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

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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 struc- ture of a protoplanetary disc undergoing magneto- spheric 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 for- mation 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.
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