Peculiar hydrogen-deficient carbon stars: strontium-rich stars and the s-process

Abstract

Context. R Coronae Borealis (RCB) variables and their non-variable counterparts, the dustless Hydrogen-deficient Carbon (dLHdC) stars have been known to exhibit enhanced s-processed material on their surfaces, especially Sr, Y, and Ba. No comprehensive work has been done to explore the s-process in these types of stars, however one particular RCB star, U Aqr, has been under scrutiny for its extraordinary Sr enhancement. Aims. We aim to identify RCB and dLHdC stars that have significantly enhanced Sr abundances, such as U Aqr, and use stellar evolution models to begin to estimate the type of neutron exposure that occurs in a typical HdC star. Methods. We compared the strength of the Sr II 4077 Å spectral line to Ca II H to identify the new subclass of Sr-rich HdCs. We additionally used the structural and abundance information from existing RCB MESA models to calculate the neutron exposure parameter, τ. Results. We identified six stars in the Sr-rich class. Two are RCBs, and four are dLHdCs. We additionally found that the preferred RCB MESA model has a neutron exposure τ 0.1 mb −1 , which is lower than the estimated τ between 0.15 and 0.6 mb −1 for the Sr-rich star U Aqr found in the literature. We found trends in the neutron exposure corresponding to He-burning shell temperature, metallicity, and assumed s-processing site. Conclusions. We have found a sub-class of six HdCs known as the Sr-rich class, which tend to lie in the halo, outside the typical distribution of RCBs and dLHdCs. We found that dLHdC stars are more likely to be Sr-rich than RCBs, with an occurrence rate of ∼13% for dLHdCs and ∼2% for RCBs. This is one of the first potential spectroscopic differences between RCBs and dLHdCs, along with dLHdCs having stronger surface abundances of 18 O. We additionally found neutron exposure trends in our RCB models that will aide in understanding the interplay between model parameters and surface s-process elements.

The dawn of a new era for dustless HdC stars withGaiaeDR3

“…The new series of Gaia RV measurements suggest that this phenomenon is more complex. Additionally, the situation is further complicated by the observed differences in chemical composition (Asplund et al 2000;Jeffery et al 2011;Crawford et al 2022) and luminosity (Tisserand et al 2022) among HdC stars, which can influence internal motions. Therefore, a more detailed analysis of the physical properties of these stars individually is necessary to elucidate the underlying mechanisms driving their RV variability and, consequently, their dust production rate.…”

An Infrared Census of R Coronae Borealis Stars II—Spectroscopic Classifications and Implications for the Rate of Low-mass White Dwarf Mergers

“…The differences between dLHdC and RCB stars have only recently started to be explored. The differences in their luminosities and chemical compositions suggest that dLHdC stars could be less massive than RCB stars (Crawford et al 2022;Karambelkar et al 2022;Tisserand et al 2022). In this picture, we would expect the maximum-light pulsation periods of these stars to differ.…”

“…As noted in Tisserand et al (2022), there could be as many, if not more, dLHdC stars in the Milky Way as RCB stars. There are several indications that the population of WDs that merge to from RCB stars have distinct properties from those that form dLHdC stars (Crawford et al 2022;Karambelkar et al 2022;Tisserand et al 2022). The lower luminosities and oxygen isotope ratios suggest that dLHdC stars could come from lower mass mergers than RCB stars.…”

“…Together, RCB and dLHdC stars constitute the class of Hydrogen-deficient Carbon (HdC) stars. The differences between RCB stars and dLHdC stars have been explored only recently, based on their positions in the HR diagram (Tisserand et al 2022), their oxygen isotope ratios (Karambelkar et al 2022), their strontium-abundances (Crawford et al 2022) and their H and Li content (Crawford et al 2023). These initial results suggest that RCB stars may be more massive than dLHdC stars.…”