499 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
BOSS Ultracool Dwarfs I: Colors and Magnetic Activity of M and L dwarfs
We present the colors and activity of ultracool (M7-L8) dwarfs from the Tenth
Data Release of the Sloan Digital Sky Survey (SDSS). We combine previous
samples of SDSS M and L dwarfs with new data obtained from the Baryon
Oscillation Sky Survey (BOSS) to produce the BOSS Ultracool Dwarf (BUD) sample
of 11820 M7-L8 dwarfs. By combining SDSS data with photometry from the Two
Micron All Sky Survey and the Wide-Field Infrared Sky Explorer mission, we
present ultracool dwarf colors from to as a function of spectral
type, and extend the SDSS-2MASS-WISE color locus to include ultracool dwarfs.
The , , and colors provide the best indication of spectral type
for M7-L3 dwarfs. We also examine ultracool dwarf chromospheric activity
through the presence and strength of H emission. The fraction of active
dwarfs rises through the M spectral sequence until it reaches 90% at
spectral type L0. The fraction of active dwarfs then declines to 50% at
spectral type L5; no H emission is observed in the late-L dwarfs in the
BUD sample. The fraction of active L0-L5 dwarfs is much higher than previously
observed. The strength of activity declines with spectral type from M7 through
L3, after which the data do not show a clear trend. Using one-dimensional
chromosphere models, we explore the range of filling factors and chromospheric
temperature structures that are consistent with H observations of M0-L7
dwarfs. M dwarf chromospheres have a similar, smoothly varying range of
temperature and surface coverage while L dwarf chromospheres are cooler and
have smaller filling factors.Comment: 24 pages and 13 figures, submitted to AJ. A short video describing
these results can be found at https://www.youtube.com/watch?v=wwX5WkuJCU
Do Kepler superflare stars really include slowly-rotating Sun-like stars ? - Results using APO 3.5m telescope spectroscopic observations and Gaia-DR2 data -
We report the latest view of Kepler solar-type (G-type main-sequence)
superflare stars, including recent updates with Apache Point Observatory (APO)
3.5m telescope spectroscopic observations and Gaia-DR2 data. First, we newly
conducted APO3.5m spectroscopic observations of 18 superflare stars found from
Kepler 1-min time cadence data. More than half (43 stars) are confirmed to be
"single" stars, among 64 superflare stars in total that have been
spectroscopically investigated so far in this APO3.5m and our previous
Subaru/HDS observations. The measurements of (projected rotational
velocity) and chromospheric lines (Ca II H\&K and Ca II 8542\AA) support the
brightness variation of superflare stars is caused by the rotation of a star
with large starspots. We then investigated the statistical properties of Kepler
solar-type superflare stars by incorporating Gaia-DR2 stellar radius estimates.
As a result, the maximum superflare energy continuously decreases as the
rotation period increases. Superflares with energies
erg occur on old, slowly-rotating Sun-like stars
(25 days) approximately once every 2000--3000 years,
while young rapidly-rotating stars with a few days have
superflares up to erg. The maximum starspot area does not depend on
the rotation period when the star is young, but as the rotation slows down, it
starts to steeply decrease at 12 days for Sun-like
stars. These two decreasing trends are consistent since the magnetic energy
stored around starspots explains the flare energy, but other factors like spot
magnetic structure should also be considered.Comment: 71 pages, 31 figures, 10 tables. Accepted for publication in The
Astrophysical Journal (on March 29, 2019
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