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The Atlas9 Stellar Atmosphere Models by Castelli and Kurucz 2004

The atlas contains about 4300 stellar atmosphere models for a wide range of metallicities, effective temperatures and gravities. These LTE models with no convective overshooting computed by Fiorella Castelli, have improved opacities and abundances upon previously used by Kurucz (1990). The main improvements from previous opacity distribution functions listed in Castelli & Kurucz 2003 (IAU Symposium 210, Modelling of Stellar Atmospheres, Uppsala, Sweden, eds. N.E. Piskunov, W.W. Weiss. and D.F. Gray, 2003, ASP-S210) are:

1. The replacement of the solar abundances from Andres & Grevesse (1989, GCA,53,197; AG89) with those from Grevesse & Sauval (1998 Space. Sci. Rev.,85,161; GS98). See Table 2 of Castelli & Kurucz 2004.
2. The replacement of the TiO lines provided by Kurucz (1993) with the TiO lines from Schwenke (1998, Faraday Discuss., 109,321). Addition of the H_2 O lines (Partridge & Schwenke 1997, J. Chem. Phys., 106, 4618) and of the HI-HI and HI-H+ quasi-molecular absorptions near 1600 A and 1400 A (Allard et al. 1998, A&A, 335,1124), as distributed by Kurucz 1999a, 1999b. Extended molecular listand corrected previous bugs in Kurucz line lists.

These models are computed with the same wavelength resolution and a smaller temperature resolution than the Kurucz 1993 models. All the models have the same number of plane parallel layers from log(tau_Ross)=-6.875 to +2.00 in steps of Delta[log(tau_Ross)] = 0.125, computed assuming a pure mixing-length convection (no oveshooting) with 1/Hp=1.25. As beforem the microturbulent velocity used is 2 km s^{-1}.

The ATLAS9 installed in CDBS is from "The Grids of ATLAS9-ODFNEW models and fluxes" from the Fiorella Castelli's web page and was created on January 2007. These grids are also available from Dr. R. Kurucz.

The ATLAS9 includes models for abundances [M/H]=0.0, -0.5, -1.0, -1.5, -2.0, -2.5, +0.5, +0.2 and gravity range from log_g= 0.0 to +5.0 in steps of +0.5. The range in effective temperature from 3500 K to 50000 K is covered with an uneven grid (see Table 1a). The model spectra cover the ultraviolet (1000A) to infrared (10 microns) spectral range with non-uniform wavelength spacing (see Table 1b).

TABLE 1a: Grid of temperatures for the models

TABLE 1b: Wavelength coverage for the models

THE HST/CDBS VERSION OF THE ATLAS9 BY CASTELLI AND KURUCZ 2004

The new atlas is divided 8 independent subdirectories, according to metallicity. Within each subdirectory the stellar atmosphere models are given in STDAS multicolumn table format. Each table consist of 12 different columns, the first one containing the wavelength grid and each of the rest containing the spectrum of a star with the same effective temperature but different gravity, ranging from log_g= 0.0 to +5.0. Columns filled with zeros indicate that the model spectrum for that particular metallicity, effective temperature and gravity combination is not covered by the atlas.

The names of the table files are given as ckszz_ttttt.tab where "ck", for Castelli & Kurucz, are the first two letters of the atlas; "szz" is the metallicity of the model (zz) with its sign (s); and "ttttt" is the model's effective temperature, using four or five digits depending on the value. For instance, models for an effective temperature of 5000 K with [M/H]= -0.5 and [M/H]= +3.5 are indicated by ttttt= 5000, s= m, zz= 05 and ttttt= 5000, s= p, zz= 35, i.e. ckm05_5000.fits and ckp35_5000.fits.

Within each individual table file, each column is named "gyy" where "yy" corresponds to 10*log_g. For example, log_g= +0.5 and log_g= +4.0 models are located in columns named g05 and g40, respectively. See the appendix for an example of a standard header of a table file.

Physical fluxes of the spectra are given in FLAM surface flux units, i.e. ergs cm^{-2} s^{-1} A^{-1}. These flux units differ from those in the Castelli & Kurucz tables by a factor of 3.336 x 10^{-19} x lambda^{2} x (4pi)^{-1}, i.e. are converted from ergs cm^{-2} s^{-1} Hz^{-1}steradian^{-1} to ergs cm^{-2} s^{-1} A^{-1} by mutiplying the Castelli & Kurucz values by 3.336 x 10^{-19} x lambda^{2} x (4pi)^{-1}, where lambda is in Angstroms. To convert to observed flux at Earth, multiply by a factor of (R/D)^2 where R is the stellar radius, and D is the distance to Earth.

The names of the files located in each metallicity subdirectory are listed in the README file located in each subdirectory. The range in gravity covered by the models for the different temperatures is also indicated.

USE OF THE ATLAS9 BY CASTELLI & KURUCZ 2004 WITH SYNPHOT

Pysynphot tasks permit the use of spectra selected from one of many columns in a single STSDAS table file. One does this by specifying as the "spectrum" parameter the name of the disk file (as before), and appending the name of the column containing the flux in brackets. Thus, to select any model spectrum characterized by a given metallicity, effective temperature, and gravity, specify a "spectrum" of the form: crgridck04$m_directory/ckszz_ttttt.tab[gyy], where m_directory is the name of the subdirectory for a given metallicity. For example, to select the spectrum of a star with a metallicity of +0.1, a temperature of 10,000 K, and log gravity of 3.0, the specification would be: crgridck04$ckp01/ckp01_10000.fits[g30].

Please note that the model spectra in the atlas are in surface flux units. Thus, if the number of counts or the calculated absolute flux is needed, the model spectrum must be renormalized appropriately. One can do this in pysynphot with the "rn" function.

Pysynphot also allows the use of the cat() and icat() functions to select Castelli & Kurucz spectra. The syntax is "cat(ckA9models,t,m,g) where "t" is the effective temperature, "m" is the log metallicity [M/H], and "g" is the log gravity. The idea is that, instead of having to remember a directory/file naming syntax, a pysynphot user could specify a spectrum from a specified catalog (the New ATLAS9 of Castelli & Kurucz, in this case) which most closely matches the specified attributes (in this case, T_{eff}, [M/H], and log_g) using an expression. The difference between the cat() and icat() functions is that cat() selects the nearest spectrum to the specified parameters and icat() interpolates between the spectra that bracket the specified parameters.

Since the entire atlas occupies close to 70MB of disk space, many applications could be satisfied by a copy of the solar metallicity spectra, only.

A list of solar metallicity stars of different spectral types and luminosity classes together with their closest Castelli & Kurucz 2004 model spectrum is presented in Table 2. The physical parameters, T_{eff} and log_g, characterizing each O stars are taken from Martins, Schaerer, & Hiller's compilation of stellar parameters of Galactic O stars (Martins, Scharer & Hiller 2005,A&A,436,1049). The physical parameters for later stars are taken from Schmidt-Kaler's compilation of physical parameters of stars (Schmidt-Kaler 1982, Landolt-Bornstein VI/2b). For the later, the U-B and B-V colors of the closest model agree with the characteristic color of each star (see Schmidt-Kaler 1982) to better than 0.06 magnitude.

TABLE 2: Suggested models for specific stellar types