The Spectra of Hydrogen-deficient Carbon Stars: The Effects of Evolution and Nucleosynthesis

Abstract

The rare class of stars known as the Hydrogen-deficient Carbon (HdC) stars includes the R Coronae Borealis (RCB) variables and the non-variable Dustless HdC (dLHdC) stars. These stars are believed to be formed via the merger of two white dwarf (WD) stars. They are known to exhibit many spectral peculiarities, such as partial helium burning products, enhancement of s-processed material and severe hydrogen-deficiency. In this work I explore many facets of HdC evolution. I begin by creating 18 HdC models in the stellar evolution code Modules for Experiments in Stellar Astrophysics (MESA) by merging two WD progenitors and evolving the post-merger star. These models span a range of metallicities and post-merger He-burning shell temperatures, and we find the best model to have 10% solar metallicity and a He-burning shell temperature of about 3.00 x 10^8 K. Next, I examine a multitude of HdC spectra for evidence of unique s-process enhancements and find a subclass of six HdC stars that I denote as Sr-rich. These stars show strongly enhanced light s-process elements such as Sr and Y, however the heavy s-process elements show significantly less enhancement, indicating a weak neutron exposure event. I also use the HdC MESA models to estimate a probable neutron exposure for a typical HdC, however this exercise does not explain the current amount of s-processed material in a typical HdC spectrum. Lastly, I use a combination of the MK process, a principal component analysis (PCA), and K-means clustering to create a spectral classification system for HdCs. This system is strongly inspired by the system used for traditional carbon stars, and notes many of the known spectral peculiarities in HdCs, including the Sr-rich class and some unique stars that show Li or H features.