Herbig Ae/Be Stars: the Missing Link in Star Formation

Astronomical News

Report on the Workshop Herbig Ae/Be Stars: The Missing Link in Star Formation

held at ESO Vitacura, Santiago, Chile, 7–11 April 2014

Willem-Jan de Wit1 Ever since the identification of the Herbig the founding father of this field, George Rene D. Oudmaijer2 Ae/Be stars (HAeBes; Herbig, 1960) as Herbig, had passed away at the age of Mario E. van den Ancker1 a class of early-type pre-main sequence 93. George Herbig pioneered the field of Nuria Calvet3 objects (Strom et al., 1972), these stars star formation, and especially that of have taken centre-stage in star and young stars and their nebulous surround- planet formation studies. They have ings. His legacy includes the identification 1 ESO masses which place them in the transi- and first description of Herbig Ae/Be 2 School of Physics & Astronomy, Univer- tional regime between Solar-type stars stars. In consultation with the director of sity of Leeds, UK and high-mass stars. In detail, the his home institute in Hawaii, it was 3 Department of Astronomy, University of change in star formation character decided to dedicate the ESO Workshop Michigan, USA between the high- and low-mass stars is to the life and works of George H. Herbig. the dichotomy between clustered and isolated formation, between boundary The workshop took place at the ESO/ The workshop highlighted the many layer accretion and magnetospheric ALMA Vitacura campus with close to one recent advances within the field of accretion, radiative and convective stellar hundred registered participants (see Fig- Herbig Ae/Be stars and the close links interiors and between fast and slow rota- ure 1). In the spotlight were the results to star and planet formation. Topics tion. Moreover, the discs around Herbig obtained with instruments like the CRyo- such as magnetospheric accretion and stars are in general brighter than around genic high-resolution InfraRed Echelle the evolution of dust in discs, the struc- their low-mass counterparts, the T-Tauri Spectrograph (CRIRES) and X-shooter, ture of circumstellar discs and the role stars. Therefore HAeBes are prime labo- and the Very Large Telescope Interfer of walls and gaps and their links to ratories in which to study the evolution ometer (VLTI) spatial imaging capabilities planet formation from many observa- from actively accreting discs towards with the MID-infrared Interferometric tional aspects were covered. The work- debris discs and mature planetary sys- instrument (MIDI), the Astronomical Multi- shop was dedicated to the life and tems. BEam combineR (AMBER) and the works of George H. Herbig, who sadly Precision Integrated Optics Near-infrared. passed away at the end of last year. Importantly, the relative proximity of Imaging ExpeRiment (PIONIER). These, HAeBes in local young star-forming and other, instruments have for the first regions allows a detailed view of their time opened up the milliarcsecond and Introduction evolving environment. As such, they are sub-milliarcsecond spatial scales where pivotal for understanding both the forma- the crucial accretion disc physics takes One of the main open questions in star tion of stars and of planets. Nonetheless, place and planet formation processes formation concerns the distinct mecha- the last large meeting purely dedicated occur. In addition, space-based advances nisms by which low- and high-mass stars to Herbig Ae/Be stars took place some by means of the Infrared Space Obser form. While it is well established that low- 20 years ago and a review on the state of vatory (ISO), the Spitzer and Herschel mass stars grow in mass by magnetically this particular field was overdue. With satellites have allowed the study of the controlled accretion, the non-magnetic, this scientific motivation in place, and dust and gas in protoplanetary discs at high-mass stars must grow by another guided by ESO’s innovative edge in infra- high sensitivities. The first Atacama Large mechanism, as yet unclear. In some criti- red instrumentation, our proposal to Millimeter/submillimeter Array (ALMA) cal mass interval, the growth mechanism organise an ESO Workshop on Herbig observations have recently revealed the changes, corresponding to the masses of Ae/Be stars was approved. Shortly after ongoing planet formation process in A- and B-type stars. approval, the sorrowful news arrived that HAeBe discs.

Figure 1. The conference participants in the garden at ESO Vitacura.

50 The Messenger 157 – September 2014 Figure 2. Sketch of the physical and chemical structure of a protoplanetary disc undergoing magnetospheric accretion. From Henning & Semenov (2013).

The scientific programme (available on grounds that, during the earliest phase Herbig Ae stars, it is not able to explain the workshop webpage1) recorded many of the star formation process, the proper- the strong observed ultraviolet (UV) breakthroughs in the field, established ties of the intermediate-mass protostars excesses in early B-type stars (talk by milestones and starting points, and are found to be similar to those of low- John Fairlamb). Further support for opened new avenues to pushing our mass protostars (contributions by Maite the magnetospheric accretion model understanding of star and planet forma- Beltran, Yuefang Wu, Fernando Comerón). comes from other observed similarities tion to higher levels. The programme was In this model, material is guided onto between Herbig Ae stars and classical arranged in sessions according to the the star via magnetic field lines stretching T-Tauri stars, such as linear polarisation physical process, roughly corresponding from the inner disc to one or more regions measurements in spectral lines (talk by to increasing length-scale domains. The on the stellar surface. The hot, shocked Jorick Vink), the geometry of the rotating highest angular resolution traces the surface regions generate excess contin- disc and the location of the inner disc inner disc, i.e., the location in the circum- uum and line emission on top of the star’s radius. Finally, the hydrogen line profiles stellar environment where there is a photospheric spectrum. In this model, are also consistent with what is expected strong interaction between the disc mate- the observed excess enables the rate of for magnetospheric accretion (contribu- rial and the star. Longer wavelengths, mass accretion onto the star to be tions by Sean Brittain, Alicia Aarnio, and lower spatial resolution, preferentially derived. The mass accretion rate is a key Ricardo Ramírez, Monika Petr-Gotzens trace the outer parts of the disc, where physical quantity in star formation as it and Mario van den Ancker). grain growth, and viscous accretion takes determines both the timescale and effi- place and giant planets form. In this sum- ciency for star growth, and can be used Despite the success of extending the for- mary we will reflect on some of the high- as a discriminator between different formation scenario of low-mass stars into lights presented at the meeting and work mation models. Figure 2 shows a sche- the intermediate-mass regime, some fun- our way from the smallest scales, close matic of a protoplanetary disc. damental problems are lurking around to the star, to the physics at larger radii of the corner. One serious problem came to the HAeBe disc. In a systematic X-shooter study of a large the fore during the meeting. The system- number of systems, the relation between atic searches for stellar magnetic fields excess emission, line luminosities and demonstrate the presence of weak or Mass accretion at the inner disc mass accretion was investigated across non-existent magnetic fields in all but a a wide mass range of young objects: small fraction (~ 6%) of the HAeBes. For The leading model for accretion onto A- from brown dwarfs to classical T-Tauri reference, a kiloGauss magnetic field is and B-type pre-main sequence stars is stars to Herbig Ae stars (talk by Ignacio required in order for the magnetospheric magnetospheric accretion. The adoption Mendigutía). Although the magneto- accretion model to work (review talk by of this model is partially justified on the spheric accretion model works well for Evelyne Alécian). Possibly connected to

The Messenger 157 – September 2014 51 Astronomical News de Wit W.-J. et al., Report on the Workshop “Herbig Ae/Be Stars”

the dearth of strong magnetic fields, the disc and also provide evidence for a have gaps (≥ 10 au) devoid of CO, which closely connected phenomenon of jet puffed-up inner rim (talks by Jacques may indicate a disc gap both devoid of formation in Herbig Ae/Be stars appears Kluska and Wing-Fai Thi). Related to the gas and dust. Yet, for other HAeBes, the to be rare (although selection effects warm dust component, located further width of the observed CO line profiles cannot yet be excluded). Nonetheless downstream from the inner dust wall, an seems consistent with formation within when they are found, their properties are extensive MIDI dataset provides evidence ~ 2 au, as in HD 163296, HD 250550 and found to be similar to those in low-mass for flaring and gaps in discs; together Hen 2-80 (talks by Sean Brittain, Gerrit young stars (talk by Catherine Dougados). with information from the spectral energy van der Plas and Rosina Hein Bertelsen). Soft X-rays emanate from these jets distribution, it shows that many discs The CO observations are extremely fer- (Christian Schneider) and they can pro- have asymmetries (talks by Jonathan tile and versatile and may even provide duce (soft) cosmic rays (talk by Tom Ray). Menu and Narges Jamialahmadi). In a hints of circumplanetary discs around new development, interferometric data at accreting planets located within the gaps This situation is markedly different for the a range of wavelengths is now combined (invited talk by Sean Brittain). What is T-Tauri stars, in which the observed to probe different regions of the disc. currently lacking is a systematic CO sur- strong fields generated in the outer con- Near-infrared, mid-infrared and millime- vey for sources with discs in different vective layers are capable of supporting tre-wave interferometry probe different evolutionary phases, i.e., primordial, pre- magnetospheric accretion (talk by Gaitee disc radii, and a variety of spectral tracers transitional and transitional. Hussain) and where jet action has a very are used to characterise both the surface high incidence. Looking for alternatives layers and the interior of the disc with the At longer wavelengths, probing the cooler to explain the strong UV excess, options aim of achieving a global, yet detailed, parts of the disc, CO continues to make were explored in which the excess comes understanding of the disc. its mark. Observations with the Photo from the gas in the inner disc. However, detector Array Camera & Spectrometer in the models presented at the meeting, Notable progress was presented regard- (PACS) and Spectral and Photometric the temperature of this gas seems to be ing the observational diagnostics of the Imaging Receiver (SPIRE) on Herschel too low to explain the observations (con- presence of dust-depleted gaps in the have detected the entire CO ladder in tributions by Stefan Kraus, Catherine radial density distribution of certain discs many flared gaseous discs and several Dougados and John Ilee). On the obser- (the known as transitional discs), a rela- non-flared systems (talks by Matthijs van vational side, a hot inner disc seems to tively new discipline with repercussions der Wiel, Gwendolyn Meeus and Peter be detected in some systems and the for planet formation. The shape of the Woitke). Detailed modelling demonstrates CO emission may be explained by UV spectral energy distribution (displaying an that the disc scale height at a radius of fluorescence (Sean Brittain, Giuseppe absence of dust within a certain temper 100 au needs to be higher for the gas Lodato). ature interval) and the absence of the than the dust in order to explain the 10 µm silicate feature was addressed in observed weak CO line emission (talks by a number of contributions. It appears Vincent Piétu and Carol Grady). However, Dust walls and gaps that all sources with discs, which are it remains difficult to simultaneously strongly flared, also have dust gaps. explain the bright [O I] emission detected Cross-fertilisation between long-baseline Nonetheless, some material is still pre- by Herschel (talks by Peter Woitke and interferometry and stellar astrophysics sent within these gaps. For example, the Christophe Pinte). Other gas diagnostics has been especially successful for the infrared spectrum of polycyclic aromatic which were explored at the meeting field of study of the Herbig stars. Probing hydrocarbons (PAHs) may be a good include OH, warm H2O and H2 (talk by the bright emission by warm and hot tracer for the material in these gaps. Ion- Davide Fedele). NASA’s Stratospheric dust particles for nearby stars, milliarc- ised PAHs appear to be present in low Observatory for Infrared Astronomy second imaging reveals that the HAeBe density, optically thin gas flowing through (SOFIA) may add to this inventory of disc displays a near vertical inner dust the gap, whereas the PAHs are neutral detected lines in the near future (talk by wall directly irradiated by the central star. in the optically thick disc (talks by Carol Hans Zinnecker). This discovery solved one of the long- Grady, Koen Maaskant). standing open questions regarding the hot thermal dust emission in Herbig stars. Intriguing disc structures from direct The dust in the wall is at the sublimation Versatile carbon monoxide imaging temperature and the observed relation between the star’s luminosity and the Valuable information on the inner disc What is revealed indirectly by spectros- distance between star and inner dust wall (≤ 10 au) can be retrieved by high spec- copy of the accretion and planet for is consistent with this finding (review by tral resolution, in particular in the near- mation processes has found a resound- Stefan Kraus). and mid-infrared using the CO to trace ing confirmation arising from advances in the physics. In this way the disc is probed direct imaging techniques. Polarimetric Recent advances in infrared interfero in a region where planet formation is differential imaging (PDI), as pioneered by metric instrumentation, like PIONIER, now suspected to occur. Observations with the Strategic Explorations of Exoplanets allow for efficient synthesis imaging at CRIRES show that some transitional and discs with Subaru (SEEDS) survey milliarcsecond resolution of the inner dust discs (e.g., around HD 100546) seem to on the Subaru telescope, and systematic

52 The Messenger 157 – September 2014 *R 1 order of these four steps is still unclear new generation of instrumentation, it will (review talk by Mark Wyatt). Infrared be possible to link together the observed spectroscopic and spectro-interferometric disc morphologies in a more systematic studies of dust in these discs show clear way. differences between pristine and secondary dust and may be able to offer more An unsolved problem remains whether insights into the order of this evolution early Be stars are to be included within (contributions by Thomas Henning, Roy the class of Herbig Ae/Be stars. Whereas van Boekel, Rodrigo Vieira). For debris there is ample evidence that HAes and discs around A-type stars, the evolu late-type HBes are indeed young stars in tionary successors of the HAeBes like the process of forming planetary sys- β Pic, the gas appears to be secondary, tems, the observational evidence for the whereas in the 30-Myr-old system early-type HBes is still absent. Future HD 21997 the gas must still be primordial facilities, such as the next generation of ǥ (Mark Wyatt, Agnes Kóspál). extremely large telescopes, will offer large gains in spatial resolution which Figure 3. Polarised light emission in Ks-band from the Although disc lifetimes are shorter for should enable us to tackle this problem disc around the Herbig star HD 135344B. These intermediate-mass stars than for low- for the more distant HBes as well. NACO polarisation differential image data clearly resolve a double spiral arm structure and an inner mass stars, no mechanism has yet been cavity around the central star (from Garufi et al., 2013). identified to explain this mass depend- The enigmatic issue which came to the ence (contributions by Chikako Yasui, fore at the workshop is what drives Massanobu Kunimoto). However, the role accretion in the HAeBes. The observed work done with the NACO camera have of binarity on stellar and disc evolution line profiles, spectral excesses and out- shown direct evidence for the existence has so far been neglected. Searches for flows show that HAeBes are accreting, of gaps, spiral arms and asymmetries companions around Herbig Ae/Be stars but they appear to have only weak, in many discs around HAeBes (review show that single Herbig stars are rare: multipolar magnetic fields. Observational talks by Carol Grady and Sascha Quanz; statistically speaking, for every star there progress in this area may come in the see Figure 3 for an example). Indeed, the is a companion at some separation near future from large monitoring pro- diversity in disc substructure, perhaps (review by Gaspard Duchêne, contribu- grammes on accretion within HAeBes, typified as somewhat overwhelming, was tions by Cesar Bric~eno, Gergely Csépány but further theoretical effort is needed considered as one the milestones pre- and Bernadette Rodgers). There also to come up with plausible alternatives to sented at the meeting. appears to be a dependence of metal the magnetospheric accretion model. licity on the accretion rate (Giacomo The leading contender for the creation Beccari). Surveys for new Herbig Ae/Be of these intriguing disc substructures is and intermediate-mass T-Tauri stars are Acknowledgements ongoing planet formation within the disc. underway in the Magellanic Clouds and We would like to thank all conference participants, Impressive ALMA Cycle 0 results were in Orion which may enable us to investi- the members of the Scientific Organising Committee presented providing further evidence for gate these questions in more detail in and the members of the Local Organising Commit- disc substructures and ongoing planet the near future (contributions by Blesson tee for their contributions in making the Herbig Ae/ formation (talks by François Ménard, Mathew, Jesús Hernández). Be workshop a resounding success. The Royal Astronomical Society is acknowledged for its finan- Sebastián Pérez, Simon Casassus and cial support. Special thanks go out to amateur Valentin Christiaens). ALMA has also observatory Roan-Jasé for a fantastic conference started to provide evidence for the exist- Outlook dinner under a starry sky. ence of dust traps, caused by a radial pressure bump within the gas once a The workshop in Vitacura highlighted References planet has grown sufficiently massive to the remarkable progress which has been open up a gap within the disc (talk by achieved in the field of Herbig Ae/Be stars Garufi, A. et al. 2013, A&A, 560, 105 Nienke van der Marel). Transitional discs in recent years. Imaging at sub-arcsecond Henning, T. & Semenov, D. 2013, ChRv, 113, 9016 Herbig, G. H. 1960, ApJS, 4, 337 may be the observational signature of down to milliarcsecond scales has given Strom, S. E. et al. 1972, ApJ, 173, 353 the existence of such dust traps. us a new view on the nature of the discs around HAeBes. The sources with flared The dust in a primordial circumstellar discs may all have cavities or gaps, and Links disc which evolves to a planetary system much structure is present in the outer 1 Workshop web page: http://www.eso.org/ would pass through the following four discs. Dust traps may be a signpost of haebe2014.html evolutionary steps: (1) the generation of planet formation in these discs. However, a dust depletion gap; (2) removal of the so far we have mainly seen snapshots millimetre-sized dust; (3) depletion of dust of individual systems. In the coming from the inner regions; and (4) concentra- years, as statistically meaningful samples tion of the planetesimals in a ring. The of Herbig stars will be observed with the

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