The following document lists the file abstract/RWATERS_BESTARS.abs
from catalogue VI/111.
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SCIENTIFIC ABSTRACT Be stars are B-type main sequence stars or giants that show or have shown H-alpha emission. They are on average rapid rotators, with vsini values above 150-200 km/s. It is generally accepted that Be stars are surrounded by a rotating circumstellar disc which is ejected by the star via a mechanism that is as yet poorly understood. In order to improve our knowledge of the mechanism which forms these discs it is essential to map their density and velocity structure, preferentially for a range of spectral types (i.e. stellar luminositites and temperatures). The aim of this proposal is to use infrared recombination lines of Hydrogen and Helium, as well as forbidden lines, to study the velocity field in Be star discs. Infrared spectroscopy of these discs has several advantadges over optical data. The IR lines are much easier to model than the optical Balmer or Paschen lines. Furthermore the IR lines have only weak photospheric components and so the correction for the underlying photospheric absorption lines introduces much less uncertainty in the emission contribution from the disc. Infrared recombination line widths depend strongly on the details of the velocity law (both rotational and radial) as a function of distance. The ISO grating spectrometer has sufficient resolution to determine the width of the lines (FWHM) as a function of wavelength and line strength, and so will allow us to map especially the rotational velocity field. Nothing is known about this rotational velocity field. Recent observations indicate that some discs rotate faster than the underlying star. If this is the case, some mechanism (possibly magnetic fields) may be responsible for spinning up these discs. The line strengths are sensitive probes of the density structure, both in the radial and in the vertical direction. In order to have sufficient constraints on models, it is important to observe many lines over a wide wavelength range and optical depth range, i.e. SWS is the ideal instrument to obtain such spectra. We also propose to investigate the high-level IR Hydrogen recombination lines in 'normal' B stars. Model calculations suggest that these lines are very senstive to the presence of small amounts of circumstellar gas, or to deviations from classical LTE model atmospheres. The recent discovery of Bralpha emission in tau Sco illustrates that normal B stars can have infrared Hydrogen recombination lines without having Balmer emission lines. Since the high-level HI recombination lines are not observable from the ground, SWS observations are the only way to obtain this information. These observations will therefore allow us to study mass loss in main sequence B stars, of which nothing is known at present. OBSERVATION SUMMARY. We propose to observe HI and HeI recombination lines and forbidden line transitions in the Be stars listed in the tables below, using the SWS in the grating mode (AOT02). We have selected a range of line strengths and optical depths to probe the circumstellar disc. In addition we request a PHT-S spectrum for all targets. For Gamma Cas we request a full SWS scan at reduced resolution (AOT01). We plan to observe the following lines with SWS AOT02: [Fe II] 5.34 [Ni II] 6.62 [Fe III] 22.94 [Fe II] 25.97 Br Alfa 4.05 Pf Alfa 7.46 Hu Bet 7.50 Hu Alfa 12.36 HI 10-7 8.76 HI 9-7 11.31 HI 8-7 19.05 HI 11-9 22.94 He I 4.03 He I 4.60 He I 23.04