Synphot Data User's Guide

A.13 Buser-Kurucz Atlas of Model Atmospheres

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This atlas has been deprecated; use of the Pickles atlas is recommended instead.

Roland Buser has provided an extensive collection of Kurucz (1979) model atmosphere spectra covering a wide range in metallicity, effective temperature, and gravity. They are organized into several atlases, A, B, C, D, M, and S, located in the directory crgridbk$.

The BK atlas is a library of stellar flux spectra calculated by Kurucz and Buser from theoretical model atmospheres. The spectra were computed for a wide range of physical parameters, covering essentially the whole HR diagram observed for galactic stars. For all the spectra, fluxes are given mostly with a resolution of 25 е on a uniform grid of wavelengths from the UV to the infrared. The BK atlas is thus especially suited for synthetic photometry applications (cf. Buser 1986, and references below), which also plays a major role in the calibration and the interpretation of HST observations (Koornneef et al. 1986). Therefore, the BK atlas has been implemented in the Calibration Data Base System (CDBS) at STScI.

The BK atlas consists of 1434 files, each of which represents a metal-line blanketed flux spectrum for a theoretical stellar model atmosphere. There files were prepared from two original data libraries, K1200 and BKLATE.

The K1200 library was computed by Kurucz (1979a,b) from his "ATLAS" model atmospheres for early-type (O to G) stars. The 1200 models were computed from three different versions of the "ATLAS" code, allowing for changes or improvements in the underlying physics, as shown in Table A.18:

Table A.18: Kurucz K1200 Library

Models	Code/Description	Reference
K1200   1- 284	A     /ATLAS6 original	Kurucz 1979a
K1200 285- 609	APR/ATLAS6 purely radiative	Kurucz 1979b
K1200 610-1200	AIC/ATLAS6 improved convective	Kurucz 1979b

K1200 models 1-284, including their flux spectra, have been fully published by Kurucz (1979a), while models 285-1200 were described by Kurucz (1979b), and have been widely distributed but not published.

The BKLATE library was computed by Buser and Kurucz (1985, 1988, 1992) for late-type (F to K) stars from published as well as unpublished "GBEN" model atmospheres calculated by Gustafsson et al. (1975) and by Eriksson et al. (1979), respectively. Line-blanketed flux spectra were computed following the same procedure and employing the same opacity source input as in Kurucz (1979a). In particular, all the spectra were calculated using the extensive "KP" atomic line lists compiled by Kurucz and Peytremann (1975), and assuming a fixed turbulent velocity, vturb=2.00 km/s, and a fixed helium abundance, N(He)=0.10 (i.e., a number fraction of 10%). The BKLATE library data have not been published yet.

For convenient use, the BK atlas flux spectra have been subdivided into separate blocks (or sub-atlases) corresponding to the physical distinctions of their underlying model atmospheres, as described above. Table A.19 summarizes the structure of the atlas.

Table A.19: BK Atlas

Block	Source of Spectra	# of Spectra	Models + Opacity	Teff	Log G	[M/H]
A	K1200	279	A    +KP	5500-50000	0.00-5.00	0/-1/-2
B	K1200	323	APR  +KP	8000-20000	1.00-4.50	1/.5/-.5/-1
C	K1200	590	AIC  +KP	5500- 8500	0.00-4.50	1 to -9.99
D	BKLATE	233	GBEN +KP	3750- 6000	0.75-5.25	.5 to -3
S	K1200	3	A/AIC+KP	5770,9400	4.44,3.95	0
	BKLATE	1	GBEN +KP	5780	4.44	0
M	K1200	5	A/APR+KP	9400-10000	3.90-4.30	0/.5/1

Notice that within each of blocks A through D, models are available for ranges of usually uniformly spaced parameter values. Models are not available, however, for all possible combinations of parameter values. Block S contains models for two of the most prominent standard stars: three models of the Sun, and one model of Vega. Block M contains five miscellaneous models with special parameter combinations.

The file names in the crgridbk$ directory are of the form bk_mnnnn.fits, where m is the block code (a, b, c, d, s, or m), and nnnn is a running sequence number within the block. Within each block, the individual models are ordered starting with the lowest metallicity, [M/H], temperature, T_eff, and surface gravity, log G, and with log G increasing fastest and [M/H] increasing most slowly. A complete listing of all the BK atlas model spectra can be found in the README file in the crgridbk$ directory.

For all 1434 models in the BK atlas, flux spectra are given for the same set of 342 wavelengths as was used with the published Kurucz (1979a) models. Most of these wavelength points cover the range between 229 е in the UV and about 2 microns in the IR, with spacings between 25 and 100 е. 

Fluxes in the BK atlas tables, tabulated in units of fnu, are surface fluxes. Therefore, calculations of absolute luminosities for the BK atlas models require additional assumptions about the radii of the stars represented by the models.

A number of synthetic photometry applications have already been made using the BK atlas, mainly for the hotter Kurucz models from the K1200 library. A few pertinent references for theoretical calibrations of photometric systems are Lub and Pel (1977) for the Walraven VBLUW system, Buser and Kurucz (1978) for the Johnson UBV system, Relyea and Kurucz (1978) and Lester et al. (1986) for the Stromgren uvby system, and Buser and Kurucz (1988, 1992) for the Johnson-Cousins UBVRI system.