Astron. Astrophys. 333, 231-250 (1998)

1. Introduction

The luminosities and temperatures of stars are most often deduced from observed broad-band colors and magnitudes. Empirical color-temperature relations and color - bolometric magnitude corrections are normally used to transform from observed standard magnitudes and colors to temperature and luminosity (eg. bolometric corrections: Schmidt-Kaler 1982; Reid & Gilmore 1984; Bessell & Wood 1984; Tinney et al. 1993; temperatures: Bessell 1979, 1995; Ridgway et al. 1980; Di Benedetto & Rabbia 1987; Blackwell & Lynas-Gray 1994 (BLG94); Tsuji et al. 1995, 1996a; Alonso et al. 1996 (AAM96); Dyck et al. 1996; Perrin et al. 1997). However, apart from AAM96 who give  - color relations for F0-K5 dwarfs with both solar and lower than solar metallicity, these empirical data are mostly defined by a restricted group of stars, namely nearby solar composition giants and dwarfs. It is therefore very important to have theoretically derived colors from model atmospheres that can cover the complete range of parameter space, temperature, gravity, composition etc. The empirical data can be compared with the near-solar composition synthetic data to check the goodness of the synthetic data for a restricted set of temperatures, gravities and compositions and by implication the viability of the full theoretical data set.

Model atmosphere grids by Gustafsson et al. (1975) (MARCS) and Kurucz (1979) (ATLAS) have been used successfully by many people over the past 20 years. In particular, discussion of MARCS synthetic photometry was given by Gustafsson & Bell (1979), and Bell & Gustafsson (1989) and of ATLAS synthetic photometry by Buser & Kurucz (1978); however, their limitations due to inadequate lists of atomic and molecular lines and treatment of semi-convection at the intermediate temperatures became more and more evident. Consequently new and improved model atmospheres have been computed making use of the greatly increased computing power and the explosion of data available on atomic and molecular lines from theoretical and experimental work. Kurucz (1993, 1994) (ATLAS9) published a new grid of models and colors for O-K stars; the convective models of these grids were later revised by Kurucz (1995a). In addition, some subgrids for convective ATLAS9 models with still a different convection than that used by Kurucz (1995a) were computed by Castelli (1997). Work is in progress to complete them. A new grid by Plez et al. (1997) (NMARCS) for A-M stars is also currently being completed.

We have preliminary published and unpublished data associated with these new grids and they form the basis of this paper. The fluxes, synthetic colors and bolometric corrections are a great improvement over the older data and deserve to be more widely available for use. They suggest that the new grids when completed should enable excellent population synthesis models to be computed from synthetic spectra and colors. They also suggest that non-standard photometric passbands, such as those from the WFPC2 on HST, Hipparcos and MACHO, can be accurately calibrated from synthetic spectra.

The UBVRIJHKL colors presented here have been computed using the passbands defined by Bessell (1990) for the Johnson-Cousins UBVRI photometry and by Bessell & Brett (1988) for the Johnson-Glass JHKL photometry.

In an extensive Appendix we discuss details associated with computing the synthetic photometry, i.e. theoretical magnitudes and colors, and describe how the zeropoints of the standard system and the bolometric corrections were adopted.

In Appendices A and B we describe how the UBVRIJHKL colors were normalised using observed and computed colors for Vega and Sirius.

The bolometric correction is defined as , where X stands for the particular passband. In Appendices C and D, the bolometric correction zeropoint was derived by assuming for the Sun and from the observed mag (Stebbins & Kron, 1957). Thus we assigned a visual bolometric correction () of -0.07 mag for the solar model. Note that this zeropoint is different to that of -0.193 mag adopted by Kurucz (1979; 1993; 1994) and followed by Schmidt-Kaler (1982).

In Appendix E the theoretical realisations of standard system passbands in general and of the UBVRI system in particular are discussed and on the basis of this discussion we decided to scale the raw computed U-B colors by 0.96 as described. The scaled U-B colors are presented in this paper. The other colors have not been adjusted, but it is suggested that they could be adjusted to fit the colors of the hottest and coolest stars, as the standard systems probably incorporate non-linear color terms.

In Appendix F the variation of the effective wavelengths of the UBVRI bands with effective temperature are discussed and theoretical interstellar reddening relations are derived.

© European Southern Observatory (ESO) 1998

Online publication: April 15, 1998
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