Why Are Rapidly Rotating M Dwarfs in the Pleiades So (Infra)red? New Period Measurements Confirm Rotation-Dependent Color Offsets From the Cluster Sequence

Abstract

Stellar rotation periods (P rot ) measured in open clusters have proved to be extremely useful for studying stars' angular momentum content and rotationally driven magnetic activity, which are both age-and mass-dependent processes. While P rot measurements have been obtained for hundreds of solar-mass members of the Pleiades, measurements exist for only a few low-mass (<0.5 M ) members of this key laboratory for stellar evolution theory. To fill this gap, we report P rot for 132 low-mass Pleiades members (including nearly 100 with M ≤ 0.45 M ), measured from photometric monitoring of the cluster conducted by the Palomar Transient Factory in late 2011 and early 2012. These periods extend the portrait of stellar rotation at 125 Myr to the lowest-mass stars and re-establish the Pleiades as a key benchmark for models of the transport and evolution of stellar angular momentum. Combining our new P rot with precise BV IJHK photometry reported by Stauffer et al. and Kamai et al., we investigate known anomalies in the photometric properties of K and M Pleiades members. We confirm the correlation detected by Kamai et al. between a star's P rot and position relative to the main sequence in the cluster's color-magnitude diagram. We find that rapid rotators have redder (V − K) colors than slower rotators at the same V , indicating that rapid and slow rotators have different binary frequencies and/or photospheric properties. We find no difference in the photometric amplitudes of rapid and slow rotators, indicating that asymmetries in the longitudinal distribution of starspots do not scale grossly with rotation rate.

The SPOTS Models: A Grid of Theoretical Stellar Evolution Tracks and Isochrones for Testing the Effects of Starspots on Structure and Colors

“…This hypothesis is supported by the work of Kamai et al (2014): these authors found a positive correlation between rotation rate and color abnormality, suggesting that the most rapidly rotating, and thus perhaps the most heavily spotted stars, show the greatest abnormalities. A rotation-color correlation was later demonstrated for the stars less massive than spectral-type K by Covey et al (2016). Moreover, Somers & Stassun (2017) found a connection between rotation rate and radius inflation within this cluster, signifying that magnetic activity is likely influencing the structure of these K-dwarfs.…”

Placing the Spotted T Tauri Star LkCa 4 on an HR Diagram

“…For example, anomalous colors of K dwarfs in the Pleiades can be explained by ∼50% fill factors of spots (Stauffer et al 2003), while LAMOST spectra of the TiO band indicates that ∼ 30% spot coverages may be common for 3500-5000 K stars (Fang et al 2016). Recent photometric monitoring of the Pleiades indicates that the anomalous colors scale with rotation rate, strengthing the case for magnetic-field induced starspots as the cause of the anomaly (Covey et al 2016). However, the Covey et al (2016) Pleiades rotation rates do not scale with the ∆V photometric amplitude at the precision of the PTF data, suggesting that while the overall starspot coverage increases with rotation rate, the longitudinally asymmetric component does not.…”

“…Recent photometric monitoring of the Pleiades indicates that the anomalous colors scale with rotation rate, strengthing the case for magnetic-field induced starspots as the cause of the anomaly (Covey et al 2016). However, the Covey et al (2016) Pleiades rotation rates do not scale with the ∆V photometric amplitude at the precision of the PTF data, suggesting that while the overall starspot coverage increases with rotation rate, the longitudinally asymmetric component does not. LkCa 4 represents an example of such an architecture, with more than 50% of the visible stellar surface covered in spots at all observational phases.…”

Planets Around Low-Mass Stars (Palms). Vi. Discovery of a Remarkably Red Planetary-Mass Companion to the Ab Dor Moving Group Candidate 2mass J22362452+4751425*

“…Hartman et al (2011) measure a period of 11.2days for 2M2236+4751; compared to the Pleaides rotation distribution, which has a comparable age to AB Dor and a well characterized stellar population, this period sits near the maximum envelope of rotation rates for stellar masses of ≈0.6M e (Hartman et al 2010;Covey et al 2016;Rebull et al 2016). K7 members in AB Dor exhibit a similarly broad range of rotation periods from 0.4-9.3 days (Table 3).…”