Temperatures and metallicities of M giants in the Galactic bulge from low-resolution K-band spectra^⋆

¹ Laboratoire Lagrange, Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Bvd de l’Observatoire, 06304 Nice, France
e-mail: mathias.schultheis@oca.eu
² Department of Astronomy and Theoretical Physics, Lund Observatory, Lund University, Box 43, 221 00 Lund, Sweden
e-mail: ryde@astro.lu.se

Received: 7 February 2016
Accepted: 7 March 2016

Abstract

Context. With the existing and upcoming large multifibre low-resolution spectrographs, the question arises of how precise stellar parameters such as T_eff and [Fe/H] can be obtained from low-resolution K-band spectra with respect to traditional photometric temperature measurements. Until now, most of the effective temperatures in Galactic bulge studies come directly from photometric techniques. Uncertainties in interstellar reddening and in the assumed extinction law could lead to large systematic errors (>200 K).

Aims. We obtain and calibrate the relation between T_eff and the ¹²CO first overtone bands for M giants in the Galactic bulge covering a wide range in metallicity.

Methods. We used low-resolution spectra for 20 M giants with well-studied parameters from photometric measurements covering the temperature range 3200 <T_eff< 4500 K and a metallicity range from 0.5 dex down to −1.2 dex and study the behaviour of T_eff and [Fe/H] on the spectral indices.

Results. We find a tight relation between T_eff and the ¹²CO(2−0) band with a dispersion of 95 K and between T_eff and the ¹²CO(3−1) with a dispersion of 120 K. We do not find any dependence of these relations on the metallicity of the star, which makes them attractive for Galactic bulge studies. This relation is also not sensitive to the spectral resolution, which allows this relation to be applied in a more general way. We also find a correlation between the combination of the Na i, Ca i, and the ¹²CO band with the metallicity of the star. However, this relation is only valid for subsolar metallicities.

Conclusions. We show that low-resolution spectra provide a powerful tool for obtaining effective temperatures of M giants. We show that this relation does not depend on the metallicity of the star within the investigated range and is also applicable to different spectral resolutions making this relation in general useful for deriving effective temperatures in high-extinction regions where photometric temperatures are not reliable.