paper

The dawn of a new era for dustless HdC stars with GAIA eDR3

arXiv:2112.07693 · doi:10.1051/0004-6361/202142916

Abstract

Decades after their discovery, only four hydrogen-deficient carbon (HdC) stars were known to have no circumstellar dust shell. This is in complete contrast to the 130 known Galactic HdC stars that are notorious for being heavy dust producers, i.e. the R Coronae Borealis (RCB) stars. Together they form a rare class of supergiant stars that are thought to originate from the merger of CO/He white dwarf binary systems, otherwise known as the double-degenerate scenario. We searched for new dustless HdC (dLHdC) stars. We used primarily the 2MASS and GAIA eDR3 catalogues to select candidates that were followed-up spectroscopically. We discovered 27 new dLHdC stars, one new RCB star and two new EHe stars. Surprisingly, 20 of the new dLHdC stars share a characteristic of the known dLHdC star HD 148839, having lower atmospheric hydrogen deficiencies. The uncovered population of dLHdC stars exhibit a Bulge-like distribution, like the RCB stars, but show multiple differences from those that indicate they are a different population of HdC stars following its own evolutionary sequence with a fainter luminosity and also a narrow range of effective temperature. We found indication of a current low dust production activity for four of the new dLHdC stars which could be typical RCB stars passing through a transition time. We have evidence for the first time of a large range of absolute magnitudes in the overall population of HdC stars, spanning over 3 mag. In the favoured formation framework, this is explained by a large range in the initial total WD binary mass which leads to a series of evolutionary sequences with distinct maximum brightness and initial temperature. The cold Galactic RCB stars are also noticeably fainter than the Magellanic ones, possibly due to a difference in metallicity resulting in different WD mass ratio. In our Galaxy, there could be as many dLHdC stars as RCB stars.

arXiv comment22 pages, 18 figures, accepted in A&A. V3: minor corrections on Table 2 and 3 related to HD stars names and (V-I)0 colours (Fig.10 changed accordingly). V4: changed the colour to light blue for the two new EHe stars indicated in Fig.10, page 13

References in corpus (66)

Cited by in corpus (8)