Using High-Resolution Optical Spectra to Measure Intrinsic Properties of Low-Mass Stars: New Properties for KOI-314 and GJ 3470 (original) (raw)
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Stellar parameters of early-M dwarfs from ratios of spectral features at optical wavelengths
Astronomy & Astrophysics, 2015
Context. Low-mass stars have been recognised as promising targets in the search for rocky, small planets with the potential of supporting life. As a consequence, Doppler search programmes using high-resolution spectrographs like HARPS or HARPS-N are providing huge quantities of optical spectra of M dwarfs. However, determining the stellar parameters of M dwarfs using optical spectra has proven to be challenging. Aims. We aim to calibrate empirical relationships to determine accurate stellar parameters for early M dwarfs (spectral types M0-M4.5) using the same spectra that are used for the radial velocity determinations, without the necessity of acquiring IR spectra or relying on atmospheric models and/or photometric calibrations. Methods. Our methodology consists in the use of ratios of pseudo equivalent widths of spectral features as a temperature diagnostic, a technique largely used in solar-type stars. Stars with effective temperatures obtained from interferometric estimates of their radii are used as calibrators. Empirical calibrations for the spectral type are also provided. Combinations of features and ratios of features are used to derive calibrations for the stellar metallicity. Our methods are then applied to a large sample of M dwarfs that are currently being observed in the framework of the HARPS GTO search for extrasolar planets. The derived temperatures and metallicities are used together with photometric estimates of mass, radius, and surface gravity to calibrate empirical relationships for these parameters. Results. A large list of spectral features in the optical spectra of early M dwarfs was identified. From this list the pseudo equivalent width of roughly 43% of the features shows a strong anticorrelation with the effective temperature. The correlation with the stellar metallicity is weaker. A total of 112 temperature sensitive ratios have been identified and calibrated over the range 3100-3950 K, providing effective temperatures with typical uncertainties of the order of 70 K. Eighty-two ratios of pseudo equivalent widths of features were calibrated to derive spectral types within 0.5 subtypes for stars with spectral types between K7V and M4.5V. Regarding stellar metallicity, 696 combinations of pseudo equivalent widths of individual features and temperature-sensitive ratios have been calibrated, over the metallicity range from -0.54 to +0.24 dex, with estimated uncertainties in the range of 0.07-0.10 dex. We provide our own empirical calibrations for stellar mass, radius, and surface gravity. These parameters are found to show a dependence on the stellar metallicity. For a given effective temperature, lower metallicities predict lower masses and radii, as well as larger gravities. Conclusions.
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.
Astronomy and Astrophysics, 2018
Context. Being the most numerous and oldest stars in the galaxy, M dwarfs are objects of great interest for exoplanet searches. The presence of molecules in their atmosphere complicates our understanding of their atmospheric properties. But great advances have recently been made in the modeling of M dwarfs due to the revision of solar abundances. Aims. We aim to determine stellar parameters of M dwarfs using high resolution spectra (R ∼ 90 000) simultaneously in the visible and the near-infrared. The high resolution spectra and broad wavelength coverage provide an unique opportunity to understand the onset of dust and cloud formation at cool temperatures. Furthermore, this study will help in understanding the physical processes which occur in a cool atmospheres, particularly, the redistribution of energy from the optical to the near-infrared. Methods. The stellar parameters of M dwarfs in our sample have been determined by comparing the high resolution spectra both in the optical and in the near-infrared simultaneously observed by CARMENES with the synthetic spectra obtained from the BT-Settl model atmosphere. The detailed spectral synthesis of these observed spectra both in the optical and in the near-infrared helps to understand the missing continuum opacity. Results. For the first time, we derive fundamental stellar parameters of M dwarfs using the high resolution optical and near-infrared spectra simultaneously. We determine T eff , log g and [M/H] for 292 M dwarfs of spectral type M0 to M9, where the formation of dust and clouds are important. The derived T eff for the sample ranges from 2300 to 4000 K, values of log g ranges from 4.5 ≤ log g ≤ 5.5 and the resulting metallicity ranges from −0.5 ≤ [M/H] ≤ +0.5. We have also explored the possible differences in T eff , log g and [M/H] by comparing them with other studies of the same sample of M dwarfs.
2011
We report stellar parameters for late-K and M-type planet-candidate host stars announced by the Kepler Mission. We obtained medium-resolution, K-band spectra of 84 cool (T eff 4400 K) Kepler Objects of Interest (KOIs) from Borucki et al. We identified one object as a giant (KOI 977); for the remaining dwarfs, we measured effective temperatures (T eff ) and metallicities [M/H] using the K-band spectral indices of Rojas-Ayala et al. We determine the masses (M ⋆ ) and radii (R ⋆ ) of the cool KOIs by interpolation onto the Dartmouth evolutionary isochrones. The resultant stellar radii are significantly less than the values reported in the Kepler Input Catalogue and, by construction, correlate better with T eff . Applying the published KOI transit parameters to our stellar radius measurements, we report new physical radii for the planet candidates. Recalculating the equilibrium temperatures of the planet-candidates assuming Earth's albedo and re-radiation fraction, we find that 3 of the planetcandidates are terrestrial-sized with orbital semi-major axes that lie within the habitable zones of their host stars (KOI 463.01, KOI 812.03 and KOI 854.01). The stellar parameters presented in this letter serve as a resource for prioritization of future follow-up efforts to validate and characterize the cool KOI planet candidates.
Spectroscopic parameters for a sample of metal-rich solar-type stars
Astronomy & Astrophysics, 2006
Aims. To date, metallicity is the only parameter of a star that appears to clearly correlate with the presence of planets and their properties. To check for new correlations between stars and the existence of an orbiting planet, we determine accurate stellar parameters for several metal-rich solar-type stars. The purpose is to fill the gap of the comparison sample presented in previous works in the high metal-content regime. Methods. The stellar parameters were determined using an LTE analysis based on equivalent widths (EW) of iron lines and by imposing excitation and ionization equilibrium. We also present a first step in determining these stellar parameters in an automatic manner by using the code DAOSPEC for the EW determination. Results. Accurate stellar parameters and metallicities are obtained for our sample composed of 64 high metal-content stars not known to harbor any planet. This sample will in the future give us the possibility of better exploring the existence of differences in the chemical abundances between planet-host stars and stars without known planets in the metal-rich domain. We also report stellar parameters for some recently discovered planet-host stars. Finally, we present an empirical calibration for DAOSPEC based on the comparison between its EW measurements and the standard "hand made" measurements for the FEROS sample presented in this paper.
Monthly Notices of the Royal Astronomical Society
We estimate effective temperature (Teff), stellar radius, and luminosity for a sample of 271 M-dwarf stars (M0V-M7V) observed as a part of CARMENES (Calar Alto high-Resolution search for M dwarfs with Exo-earths with Near-infrared and optical Echelle Spectrographs) radial-velocity planet survey. For the first time, using the simultaneously observed high-resolution ($R\sim 90\, 000$) spectra in the optical (0.52–0.96 mu\mumum) and near-infrared (0.96–1.71 mu\mumum) bands, we derive empirical calibration relationships to estimate the fundamental parameters of these low-mass stars. We select a sample of nearby and bright M-dwarfs as our calibrators for which the physical parameters are acquired from high-precision interferometric measurements. To identify the most suitable indicators of Teff, radius, and luminosity (log L/L⊙), we inspect a range of spectral features and assess them for reliable correlations. We perform multivariate linear regression and find that the combination of pseudo-...
Spectroscopic Validation of Low-metallicity Stars from RAVE
The Astronomical Journal, 2018
We present results from a medium-resolution (R∼2000) spectroscopic follow-up campaign of 1694 bright (V<13.5), very metal-poor star candidates from the RAdial Velocity Experiment (RAVE). Initial selection of the low-metallicity targets was based on the stellar parameters published in RAVE Data Releases 4 and 5. Follow up was accomplished with the Gemini-N and Gemini-S, the ESO/NTT, the KPNO/Mayall, and the SOAR telescopes. The wavelength coverage for most of the observed spectra allows for the determination of carbon and α-element abundances, which are crucial for considering the nature and frequency of the carbon-enhanced metalpoor (CEMP) stars in this sample. We find that 88% of the observed stars have Fe H [ ]−1.0, 61% have Fe H [ ]−2.0, and 3% have Fe H [ ]−3.0 (with four stars at Fe H [ ]−3.5). There are 306 CEMP star candidates in this sample, and we identify 169 CEMP GroupI, 131 CEMP GroupII, and 6 CEMP GroupIII stars from the A(C) versus [Fe/H] diagram. Inspection of the C a [ ] abundance ratios reveals that five of the CEMP GroupII stars can be classified as "mono-enriched second-generation" stars. Gaia DR1 matches were found for 734 stars, and we show that transverse velocities can be used as a confirmatory selection criteria for low-metallicity candidates. Selected stars from our validated list are being followed-up with high-resolution spectroscopy to reveal their full chemical-abundance patterns for further studies.
Binarity, activity and metallicity among late-type stars
Astronomy and Astrophysics, 2005
We present the first in a series of papers that attempt to investigate the relation between binarity, magnetic activity, and chemical surface abundances of cool stars. In the current paper, we lay out and test two abundance analysis methods and apply them to two well-known, active, single stars, HD 27536 (G8IV-III) and HD 216803 (K5V), presenting photospheric fundamental parameters and abundances of Li, Al, Ca, Si, Sc, Ti, V, Cr, Fe, Co and Ni. The abundances from the two methods agree within the errors for all elements except calcium in HD 216803, which means that either method yields the same fundamental model parameters and the same abundances. Activity is described by the radiative loss in the Ca H&K lines with respect to the bolometric luminosity, through the activity index R HK . Binarity is established by very precise radial velocity (RV) measurements using HARPS spectra. The spectral line bisectors are examined for correlations between RV and bisector shape to distinguish between the effects of stellar activity and unseen companions. We show that HD 27536 exhibit RV variations mimicking the effect of a low-mass (m ∼ 4 M J ) companion in a relatively close (a ∼ 1 AU) orbit. The variation is strongly correlated with the activity, and consistent with the known photometric period P = 306.9 d, demonstrating a remarkable coherence between R HK and the bisector shape, i.e. between the photosphere and the chromosphere. We discuss the complications involved in distinguishing between companion and activity induced RV variations.
M dwarf spectral indices at moderate resolution: accurate Teff and [Fe/H] for 178 southern stars
Monthly Notices of the Royal Astronomical Society, 2021
We present a spectroscopic and photometric calibration to derive effective temperatures Teff and metallicities [Fe/H] for M dwarfs, based on a principal component analysis (PCA) of 147 spectral indices measured off moderate resolution (R∼11 000), high S/N (>100) spectra in the λλ 8390–8834 region, plus the J−H colour. Internal uncertainties, estimated by the residuals, are 81 K and 0.12 dex, respectively, for Teff and [Fe/H], the calibrations being valid for 3050 K < T$_\text{eff}\, \lt $ 4100 K and −0.45 < [Fe/H] lt,+\lt \, +lt,+0.50 dex. The PCA calibration is a competitive model-independent method to derive Teff and [Fe/H] for large samples of M dwarfs, well suited to the available database of far-red spectra. The median uncertainties are 105 K and 0.23 dex for Teff and [Fe/H], respectively, estimated by Monte Carlo simulations. We compare our values to other works based on photometric and spectroscopic techniques and find median differences 75 ± 273 K and 0.02 ± 0.31 dex for ...