The Rotation of M Dwarfs Observed by the Apache Point Galactic Evolution Experiment

TL;DR

This study analyzes the rotational velocities of 714 M dwarf stars using APOGEE spectroscopic data, revealing increased rapid rotation in fully convective stars and comparing spectroscopic and photometric rotation measurements.

Contribution

It provides the first large-scale spectroscopic analysis of M dwarf rotation velocities, highlighting the transition to fully convective interiors and comparing results with photometric surveys.

Findings

Rapid rotators increase near the M4 transition.

Spectroscopic and photometric rotation fractions are broadly consistent.

Photometric surveys show fewer slow rotators beyond M4 by a factor of two.

Abstract

We present the results of a spectroscopic analysis of rotational velocities in 714 M dwarf stars observed by the SDSS III Apache Point Galactic Evolution Experiment (APOGEE) survey. We use a template fitting technique to estimate $v\sin{i}$ while simultaneously estimating $\log{g}$, $[\text{M}/\text{H}]$, and $T_{\text{eff}}$. We conservatively estimate that our detection limit is 8 km s$^{-1}$. We compare our results to M dwarf rotation studies in the literature based on both spectroscopic and photometric measurements. Like other authors, we find an increase in the fraction of rapid rotators with decreasing stellar temperature, exemplified by a sharp increase in rotation near the M$4$ transition to fully convective stellar interiors, which is consistent with the hypothesis that fully convective stars are unable to shed angular momentum as efficiently as those with radiative cores. We compare a sample of targets observed both by APOGEE and the MEarth transiting planet survey and find no cases were the measured $v\sin{i}$ and rotation period are physically inconsistent, requiring $\sin{i}>1$. We compare our spectroscopic results to the fraction of rotators inferred from photometric surveys and find that while the results are broadly consistent, the photometric surveys exhibit a smaller fraction of rotators beyond the M$4$ transition by a factor of $\sim 2$. We discuss possible reasons for this discrepancy. Given our detection limit, our results are consistent with a bi-modal distribution in rotation that is seen in photometric surveys.