Other
Scientific paper
Nov 1990
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990mnras.247...91l&link_type=abstract
Monthly Notices of the Royal Astronomical Society, Vol. 247, NO. 1/NOV1, P. 91, 1990
Other
96
Scientific paper
A photometric survey of cool hydrogen-deficient carbon (HdC) stars, including many R Coronae Borealis (RCB) stars, has been carried out in order to more fully investigate the properties of these stars. Photometry has been obtained over intervals of 700-1100 d for many of these stars. This has enabled a more accurate analysis of the light and colour curves of these objects than has been possible with previously published data. More than 1100 nightly mean observations are reported for 16 stars. The objects surveyed include the RCB stars in the Large Magellanic Cloud. Our discussion of the RCB stars emphasizes the properties of these stars when they are at maximum light. Properties of the low-amplitude variations observed in all of these stars are described and analysed using Fourier techniques. Synthetic light curves are produced for some stars.
Preliminary results of a spectroscopic survey of these stars are also discussed, in so far as these results influence the interpretation of the photometry. For the 20 galactic stars for which spectra are available, 13 stars appear similar to the RCB stars R CrB and RY Sgr, which have Teff ≍ 7000 K. By combining the observed periods and estimated temperatures, we produce a period-temperature relationship for these stars. Most of these stars have periods that compare with theoretical periods for HdC stars in this temperature range.
Theoretical period decrease rates for HdC stars indicate that cooler (Teff ≍ 5000 K) stars should dominate the temperature distribution. This is not observed. We discuss the reasons for this apparent lack of cooler stars and implications for the numbers of these objects in the Galaxy. If the evolutionary models are correct, we predict that there should be many more cool HdC stars still to be discovered.
Another implication of the estimated temperatures is that many of these stars are not as cool as suggested by their photometric colour indices. Consequently, there must be large amounts of reddening in the directions of these stars, many of which have low galactic latitudes. This is not surprising given their assumed luminosity Mbol ≍ -5. From the general appearance of the spectra, we de-redden available photometry for these stars to produce intrinsic magnitudes and, hence, distances.
Cottrell Peter L.
Gilmore Alan C.
Kilmartin Pam M.
Lawson Warrick A.
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