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arXiv · 2110.00013

New Clues to the Evolution of Dwarf Carbon Stars From Their Variability and X-ray Emission

Benjamin R. Roulston·Paul J. Green·Rodolfo Montez·Joseph Filippazzo·Jeremy J. Drake·Silvia Toonen·Scott F. Anderson·Michael Eracleous·Adam Frank
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Abstract

As main-sequence stars with C$>$O, dwarf carbon (dC) stars are never born alone but inherit carbon-enriched material from a former asymptotic giant branch (AGB) companion. In contrast to M dwarfs in post-mass transfer binaries, C$_2$ and/or CN molecular bands allow dCs to be identified with modest-resolution optical spectroscopy, even after the AGB remnant has cooled beyond detectability. Accretion of substantial material from the AGB stars should spin up the dCs, potentially causing a rejuvenation of activity detectable in X-rays. Indeed, a few dozen dCs have recently been found to have photometric variability with periods under a day. However, most of those are likely post-common-envelope binaries (PCEBs), spin-orbit locked by tidal forces, rather than solely spun-up by accretion. Here, we study the X-ray properties of a sample of the five nearest known dCs with $Chandra$. Two are detected in X-rays, the only two for which we also detected short-period photometric variability. We suggest that the coronal activity detected so far in dCs is attributable to rapid rotation due to tidal locking in short binary orbits after a common-envelope phase, late in the thermally pulsing (TP) phase of the former C-AGB primary (TP-AGB).

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Benjamin R. Roulston, Paul J. Green, Rodolfo Montez, Joseph Filippazzo, Jeremy J. Drake, Silvia Toonen, Scott F. Anderson, Michael Eracleous, Adam Frank. 2021-09-30. New Clues to the Evolution of Dwarf Carbon Stars From Their Variability and X-ray Emission. https://doi.org/10.3847/1538-4357%2Fac4706

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