Synphot Data User's Guide

A.3 The HST/CDBS Version of the 1993 Kurucz Atlas

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The atlas is divided in 19 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 kszz_ttttt.fits where "k", for Kurucz, is the first letter 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 log_Z= -0.5 and log_Z= +3.5 are indicated by ttttt= 5000, s= m, zz= 05 and ttttt= 5000, s= p, zz= 35, i.e. km05_5000.fits and kp35_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 Table A.5 for an example of a standard header of a table file. In this example the name of the file is kp00_8000.fits and contains all the models for a star of metallicity log_Z= 0.0 and effective temperature T_eff= 8000 K. 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 do not appear listed in the header. Their corresponding columns (g00 and g05) are filled with zeros. The models are in FLAM surface flux units, i.e. ergs cm^-2 s^-1 A^-1.

Table A.5: Sample FITS Header from Kurucz 93 Model

SIMPLE = T / file does conform to FITS standard BITPIX = 16 / number of bits per data pixel NAXIS = 0 / number of data axes EXTEND = T / FITS dataset may contain extensions COMMENT FITS (Flexible Image Transport System) format defined in Astronomy and COMMENT Astrophysics Supplement Series v44/p363, v44/p371, v73/p359, v73/p365. COMMENT Contact the NASA Science Office of Standards and Technology for the COMMENT FITS Definition document #100 and other FITS information. ORIGIN = 'STScI-STSDAS/TABLES' / Tables version 1999-03-22 FILENAME= 'kp00_8000.fits' / name of file TEFF = 8000 LOG_Z = 0.00000000000000E+00 HISTORY g10 HISTORY g15 HISTORY g20 HISTORY g25 HISTORY g30 HISTORY g35 HISTORY g40 HISTORY g45 HISTORY g50 HISTORY Kurucz model atmospheres (1993) HISTORY Fluxes tabulated in units of erg/s/cm^2/A HISTORY are surface fluxes. To transform to observed HISTORY fluxes multiply by (R/D)^2 where R is the HISTORY radius of the star and D the distance. HISTORY Each column in the table represents the HISTORY spectrum of a star for the same metallicity HISTORY and effective temperature but different gravity. END

 

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 Kurucz CD-ROM by a factor of 3.336 x 10^-19 x lambda² 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. 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.