Probing Accretion Spin-Up in Binary Main-Sequence Carbon Stars

Main-Sequence (dwarf) carbon stars, with C/O>1 and strong molecular C₂ and CN bands in their optical spectra, are a prime population for testing a variety of the most significant uncertainties in binary evolution. As main-sequence stars, their carbon enhancement must have come from substantial mass accretion from a previous AGB companion. In most cases, the remnant white dwarf has cooled beyond detectability. Along with the mass, the dwarf carbon star should have accreted significant amounts of angular momentum, spinning up the dwarf carbon star into a few Gyr-long rejuvenated active phase. This active phase should result in increased stellar activity detectable in X-rays. We present our Chandra studies of three samples of dwarf carbon stars for which we detect X-ray emission. Using light curves from the ZTF survey, we find that the X-ray-detected dwarf carbon stars all are in short-period post-common-envelope binaries. These systems are tidally locked, so their fast rotations can not be attributed to accretion-induced spin-up alone. This motivates future studies to observe wide binary dwarf carbon stars to search for X-ray detections that will unambiguously probe accretion spin-up in these systems.