Introduction

The hydrogen-deficient carbon stars consist of several groups of probably-related objects with a combined known population in the Galaxy of about 100 stars. They are believed to be examples of post-AGB objects. Most exhibit a high degree of hydrogen-depletion (up to factors of 10), although their metal abundances range from 0.1 solar to solar (Lambert & Rao 1994). The cooler hydrogen-deficient stars are generally separated into (1) the RCB stars, highly variable objects well-known for their large amplitude dust-formation events (declines) (Cottrell 1996; Clayton 1996) and (2) the HdC stars, which may be low-activity RCB stars on account of their lower pulsational activity (Lawson & Cottrell 1997) and evidence for limited mass-loss (Walker 1986). The distinction between these two groups is often arbitrary, eg, XX Cam has been variously classified as RCB or HdC depending upon different characteristics of the star. The eHe stars are distinguished from the cool RCB/HdC stars in terms of ; the cool RCB/HdC stars have 5000-7500 K whereas the eHe stars have exceeding 8000 K.

Spectroscopic analyses of these stars (Cottrell & Lambert 1982; Pollard, Cottrell & Lawson 1994; Lambert & Rao 1994) indicate that these are low-gravity objects (log g 0-1). The only distance estimates, and therefore observationally determined values for or , are for RCB stars in the LMC.

Three LMC RCB stars have been known since discovery on the Harvard Observatory survey plates during the early decades of this century, but a larger population is currently emerging from the MACHO inventory of variable stars in the LMC. W Men and HV 12842 are F-type RCB stars, spectroscopically similar to the prototype galactic RCB/HdC stars R CrB and RY Sgr (Pollard et al. 1994). Assuming a distance modulus to the LMC of 18.7 (Feast & Catchpole 1997), W Men and HV 12842 have of -4.9 and -5.0 at maximum light, respectively. The cooler (K spectral type) LMC RCB star HV 5637 (Lawson et al. 1990; Glass, Lawson & Laney 1994) has = -4.2. Early results from the MACHO survey (Alcock et al. 1996; Clayton 1997) suggest a larger population (10-20 confirmed or probable RCB stars in fields examined to date) dominated by cooler, less-luminious, stars with = -3 to -4. After bolometric corrections are accounted for, these objects have mean luminosities at maximum light ranging from -4 to -5, with some probable scatter in for stars of similar spectral type.

Distances to the galactic hydrogen-deficient carbon stars would usefully contribute to a number of outstanding issues concerning these types of objects, eg, whether the different metallicity environments of the LMC and the Galaxy result in different luminosities, support the possibility of a period-luminosity relationship as suggested in the LMC RCB stars, in conjunction with atmospheric models provide intrinsic colours and values for circumstellar reddening due to the presence of dust, and provide a direct measure of the extent of large circumstellar features known to surround some RCB stars.

© Copyright Astronomical Society of Australia 1997