A standard stellar library for evolutionary synthesis - II. The M dwarf extension

Astron. Astrophys. Suppl. Ser. A standard stellar library for evolutionary synthesis II. The M dwarf extension? T. Lejeune 1 2 F. Cuisinier 0 R. Buser 2 0 Universidade de S~ao Paulo, IAG, Dept. de Astronomia , C.P. 9638, Sa~o Paulo 01065-970 , Brasil 1 Observatoire de Strasbourg , 11 rue de l'Universite, F 2 Astronomisches Institut der Universita ̈t Basel , Venusstr. 7, CH 3 67000 Strasbourg , France A standard library of theoretical stellar spectra intended for multiple synthetic photometry applications including spectral evolutionary synthesis is presented. The grid includes M dwarf model spectra, hence complementing the rst library version established in Paper I (Lejeune et al. 1997). It covers wide ranges of fundamental parameters: Te : 50 000 K 2000 K, log g: 5.5 −1:02, and [M=H]: +1:0 −5:0. A correction procedure is also applied to the theoretical spectra in order to provide color-calibrated flux distributions over a large domain of e ective temperatures. For this purpose, empirical Te {color calibrations are constructed between 11500 K and 2000 K, and semi-empirical calibrations for non-solar abundances ([M=H] = −3:5 to +1.0) are established. Model colors and bolometric corrections for both the original and the corrected spectra, synthesized in the (U BV )J(RI)CJ HKLL0M system, are given for the full range of stellar parameters. We nd that the corrected spectra provide a more realistic representation of empirical stellar colors, though the method employed is not completely adapted to the lowest temperature models. In particular the original di erential colors of the grid implied by metallicity and/or luminosity changes are not preserved below 2500 K. Limitations of the correction method used are also discussed. stars; fundamental parameters | stars; late type - Send o print requests to: T. Lejeune ? Tables 1 to 10 only available in electronic form at CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html 1. Introduction Grids of theoretical stellar spectra providing models at low and high metallicities are indispensable for modelling the chemical evolution of the integrated light of stellar systems from theoretical isochrones used to describe the timedependent stellar population. However, even the combined uses of available modern stellar libraries in evolutionary synthesis studies (e.g. Buzzoni 1989; Worthey 1994; Bressan et al. 1994) have been handicapped by intrinsic inhomogeneities and incompleteness. Furthermore, one of the most serious di culties arises from the fact that the synthetic spectra and colors provided by most theoretical libraries still show large systematic discrepancies with calibrations based on spectroscopic and photometric observations. This is particularly true at low e ective temperatures for which an accurate modelling of the stellar spectra requires important molecular opacity data which are not yet completely available. Ultimately, these limitations lead unavoidably to serious uncertainties in the interpretation of the population model. In order to overcome these major shortcomings, we have undertaken the construction of a comprehensive combined library of realistic stellar flux distributions intended for population and evolutionary synthesis studies. A preliminary version of such a standard grid was presented in Lejeune et al. (1997, Paper I, referred to hereafter as LCB97), along with an algorithm developed for correction and calibration of the (theoretical) spectra. In this previous grid, M dwarf models, which are important for the determination of mass-to-light ratios in stellar populations, were missing. We present here a more comprehensive library which incorporates these dwarf spectra. The construction of this basic combined library is presented in the following section. The empirical Te {color relations in U BV RIJ HKL photometry, required to calibrate the spectra, are presented in Sect. 3. Section 4 is dedicated to the correction of the theoretical spectra; in particular, 2000 2500 3500 . . . . we also examine the properties and the limitations of the method used when applied to the M dwarf models. Finally, we summarize the main properties of this new standard library in view of its synthetic photometry applications. Proceeding with the work undertaken in LCB97, we have built a more extensive library providing now almost complete coverage of the stellar parameter ranges in Te , log g, [M=H] which are required for population and evolutionary synthesis studies. This new grid is complementing the previous version by the addition of M dwarf model spectra. Di erent basic libraries have been assembled: the Kurucz (1995 , [K95]) models provide wide coverage in Te (50 000 K 3500 K), log g (5.0 0.0), and [M=H] (+1.0 −5.0), whereas in the temperature range 3500 K 2500 K the M giants spectra are represented by the hybrid \B+F" models constructed from spectra of Fluks et al. (1994) and Bessell et al. (1989 (...truncated)