3,376 research outputs found
KOI-1003: A new spotted, eclipsing RS CVn binary in the Kepler field
Using the high-precision photometry from the Kepler space telescope,
thousands of stars with stellar and planetary companions have been observed.
The characterization of stars with companions is not always straightforward and
can be contaminated by systematic and stellar influences on the light curves.
Here, through a detailed analysis of starspots and eclipses, we identify
KOI-1003 as a new, active RS CVn star---the first identified with data from
Kepler. The Kepler light curve of this close binary system exhibits the
system's primary transit, secondary eclipse, and starspot evolution of two
persistent active longitudes. The near equality of the system's orbital and
rotation periods indicates the orbit and primary star's rotation are nearly
synchronized ( days; days). By assuming the secondary star is on the main sequence, we suggest
the system consists of a subgiant primary and
a main-sequence companion. Our work gives a
distance of pc and an age of Gyr,
parameters which are discrepant with previous studies that included the star as
a member of the open cluster NGC 6791.Comment: 21 pages, 19 figures, accepted to Ap
Differential rotation of Kepler-71 via transit photometry mapping of faculae and starspots
Knowledge of dynamo evolution in solar-type stars is limited by the difficulty of using active region monitoring to measure stellar differential rotation, a key probe of stellar dynamo physics. This paper addresses the problem by presenting the first ever measurement of stellar differential rotation for a main-sequence solar-type star using starspots and faculae to provide complementary information. Our analysis uses modelling of light curves of multiple exoplanet transits for the young solar-type star Kepler-71, utilizing archival data from the Kepler mission. We estimate the physical characteristics of starspots and faculae on Kepler-71 from the characteristic amplitude variations they produce in the transit light curves and measure differential rotation from derived longitudes. Despite the higher contrast of faculae than those in the Sun, the bright features on Kepler-71 have similar properties such as increasing contrast towards the limb and larger sizes than sunspots. Adopting a solar-type differential rotation profile (faster rotation at the equator than the poles), the results from both starspot and facula analysis indicate a rotational shear less than about 0.005 rad d-1, or a relative differential rotation less than 2 per cent, and hence almost rigid rotation. This rotational shear contrasts with the strong rotational shear of zero-age main-sequence stars and the modest but significant shear of the modern-day Sun. Various explanations for the likely rigid rotation are considered
Molecules as magnetic probes of starspots
Stellar dynamo processes can be explored by measuring the magnetic field.
This is usually obtained using the atomic and molecular Zeeman effect in
spectral lines. While the atomic Zeeman effect can only access warmer regions,
the use of molecular lines is of advantage for studying cool objects. The
molecules MgH, TiO, CaH, and FeH are suited to probe stellar magnetic fields,
each one for a different range of spectral types, by considering the signal
that is obtained from modeling various spectral types. We have analyzed the
usefulness of different molecules (MgH, TiO, CaH, and FeH) as diagnostic tools
for studying stellar magnetism on active G-K-M dwarfs. We investigate the
temperature range in which the selected molecules can serve as indicators for
magnetic fields on highly active cool stars and present synthetic Stokes
profiles for the modeled spectral type. We modeled a star with a spot size of
10% of the stellar disk and a spot comprising either only longitudinal or only
transverse magnetic fields and estimated the strengths of the polarization
Stokes V and Q signals for the molecules MgH, TiO, CaH, and FeH. We combined
various photosphere and spot models according to realistic scenarios. In G
dwarfs, the molecules MgH and FeH show overall the strongest Stokes V and Q
signals from the starspot, whereas FeH has a stronger Stokes V signal in all G
dwarfs, with a spot temperature of 3800K. In K dwarfs, CaH signals are
generally stronger, and the TiO signature is most prominent in M dwarfs.
Modeling synthetic polarization signals from starspots for a range of G-K-M
dwarfs leads to differences in the prominence of various molecular signatures
in different wavelength regions, which helps to efficiently select targets and
exposure times for observations.Comment: 9 pages, 5 figures, 1 tabl
The effect of starspots on the radii of low-mass pre-main sequence stars
A polytropic model is used to investigate the effects of dark photospheric
spots on the evolution and radii of magnetically active, low-mass (M<0.5Msun),
pre-main sequence (PMS) stars. Spots slow the contraction along Hayashi tracks
and inflate the radii of PMS stars by a factor of (1-beta)^{-N} compared to
unspotted stars of the same luminosity, where beta is the equivalent covering
fraction of dark starspots and N \simeq 0.45+/-0.05. This is a much stronger
inflation than predicted by the models of Spruit & Weiss (1986) for main
sequence stars with the same beta, where N \sim 0.2 to 0.3. These models have
been compared to radii determined for very magnetically active K- and M-dwarfs
in the young Pleiades and NGC 2516 clusters, and the radii of tidally-locked,
low-mass eclipsing binary components. The binary components and ZAMS K-dwarfs
have radii inflated by \sim 10 per cent compared to an empirical
radius-luminosity relation that is defined by magnetically inactive field
dwarfs with interferometrically measured radii; low-mass M-type PMS stars, that
are still on their Hayashi tracks, are inflated by up to \sim 40 per cent. If
this were attributable to starspots alone, we estimate that an effective spot
coverage of 0.35 < beta < 0.51 is required. Alternatively, global inhibition of
convective flux transport by dynamo-generated fields may play a role. However,
we find greater consistency with the starspot models when comparing the loci of
active young stars and inactive field stars in colour-magnitude diagrams,
particularly for the highly inflated PMS stars, where the large, uniform
temperature reduction required in globally inhibited convection models would
cause the stars to be much redder than observed.Comment: MNRAS in press, 13 page
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
Formation of starspots in self-consistent global dynamo models: Polar spots on cool stars
Observations of cool stars reveal dark spot-like features on their surfaces.
Compared to sunspots, starspots can be bigger or cover a larger fraction of the
stellar surface. While sunspots appear only at low latitudes, starspots are
also found in polar regions, in particular on rapidly rotating stars. Sunspots
are believed to result from the eruption of magnetic flux-tubes rising from the
deep interior of the Sun. The strong magnetic field locally reduces convective
heat transport to the solar surface. Such flux-tube models have also been
invoked to explain starspot properties. However, these models use several
simplifications and so far the generation of either sunspots or starspots has
not been demonstrated in a self-consistent simulation of stellar magnetic
convection. Here we show that direct numerical simulations of a distributed
dynamo operating in a density-stratified rotating spherical shell can
spontaneously generate cool spots. Convection in the interior of the model
produces a large scale magnetic field which interacts with near surface
granular convection leading to strong concentrations of magnetic flux and
formation of starspots. Prerequisites for the formation of sizeable
high-latitude spots in the model are sufficiently strong density stratification
and rapid rotation. Our model presents an alternate mechanism for starspot
formation by distributed dynamo action.Comment: 14 pages; Important additions in version 2; To appear in A&
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