Synphot Data User's Guide
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| Synphot Data User's Guide | ||||
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A.3 The Castelli and Kurucz 2004 ATLAS9 of Model Atmospheres
The atlas contains 476 stellar atmosphere models for each atmosphere grid. The models were computed for 76 effective temperature values, 11 gravity values (although for a given temperature not all fluxes for all gravity values were calculated), and for 8 different metallicities. Table 1 in Castelli and Kurucz 2003 summarizes the available models for all grids. These LTE models with no convective overshooting computed by Fiorella Castelli, use improved opacity distribution functions (ODFs) and updated abundances upon previously used by Kurucz (1990).
The main improvements as summarized in Castelli & Kurucz 2003, are:
- solar abundances from Grevesse & Sauval 1998, in place of the older ones from Andres & Grevesse (1989, GCA,53,197). See Table A.3 of Castelli & Kurucz 2003;
- TiO lines from Schwenke 1998 in place of those from Kurucz (1993);
- The addition of the H_2 O lines (Partridge & Schwenke 1997) as distributed by Kurucz (1999a, 1999b), and the HI-HI and HI-H+ quasi-molecular absorptions near 1600 A and 1400 A computed according to Allard et al. 1998; plus additional changes to atomic and molecular data.
These models are computed with the same wavelength resolution and a smaller temperature resolution than the Kurucz 1993 models. Most 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, and all are computed assuming a mixing-length convection (no overshooting) with 1/Hp=1.25. As for earlier models, a microturbulent velocity of 2 km s-1 is used.
These models are for metallicities [M/H]=0.0, -0.5, -1.0, -1.5, -2.0, -2.5, +0.5, +0.2 and gravity values 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 A.3). The model spectra cover the ultraviolet (1000A) to infrared (10 microns) spectral range with non-uniform wavelength spacing (see Table A.4).
Table A.3: Grid of Temperatures for the Castelli-Kurucz Models
Table A.4: Wavelength coverage for the Castelli-Kurucz Models
Wavelength Range Grid Step microns A 0.09 - 0.29 10 0.29 - 1.00 20 1.00 - 1.60 50 1.60 - 3.20 100 3.20 - 6.4 200 6.4 - 10.0 400
A.3.1 The HST/CDBS Version of the Castelli and Kurucz models
The new atlas is divided in 8 independent subdirectories, according to metallicity (the CDBS tables were created in January 2007). Within each subdirectory the stellar atmosphere models are given in FITS multicolumn table format. Each table consists of 12 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 tables are given as ckszz_ttttt.fits where "ck" stand for Castelli & Kurucz; "szz" is the metallicity ([M/H] or log_Z) of the model (zz) with its sign (s); and "ttttt" is the model effective temperature (Teff), 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, respectively, and correspond to the files 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.
In Table A.5, an example of a standard header for the file ckp00_8000.fits which gathers all calculated models for a star of metallicity log_Z= 0.0 and effective temperature Teff= 8000 K is given. Models cover a range of gravities from log_g= +1.0 (g10 in the header) to log_g= +5.0 (g50 in the header). Models for gravities log_g= +0.0 and +0.5 are not available for this particular metallicity and effective temperature combination, and therefore are not listed in the header.Their corresponding columns (g00 and g05) are filled with zeros.
Table A.5: Sample FITS Header for Castelli and Kurucz Model
As indicated in the header file, physical fluxes of the spectra are given in FLAM surface flux units, i.e. ergs cm-2 s-1 Å-1. These flux units differ from those in the Castelli & Kurucz tables by a factor of (3.336 x 10-19 x
-2 x (4
)-1)-1, i.e. are converted from the original units of ergs cm-2 s-1 Hz-1 steradian-1 to ergs cm-2 s-1 Å-1 by multiplying the Castelli & Kurucz values by (3.336 x 10-19 x
The files located in each metallicity subdirectory are listed in a file located in the main subdirectory called catalog.fits: for a given temperature a number of models are available for different gravity values.
In the following an excerpt of the file is provided (Table A.6). The first column provides temperature, metallicity, and gravity value, while the second column lists the corresponding file with the gravity column in square brackets. In this case, the two sets of models belong to the same directory, ckm05, for a [M/H]= -0.5, for two temperatures (10000 and 10250 respectively), and for gravities between log_g= +0.0 and log= +5.0. By attaching to the filename a gravity value between square brackets a specific column, with flux values for that specific gravity value, is going to be read and used by synphot. See next paragraph about the use of the catalog with Synphot tasks.
Table A.6: Excerpt from catalog.fits in crgrid$ck04models/
A.3.2 Use of the Castelli and Kurucz models with Synphot
Synphot tasks permit the use of spectra selected from one of many columns in a single FITS file. One does this by specifying as the "spectrum" parameter the name of the disk file (as before), and appending the name of the column containing the flux in brackets. Thus, to select any model spectrum characterized by a given metallicity, effective temperature, and gravity, one needs to specify a "spectrum" of the form: crgridck04$m_directory/ckszz_ttttt.fits[gyy], where m_directory is the name of the subdirectory for a given metallicity, szz, ttttt, and gyy are as above. For example, to select the spectrum of a star with a metallicity of +0.1, a temperature of 10,000 K, and log_g 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 synphot with the "rn" function.
Synphot also allows the use of the cat() and icat() functions to select Castelli & Kurucz spectra. The syntax is "cat(ck04models,t,m,g) (or icat(ck04models,t,m,g)) where "t" is the effective temperature, "m" is the log metallicity [M/H], and "g" is the log_g.
Since the entire atlas is very large, and many cases can be explored by use of the solar metallicity models, only these are made available.
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 A.7. The physical parameters, Teff and log_g, characterizing each O stars are taken from Martins, Schaerer, & Hiller's (2005) compilation of stellar parameters of Galactic O stars. The physical parameters for later stars are taken from Schmidt-Kaler's (1982) compilation of physical parameters of stars); for these, 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 A.7: Suggested Models for Specific Stellar Types
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