The dawn of a new era for dustless HdC stars withGaiaeDR3

Abstract

Context. 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 (WD) binary systems, otherwise known as the double degenerate scenario. Aims. We searched for new dustless HdC (dLHdC) stars to understand their Galactic distribution, to estimate their total number in the Milky Way and to study their evolutionary link with RCB stars and extreme Helium stars, the final phase of HdC stars. Methods. We used primarily the 2MASS and GAIA eDR3 all-sky catalogues to select candidates that were then followed up spectroscopically. We studied the distribution of known and newly discovered stars in the HR diagram. Results. 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 temperatures, between 5000 and 8000 K. Not all the new dLHdC stars belong to that new population, as we found indication of a current low dust production activity around four of them: the warm F75, F152, C526 and the cold A166. Those could in fact be typical RCB stars passing through a transition time, going in or out of the RCB phase. Conclusions. For the first time we have evidence of a large range of absolute magnitude in the overall population of HdC stars, spanning more than 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 between the original population of stars resulting in different WD mass ratio. The unveiled population of dLHdC stars indicates that the ability to create dust could be linked to the initial total mass. In our Galaxy, there could be as many dLHdC stars as RCB stars.

The Asteroseismological Richness of RCB and dLHdC Stars

“…In addition, dLHdC stars have lower absolute magnitudes than RCB stars with the same color and shorter pulsation periods (Tisserand et al 2022), which as our models show, hints that dLHdC stars may have lower masses. If true, then dLHdC stars are ideal for studying pulsations in HdC stars, not just because they do not show dust ejection episodes but also because nonadiabatic effects are less important.…”

“…Furthermore, the inferred number of galactic RCB stars (e.g., Tisserand et al 2020) is broadly consistent with that expected from the He WD-CO WD merger rate (e.g., Ruiter et al 2009;Karakas et al 2015), whereas the final flash stars spend too little time in the RCB phase to explain the number of known RCB stars (Clayton et al 2006). It has been suggested that dLHdC stars are similarly formed through mergers but with different mass ratios (e.g., Karambelkar et al 2022;Tisserand et al 2022).…”

“…Similar periods are found in the infrared light curves of five RCB stars (Karambelkar et al 2021). Only 13 out of 32 dLHdC stars have detectable variability from ASAS-SN (Tisserand et al 2022), with lower peak-to-peak photometric amplitudes of 0.05-0.15 mag, shorter periods of 10-40 days, and lower radial velocity amplitudes (Lawson & Cottrell 1997;Tisserand et al 2023).…”

“…Because of its smaller radius, its pulsation periods (and period corresponding to n max ) are also shorter. This points to a smaller mass for dLHdC stars, which have shorter pulsation periods as well as lower absolute magnitudes than RCB stars (≈10-40 days; Tisserand et al 2022).…”

“…While RCB stars normally have M V between −3 and −5 (e.g., Alcock et al 2001;Tisserand et al 2009), they often show large optical photometric declines (sometimes reaching 9 mag) lasting years, caused by dust ejection episodes. Closely related to RCB stars are dustless hydrogen-deficient carbon (dLHdC) stars, which show RCB-like spectra but do not show dust ejection episodes (e.g., Warner 1967;Tisserand et al 2022). In this work, we do not distinguish between RCB and dLHdC stars but use HdC as an umbrella term encasing both.…”