# The Time-Domain Spectroscopic Survey: Radial Velocity Variability in Dwarf Carbon Stars

## TL;DR

This study investigates the binary nature of dwarf carbon stars using radial velocity data, revealing a high binary fraction and supporting their formation through mass transfer in close binary systems.

## Contribution

It provides the first comprehensive analysis of binary frequency and orbital properties of dwarf carbon stars using time-domain spectroscopy.

## Key findings

- Dwarf carbon stars have a binary fraction of approximately 95%.
- Most dC binaries have separations less than 1 AU, with periods around 1 year.
- Radial velocity variability confirms mass transfer as a key formation mechanism.

## Abstract

Dwarf carbon (dC) stars, main sequence stars showing carbon molecular bands, were initially thought to be an oxymoron since only AGB stars dredge carbon into their atmospheres. Mass transfer from a former AGB companion that has since faded to a white dwarf seems the most likely explanation. Indeed, a few types of giants known to show anomalous abundances --- notably, the CH, Ba and CEMP-s stars --- are known to have a high binary frequency. The dC stars may be the enhanced-abundance progenitors of most, if not all, of these systems, but this requires demonstrating a high binary frequency for dCs. Here, for a sample of 240 dC stars targeted for repeat spectroscopy by the SDSS-IV's Time Domain Spectroscopic Survey, we analyze radial velocity variability to constrain the binary frequency and orbital properties. A handful of dC systems show large velocity variability ($>$100 km s$^{-1}$). We compare the dCs to a control sample with a similar distribution of magnitude, color, proper motion, and parallax. Using MCMC methods, we use the measured $\Delta$RV distribution to estimate the binary fraction and the separation distribution assuming both a unimodal and bimodal distribution. We find the dC stars have an enhanced binary fraction of 95\%, consistent with them being products of mass transfer. These models result in mean separations of less than 1 AU corresponding to periods on the order of 1 year. Our results support the conclusion that dC stars form from close binary systems via mass transfer.