Ultraviolet Spectroscopy of R Coronae Borealis Stars -- Broad Lines from an Accretion Disc?
Abstract
STIS spectra of three R Coronae Borealis {RCB} stars will provide novel data on hot gas near these stars; such data can be obtained only from HST. RCBs are hydrogen-deficient and fade at unpredictable times as a carbon soot cloud obscures the star. Optical spectra taken when a star has faded reveal an emission line spectrum containing sharp {FWHM 15 km s^-1} and broad {FWHM 250 km s^-1} lines. The latter are of much higher excitation than the former. For R CrB, it has been suggested that the broad lines arise from an accretion disk around a compact secondary. These optical lines are visible only during the infrequent extreme fadings of a RCB. Although IUE spectra reveal high-excitation emission lines at maximum light, the IUE spectra lack the spectral resolution to distinguish broad from sharp lines. STIS spectra will resolve the broad-line from a sharp-line component, and, in the case, of R CrB, where repeat visits are requested, be used to search for velocity variations as the companion orbits the RCB. In addition, the line intensity ratios will provide new estimates of the physical conditions of the gas emitting the broad lines. This proposal may help to answer the question - How are RCBs formed? And this answer may show that, although RCBs are rare, many low mass stars evolve through the RCB on their path to extinction as a white dwarf.
- Publication:
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HST Proposal
- Pub Date:
- July 2000
- Bibcode:
- 2000hst..prop.8615L