227 research outputs found
An empirical calibration to estimate cool dwarf fundamental parameters from H-band spectra
Interferometric radius measurements provide a direct probe of the fundamental
parameters of M dwarfs, but is within reach for only a limited sample of
nearby, bright stars. We use interferometrically-measured radii, bolometric
luminosities, and effective temperatures to develop new empirical calibrations
based on low-resolution, near-infrared spectra. We use H-band Mg and Al
features to derive calibrations for effective temperature, radius and log
luminosity; the standard deviations in the residuals of our best fits are,
respectively, 73K, 0.027Rsun, and 0.049 dex (11% error on luminosity). These
relationships are valid for mid K to mid M dwarf stars, roughly corresponding
to temperatures between 3100 and 4800K. We apply our calibrations to M dwarfs
targeted by the MEarth transiting planet survey and to the cool Kepler Objects
of Interest (KOIs). We independently validate our calibrations by demonstrating
a clear relationship between our inferred parameters and the absolute K
magnitudes of the MEarth stars, and we identify objects with magnitudes too
bright for their estimated luminosities as candidate multiple systems. We also
use our inferred luminosities to address the applicability of near-infrared
metallicity calibrations to mid and late M dwarfs. The temperatures we infer
for the KOIs agree remarkably well with those from the literature; however, our
stellar radii are systematically larger than those presented in previous works
that derive radii from model isochrones. This results in a mean planet radius
that is 15% larger than one would infer using the stellar properties from
recent catalogs. Our results confirm those of previous in-depth studies of
Kepler-42, Kepler-45, and Kepler-186.Comment: Accepted to ApJ. Tables 4 and 5, and machine readable versions of
Tables 5 and 7 are available in the ApJ journal articl
A Search for Additional Bodies in the GJ 1132 Planetary System from 21 Ground-based Transits and a 100 Hour Spitzer Campaign
We present the results of a search for additional bodies in the GJ 1132
system through two methods: photometric transits and transit timing variations
of the known planet. We collected 21 transit observations of GJ 1132b with the
MEarth-South array since 2015. We obtained 100 near-continuous hours of
observations with the Space Telescope, including two transits of GJ
1132b and spanning 60\% of the orbital phase of the maximum period at which
bodies coplanar with GJ 1132b would pass in front of the star. We exclude
transits of additional Mars-sized bodies, such as a second planet or a moon,
with a confidence of 99.7\%. When we combine the mass estimate of the star
(obtained from its parallax and apparent band magnitude) with the stellar
density inferred from our high-cadence light curve (assuming zero
eccentricity), we measure the stellar radius of GJ 1132 to be
, and we refine the radius measurement of
GJ 1132b to . Combined with HARPS RV measurements, we
determine the density of GJ 1132b to be \ g cm, with the
mass determination dominating this uncertainty. We refine the ephemeris of the
system and find no evidence for transit timing variations, which would be
expected if there was a second planet near an orbital resonance with GJ 1132b.Comment: 29 pages, 4 Tables, 8 Figures, Submitted to ApJ. Comments welcom
Magnetic inflation and stellar mass. V. Intensification and saturation of M-dwarf absorption lines with Rossby number
In young Sun-like stars and field M-dwarf stars, chromospheric and coronal magnetic activity indicators such as Hα, X-ray, and radio emission are known to saturate with low Rossby number (Ro lesssim 0.1), defined as the ratio of rotation period to convective turnover time. The mechanism for the saturation is unclear. In this paper, we use photospheric Ti i and Ca i absorption lines in the Y band to investigate magnetic field strength in M dwarfs for Rossby numbers between 0.01 and 1.0. The equivalent widths of the lines are magnetically enhanced by photospheric spots, a global field, or a combination of the two. The equivalent widths behave qualitatively similar to the chromospheric and coronal indicators: we see increasing equivalent widths (increasing absorption) with decreasing Ro and saturation of the equivalent widths for Ro lesssim 0.1. The majority of M dwarfs in this study are fully convective. The results add to mounting evidence that the magnetic saturation mechanism occurs at or beneath the stellar photosphere.Published versio
The rotation and Galactic kinematics of mid M dwarfs in the Solar Neighborhood
Rotation is a directly-observable stellar property, and drives magnetic field
generation and activity through a magnetic dynamo. Main sequence stars with
masses below approximately 0.35Msun (mid-to-late M dwarfs) are
fully-convective, and are expected to have a different type of dynamo mechanism
than solar-type stars. Measurements of their rotation rates provide insights
into these mechanisms, but few rotation periods are available for these stars
at field ages. Using photometry from the MEarth transit survey, we measure
rotation periods for 387 nearby, mid-to-late M dwarfs in the Northern
hemisphere, finding periods from 0.1 to 140 days. The typical detected rotator
has stable, sinusoidal photometric modulations at a semi-amplitude of 0.5 to
1%. We find no period-amplitude relation for stars below 0.25Msun and an
anti-correlation between period and amplitude for higher-mass M dwarfs. We
highlight the existence of older, slowly-rotating stars without H{\alpha}
emission that nevertheless have strong photometric variability. The Galactic
kinematics of our sample is consistent with the local population of G and K
dwarfs, and rotators have metallicities characteristic of the Solar
Neighborhood. We use the W space velocities and established age-velocity
relations to estimate that stars with P<10 days are on average <2 Gyrs, and
that those with P>70 days are about 5 Gyrs. The period distribution is mass
dependent: as the mass decreases, the slowest rotators at a given mass have
longer periods, and the fastest rotators have shorter periods. We find a lack
of stars with intermediate rotation periods. [Abridged]Comment: Accepted to ApJ. Machine readable tables and additional figures are
available in the published article or on reques
Orbital Parameter Determination for Wide Stellar Binary Systems in the Age of Gaia
The orbits of binary stars and planets, particularly eccentricities and
inclinations, encode the angular momentum within these systems. Within stellar
multiple systems, the magnitude and (mis)alignment of angular momentum vectors
among stars, disks, and planets probes the complex dynamical processes guiding
their formation and evolution. The accuracy of the \textit{Gaia} catalog can be
exploited to enable comparison of binary orbits with known planet or disk
inclinations without costly long-term astrometric campaigns. We show that
\textit{Gaia} astrometry can place meaningful limits on orbital elements in
cases with reliable astrometry, and discuss metrics for assessing the
reliability of \textit{Gaia} DR2 solutions for orbit fitting. We demonstrate
our method by determining orbital elements for three systems (DS Tuc AB, GK/GI
Tau, and Kepler-25/KOI-1803) using \textit{Gaia} astrometry alone. We show that
DS Tuc AB's orbit is nearly aligned with the orbit of DS Tuc Ab, GK/GI Tau's
orbit might be misaligned with their respective protoplanetary disks, and the
Kepler-25/KOI-1803 orbit is not aligned with either component's transiting
planetary system. We also demonstrate cases where \textit{Gaia} astrometry
alone fails to provide useful constraints on orbital elements. To enable
broader application of this technique, we introduce the python tool
\texttt{lofti\_gaiaDR2} to allow users to easily determine orbital element
posteriors.Comment: 18 pages, 10 figures, accepted for publication in Ap
An early giant planet instability recorded in asteroidal meteorites
Giant planet migration appears widespread among planetary systems in our
Galaxy. However, the timescales of this process, which reflect the underlying
dynamical mechanisms, are not well constrained, even within the solar system.
Since planetary migration scatters smaller bodies onto intersecting orbits, it
would have resulted in an epoch of enhanced bombardment in the solar system's
asteroid belt. To accurately and precisely quantify the timescales of
migration, we interrogate thermochronologic data from asteroidal meteorites,
which record the thermal imprint of energetic collisions. We present a database
of 40K-40Ar system ages from chondrite meteorites and evaluate it with an
asteroid-scale thermal code coupled to a Markov chain Monte Carlo inversion.
Simulations require bombardment in order to reproduce the observed age
distribution and identify a bombardment event beginning ~11 million years after
the Sun formed. Our results associate a giant planet instability in our solar
system with the dissipation of the gaseous protoplanetary disk.Comment: 24 pages, 4 figures, 2 tables, 10 extended data items (8 figures, 2
tables). Under review at Nature Astronom
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