars slow events and of long soft gamma ray masses are quite well known. This down during their lifetime on account of bursts. The range of initial masses that favourable situation arises mostly from the presence of winds, mass loss and give birth to white dwarfs, neutron stars the results of the CoRoT and Kepler magnetic fields. Sydney Barnes reviewed and black holes will probably also depend ­satellites, which were reviewed by Annie the prospects for and limitations of this to some extent on rotation. How the rota- Baglin and Soeren Meibom respectively. technique. Interestingly enough, while tion of a star depends on the presence However ground-based photometric a regular rotational period–age pattern of planets, a question already addressed ­surveys of thousands of stars with meas- is observed in open cluster stars, the by Struve, was discussed during the con- ured rotational periods all over the HR results are not so clear for field stars with ference, as well as what may happen diagram, including stars in many open ages derived from asteroseismology. In when a planet is engulfed by its host star. clusters, also contribute strongly. Jerome general, the debate on whether gyro­ As demonstrated by these few points, Bouvier reviewed how this wealth of chronology is a viable way of measuring rotation is at the crossroads of many topi- data has allowed the modelling of the stellar ages appears to be quite lively. cal subjects in modern astrophysics, early angular momentum evolution taking An associated question is whether the from planet and star formation to the evo- place in stars from the pre-main sequence rotation of the Sun is anomalous for its lution of galaxies passing through the first (PMS) up to Solar age. Although the mass and age. The answer appears to stellar generations in the Universe. main ingredients of early angular momen- be: probably not. tum evolution have been known for quite Stellar rotation studies discriminate some time, the amount, and mainly the For some stars even differential stellar between Solar-type stars and those, hot- quality, of the data have now allowed a rotation can be obtained, showing that, ter than the Sun, without an external con- much more refined modelling of the phys- although the Solar case is not common, vective zone. It is no exaggeration to ics involved. Luiza Rebull reviewed the differential rotation behaviour generally state that “the Universe without rotation legacy of the Spitzer Space Telescope for agrees with models. Indeed, in his didac- would have been much less interesting”. our understanding of stellar discs; more tic review, Nuccio Lanza presented spec- Thus Georges Meynet concluded his detailed studies of the young stellar disc– troscopic and photometric methods to vibrant review in which he explained that, rotation interaction in the infrared have measure surface differential rotation, also for massive stars, rotation explains a been enabled for a wider range of ages considering recent contributions from number of observed phenomena, from and masses than ever before. high-precision photometric time series unexpected surface chemical composi- from the MOST, CoRoT and Kepler space

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missions. New, large, spectroscopic sur- On the other hand, rotation-induced Acknowledgements veys are complementing the satellite magnetic activity of stars plays a key role Galileo would likely have been delighted to discover data, producing precise stellar parame- in determining whether planets can retain the long trail that has been explored in Solar and ters and measured Vsini. Steve Kawaler a magnetosphere, which is essential for stellar rotation since his time! The conference was and Kepler Oliveira reviewed the present life as we know it. As Edward Guinan superbly organised, and the attendees could enjoy status of methods for determining the emphasised, “the Sun is still a dangerous the beautiful beach of Tabatinga, close to Natal, on the social trip (see Figure). The Local Organising rotation velocities of white dwarf stars via star”. In the same vein, Nuno Santos Committee, and in particular Bruno Canto Martins, asteroseismology and spectroscopy, remarked on the analysis of rotation from Sanzia Alves and Bia Pessoa, had a hard time which will open up the study of rotation in the point of view of the research on exo- ­making everything run smoothly, or perhaps more the final stages of stellar evolution. planets; he noted that “astronomers appropriately, “rotate swiftly”, but succeeded admi- rably. working on exoplanets don’t like the fact Stellar rotation and angular momentum that stars rotate”, since the rotation–­ evolution are also intimately coupled activity connection is the main source of References to proto-planetary discs and to the pres- stellar noise in planet searches. Molaro, P. 2012, AN, 333, 186 ence of planetary systems. The interac- Righini, A. 2008, Galileo. Tra scienza fede e politica, tion between planets and stars is, how- Several talks (e.g., Reiners, Gouveia dal (Bologna: Editrice Compositori) ever, not limited to the dynamics in the Pino, Vishniac and Lazarian) reminded early stages, but continues all through us that while magnetic activity and mag- Links the stellar lifetime. On the one hand, evi- netic fields can now be measured well dence is being sought for the presence in many stars and qualitatively under- 1 Conference web page: http://www.dfte.ufrn. of chemical abundance peculiarities in stood, detailed modelling of the (Solar) br/400rotation/index.htm planet-hosting stars. R. Similjanic and A. dynamo still encounters several chal- Recio-Blanco reinforced the evidence lenges in reproducing the observations. for a link between rotation history and For binary stars in particular, as stressed light-element abundances. This connec- by Klaus Strassmeier in his review, the tion is not new, but in spite of the amount situation is significantly more complex. of data available, its nature is not yet fully understood.

Report on the ALMA Community Days: Preparing for Cycle 2

held at ESO Headquarters, Garching, Germany, 19–20 November 2013

Suzanna Randall1 tions. The Cycle 2 ALMA Community level, the final array will comprise at least Andy Biggs1 Days, summarised here, were designed 66 high precision antennas equipped to Tim Davis1 to optimally prepare the European observe in the 30 GHz to 1 THz frequency Claudia Lagos1 ALMA Community for proposal submis- range. The antennas are grouped into Thomas Stanke1 sion and were held just a couple of the main array, comprising 50 12-metre Leonardo Testi1 weeks before the Cycle 2 deadline. dishes, and the Atacama Compact Array Martin Zwaan1 (ACA, also known as the Morita Array), containing 12 closely placed 7-metre The Atacama Large Millimeter/submil­ antennas together with four total-power 1 ESO limeter Array (ALMA), is the world’s lead- 12-metre antennas used to recover large- ing observatory at millimetre and sub­ scale structures on the sky. By combin- millimetre wavelengths. It is the result of ing data obtained with different configu- ALMA has now been obtaining scientific a global cooperation involving Europe rations of the main array and the ACA, observations for the astronomical (through the ESO Member States), North complex and extended sources can be ­community for over two years. While America and East Asia, as well as the accurately imaged. commissioning is still continuing, the host country Chile. Located at the unique upcoming Cycle 2 should allow for site on the Chajnantor Plateau in northern ALMA has been used, at least part of the nearly 2000 hours of science observa- Chile at over 5000 metres above sea time, for observations proposed by the

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