285 research outputs found
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 day rotation period is measured due to the 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, which
corresponds to a differential rotation rate of
rad day. 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
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
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