The H$α$ emission of nearby M dwarfs and its relation to stellar rotation (1611.03509v1)

Abstract: The high-energy emission from low-mass stars is mediated by the magnetic dynamo. Although the mechanisms by which fully convective stars generate large-scale magnetic fields are not well understood, it is clear that, as for solar-type stars, stellar rotation plays a pivotal role. We present 270 new optical spectra of low-mass stars in the Solar Neighborhood. Combining our observations with those from the literature, our sample comprises 2202 measurements or non-detections of H$\alpha$ emission in nearby M dwarfs. This includes 466 with photometric rotation periods. Stars with masses between 0.1 and 0.6 solar masses are well-represented in our sample, with fast and slow rotators of all masses. We observe a threshold in the mass-period plane that separates active and inactive M dwarfs. The threshold coincides with the fast-period edge of the slowly rotating population, at approximately the rotation period at which an era of rapid rotational evolution appears to cease. The well- defined active/inactive boundary indicates that H$\alpha$ activity is a useful diagnostic for stellar rotation period, e.g. for target selection for exoplanet surveys, and we present a mass-period relation for inactive M dwarfs. We also find a significant, moderate correlation between $L_{\mathrm{H}\alpha}/L_{\mathrm{bol}}$ and variability amplitude: more active stars display higher levels of photometric variability. Consistent with previous work, our data show that rapid rotators maintain a saturated value of $L_{\mathrm{H}\alpha}/L_{\mathrm{bol}}$. Our data also show a clear power-law decay in $L_{\mathrm{H}\alpha}/L_{\mathrm{bol}}$ with Rossby number for slow rotators, with an index of $-1.7 \pm 0.1$.

Insights into Hα Emission and Stellar Rotation in M Dwarfs

The paper "The Hα emission of nearby M dwarfs and its relation to stellar rotation," authored by Elisabeth R. Newton et al., provides an expansive paper on the correlation between Hα emission, a proxy for stellar magnetic activity, and rotation periods in M dwarfs. Through their comprehensive dataset, Newton et al. investigate the underlying magnetic dynamo mechanisms as manifested in low-mass stars within the Solar Neighborhood.

Core Findings

• Data Collection and Sample Composition: The research incorporates 270 newly acquired optical spectra of M dwarfs combined with extant literature measurements, yielding a robust sample size of 2202 data points. Among these, 466 data points have well-determined photometric rotation periods, with a broad mass range coverage from 0.1 to 0.6 solar masses.
• Active/Inactive Threshold: The analysis delineates a clear mass-dependent boundary in the mass-period plane separating active and inactive M dwarfs. This boundary corresponds to a transition in stellar rotation, suggesting that the presence of Hα emission correlates with rapid stellar rotation, a finding relevant to exoplanet target selection processes. Notably, stars with rotation periods beyond ≈ 30 days for masses around 0.3 solar masses tend towards inactivity.
• Magnetic Activity Diagnostic: Hα activity serves as a reliable diagnostic for discerning rotational periods, especially in M dwarfs. The empirical mass-period relation provided for inactive stars contributes significant insights for future gyrochronology models.
• Saturation and Decay in Activity: The paper confirms that rapid rotators exhibit saturated Hα emission levels, while a power-law decay in activity (index of -1.7 ± 0.1) is apparent for slower rotators when analyzed against the Rossby number (Ro). The saturation occurs at Ro ≈ 0.2, offering a distinct comparative framework for M dwarfs against solar analogs.

Implications and Theoretical Considerations

The findings revisit the rotationally active domain in fully convective stars, positing that these stars retain an analogous rotation-activity relationship seen in solar analogs despite lacking a tachocline. The research substantiates the presence of a robust magnetic dynamo even in stars traditionally classified as fully convective, potentially driven by alternative dynamo processes rather than the classical αΩ mechanism.

Future Directions

This research opens avenues for further scrutiny on how stellar activity indices, like Hα emission, could enhance our understanding of stellar evolution, especially in the late evolutionary stages of M dwarfs. The research implications extend to refining exoplanet survey methodologies where rotational activity indicators act as proxies for determining stellar age, rotation period, and consequently the habitability potential of orbiting weakly irradiated planets.

Continuous spectroscopic monitoring could elucidate stellar magnetic field geometry intricacies and spot cycles, contributing richer datasets for calibration against theoretical dynamo models. Future efforts might integrate high-precision photometric data from space-based observatories to reduce systematic errors inherent in ground-based variability studies, thereby refining the accuracy of period determinations for extremely slow rotators. Overall, Newton et al.'s work forms a foundational basis for extending rotational and activity studies across other spectral classes, potentially influencing broader astrophysical paradigms concerning stellar magnetic dynamics.