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ATLAS Versus NextGen Model Atmospheres: A Combined Analysis of Synthetic Spectral Energy Distributions
We carried out a critical appraisal of the two theoretical models,Kurucz' ATLAS9 and PHOENIX/NextGen, for stellar atmosphere synthesis.Our tests relied on the theoretical fit of spectral energy distributions(SEDs) for a sample of 334 target stars along the whole spectral-typesequence, from the classical optical catalogs of Gunn & Stryker andJacoby et al. The best-fitting physical parameters (Teff,logg) of stars allowed an independent calibration of the temperature andbolometric scale versus empirical classification parameters (i.e.,spectral type and MK luminosity class); in addition, the comparison ofthe synthetic templates from the ATLAS and NextGen grids allowed us toprobe the capability of the models to match spectrophotometricproperties of real stars and assess the impact of the different inputphysics. We can sketch the following main conclusions of our analysis:(1) Fitting accuracy of both theoretical libraries drastically degradesat low Teff at which both ATLAS and NextGen models still failto properly account for the contribution of molecular features in theobserved SED of K-M stars. (2) Compared with empirical calibrations,both ATLAS and NextGen fits tend, on average, to predict slightly warmer(by 4%-8%) Teff for both giant and dwarf stars of fixedspectral type, but ATLAS provides, in general, a sensibly better fit (afactor of 2 lower σ of flux residuals) than NextGen. (3) There isa striking tendency of NextGen to label target stars with an effectivetemperature and surface gravity higher than that of ATLAS. The effect isespecially evident for MK I-III objects for which about one in fourstars is clearly misclassified by NextGen in logg. This is a consequenceof some ``degeneracy'' in the solution space, partly induced by thedifferent input physics and geometry constraints in the computation ofthe integrated emerging flux (ATLAS model atmospheres assume standardplane-parallel layers, while NextGen adopts, for low-gravity stars, aspherical-shell geometry). A different T(τ) vertical structure ofstellar atmosphere seems also required for NextGen synthetic SEDs inorder to better account for limb-darkening effects in cool stars, assupported by the recent observations of the EROS BLG2000-5 microlensingevent.
| Photometric Separation of Stellar Properties Using SDSS Filters
Using synthetic photometry of Kurucz model spectra, we explore thecolors of stars as a function of temperature, metallicity, and surfacegravity with Sloan Digital Sky Survey (SDSS) filters, u'g'r'i'z'. Thesynthetic colors show qualitative agreement with the few publishedobservations in these filters. We find that the locus of synthetic starsis basically two-dimensional for 4500 < T < 8000 K, whichprecludes simultaneous color separation of the three basic stellarcharacteristics we consider. Colors including u' contain the mostinformation about normal stellar properties; measurements in this filterare also important for selecting white dwarfs. We identify two differentsubsets of the locus in which the loci separate by either metallicity orsurface gravity. For 0.5 < g' - r' < 0.8 (corresponding roughly toG stars), the locus separates by metallicity; for photometric error of afew percent, we estimate metallicity to within ~0.5 dex in this range.In the range -0.15 < g' - r' < 0.00 (corresponding roughly to Astars), the locus shows separation by surface gravity. In both cases, weshow that it is advantageous to use more than two colors whendetermining stellar properties by color. Strategic observations in SDSSfilters are required to resolve the source of a ~5% discrepancy betweensynthetic colors of Gunn-Stryker stars, Kurucz models, and externaldeterminations of the metallicities and surface gravities. The syntheticstar colors can be used to investigate the properties of any normal starand to construct analytic expressions for the photometric prediction ofstellar properties in special cases.
| Far-Ultraviolet Stellar Photometry: Fields Centered on rho Ophiuchi and the Galactic Center
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJS..104..101S&db_key=AST
| CCD photometry and astrometry of visual double stars
We present magnitude and color differences as well as separations andposition angles of the component of 30 visual double and multiplesystems with A-type primaries measured by means of a CCD detectorattached to the 61 cm telescope on Cerro Las Campanas. The angularseparation measured using the V filter rho(V) is systematically largerthan the angular separation measured using the B filter rho(B). At least20 percent of them have very similar astrophysical parameters, but thereal separation of their components is larger than 0.01 parsecs. Thatmeans that these stars are not gravitationally bound, but they have acommon origin.
| Stellar spectrophotometric atlas, wavelengths from 3130 to 10800 A
Over the course of the past 10 years, a large body of homogeneousspectrophotometric data on relatively bright stars has been gathered toserve as a library with which to synthesize galaxy spectra in populationstudies. As the data might be generally useful, they are presented inthree tables. One hundred seventy-five (175) stars were selected, mostfrom the Navy photometric catalog. Covered are complete ranges ofspectral type and luminosity class. Normalized spectral energydistributions of these stars in pictorial form are provided along withtables of calculated colors, scan line and continuum indices, and someinformation on the relation of the scan colors to real photometricsystems.
| Local photometric standards for CaII emission stars
UBV data are given for 108 stars which are suitable local standards for52 stars with strong Ca2 emissions. An additional eight stars wererejected as possible standards because of suspected variability.
| Visual multiples. VII - MK classifications
Classifications are given for 865 components of visual multiples; theyshow no systematic differences from the MK system, and the random errorsare one subclass in type and two-thirds of a luminosity class. It isfound that at least 1% of the F-type IV and V stars are weak-lined, 32%of the A4-F1 IV and V stars are Am, and 5% of the A0-A3 IV and V starsare early-type Am. Attention is called to the large fraction (55%) ofthe A3-A9 III-V stars that are of luminosity classes III or IV, unlikethe percentage (16%) at neighboring types.
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