arXiv subjects

Barbara Rojas-Ayala

Publications and source records attributed to Barbara Rojas-Ayala.

17 recordsLinked to original sources

arXiv2025

Chemical Abundances of M and G Dwarfs in the Hyades and Coma Berenices Open Clusters from APOGEE Spectra

Open clusters are one of the best astrophysical laboratories we have available for stellar astrophysics studies. This work presents metallicities and individual abundances for fourteen M dwarfs and six G dwarfs from two well-known open clusters: Hyades and Coma Berenices. Our analysis is based on near-infrared (1.51--1.69 $μ$m), high-resolution ($R \sim 22,500$) spectra obtained from the SDSS IV/APOGEE Survey. Using one-dimensional, plane-parallel MARCS model atmospheres, the APOGEE line list, and the Turbospectrum radiative transfer code in local thermodynamic equilibrium, we derived spectroscopic stellar parameters for the M dwarfs, along with abundances of 13 elements (C, O, Na, Mg, Al, Si, K, Ca, Ti, V, Cr, Mn, and Fe) for both M and G dwarfs. We find a high degree of chemical homogeneity within each cluster when comparing abundances derived from M and G dwarfs: $δ$[M/H] (M dwarfs -- G dwarfs) of 0.01$\pm$0.04, and 0.02$\pm$0.03 for the Hyades and Coma Berenices, respectively. The overall cluster metallicities derived from M dwarfs (Hyades: 0.16$\pm$0.03 and Coma Berenices: 0.02$\pm$0.06) are consistent with previous literature determinations. Finally, we demonstrate the value of M dwarfs as key tracers in galactic archaeology, emphasizing their potential for studying galactic metallicity gradients and chemical evolution.

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arXiv2020

Stellar Characterization of M-dwarfs from the APOGEE Survey: A Calibrator Sample for the M-dwarf Metallicities

We present spectroscopic determinations of the effective temperatures, surface gravities and metallicities for 21 M-dwarfs observed at high-resolution (R $\sim$ 22,500) in the \textit{H}-band as part of the SDSS-IV APOGEE survey. The atmospheric parameters and metallicities are derived from spectral syntheses with 1-D LTE plane parallel MARCS models and the APOGEE atomic/molecular line list, together with up-to-date H$_{2}$O and FeH molecular line lists. Our sample range in $T_{\rm eff}$ from $\sim$ 3200 to 3800K, where eleven stars are in binary systems with a warmer (FGK) primary, while the other 10 M-dwarfs have interferometric radii in the literature. We define an $M_{K_{S}}$--Radius calibration based on our M-dwarf radii derived from the detailed analysis of APOGEE spectra and Gaia DR2 distances, as well as a mass-radius relation using the spectroscopically-derived surface gravities. A comparison of the derived radii with interferometric values from the literature finds that the spectroscopic radii are slightly offset towards smaller values, with $Δ$ = -0.01 $\pm$ 0.02 $R{\star}$/$R_{\odot}$. In addition, the derived M-dwarf masses based upon the radii and surface gravities tend to be slightly smaller (by $\sim$5-10\%) than masses derived for M-dwarf members of eclipsing binary systems for a given stellar radius. The metallicities derived for the 11 M-dwarfs in binary systems, compared to metallicities obtained for their hotter FGK main-sequence primary stars from the literature, shows excellent agreement, with a mean difference of [Fe/H](M-dwarf - FGK primary) = +0.04 $\pm$ 0.18 dex, confirming the APOGEE metallicity scale derived here for M-dwarfs.

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arXiv2019

Fundamental Parameters of $\sim$30,000 M dwarfs in LAMOST DR1 Using Data-driven Spectral Modeling

M dwarfs are the most common type of star in the Galaxy, and because of their small size are favored targets for searches of Earth-sized transiting exoplanets. Current and upcoming all-sky spectroscopic surveys, such as the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), offer an opportunity to systematically determine physical properties of many more M dwarfs than has been previously possible. Here we present new effective temperatures, radii, masses, and luminosities for 29,678 M dwarfs with spectral types M0-M6 in the first data release (DR1) of LAMOST. We derived these parameters from the supervised machine learning code, The Cannon, trained with 1,388 M dwarfs in the Transiting Exoplanet Survey Satellite (TESS) Cool Dwarf Catalog that were also present in LAMOST with high signal-to-noise ratio ($>$250) spectra. Our validation tests show that the output parameter uncertainties are strongly correlated with the signal-to-noise of the LAMOST spectra, and we achieve typical uncertainties of 110 K in Teff ($\sim$3%), 0.065 $R_{\odot}$ ($\sim$14%) in radius, 0.054 $M_{\odot}$ ($\sim$12%) in mass, and 0.012 $L_{\odot}$ ($\sim$20%) in luminosity. The model presented here can be rapidly applied to future LAMOST data releases, significantly extending the samples of well-characterized M dwarfs across the sky using new and exclusively data-based modeling methods.

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arXiv2019

A super-Earth and sub-Neptune transiting the late-type M dwarf LP 791-18

Planets occur most frequently around cool dwarfs, but only a handful of specific examples are known to orbit the latest-type M stars. Using TESS photometry, we report the discovery of two planets transiting the low-mass star called LP 791-18 (identified by TESS as TOI 736). This star has spectral type M6V, effective temperature 2960 K, and radius 0.17 R_Sun, making it the third-coolest star known to host planets. The two planets straddle the radius gap seen for smaller exoplanets; they include a 1.1 R_Earth planet on a 0.95 day orbit and a 2.3 R_Earth planet on a 5 day orbit. Because the host star is small the loss of light during these planets' transits is fairly large (0.4% and 1.7%). This has allowed us to detect both planets' transits from ground-based photometry, refining their radii and orbital ephemerides. In the future, radial velocity observations and transmission spectroscopy can both probe these planets' bulk interior and atmospheric compositions, and additional photometric monitoring would be sensitive to even smaller transiting planets.

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arXiv2019

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.

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arXiv2018

The TESS Input Catalog and Candidate Target List

The Transiting Exoplanet Survey Satellite (TESS) will be conducting a nearly all-sky photometric survey over two years, with a core mission goal to discover small transiting exoplanets orbiting nearby bright stars. It will obtain 30-minute cadence observations of all objects in the TESS fields of view, along with 2-minute cadence observations of 200,000 to 400,000 selected stars. The choice of which stars to observe at the 2-min cadence is driven by the need to detect small transiting planets, which leads to the selection of primarily bright, cool dwarfs. We describe the catalogs assembled and the algorithms used to populate the TESS Input Catalog (TIC). We also describe a ranking system for prioritizing stars according to the smallest transiting planet detectable, and assemble a Candidate Target List (CTL) using that ranking. We discuss additional factors that affect the ability to photometrically detect and dynamically confirm small planets, and we note additional stellar populations of interest that may be added to the final target list. The TIC is available on the STScI MAST server, and an enhanced CTL is available through the Filtergraph data visualization portal system at the URL https://filtergraph.vanderbilt.edu/tess_ctl .

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arXiv2018

Stellar and Planetary Characterization of the Ross 128 Exoplanetary System from APOGEE Spectra

The first detailed chemical abundance analysis of the M dwarf (M4.0) exoplanet-hosting star Ross 128 is presented here, based upon near-infrared (1.5--1.7 \micron) high-resolution ($R$$\sim$22,500) spectra from the SDSS-APOGEE survey. We determined precise atmospheric parameters $T_{\rm eff}$=3231$\pm$100K, log$g$=4.96$\pm$0.11 dex and chemical abundances of eight elements (C, O, Mg, Al, K, Ca, Ti, and Fe), finding Ross 128 to have near solar metallicity ([Fe/H] = +0.03$\pm$0.09 dex). The derived results were obtained via spectral synthesis (1-D LTE) adopting both MARCS and PHOENIX model atmospheres; stellar parameters and chemical abundances derived from the different adopted models do not show significant offsets. Mass-radius modeling of Ross 128b indicate that it lies below the pure rock composition curve, suggesting that it contains a mixture of rock and iron, with the relative amounts of each set by the ratio of Fe/Mg. If Ross 128b formed with a sub-solar Si/Mg ratio, and assuming the planet's composition matches that of the host-star, it likely has a larger core size relative to the Earth. The derived planetary parameters -- insolation flux (S$_{\rm Earth}$=1.79$\pm$0.26) and equilibrium temperature ($T_{\rm eq}$=294$\pm$10K) -- support previous findings that Ross 128b is a temperate exoplanet in the inner edge of the habitable zone.

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arXiv2013

Near-infrared Metallicities, Radial Velocities and Spectral Types for 447 Nearby M Dwarfs

We present metallicities, radial velocities and near-infrared spectral types for 447 M dwarfs determined from moderate resolution (R~2000) near-infrared (NIR) spectra obtained with IRTF/SpeX. These M dwarfs are targets of the MEarth Survey, a transiting planet survey searching for super Earths around mid-to-late M dwarfs within 33pc. We present NIR spectral types and new IRTF spectral templates in the Z, J, H and K-bands, created using M dwarfs with near-solar metallicities. We developed two spectroscopic distance calibrations that use NIR spectral type or an index based on the curvature of the K-band continuum. Our distance calibration has a scatter of 14%. We searched 27 NIR spectral lines and 10 spectral indices for metallicity sensitive features, taking into account correlated noise in our estimates of the errors on these parameters. We calibrated our relation using 36 M dwarfs in common proper pairs with an F, G or K-type star of known metallicity. We validated the physical association of these pairs using proper motions, radial velocities and spectroscopic distance estimates. Our resulting metallicity calibration uses the sodium doublet at 2.2um as the sole indicator for metallicity. It has an accuracy of 0.12dex inferred from the scatter between the metallicities of the primaries and the estimated metallicities of the secondaries. Our relation is valid for NIR spectral types from M1V to M5V and for -1.0<[Fe/H]<+0.35dex. We present a new metallicity relation using J-H and J-K colors that directly relates two observables: the distance from the M dwarf main sequence and equivalent width of the sodium line at 2.2um. We measured radial velocities by modeling telluric features to determine the absolute wavelength calibration of our spectra, and used M dwarf binaries, observations at different epochs, and comparison to precisely measured radial velocities to demonstrate 4.4km/s accuracy.

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arXiv2013

A Spectroscopic Catalog of the Brightest (J<9) M Dwarfs in the Northern Sky

We present a spectroscopic catalog of the 1,564 brightest (J<9) M dwarf candidates in the northern sky, as selected from the SUPERBLINK proper motion catalog. Observations confirm 1,408 of the candidates to be late-K and M dwarfs with spectral subtypes K7-M6. From the low μ>40 mas/yr proper motion limit and high level of completeness of the SUPERBLINK catalog in that magnitude range, we estimate that our spectroscopic census most likely includes >90% of all existing, northern-sky M dwarfs with apparent magnitude J<9. Only 682 stars in our sample are listed in the Third Catalog of Nearby Stars (CNS3); most others are relative unknowns and have spectroscopic data presented here for the first time. Spectral subtypes are assigned based on spectral index measurements of CaH and TiO molecular bands. A comparison of spectra from the same stars obtained at different observatories however reveals that spectral band index measurements are dependent on spectral resolution, spectrophotometric calibration, and other instrumental factors. After systematic corrections and a recalibration of the subtype-index relationships, we find that we can consistently and reliably classify all our stars to a half-subtype precision. The use of corrected spectral indices further requires us to recalibrate the ζparameter, from which we estimate metallicities +/-0.5dex accuracy in dwarfs of subtypes M2 and earlier, and +/-0.2dex accuracy for later (M3-M5) subtypes. Existing geometric parallax measurements are extracted from the literature for 624 stars, and are used to determine spectroscopic and photometric distances for all the other stars. Our catalog will be most useful to guide the selection of the best M dwarf targets for exoplanet searches, in particular those using high-precision radial velocity measurements.

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arXiv2012

Characterizing the Cool KOIs II. The M Dwarf KOI-254 and its Hot Jupiter

We report the confirmation and characterization of a transiting gas giant planet orbiting the M dwarf KOI-254 every 2.455239 days, which was originally discovered by the Kepler mission. We use radial velocity measurements, adaptive optics imaging and near infrared spectroscopy to confirm the planetary nature of the transit events. KOI-254b is the first hot Jupiter discovered around an M-type dwarf star. We also present a new model-independent method of using broadband photometry to estimate the mass and metallicity of an M dwarf without relying on a direct distance measurement. Included in this methodology is a new photometric metallicity calibration based on J-K colors. We use this technique to measure the physical properties of KOI-254 and its planet. We measure a planet mass of Mp = 0.505 Mjup, radius Rp = 0.96 Rjup and semimajor axis a = 0.03 AU, based on our measured stellar mass Mstar = 0.59 Msun and radius Rstar = 0.55 Rsun. We also find that the host star is metal-rich, which is consistent with the sample of M-type stars known to harbor giant planets.

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arXiv2012

Characterizing the Cool KOIs III. KOI-961: A Small Star with Large Proper Motion and Three Small Planets

We present the characterization of the star KOI 961, an M dwarf with transit signals indicative of three short-period exoplanets, originally discovered by the Kepler Mission. We proceed by comparing KOI 961 to Barnard's Star, a nearby, well-characterized mid-M dwarf. By comparing colors, optical and near-infrared spectra, we find remarkable agreement between the two, implying similar effective temperatures and metallicities. Both are metal-poor compared to the Solar neighborhood, have low projected rotational velocity, high absolute radial velocity, large proper motion and no quiescent H-alpha emission--all of which is consistent with being old M dwarfs. We combine empirical measurements of Barnard's Star and expectations from evolutionary isochrones to estimate KOI 961's mass (0.13 +/- 0.05 Msun), radius (0.17 +/- 0.04 Rsun) and luminosity (2.40 x 10^(-3.0 +/- 0.3) Lsun). We calculate KOI 961's distance (38.7 +/- 6.3 pc) and space motions, which, like Barnard's Star, are consistent with a high scale-height population in the Milky Way. We perform an independent multi-transit fit to the public Kepler light curve and significantly revise the transit parameters for the three planets. We calculate the false-positive probability for each planet-candidate, and find a less than 1% chance that any one of the transiting signals is due to a background or hierarchical eclipsing binary, validating the planetary nature of the transits. The best-fitting radii for all three planets are less than 1 Rearth, with KOI 961.03 being Mars-sized (Rp = 0.57 +/- 0.18 Rearth), and they represent some of the smallest exoplanets detected to date.

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arXiv2011

Evidence for an FU Orionis-like Outburst from a Classical T Tauri Star

We present pre- and post-outburst observations of the new FU Orionis-like young stellar object PTF 10qpf (also known as LkHa 188-G4 and HBC 722). Prior to this outburst, LkHa 188-G4 was classified as a classical T Tauri star on the basis of its optical emission-line spectrum superposed on a K8-type photosphere, and its photometric variability. The mid-infrared spectral index of LkHa 188-G4 indicates a Class II-type object. LkHa 188-G4 exhibited a steady rise by ~1 mag over ~11 months starting in Aug. 2009, before a subsequent more abrupt rise of > 3 mag on a time scale of ~2 months. Observations taken during the eruption exhibit the defining characteristics of FU Orionis variables: (i) an increase in brightness by > 4 mag, (ii) a bright optical/near-infrared reflection nebula appeared, (iii) optical spectra are consistent with a G supergiant and dominated by absorption lines, the only exception being Halpha which is characterized by a P Cygni profile, (iv) near-infrared spectra resemble those of late K--M giants/supergiants with enhanced absorption seen in the molecular bands of CO and H_2O, and (v) outflow signatures in H and He are seen in the form of blueshifted absorption profiles. LkHa 188-G4 is the first member of the FU Orionis-like class with a well-sampled optical to mid-infrared spectral energy distribution in the pre-outburst phase. The association of the PTF 10qpf outburst with the previously identified classical T Tauri star LkHa 188-G4 (HBC 722) provides strong evidence that FU Orionis-like eruptions represent periods of enhanced disk accretion and outflow, likely triggered by instabilities in the disk. The early identification of PTF 10qpf as an FU Orionis-like variable will enable detailed photometric and spectroscopic observations during its post-outburst evolution for comparison with other known outbursting objects.

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arXiv2011

The Sloan Digital Sky Survey Data Release 7 Spectroscopic M Dwarf Catalog I: Data

We present a spectroscopic catalog of 70,841 visually inspected M dwarfs from the seventh data release of the Sloan Digital Sky Survey (SDSS). For each spectrum, we provide measurements of the spectral type, a number of molecular bandheads, and the H-alpha, H-beta, H-gamma, H-delta and Ca II K emission lines. In addition, we calculate the metallicity-sensitive parameter zeta and identify a relationship between zeta and the g-r and r-z colors of M dwarfs. We assess the precision of our spectral types (which were assigned by individual examination), review the bulk attributes of the sample, and examine the magnetic activity properties of M dwarfs, in particular those traced by the higher order Balmer transitions. Our catalog is cross-matched to Two Micron All Sky Survey (2MASS) infrared data, and contains photometric distances for each star. Lastly, we identify eight new late-type M dwarfs that are possibly within 25 pc of the Sun. Future studies will use these data to thoroughly examine magnetic activity and kinematics in late-type M dwarfs and examine the chemical and dynamical history of the local Milky Way.

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arXiv2011

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.

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arXiv2010

The Sloan Digital Sky Survey Data Release 7 M Dwarf Spectroscopic Catalog

We present a spectroscopic catalog of 70,841 visually inspected M dwarfs from the seventh data release (DR7) of the Sloan Digital Sky Survey (SDSS). For each spectrum, we provide measurements of the spectral type, a number of molecular bandheads, and the H-alpha, H-beta, H-gamma, H-delta and Ca II K emission lines. In addition, we calculate the metallicity-sensitive parameter zeta and 3D space motions for most of the stars in the sample. Our catalog is cross-matched to Two Micron All Sky Survey (2MASS) infrared data, and contains photometric distances for each star. Future studies will use these data to thoroughly examine magnetic activity and kinematics in late-type M dwarfs and examine the chemical and dynamical history of the local Milky Way.

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arXiv2010

Metal-Rich M-dwarf Planet Hosts: Metallicities with K-Band Spectra

A metal-rich environment facilitates planet formation, making metal-rich stars the most favorable targets for surveys seeking to detect new exoplanets. Using this advantage to identify likely low-mass planet hosts, however, has been difficult: until now, methods to determine M-dwarf metallicities required observationally expensive data (such as parallaxes and high-resolution spectra), and were limited to a few bright cool stars. We have obtained moderate (R~2700) resolution K-band spectra of 17 M-dwarfs with metallicity estimates derived from their FGK companions. Analysis of these spectra, and inspection of theoretical synthetic spectra, reveal that an M-dwarf's metallicity can be inferred from the strength of its Na I doublet (2.206 μm & 2.209 μm) and Ca I triplet (2.261 μm, 2.263 μm & 2.265 μm) absorption lines. We use these features, and a temperature-sensitive water index, to construct an empirical metallicity indicator applicable for M-dwarfs with near-solar metallicities (-0.5<[Fe/H]<+0.5). This indicator has an accuracy of +/- 0.15 dex, comparable to that of existing techniques for estimating M-dwarf metallicities, but is more observationally accessible, requiring only a moderate resolution K-band spectrum. Applying this method to 8 known M-dwarf planet hosts, we estimate metallicities ([Fe/H]) in excess of the mean metallicity of M-dwarfs in the solar neighborhood, consistent with the metallicity distribution of FGK planet hosts.

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arXiv2008

Visual orbit of GJ 164 AB

We report seven successful observations of the astrometric binary GJ 164 AB system with aperture masking interferometry. The companion, with a near infrared contrast of 5:1 was detected beyond the formal diffraction limit. Combined with astrometric observations from the literature, these observations fix the parallax of the system, and allow a model-independent mass determination of both components. We find the mass of GJ 164B to be $ 0.086 \pm 0.007 M_{\sun}$. An infrared spectroscopic study of a sample of M-Dwarfs outlines a method for calibrating metallicity of M-Dwarfs. Results from the newly commissionned TripleSpec spectrograph reveal that the GJ 164 system is at least of Solar metallicity. Models are not consistent with color and mass, requiring a very young age to accommodate a secondary too luminous, a scenario ruled out by the kinematics.

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