# A Chandra Study: Are Dwarf Carbon Stars Spun Up and Rejuvenated by Mass Transfer?

## TL;DR

This study investigates whether dwarf carbon stars, formed through mass transfer in binary systems, show signs of magnetic activity rejuvenation via X-ray emissions, challenging expectations based on their age and activity correlations.

## Contribution

First X-ray observational evidence suggesting dwarf carbon stars may have rejuvenated magnetic dynamos due to accretion spin-up from binary interactions.

## Key findings

- All six studied dCs detected in X-ray, indicating possible magnetic activity.
- X-ray luminosities suggest short rotation periods, consistent with dynamo rejuvenation.
- Further observations needed for detailed plasma temperature analysis.

## Abstract

Carbon stars (with C/O> 1) were long assumed to all be giants, because only AGB stars dredge up significant carbon into their atmospheres. The case is nearly iron-clad now that the formerly mysterious dwarf carbon (dC) stars are actually far more common than C giants, and have accreted carbon-rich material from a former AGB companion, yielding a white dwarf and a dC star that has gained both significant mass and angular momentum. Some such dC systems have undergone a planetary nebula phase, and some may evolve to become CH, CEMP, or Ba giants. Recent studies indicate that most dCs are likely from older, metal-poor kinematic populations. Given the well-known anti-correlation of age and activity, dCs would not be expected to show significant X-ray emission related to coronal activity. However, accretion spin-up might be expected to rejuvenate magnetic dynamos in these post mass-transfer binary systems. We describe our Chandra pilot study of six dCs selected from the SDSS for Halpha emission and/or a hot white dwarf companion, to test whether their X-ray emission strength and spectral properties are consistent with a rejuvenated dynamo. We detect all 6 dCs in the sample, which have X-ray luminosities ranging from logLx= 28.5 - 29.7, preliminary evidence that dCs may be active at a level consistent with stars that have short rotation periods of several days or less. More definitive results require a sample of typical dCs with deeper X-ray observations to better constrain their plasma temperatures.