Laszlo Sturmann

Stellar Diameters and Temperatures. II. Main-sequence K- and M-stars

We present interferometric angular diameter measurements of 21 low-mass, K- and M-dwarfs made with the CHARA Array. This sample is enhanced by adding a collection of radius measurements published in the literature to form a total data set of 33 K-M-dwarfs with diameters measured to better than 5%. We use these data in combination with the Hipparcos parallax and new measurements of the stars bolometric flux to compute absolute luminosities, linear radii, and effective temperatures for the stars. We develop empirical relations for ~K0 to M4 main-sequence stars that link the stellar temperature, radius, and luminosity to the observed (B – V), (V – R), (V – I), (V – J), (V – H), and (V – K) broadband color index and stellar metallicity [Fe/H]. These relations are valid for metallicities ranging from [Fe/H] = –0.5 to +0.1 dex and are accurate to ~2%, ~5%, and ~4% for temperature, radius, and luminosity, respectively. Our results show that it is necessary to use metallicity-dependent transformations in order to properly convert colors into stellar temperatures, radii, and luminosities. Alternatively, we find no sensitivity to metallicity on relations we construct to the global properties of a star omitting color information, e.g., temperature-radius and temperature-luminosity. Thus, we are able to empirically quantify to what order the stars observed color index is impacted by the stellar iron abundance. In addition to the empirical relations, we also provide a representative look-up table via stellar spectral classifications using this collection of data. Robust examinations of single star temperatures and radii compared to evolutionary model predictions on the luminosity-temperature and luminosity-radius planes reveal that models overestimate the temperatures of stars with surface temperatures <5000 K by ~3%, and underestimate the radii of stars with radii <0.7 R_☉ by ~5%. These conclusions additionally suggest that the models over account for the effects that the stellar metallicity may have on the astrophysical properties of an object. By comparing the interferometrically measured radii for the single star population to those of eclipsing binaries, we find that for a given mass, single and binary star radii are indistinguishable. However, we also find that for a given radius, the literature temperatures for binary stars are systematically lower compared to our interferometrically derived temperatures of single stars by ~200 to 300 K. The nature of this offset is dependent on the validation of binary star temperatures, where bringing all measurements to a uniform and correctly calibrated temperature scale is needed to identify any influence stellar activity may have on the physical properties of a star. Lastly, we present an empirically determined H-R diagram using fundamental properties presented here in combination with those in Boyajian et al. 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.

Stellar Diameters and Temperatures. III. Main-sequence A, F, G, and K Stars: Additional High-precision Measurements and Empirical Relations

Based on CHARA Array measurements, we present the angular diameters of 23 nearby, main-sequence stars, ranging from spectral types A7 to K0, 5 of which are exoplanet host stars. We derive linear radii, effective temperatures, and absolute luminosities of the stars using Hipparcos parallaxes and measured bolometric fluxes. The new data are combined with previously published values to create an Angular Diameter Anthology of measured angular diameters to main-sequence stars (luminosity classes V and IV). This compilation consists of 125 stars with diameter uncertainties of less than 5%, ranging in spectral types from A to M. The large quantity of empirical data is used to derive color-temperature relations to an assortment of color indices in the Johnson (BVR_(J)I_(J)JHK), Cousins (R_(C)I_(C)), Kron (R_(K)I_(K)), Sloan (griz), and WISE (W_(3)W_(4)) photometric systems. These relations have an average standard deviation of ~3% and are valid for stars with spectral types A0-M4. To derive even more accurate relations for Sun-like stars, we also determined these temperature relations omitting early-type stars (T_eff > 6750 K) that may have biased luminosity estimates because of rapid rotation; for this subset the dispersion is only ~2.5%. We find effective temperatures in agreement within a couple of percent for the interferometrically characterized sample of main-sequence stars compared to those derived via the infrared flux method and spectroscopic analysis.

STELLAR DIAMETERS AND TEMPERATURES. I. MAIN-SEQUENCE A, F, AND G STARS

We have executed a survey of nearby, main-sequence A-, F-, and G-type stars with the CHARA Array, successfully measuring the angular diameters of forty-four stars with an average precision of ~1.5%. We present new measures of the bolometric flux, which in turn leads to an empirical determination of the effective temperature for the stars observed. In addition, these CHARA-determined temperatures, radii, and luminosities are fit to Yonsei-Yale model isochrones to constrain the masses and ages of the stars. These results are compared to indirect estimates of these quantities obtained by collecting photometry of the stars and applying them to model atmospheres and evolutionary isochrones. We find that for most cases, the models overestimate the effective temperature by ~1.5%-4% when compared to our directly measured values. The overestimated temperatures and underestimated radii in these works appear to cause an additional offset in the stars surface gravity measurements, which consequently yield higher masses and younger ages, in particular for stars with masses greater than ~1.3 M_☉. Additionally, we compare our measurements to a large sample of eclipsing binary stars, and excellent agreement is seen within both data sets. Finally, we present temperature relations with respect to (B – V) and (V – K) colors as well as spectral type, showing that calibration of effective temperatures with errors ~1% is now possible from interferometric angular diameters of stars.

55 CANCRI: STELLAR ASTROPHYSICAL PARAMETERS, A PLANET IN THE HABITABLE ZONE, AND IMPLICATIONS FOR THE RADIUS OF A TRANSITING SUPER-EARTH

The bright star 55 Cancri is known to host five planets, including a transiting super-Earth. The study presented here yields directly determined values for 55 Cncs stellar astrophysical parameters based on improved interferometry: R = 0.943 ± 0.010 R_☉, T EFF = 5196 ± 24 K. We use isochrone fitting to determine 55 Cncs age to be 10.2 ± 2.5 Gyr, implying a stellar mass of 0.905 ± 0.015 M_☉. Our analysis of the location and extent of the systems habitable zone (HZ; 0.67-1.32 AU) shows that planet f, with period ~260 days and M sin i = 0.155 M_(Jupiter), spends the majority of the duration of its elliptical orbit in the circumstellar HZ. Though planet f is too massive to harbor liquid water on any planetary surface, we elaborate on the potential of alternative low-mass objects in planet fs vicinity: a large moon and a low-mass planet on a dynamically stable orbit within the HZ. Finally, our direct value for 55 Cancris stellar radius allows for a model-independent calculation of the physical diameter of the transiting super-Earth 55 Cnc e (~2.05 ± 0.15 R_⊕), which, depending on the planetary mass assumed, implies a bulk density of 0.76 ρ_⊕ or 1.07 ρ_⊕.

ASTROPHYSICAL PARAMETERS AND HABITABLE ZONE OF THE EXOPLANET HOSTING STAR GJ 581

GJ 581 is an M dwarf host of a multiplanet system. We use long-baseline interferometric measurements from the CHARA Array, coupled with trigonometric parallax information, to directly determine its physical radius to be 0.299 ± 0.010 R_☉. Literature photometry data are used to perform spectral energy distribution fitting in order to determine GJ 581s effective surface temperature T_(EFF) = 3498 ± 56 K and its luminosity L = 0.01205 ± 0.00024 L_☉. From these measurements, we recompute the location and extent of the systems habitable zone and conclude that two of the planets orbiting GJ 581, planets d and g, spend all or part of their orbit within or just on the edge of the habitable zone.

CHARA ARRAY MEASUREMENTS OF THE ANGULAR DIAMETERS OF EXOPLANET HOST STARS

Wehavemeasuredtheangulardiametersforasampleof 24exoplanethoststarsusingGeorgiaStateUniversity’s CHARA Array interferometer. We use these improved angular diameters together with Hipparcos parallax measurements to derive linear radii and to estimate the stars’ evolutionary states. Subjectheadingginfrared:stars — planetarysystems — stars:fundamentalparameters — techniques:interferometric Online material: extended figure, machine-readable table

The GJ 436 System: Directly Determined Astrophysical Parameters of an M Dwarf and Implications for the Transiting Hot Neptune

The late-type dwarf GJ 436 is known to host a transiting Neptune-mass planet in a 2.6 day orbit. We present results of our interferometric measurements to directly determine the stellar diameter (R_* = 0.455 ± 0.018 R_☉) and effective temperature (T_(EFF) = 3416 ± 54 K). We combine our stellar parameters with literature time-series data, which allows us to calculate physical and orbital system parameters, including GJ 436s stellar mass (M_* = 0.507^(+0.071)_(– 0.062) M_☉), stellar density (ρ_* = 5.37^(+0.30)_(–0.27) ρ_☉), planetary radius (R_p = 0.369^(+0.015)_(–0.015)R _(Jupiter)), and planetary mass (M_p = 0.078^(+0.007)_(–0.008) M_(Jupiter)), implying a mean planetary density of ρ_p = 1.55^(+0.12)_(–0.10) ρ_(Jupiter). These values are generally in good agreement with previous literature estimates based on assumed stellar mass and photometric light curve fitting. Finally, we examine the expected phase curves of the hot Neptune GJ 436b, based on various assumptions concerning the efficiency of energy redistribution in the planetary atmosphere, and find that it could be constrained with Spitzer monitoring observations.

Stellar diameters and temperatures - V. 11. Newly characterized exoplanet host stars

We use near-infrared interferometric data coupled with trigonometric parallax values and spectral energy distribution fitting to directly determine stellar radii, effective temperatures and luminosities for the exoplanet host stars 61 Vir, ρ CrB, GJ 176, GJ 614, GJ 649, GJ 876, HD 1461, HD 7924, HD 33564, HD 107383 and HD 210702. Three of these targets are M dwarfs. Statistical uncertainties in the stellar radii and effective temperatures range from 0.5 to 5 per cent and from 0.2 to 2 per cent, respectively. For eight of these targets, this work presents the first directly determined values of radius and temperature; for the other three, we provide updates to their properties. The stellar fundamental parameters are used to estimate stellar mass and calculate the location and extent of each system’s circumstellar habitable zone. Two of these systems have planets that spend at least parts of their respective orbits in the system habitable zone: two of GJ 876’s four planets and the planet that orbits HD 33564. We find that our value for GJ 876’s stellar radius is more than 20 per cent larger than previous estimates and frequently used values in the astronomical literature.

THE CHARA ARRAY ANGULAR DIAMETER OF HR 8799 FAVORS PLANETARY MASSES FOR ITS IMAGED COMPANIONS

HR 8799 is an hF0 mA5 γ Doradus-, λ Bootis-, Vega-type star best known for hosting four directly imaged candidate planetary companions. Using the CHARA Array interferometer, we measure HR 8799s limb-darkened angular diameter to be 0.342 ± 0.008 mas (an error of only 2%). By combining our measurement with the stars parallax and photometry from the literature, we greatly improve upon previous estimates of its fundamental parameters, including stellar radius (1.44 ± 0.06 R_☉), effective temperature (7193 ± 87 K, consistent with F0), luminosity (5.05 ± 0.29 L_☉), and the extent of the habitable zone (HZ; 1.62-3.32 AU). These improved stellar properties permit much more precise comparisons with stellar evolutionary models, from which a mass and age can be determined, once the metallicity of the star is known. Considering the observational properties of other λ Bootis stars and the indirect evidence for youth of HR 8799, we argue that the internal abundance, and what we refer to as the effective abundance, is most likely near solar. Finally, using the Yonsei-Yale evolutionary models with uniformly scaled solar-like abundances, we estimate HR 8799s mass and age considering two possibilities: 1.516^(+0.038)_(–0.024) M_☉ and 33^(+7)_(–13.2) Myr if the star is contracting toward the zero-age main sequence or 1.513^(+0.023)_(–0.024_ M_☉ and 90^(+381)_(–50) Myr if it is expanding from it. This improved estimate of HR 8799s age with realistic uncertainties provides the best constraints to date on the masses of its orbiting companions, and strongly suggests they are indeed planets. They nevertheless all appear to orbit well outside the HZ of this young star.

Stellar diameters and temperatures – VI. High angular resolution measurements of the transiting exoplanet host stars HD 189733 and HD 209458 and implications for models of cool dwarfs

We present direct radii measurements of the well-known transiting exoplanet host stars HD 189733 and HD 209458 using the CHARA Array interferometer. We find the limbdarkened angular diameters to be θLD = 0.3848 ± 0.0055 and 0.2254 ± 0.0072 mas for HD 189733 and HD 209458, respectively. HD 189733 and HD 209458 are currently the only two transiting exoplanet systems where detection of the respective planetary companion’s orbital motion from high-resolution spectroscopy has revealed absolute masses for both star and planet. We use our new measurements together with the orbital information from radial velocity and photometric time series data, Hipparcos distances, and newly measured bolometric fluxes to determine the stellar effective temperatures (Teff = 4875 ± 43, 6092 ± 103K), stellar linearradii(R∗ =0.805 ±0.016,1.203 ±0.061R� ),meanstellardensities(ρ∗ =1.62 ±0.11, 0.58 ± 0.14 ρ� ), planetary radii (Rp = 1.216 ± 0.024, 1.451 ± 0.074 RJup), and mean planetary densities (ρp = 0.605 ± 0.029, 0.196 ± 0.033 ρJup) for HD 189733b and HD 209458b, respectively. The stellar parameters for HD 209458, an F9 dwarf, are consistent with indirect estimates derived from spectroscopic and evolutionary modelling. However, we find that models are unable to reproduce the observational results for the K2 dwarf, HD 189733. We show that, for stellar evolutionary models to match the observed stellar properties of HD 189733, adjustments lowering the solar-calibrated mixing-length parameter to αMLT =1.34 need to be