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Session 81 - Variable Cool and Late Type Stars.
Display session, Wednesday, January 15
Metropolitan Ballroom,
The determination of fundamental parameters such as T_eff, [M/H], and log\ g_cgs is difficult to do with precision for late-type dwarfs, since they are compact (4.5 < log\ g_cgs < 5.5) objects with a low effective temperature (T_eff \simeq 2000 - 4000 K). Therefore i) the stars are intrinsically faint (M_v \simeq +11^m ), making spectroscopy for all but the closest dwarfs impractical, and ii) the spectral region about \lambda_peak \simeq 1.1\ \mum is strongly affected by telluric water vapor, making the extraction of the intrinsic spectrum non-trivial. In addition, the atmospheres of cool dwarfs are difficult to model because the line opacities include large contributions from relatively complex molecules; only recently have realistic modeling techniques emerged.
Here we present methods based on broad-band photometry which circumvent many spectroscopic limitations. Synthetic photometry of the theoretical models of Allard (1995) has been performed to produce various color-color diagrams, such as (I-J) vs. (J-H). It is shown that loci of constant [M/H] and T_eff\ on this diagram are widely separated for 2000 K \leq T_eff \leq 4000 K, indicating its excellent utility as a metallicity-temperature diagnostic for late-type subdwarfs.
The empirical calibration of the (I-J), (J-H) diagram for these dwarf stars is done as follows. The LHS and NLTT catalogs prove to be convenient filters for subdwarf stars since they preferentially contain Galactic halo stars. 30 program stars were selected from the catalogs such that i) the star was identified as an M-type subdwarf, ii) it had a common proper motion with an F, G, or an early K-type companion, and iii) the star is brighter than V=+16^m (for high-quality photometry). Detailed information (particularly abundances) on the primary is available in the literature, so one can immediately infer the composition and distance of the subdwarf. Observed IJH photometry for the program stars was plotted on a cubic spline-interpolated (I-J), (J-H) diagram, and the derived [M/H] and T_eff\ were calculated from the theoretical colors. The derived metallicities were then compared with those from the literature.
