New atmospheric parameters and spectral interpolator for the MILES cool stars^⋆

¹ Université de Lyon, Université Lyon 1, 69622 Villeurbanne, France
e-mail: philippe.prugniel@univ-lyon1.fr
² CRAL, Observatoire de Lyon, CNRS UMR 5574, 69561 Saint-Genis Laval, France
³ Department of Physics and Astrophysics, University of Delhi, 110007 Delhi, India
e-mail: kaushal@physics.du.ac.in; hpsingh@physics.du.ac.in

Received: 17 March 2015
Accepted: 18 October 2015

Abstract

Context. The full spectrum fitting of stellar spectra against a library of empirical spectra is a well-established approach to measure the atmospheric parameters of FGK stars with a high internal consistency. Extending it towards cooler stars still remains a challenge.

Aims. We address this question by improving the interpolator of the Medium-resolution INT Library of Empirical Spectra (MILES) library in the low effective temperature regime (T_eff < 4800 K), and we refine the determination of the parameters of the cool MILES stars.

Methods. We use the ULySS package to determine the atmospheric parameters (T_eff, log  g and [Fe/H]), and measure the biases of the results with respect to our updated compilation of parameters calibrated against theoretical spectra. After correcting some systematic effects, we compute a new interpolator that we finally use to redetermine the atmospheric parameters homogeneously and assess the biases.

Results. Based on an updated literature compilation, we determine T_eff in a more accurate and unbiased manner compared to those determined with the original interpolator. The validity range is extended downwards to about T_eff= 2900 K compared to 3500 K previously. The mean residual biases on T_eff, log  g, and [Fe/H], with respect to the literature compilation for the coolest stars (T_eff ≤ 3800 K) computed using the new interpolator, are −15 K, −0.02 dex, and 0.02 dex respectively. The corresponding estimations of the external precision are 63 K, 0.23 dex, and 0.15 dex respectively. For the stars with T_eff in the range 3800−4200 K, the determinations of T_eff and [Fe/H] have been slightly improved. At higher temperatures, the new interpolator is comparable to the original one. The new version of the interpolator is publicly available.