SPECTRAL-SYNTHESIS OF TIO LINES

We explore the extent to which current titanium oxide (TiO) line data and M dwarf model atmospheres can be used to reproduce an R = 120,000 optical spectrum of the relatively inactive star Gliese 725B (M3.5 V). We find that tabulated TiO wavelengths have errors large enough to co mplicate line identification, especially for transitions involving hig her vibrational states. We determine empirical wavelength corrections for 12 strong gamma-bands near 6680 and 7090 Angstrom. For the sequenc e of orbital quantum numbers, J, within any one of these bands, our ob servations confirm the predicted line spacing, thereby validating the rotational constants for low vibrational levels. However, the predicte d wavelengths have zero-point errors that differ for each overlapping band. Next, we compare observed and synthetic spectra near 8463 Angstr om, where an epsilon Q(3) 0-0 band head is expected, demonstrating tha t the electronic oscillator strength of 0.014 advocated by Jorgensen i s too large by at least a factor of 5. This has a minor effect on the structure of theoretical model atmospheres. Using our empirically corr ected TiO wavelengths, we compute a grid of synthetic spectra for Alla rd & Hauschildt models spanning a range in effective temperature (T-ef f), surface gravity (log g), and metallicity ([M/H]). Interpolating in this grid of synthetic spectra, we simultaneously fit observations of the TiO band head region near 7088 A and five Ti I and Fe I lines nea r 8683 Angstrom. For G1 725B, we find T-eff=3170+/-71 K, log g=4.77+/- 0.14, [M/H] = -0.92 +/- 0.07, and v(mac) = 1.1 +/- 0.7 km s(-1). We sh ow that by using both atomic and molecular lines as constraints, syste matic uncertainties in derived stellar parameters can be reduced. Thes e parameters are consistent with published values obtained by other me ans, but more stringent tests would be useful. In the Appendix, we tab ulate wavelengths, identifications, relative line strengths, and other properties of the strongest band heads in the alpha, beta, gamma, gam ma', delta, epsilon, and phi electronic systems of TiO.