We present the stellar atmospheric parameters (effective temperature, surface gravity, overall metallicity), radial velocities, individual abundances, and distances determined for 425,561 stars, which constitute the fourth public data release of the RAdial Velocity Experiment (RAVE). The stellar atmospheric parameters are computed using a new pipeline, based on the algorithms of MATISSE and DEGAS. The spectral degeneracies and the Two Micron All Sky Survey photometric information are now better taken into consideration, improving the parameter determination compared to the previous RAVE data releases. The individual abundances for six elements (magnesium, aluminum, silicon, titanium, iron, and nickel) are also given, based on a special-purpose pipeline that is also improved compared to that available for the RAVE DR3 and Chemical DR1 data releases. Together with photometric information and proper motions, these data can be retrieved from the RAVE collaboration Web site and the Vizier database.

/pdf/the-radial-velocity-experiment-rave-fourth-data-release-1v553gii0c.pdf

The Radial Velocity Experiment (RAVE): Fourth Data Release

We validate the accuracy and precision of the current SEGUE (Sloan Extension for Galactic Understanding and Exploration) Stellar Parameter Pipeline (SSPP), which determines stellar atmospheric parameters (effective temperature, surface gravity, and metallicity) and radial velocities (RVs), by comparing these estimates for selected members of three globular clusters (M 13, M 15, and M 2) and two open clusters (NGC 2420 and M 67) to the literature values. Spectroscopic and photometric data obtained during the course of the original Sloan Digital Sky Survey (SDSS-I) and its first extension (SDSS-II/SEGUE) are used to determine atmospheric parameter and RV estimates for stars in these clusters. Based on the scatter in the metallicities derived for the members of each cluster, we quantify the typical uncertainty of the SSPP values, σ ([Fe/H]) = 0.13 dex for stars in the range of –0.3 ≤ g – r ≤ 1.3 and 2.0 ≤ log g ≤ 5.0, at least over the metallicity interval spanned by the clusters studied (–2.3 ≤ [Fe/H] ≤ 0). The surface gravities and effective temperatures derived by the SSPP are also compared with those estimated from the comparison of color-magnitude diagrams with stellar evolution models; we find satisfactory agreement (σ(T eff)< 200 K and σ(log g) ≤ 0.4 dex).

https://digital.library.unt.edu/ark:/67531/metadc893670/m2/1/high_res_d/922062.pdf

The SEGUE Stellar Parameter Pipeline. II. Validation with Galactic Globular and Open Clusters

The weak radio source LBDS 53W091 is associated with a very faint (R ? 24.5) red (R - K ? 5.8) galaxy. Long spectroscopic integrations with the W. M. Keck telescope have provided an absorption-line redshift, z = 1.552 ? 0.002. The galaxy has a rest frame ultraviolet spectrum very similar to that of an F6 V star, and a single-burst old stellar population that matches the IR colors, the optical energy distribution and the spectral discontinuities has a minimum age of 3.5 Gyr. We present detailed population synthesis analyses of the observed spectrum in order to estimate the time since the last major epoch of star formation. We discuss the discrepancies in these estimates resulting from using different models, subjecting the UV spectrum of M32 to the same tests as a measure of robustness of these techniques. The models most consistent with the data tend to yield ages at z = 1.55 of 3.5 Gyr, similar to that inferred for the intermediate-age population in M32. Depending upon the assumed Hubble constant and the value of ?0, only certain cosmological expansion times are consistent with the age of LBDS 53W091; in particular, for ?0 = 1, only models with H0 45 km s-1 Mpc-1 are permitted. For H0 = 50 km s-1 Mpc-1 and ?0 = 0.2, we derive a formation redshift, zf ? 5.

http://iopscience.iop.org/article/10.1086/304381/pdf

LBDS 53W091: An Old, Red Galaxy at z = 1.552*

We present a new method, using Bayesian estimation, to determine stellar ages and their uncertainties from observational data and theoretical isochrones. The result for an individual star is obtained as the relative posterior probability density as function of the age ("G function"). From this can be derived the most probable age and confidence intervals. The convoluted morphology of isochrones and strong non-linearities make the age determination by any method difficult and susceptible to statistical biases, and as a result age uncertainties havee often been underestimated in the literature. From simulations we find that the G functions provide a general, robust and reliable way to quantify age information. Resulting age estimates are at least as accurate as those obtained with conventional isochrone fitting methods, and in some cases much better, especially when the observational uncertainties are large. We also find that undetected binaries, on the whole, have a surprisingly small effect on the age determinations. For a stellar sample, the individual G functions can be combined to derive the star formation history of the population; this will be developed in a forthcoming paper. For a coeval population the combination simplifies to computing the product of the individual G functions, and we apply that method to estimate the ages of the two open clusters IC 4651 and M67, using Padova isochrones and photometric data from the literature. For IC 4651 we find an estimated age of 1.56 +/- 0.03 Gyr, assuming a true distance modulus of 9.80. For M67 we find 4.05 +/- 0.05 Gyr for true distance modulus 9.48. The small formal errors of these age estimates do not include the ( much larger) uncertainties from calibration and model errors, but illustrate the statistical power of combining G functions. Our statistical approach to the age determination problem is well suited for the mass treatment of data resulting from large-scale surveys such as the Gaia mission. (Less)

/pdf/determination-of-stellar-ages-from-isochrones-bayesian-4xmpos2v2a.pdf

Determination of stellar ages from isochrones: Bayesian estimation versus isochrone fitting

We present new grids of colors and bolometric corrections for F-K stars having 4000 K ≤ Teff ≤ 6500 K, 0.0 ≤ log g ≤ 4.5, and -3.0 ≤ [Fe/H] ≤ 0.0. A companion paper extends these calculations into the M giant regime (3000 K ≤ Teff ≤ 4000 K). Colors are tabulated for Johnson U-V and B-V, Cousins V-R and V-I, Johnson-Glass V-K, J-K, and H-K, and CIT/CTIO V-K, J-K, H-K, and CO. We have developed these color-temperature relations by convolving synthetic spectra with the best-determined, photometric filter transmission profiles. The synthetic spectra have been computed with the SSG spectral synthesis code using MARCS stellar atmosphere models as input. Both of these codes have been improved substantially, especially at low temperatures, through the incorporation of new opacity data. The resulting synthetic colors have been put onto the observational systems by applying color calibrations derived from models and photometry of field stars that have effective temperatures determined by the infrared flux method. These color calibrations have zero points that change most of the original synthetic colors by less than 0.02 mag, and the corresponding slopes generally alter the colors by less than 5%. The adopted temperature scale, that of Bell & Gustafsson, is confirmed by the extraordinary agreement between the predicted and observed angular diameters of these field stars, indicating that the differences between the synthetic colors and the photometry of the field stars are not due to errors in the effective temperatures adopted for these stars. Thus, we have derived empirical color-temperature relations from the field star photometry, which we use as one test of our calibrated, theoretical, solar-metallicity color-temperature relations. Except for the coolest dwarfs (Teff < 5000 K), our calibrated model colors are found to match these relations, as well as the empirical relations of others, quite well, and our calibrated, 4 Gyr, solar-metallicity isochrone also provides a good match to color-magnitude diagrams of M67. We regard this as evidence that our calibrated colors can be applied to many astrophysical problems, including modeling the integrated light of galaxies. Because there are indications that the dwarfs cooler than 5000 K may require different optical color calibrations than the other stars, we present additional colors for our coolest dwarf models that account for this possibility.

https://iopscience.iop.org/article/10.1086/301243/pdf

Chih Kang Chou

Papers

Deep wide-field spectrophotometry of the open cluster M67