Detecting Differential Rotation and Starspot Evolution on the M dwarf GJ 1243 with Kepler

We present an analysis of the starspots on the active M4 dwarf GJ 1243, using four years of time series photometry from Kepler. A rapid $P = 0.592596\pm0.00021$ day rotation period is measured due to the $\sim$2.2\% starspot-induced flux modulations in the light curve. We first use a light curve modeling approach, using a Monte Carlo Markov Chain sampler to solve for the longitudes and radii of the two spots within 5-day windows of data. Within each window of time the starspots are assumed to be unchanging. Only a weak constraint on the starspot latitudes can be implied from our modeling. The primary spot is found to be very stable over many years. A secondary spot feature is present in three portions of the light curve, decays on 100-500 day timescales, and moves in longitude over time. We interpret this longitude shearing as the signature of differential rotation. Using our models we measure an average shear between the starspots of 0.0047 rad day$^{-1}$, which corresponds to a differential rotation rate of $\Delta\Omega = 0.012 \pm 0.002$ rad day$^{-1}$. We also fit this starspot phase evolution using a series of bivariate Gaussian functions, which provides a consistent shear measurement. This is among the slowest differential rotation shear measurements yet measured for a star in this temperature regime, and provides an important constraint for dynamo models of low mass stars.Comment: 13 pages, 7 figures, ApJ Accepte

M Dwarf Flares from Time-Resolved SDSS Spectra

We have identified 63 flares on M dwarfs from the individual component spectra in the Sloan Digital Sky Survey using a novel measurement of emission line strength called the Flare Line Index. Each of the ~38,000 M dwarfs in the SDSS low mass star spectroscopic sample of West et al. was observed several times (usually 3-5) in exposures that were typically 9-25 minutes in duration. Our criteria allowed us to identify flares that exhibit very strong H-alpha and H-beta emission line strength and/or significant variability in those lines throughout the course of the exposures. The flares we identified have characteristics consistent with flares observed by classical spectroscopic monitoring. The flare duty cycle for the objects in our sample is found to increase from 0.02% for early M dwarfs to 3% for late M dwarfs. We find that the flare duty cycle is larger in the population near the Galactic plane and that the flare stars are more spatially restricted than the magnetically active but non-flaring stars. This suggests that flare frequency may be related to stellar age (younger stars are more likely to flare) and that the flare stars are younger than the mean active population.Comment: 38 pages, 10 figures, Accepted for publication in AJ. Note that Figure 6 is shown here at lower resolutio

The \chi Factor: Determining the Strength of Activity in Low Mass Dwarfs

We describe a new, distance-independent method for calculating the magnetic activity strength in low mass dwarfs, L_{H\alpha}/L_{bol}. Using a well-observed sample of nearby stars and cool standards spanning spectral type M0.5 to L0, we compute ``\chi'', the ratio between the continuum flux near H-alpha and the bolometric flux, f_{\lambda6560}/f_{bol}. This ratio may be multiplied by the measured equivalent width of the H-alpha emission line to yield L_{H\alpha}/L_{bol}. We provide \chi values for all objects in our sample, as well as fits to \chi as a function of color and average values by spectral type. This method was used by West et al.(2004) to examine trends in magnetic activity strength in low mass stars.Comment: 11 pages, 5 figures. Accepted for publication in PAS