* Astronomy

Blobrana · L

RE: Herbig Ae/Be stars

Blobrana · L

Title: The origin of hydrogen line emission for five Herbig Ae/Be stars spatially resolved by VLTI/AMBER spectro-interferometry
Authors: S. Kraus, K.-H. Hofmann, M. Benisty, J.-P. Berger, O. Chesneau, A. Isella, F. Malbet, A. Meilland, N. Nardetto, A. Natta, T. Preibisch, D. Schertl, M. Smith, P. Stee, E. Tatulli, L. Testi, G. Weigelt

To trace the accretion and outflow processes around YSOs, diagnostic spectral lines such as the BrG 2.166 micron line are widely used, although due to a lack of spatial resolution, the origin of the line emission is still unclear. Employing the AU-scale spatial resolution which can be achieved with infrared long-baseline interferometry, we aim to distinguish between theoretical models which associate the BrG line emission with mass infall or mass outflow processes. Using the VLTI/AMBER instrument, we spatially and spectrally (R=1500) resolved the inner environment of five Herbig Ae/Be stars (HD163296, HD104237, HD98922, MWC297, V921Sco) in the BrG emission line as well as in the adjacent continuum. All objects (except MWC297) show an increase of visibility within the BrG emission line, indicating that the BrG-emitting region in these objects is more compact than the dust sublimation radius. For HD98922, our quantitative analysis reveals that the line-emitting region is compact enough to be consistent with the magnetospheric accretion scenario. For HD163296, HD104237, MWC297, and V921Sco we identify a stellar wind or a disk wind as the most likely line-emitting mechanism. We search for general trends and find that the size of the BrG-emitting region does not seem to depend on the basic stellar parameters, but correlates with the H-alpha line profile shape. We find evidence for at least two distinct BrG line-formation mechanisms. Stars with a P-Cygni H-alpha line profile and a high mass-accretion rate seem to show particularly compact BrG-emitting regions (R_BrG/R_cont<0.2), while stars with a double-peaked or single-peaked H-alpha-line profile show a significantly more extended BrG-emitting region (0.6<R_BrG/R_cont<1.4), possibly tracing a stellar wind or a disk wind.

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Blobrana · L

Title: Magnetism in Herbig Ae/Be stars and the link to the Ap/Bp stars
Authors: E. Alecian (RMC/LESIA), C. Catala (LESIA), G.A. Wade (RMC), S. Bagnulo (Armagh Observatory), T. Boehm (LATT), J.-C. Bouret (LAM), J.-F. Donati (LATT), C. Folsom (Armagh Observatory), J. Grunhut (RMC), J.D. Landstreet (UWO), S.C. Marsden (AAO), P. Petit (LATT), J. Ramirez (LESIA), J. Silvester (RMC)

Among the A/B stars, about 5% host large-scale organised magnetic fields. These magnetic stars show also abundance anomalies in their spectra, and are therefore called the magnetic Ap/Bp stars. Most of these stars are also slow rotators compared to the normal A and B stars. Today, one of the greatest challenges concerning the Ap/Bp stars is to understand the origin of their slow rotation and their magnetic fields. The favoured hypothesis for the latter is that the fields are fossils, which implies that the magnetic fields subsist throughout the different evolutionary phases, and in particular during the pre-main sequence phase. The existence of magnetic fields at the pre-main sequence phase is also required to explain the slow rotation of Ap/Bp stars. During the last 3 years we performed a spectropolarimetric survey of the Herbig Ae/Be stars in the field and in young clusters, in order to investigate their magnetism and rotation. These investigations have resulted in the detection and/or confirmation of magnetic fields in 8 Herbig Ae/Be stars, ranging in mass from 2 to nearly 15 solar masses. In this paper I will present the results of our survey, as well as their implications for the origin and evolution of the magnetic fields and rotation of the A and B stars.

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Title: Discovery of magnetic fields in the very young, massive stars W601 (NGC 6611) and OI 201 (NGC 2244)
Authors: E. Alecian (RMC, LESIA), G.A. Wade (RMC), C. Catala (LESIA), S. Bagnulo (Armagh Observatory), T. Boehm (LATT), D. Bohlender (NRC), J.-C. Bouret (LAM), J.-F. Donati (LATT), C.P. Folsom (Armagh Observatory), J. Grunhut (RMC), J.D. Landstreet (UWO)

Context: Recent spectropolarimetric observations of Herbig Ae/Be stars have yielded new arguments in favour of a fossil origin for the magnetic fields of intermediate mass stars.
Aims: To study the evolution of these magnetic fields, and their impact on the evolution of the angular momentum of these stars during the pre-main sequence phase, we observed Herbig Ae/Be members of young open clusters of various ages.
Methods: We obtained high-resolution spectropolarimetric observations of Herbig Ae/Be stars belonging to the young open clusters NGC 6611 (< 6 Myr), NGC 2244 (~1.9 Myr), and NGC 2264 (~8 Myr), using ESPaDOnS at theCanada-France-Hawaii Telescope.
Results: Here we report the discovery of strong magnetic fields in two massive pre-main sequence cluster stars. We detected, for the first time, a magnetic field in a pre-main sequence rapid rotator: the 10.2 Msun Herbig B1.5e star W601 (NGC 6611; v sin i ~ 190 km/s). Our spectropolarimetric observations yield a longitudinal magnetic field larger than 1 kG, and imply a rotational period shorter than 1.7 days. The spectrum of this very young object (age ~ 0.017 Myr) shows strong and variable lines of He and Si. We also detected a magnetic field in the 12.1 Msun B1 star OI 201 (NGC 2244; v sin i = 23.5 km/s). The Stokes V profile of this star does not vary over 5 days, suggesting a long rotational period, a pole-on orientation, or aligned magnetic and rotation axes. OI 201 is situated near the Zero-Age Main Sequence on the HR diagram, and exhibits normal chemical abundances and no spectrum variability.

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Title: Young Stellar Groups Around Herbig Ae/Be Stars: A Low-Mass YSO Census
Authors: Shiya Wang, Leslie W. Looney

We present NIR and MIR observations of eight embedded young stellar groups around Herbig Ae/Be stars (HAEBEs) using archived Spitzer IRAC data and 2MASS data. These young stellar groups are nearby (< = 1 kpc) and still embedded within their molecular clouds. In order to identify the young stellar objects in our sample, we use the colour-colour diagram of J - (3.6) vs. Ks - (4.5). The Spitzer images of our sample show that the groups around HAEBEs, spectral types earlier than B8, are usually associated with bright infrared nebulosity. Within this, there are normally 10 - 50 young stars distributed close to the HAEBEs (< 1 pc). Not only are there young stars around the HAEBEs, there are also young stellar populations throughout the whole cloud, some are distributed and some are clumped. The groups around the HAEBEs are sub-structures of the large young population within the molecular cloud. The sizes of groups are also comparable with those sub-structures seen in massive clusters. Young stars in groups around HAEBEs have generally larger SED slopes compared to those outside, which suggests that the young stars in groups are probably younger than the distributed systems. This might imply that there is usually a higher and more continuous star forming rate in groups, that the formation of groups initiates later, or that low mass stars in groups form slower than those outside. Finally, there is no obvious trend between the SED slopes and the distance to the HAEBEs for those young stars within the groups. This suggests that the clustering of young stars dominates over the effect of massive stars on the low-mass young stars at the scale of our study.

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Blobrana · L

Title: Magnetic fields in Herbig Ae/Be stars
Authors: G.A. Wade, C. Catala, E. Alecian, C. Folsom, S. Bagnulo, T. Boehm, J.-C. Bouret, J.-F. Donati, D. Drouin, J.D. Landstreet

Studies of stellar magnetism at the pre-main sequence phase can provide important new insights into the detailed physics of the late stages of star formation, and into the observed properties of main sequence stars. This is especially true at intermediate stellar masses, where magnetic fields are strong and globally organised, and therefore most amenable to direct study. This paper reviews recent high-precision spectropolarimetric observations of pre-main sequence Herbig Ae/Be stars, which are yielding qualitatively new information about intermediate-mass stars: the origin and evolution of their magnetic fields; the role of magnetic fields in generating their spectroscopic activity and in mediating accretion in their late formative stages; the factors influencing their rotational angular momentum; and the development and evolution of chemical peculiarity in their photospheres.

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