Presentation #225.03D in the session Fundamental Properties II.
Dwarf carbon (dC) stars have been an astrophysical enigma since their discovery. These main-sequence stars, which show carbon molecular (C2, CN, CH) bands in their optical spectra, represent an exciting class of post-mass transfer binaries. As main-sequence stars, dCs cannot have produced carbon themselves. Instead, this carbon excess must have come from an evolved companion, now a white dwarf, via mass transfer. dC stars, therefore, represent an excellent sample for testing stellar physics, including common-envelope evolution, wind accretion, mass transfer efficiencies, and AGB mass-loss rates, as well as accretion spin-up and mixing in stellar envelopes. However, there is a lack of the fundamental properties of dCs in the literature, which will impede any effort to use dCs to study these phenomena. I aimed to fill these gaps by investigating the properties and formation pathways of dC stars. First, I determined the binary fraction of dC stars to be consistent with 100% binarity. Second, I fit orbital separation distributions by using radial velocity variations from a large sample of dC stars observed in the Sloan Digital Sky Survey. Additionally, I found 34 new periodic dC stars, which dramatically increases the number of measured periods, adding to the understanding of which mass transfer mechanisms are likely to be important in dC star formation. Interestingly, ≈82% of these photometrically periodic dC stars have short periods (P < 2d), marking them as post-common-envelope binaries. I have also searched for signs of activity in dC stars using Chandra and two different dC samples. I found that the X-ray emission from dwarf carbon stars is consistent with rapid rotation seen in samples of normal GKM dwarfs.