33 research outputs found
M dwarfs in the b201 tile of the VVV survey: Colour-based Selection, Spectral Types and Light Curves
The intrinsically faint M dwarfs are the most numerous stars in the Galaxy,
have main-sequence lifetimes longer than the Hubble time, and host some of the
most interesting planetary systems known to date. Their identification and
classification throughout the Galaxy is crucial to unravel the processes
involved in the formation of planets, stars and the Milky Way. The ESO Public
Survey VVV is a deep near-IR survey mapping the Galactic bulge and southern
plane. The VVV b201 tile, located in the border of the bulge, was specifically
selected for the characterisation of M dwarfs. We used VISTA photometry to
identify M dwarfs in the VVV b201 tile, to estimate their subtypes, and to
search for transit-like light curves from the first 26 epochs of the survey.
UKIDSS photometry from SDSS spectroscopically identified M dwarfs was used to
calculate their expected colours in the VISTA system. A colour-based
spectral subtype calibration was computed. Possible giants were identified by a
reduced proper motion diagram. The light curves of
12.8<<15.8 colour-selected M dwarfs were inspected for signals consistent
with transiting objects. We identified 23,345 objects in VVV b201 with colours
consistent with M dwarfs. We provided their spectral types and photometric
distances, up to 300 pc for M9s and 1.2 kpc for M4s, from
photometry. In the range 12<<16, we identified 753 stars as possible
giants out of 9,232 M dwarf candidates. While only the first 26 epochs of VVV
were available, and 1 epoch was excluded, we were already able to identify
transit-like signals in the light curves of 95 M dwarfs and of 12 possible
giants. Thanks to its deeper photometry (4 magnitudes deeper than 2MASS),
the VVV survey will be a major contributor to the discovery and study of M
dwarfs and possible companions towards the center of the Milky Way.Comment: 11 pages, 4 figures. Accepted for publication in Catalogs and data of
Astronomy and Astrophysic
A Catalog of Cool Dwarf Targets for the Transiting Exoplanet Survey Satellite
We present a catalog of cool dwarf targets (, ) and their stellar properties for the upcoming Transiting Exoplanet
Survey Satellite (TESS), for the purpose of determining which cool dwarfs
should be observed using two-minute observations. TESS has the opportunity to
search tens of thousands of nearby, cool, late K and M-type dwarfs for
transiting exoplanets, an order of magnitude more than current or previous
transiting exoplanet surveys, such as {\it Kepler}, K2 and ground-based
programs. This necessitates a new approach to choosing cool dwarf targets. Cool
dwarfs were chosen by collating parallax and proper motion catalogs from the
literature and subjecting them to a variety of selection criteria. We calculate
stellar parameters and TESS magnitudes using the best possible relations from
the literature while maintaining uniformity of methods for the sake of
reproducibility. We estimate the expected planet yield from TESS observations
using statistical results from the Kepler Mission, and use these results to
choose the best targets for two-minute observations, optimizing for small
planets for which masses can conceivably be measured using follow up Doppler
spectroscopy by current and future Doppler spectrometers. The catalog is
incorporated into the TESS Input Catalog and TESS Candidate Target List until a
more complete and accurate cool dwarf catalog identified by ESA's Gaia Mission
can be incorporated.Comment: Accepted to The Astronomical Journal. For the full catalog, please
contact the corresponding autho
Portraying the hosts: Stellar science from planet searches
Information on the full session can be found on this website: https://sites.google.com/site/portrayingthehostscs18/We present a compendium of the splinter session on stellar science from planet searches that was organized as part of the Cool Stars 18 conference. Seven speakers discussed techniques to infer stellar information from radial velocity, transit and microlensing data, as well as new instrumentation and missions designed for planet searches that will provide useful for the study of the cool stars
Determining the Metallicity of Low-Mass Stars and Brown Dwarfs: Tools for Probing Fundamental Stellar Astrophysics, Tracing Chemical Evolution of the Milky Way and Identifying the Hosts of Extrasolar Planets
We present a brief overview of a splinter session on determining the metallicity of low mass dwarfs that was organized as part of the Cool Stars 16 conference. We review contemporary spectroscopic and photometric techniques for estimating metallicity in low mass dwarfs and discuss the importance of measuring accurate metallicities for studies of Galactic and chemical evolution using subdwarfs, creating metallicity benchmarks for brown dwarfs, and searching for extrasolar planets that are orbiting around low mass dwarfs. In addition, we present the current understanding of the effects of metallicity on stellar evolution and atmosphere models and discuss some of the limitations that are important to consider when comparing theoretical models to data
Characterizing the Cool KOIs. VI. H- and K-band Spectra of Kepler M Dwarf Planet-Candidate Hosts
We present H- and K-band spectra for late-type Kepler Objects of Interest
(the "Cool KOIs"): low-mass stars with transiting-planet candidates discovered
by NASA's Kepler Mission that are listed on the NASA Exoplanet Archive. We
acquired spectra of 103 Cool KOIs and used the indices and calibrations of
Rojas-Ayala et al. to determine their spectral types, stellar effective
temperatures and metallicities, significantly augmenting previously published
values. We interpolate our measured effective temperatures and metallicities
onto evolutionary isochrones to determine stellar masses, radii, luminosities
and distances, assuming the stars have settled onto the main-sequence. As a
choice of isochrones, we use a new suite of Dartmouth predictions that reliably
include mid-to-late M dwarf stars. We identify five M4V stars: KOI-961
(confirmed as Kepler 42), KOI-2704, KOI-2842, KOI-4290, and the secondary
component to visual binary KOI-1725, which we call KOI-1725 B. We also identify
a peculiar star, KOI-3497, which has a Na and Ca lines consistent with a dwarf
star but CO lines consistent with a giant. Visible-wavelength adaptive optics
imaging reveals two objects within a 1 arc second diameter; however, the
objects' colors are peculiar. The spectra and properties presented in this
paper serve as a resource for prioritizing follow-up observations and planet
validation efforts for the Cool KOIs, and are all available for download online
using the "data behind the figure" feature.Comment: Accepted for publication in the Astrophysical Journal Supplement
Series (ApJS). Data and table are available in the sourc
Stellar Diameters and Temperatures II. Main Sequence K & M Stars
We present interferometric diameter measurements of 21 K- and M- dwarfs made
with the CHARA Array. This sample is enhanced by literature radii measurements
to form a data set of 33 K-M dwarfs with diameters measured to better than 5%.
For all 33 stars, we compute absolute luminosities, linear radii, and effective
temperatures (Teff). We develop empirical relations for \simK0 to M4 main-
sequence stars between the stellar Teff, radius, and luminosity to broad-band
color indices and metallicity. These relations are valid for metallicities
between [Fe/H] = -0.5 and +0.1 dex, and are accurate to ~2%, ~5%, and ~4% for
Teff, radius, and luminosity, respectively. Our results show that it is
necessary to use metallicity dependent transformations to convert colors into
stellar Teffs, radii, and luminosities. We find no sensitivity to metallicity
on relations between global stellar properties, e.g., Teff-radius and
Teff-luminosity. Robust examinations of single star Teffs and radii compared to
evolutionary model predictions on the luminosity-Teff and luminosity-radius
planes reveals that models overestimate the Teffs of stars with Teff < 5000 K
by ~3%, and underestimate the radii of stars with radii < 0.7 R\odot by ~5%.
These conclusions additionally suggest that the models overestimate the effects
that the stellar metallicity may have on the astrophysical properties of an
object. By comparing the interferometrically measured radii for single stars to
those of eclipsing binaries, we find that single and binary star radii are
consistent. However, the literature Teffs for binary stars are systematically
lower compared to Teffs of single stars by ~ 200 to 300 K. Lastly, we present a
empirically determined HR diagram for a total of 74 nearby, main-sequence, A-
to M-type stars, and define regions of habitability for the potential existence
of sub-stellar mass companions in each system. [abridged]Comment: 73 pages, 12 Tables, 18 Figures. Accepted for publication in The
Astrophysical Journa
The Revised TESS Input Catalog and Candidate Target List
We describe the catalogs assembled and the algorithms used to populate the
revised TESS Input Catalog (TIC), based on the incorporation of the Gaia second
data release. We also describe a revised ranking system for prioritizing stars
for 2-minute cadence observations, and assemble a revised Candidate Target List
(CTL) using that ranking. The TIC is available on the Mikulski Archive for
Space Telescopes (MAST) server, and an enhanced CTL is available through the
Filtergraph data visualization portal system at the URL
http://filtergraph.vanderbilt.edu/tess_ctl.Comment: 30 pages, 16 figures, submitted to AAS Journals; provided to the
community in advance of publication in conjunction with public release of the
TIC/CTL on 28 May 201
Metallicity and Temperature Indicators in M dwarf K band Spectra: Testing New & Updated Calibrations With Observations of 133 Solar Neighborhood M dwarfs
We present K band spectra for 133 nearby (d < 33 parsecs) M dwarfs, including
18 M dwarfs with reliable metallicity estimates (as inferred from an FGK type
companion), 11 M dwarf planet hosts, more than 2/3 of the M dwarfs in the
Northern 8 pc sample, and several M dwarfs from the LSPM catalog. From these
spectra, we measure equivalent widths of the Ca and Na lines, and a spectral
index quantifying the absorption due to H2O opacity (the H2O-K2 index). Using
empirical spectral types standards and synthetic models, we calibrate the
H2O-K2 index as an indicator of an M dwarf's spectral type and effective
temperature. We also present a revised relationship that estimates the [Fe/H]
and [M/H] metallicities of M dwarfs from their Na I, Ca I, and H2O-K2
measurements. Comparisons to model atmosphere provide a qualitative validation
of our approach, but also reveal an overall offset between the atomic line
strengths predicted by models as compared to actual observations. Our
metallicity estimates also reproduce expected correlations with Galactic space
motions and H alpha emission line strengths, and return statistically identical
metallicities for M dwarfs within a common multiple system. Finally, we find
systematic residuals between our H2O-based spectral types and those derived
from optical spectral features with previously known sensitivity to stellar
metallicity, such as TiO, and identify the CaH1 index as a promising optical
index for diagnosing the metallicities of near-solar M dwarfs.Comment: 132 pages, 48 figures. Paper resubmitted to Ap