Synphot Data User's Guide


A.8 Kurucz 1993 Atlas of Model Atmospheres

This atlas has been deprecated; use of the Castelli & Kurucz atlas is recommended instead.

The atlas contains about 7600 stellar atmosphere models for a wide range of metallicities, effective temperatures and gravities. These LTE models have improved opacities and are computed with a finer wavelength and temperature resolution than the previous Buser-Kurucz atlas installed in the CDBS (crgridbk). The micro-turbulent velocity is 2 km s-1.

The new atlas installed in the CDBS is from the Kurucz database at Goddard Space Flight Center. The original atlas (CD-ROM No. 13) was created on August 22, 1993 and can be obtained from Dr. R. Kurucz.

The atlas includes models of abundances (log_Z) relative to solar of +1.0, +0.5, +0.3, +0.2, +0.1, +0.0, -0.1, -0.2, -0.3, -0.5, -1.0, -1.5, -2.0, -2.5,-3.0, -3.5, -4.0, -4.5, and -5.0. The grid of models cover the 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 A.11). The model spectra cover the ultraviolet (1000A) to infrared (10 microns) spectral range with non-uniform wavelength spacing (see Table A.12).

Table A.11: Grid of Temperatures for the Models
Temperature Range Grid Step
3000 - 10000 250
10000 - 13000 500
13000 - 35000 1000
35000 - 50000 2500

Table A.12: Wavelength Coverage for the Models
Wavelength Range Grid Step
microns A
0.10 - 0.29 10
0.29 - 1.00 20
1.00 - 1.60 50
1.60 - 3.20 100
3.20 - 8.35 200
8.35 - 10.0 400

A.8.1 The HST/CDBS Version of the 1993 Kurucz Atlas

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.13 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 Teff= 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.13: Sample FITS Header for a Pickles Spectrum
 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.

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.

A.8.2 Use of Kurucz Atlas with SYNPHOT

Synphot tasks now 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, specify a "spectrum" of the form: crgridk93$m_directory/kszz_ttttt.fits[gyy], where m_directory is the name of the subdirectory for a given metallicity. For example, to select the spectrum of a star with a metallicity of +0.1, a temperature of 10,000 K, and log gravity of 3.0, the specification would be: crgridk93$kp01/kp01_10000.fits[g30].

Alternatively, one may select a model spectrum within a synphot expression using the function icat (which interpolates between the nearest matching models) or cat (which simply selects the closest match). The syntax for the Kurucz models is icat(k93models,Teff,metallicity,logG).

The icat task can give incorrect results without warning if it is used at the edges of the defined grid of models. See above for a discussion of the available ranges for these values, and exercise caution when selecting a value near one of the extremes.

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.

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 Kurucz model spectrum is presented in Table A.14. The physical parameters, Teff and log_g, characterizing each star are taken from Schmidt-Kaler's compilation of physical parameters of stars (Schmidt-Kaler 1982, Landolt-Bornstein VI/2b). 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.14: Suggested Models for Specific Stellar Types
Type Teff log_g Kurucz model
O3V 52500 +4.14 kp00_50000[g50]
O5V 44500 +4.04 kp00_45000[g50]
O6V 41000 +3.99 kp00_40000[g45]
O8V 35800 +3.94 kp00_35000[g40]
B0V 30000 +3.9 kp00_30000[g40]
B3V 18700 +3.94 kp00_19000[g40]
B5V 15400 +4.04 kp00_15000[g40]
B8V 11900 +4.04 kp00_12000[g40]
A0V 9520 +4.14 kp00_9500[g40]
A5V 8200 +4.29 kp00_8250[g45]
F0V 7200 +4.34 kp00_7250[g45]
F5V 6440 +4.34 kp00_6500[g45]
G0V 6030 +4.39 kp00_6000[g45]
G5V 5770 +4.49 kp00_5750[g45]
K0V 5250 +4.49 kp00_5250[g45]
K5V 4350 +4.54 kp00_4250[g45]
M0V 3850 +4.59 kp00_3750[g45]
M2V 3580 +4.64 kp00_3500[g45]
M5V 3240 +4.94 kp00_3500[g50]
B0III 29000 +3.34 kp00_29000[g35]
B5III 15000 +3.49 kp00_15000[g35]
G0III 5850 +2.94 kp00_5750[g30]
G5III 5150 +2.54 kp00_5250[g25]
K0III 4750 +2.14 kp00_4750[g20]
K5III 3950 +1.74 kp00_4000[g15]
M0III 3800 +1.34 kp00_3750[g15]
O5I 40300 +3.34 kp00_40000[g45]
O6I 39000 +3.24 kp00_40000[g45]
O8I 34200 +3.24 kp00_34000[g40]
BOI 26000 +2.84 kp00_26000[g30]
B5I 13600 +2.44 kp00_14000[g25]
AOI 9730 +2.14 kp00_9750[g20]
A5I 8510 +2.04 kp00_8500[g20]
F0I 7700 +1.74 kp00_7750[g20]
F5I 6900 +1.44 kp00_7000[g15]
G0I 5550 +1.34 kp00_5500[g15]
G5I 4850 +1.14 kp00_4750[g10]
K0I 4420 +0.94 kp00_4500[g10]
K5I 3850 +0.34 kp00_3750[g05]
M0I 3650 +0.14 kp00_3750[g00]
M2I 3450 -0.06 kp00_3500[g00]

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