Stellar SEDs from 0.3-2.5 Microns: Tracing the Stellar Locus and Searching for Color Outliers in SDSS and 2MASS

The Sloan Digital Sky Survey (SDSS) and Two Micron All Sky Survey (2MASS) are rich resources for studying stellar astrophysics and the structure and formation history of the Galaxy. As new surveys and instruments adopt similar filter sets, it is increasingly important to understand the properties of the ugrizJHKs stellar locus, both to inform studies of `normal' main sequence stars as well as for robust searches for point sources with unusual colors. Using a sample of ~600,000 point sources detected by SDSS and 2MASS, we tabulate the position and width of the ugrizJHKs stellar locus as a function of g-i color, and provide accurate polynomial fits. We map the Morgan-Keenan spectral type sequence to the median stellar locus by using synthetic photometry of spectral standards and by analyzing 3000 SDSS stellar spectra with a custom spectral typing pipeline. We develop an algorithm to calculate a point source's minimum separation from the stellar locus in a seven-dimensional color space, and use it to robustly identify objects with unusual colors, as well as spurious SDSS/2MASS matches. Analysis of a final catalog of 2117 color outliers identifies 370 white-dwarf/M dwarf (WDMD) pairs, 93 QSOs, and 90 M giant/carbon star candidates, and demonstrates that WDMD pairs and QSOs can be distinguished on the basis of their J-Ks and r-z colors. We also identify a group of objects with correlated offsets in the u-g vs. g-r and g-r vs. r-i color-color spaces, but subsequent follow-up is required to reveal the nature of these objects. Future applications of this algorithm to a matched SDSS-UKIDSS catalog may well identify additional classes of objects with unusual colors by probing new areas of color-magnitude space.Comment: 23 pages in emulateapj format, 17 figures, 7 tables. Accepted for publication in the Astronomical Journal. To access a high-resolution version of this paper, as well as machine readable tables and an archive of 'The Hammer' spectral typing suite, see http://www.cfa.harvard.edu/~kcovey v2 -- fixed typos in Table 7 (mainly affecting lines for M8-M10 III stars

Sub-Subgiants in the Old Open Cluster M67?

We report the discovery of two spectroscopic binaries in the field of the old open cluster M67 -- S1063 and S1113 -- whose positions in the color-magnitude diagram place them approximately 1 mag below the subgiant branch. A ROSAT study of M67 independently discovered these stars to be X-ray sources. Both have proper-motion membership probabilities greater than 97%; precise center-of-mass velocities are consistent with the cluster mean radial velocity. S1063 is also projected within one core radius of the cluster center. S1063 is a single-lined binary with a period of 18.396 days and an orbital eccentricity of 0.206. S1113 is a double-lined system with a circular orbit having a period of 2.823094 days. The primary stars of both binaries are subgiants. The secondary of S1113 is likely a 0.9 Mo main-sequence star, which implies a 1.3 Mo primary star. We have been unable to explain securely the low apparent luminosities of the primary stars; neither binary contain stars presently limited in radius by their Roche lobes. We speculate that S1063 and S1113 may be the products of close stellar encounters involving binaries in the cluster environment, and may define alternative stellar evolutionary tracks associated with mass-transfer episodes, mergers, and/or dynamical stellar exchanges

The design and performance of the Gaia photometric system

The European Gaia astrometry mission is due for launch in 2011. Gaia will rely on the proven principles of the ESA Hipparcos mission to create an all-sky survey of about one billion stars throughout our Galaxy and beyond, by observing all objects down to 20 mag. Through its massive measurement of stellar distances, motions and multicolour photometry, it will provide fundamental data necessary for unravelling the structure, formation and evolution of the Galaxy. This paper presents the design and performance of the broad- and medium-band set of photometric filters adopted as the baseline for Gaia. The 19 selected passbands (extending from the UV to the far-red), the criteria and the methodology on which this choice has been based are discussed in detail. We analyse the photometric capabilities for characterizing the luminosity, temperature, gravity and chemical composition of stars. We also discuss the automatic determination of these physical parameters for the large number of observations involved, for objects located throughout the entire Hertzsprung-Russell diagram. Finally, the capability of the photometric system (PS) to deal with the main Gaia science case is outline

HOMOGENIZATION OF STELLAR CATALOGUES THROUGH DATA INTERCOMPARISON

The accuracies of some selected stellar catalogues of Teff values have been estimated through data intercomparison. The technique of such estimating developed earlier for triples of catalogues has been adapted to a set of catalogues. A homogenized catalogue of Teff values has been produced by weighted data averaging and compared with some available data

Temperature and luminosity effects in photoelectric spectrum scanner measurements of F0-K5 stars

We study the possibility to classify the stars with the use of the photoelectric spectrum scanner designed at Astronomisches Institut at Bochum using a spectral resolution of about 10 Å. We analyse temperature and luminosity effects in two spectral indices which were expected to be promising as spectral classification criteria from previous studies

Spectral classification systems and photometry needed in calculations for atmospheric refraction


In ground-based measurements of exact positions of celestial objects on the sky the astronomical atmospheric refraction should be taken into account. In our calculations for refraction we use the Stone code (Stone 1996) where the mean refraction is computed by integrating numerically across the passband with Simpson's Rule. The code has been modified for the case of the 10″ LX200 telescope installed at the Lohrmann Observatory in Dresden. Tabulations of stellar spectral energy from Sviderskiene (1988) together with the quantum efficiency of the CCD detector being used, are applied. We have estimated the accuracies of stellar classification required to achieve the desirable accuracies in calculated refraction, the influence of ground-based atmospheric parameters has been discussed too. Some examples of the spectral classification systems which may provide the required classification accuracies, are given. The special case of computing differential refraction has also been considered. We justify the usefulness of photometric $V-R$ and $V-I$ indices in calculating refraction. Observations are planned to compare calculated and true refraction.


Tartu Astrophysical Observatory, Toravere, EE2444, Estonia

Proposal No. 133 for the Hipparcos program contains a sample of 111 apparently bright stars of spectral classes F--G--K, different luminosity classes and metallicities. Stars of solar chemical composition, supplemented by stars with reliable ground-base parallaxes, Hyades cluster stars, etc. are used for the calibration of photometric criteria for two-dimensional classification of stars in the Vilnius photometric system in terms of M V . New calibration provides a significant accuracy improvement in comparison to earlier calibrations. Key words: multicolor photometry; photometric classification; absolute magnitudes. 1. INTRODUCTION The seven-color photometric system, developed at the Vilnius Observatory, is being used already three decades for the photometric classification of stars. The system classifies stars of all MK types in spectral classes and absolute magnitudes in the presence of interstellar reddening. Also, it makes possible to identify photometrically the stars with differe..