17th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun

June 24-29, 2012
Contribution Abstract

Full Metal Bracket: a calibration of infrared and optical spectroscopic metallicities of M dwarfs over 1.5 dex
Andrew W. Mann, Institute for Astronomy

John M. Brewer Department of Astronomy, Yale University
Eric Gaidos Department of Geology and Geophysics, University of Hawaii
Sebastien Lepine Department of Astrophysics, American Museum of Natural History
Eric J. Hilton Department of Geology and Geophysics, University of Hawaii

Type: Oral

Topic: Fundamental Parameters of Cool Stars and Brown Dwarfs

We present a new technique, and improved calibrations of existing techniques, to determine M dwarf metallicities from optical and/or NIR spectra. Deriving metallicities of M dwarfs from stellar spectra is difficult due to the presence of complex molecules in their atmospheres. Recently, several methods have been developed using either NIR (Rojas-Ayala et al. 2011; Terrien et al. 2012) or visible wavelength (Woolf et al. 2009) spectra. These techniques use atomic (for NIR) or molecular (for visible wavelengths) lines to produce models that approximate M-dwarf metallicities. The models are calibrated using well-separated M dwarf companions to FGK stars with precise metallicities. Unfortunately, most of these studies have made use of a relatively small number of calibrator pairs ($\lesssim 30$). As a result, they rely heavily on stars in a narrow range of metallicities ($-0.4 \lesssim $[Fe/H]$ \lesssim +0.4$) and spectral types ($\simeq$ M0 to M4). We improve on this by identifying $>$120 FGK-M suitable stellar pairs, more than 70 of which have primaries with published metallicities. For the remaining primary stars, we derive metallicities from high-resolution spectra taken with ESPaDOnS on CFHT. We obtain spectra for each M dwarf companion with SNIFS on the UH2.2m and SpeX on the IRTF in the visible and NIR respectively. Our resulting sample spans a wide range of metallicities ($-1.1 < $[Fe/H]$ < +0.5$) and spectral types (K7 to M7). We then use this sample to test and recalibrate known metallicity estimators as well as to identify new methods of determining M dwarf metallicities. We find that metallicities derived from NIR spectra are precise (RMSE $\simeq 0.12$~dex) near solar metallicity, but are unreliable at extremely low metallicities ([Fe/H]$<-0.5$). We find the most accurate metallicities can be determined using a joint fit of visible wavelength and NIR spectroscopic features Although our results are preliminary, this new method already shows promising results (RMSE $\simeq 0.07$) over the full range of metallicities in our sample.