Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993hst..prop.4328m&link_type=abstract
HST Proposal ID #4328
Other
Hst Proposal Id #4328
Scientific paper
Until very recently, virtually all carbon (C) stars were thought to have dredged up their carbon during post main sequence evolution. The recent recognition of dwarf C (dC) stars as a class unto themselves presents a major puzzle for stellar evolution theory unless they can all be explained as the result of mass transfer from a former giant companion that has since evolved into a white dwarf (WD). Strongly suggestive evidence for this scenario is presented by two examples. The prototype dC star, G77-61, is a spectroscopic binary with known parallax. This system is only weakly if at all detected in IUE spectra, setting an upper limit of T=7000 K to the temperature of the WD companion. PG 0824+289, on the other hand, is a hot DA that has been shown by optical spectroscopy to be the companion to a dC star. We have observed that other dCs show no evidence in the optical for a WD companion. If they are systems with long periods similar to G77-61, determination of binarity from radial velocity variations is impractical for these fainter dC stars. G77-61 and PG 0824+289 illustrate that dC/WD systems trace a wide range of evolutionary histories in the Galaxy. A low resolution UV survey with the FOS simultaneously tests for the existence of white dwarf companions as mandated by the progenitor hypothesis of mass transfer, and further serves to define the most basic properties of these companions, namely temperature and age.
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