A new stellar library in the region of the CO index at 2.3 m

New index definition and empirical fitting functions

¹ Departamento de Astrofísica y CC. de la Atmósfera, Universidad Complutense de Madrid, 28040 Madrid, Spain e-mail: emq@astrax.fis.ucm.es
² Instituto de Astrofísica de Canarias, vía Láctea s/n, 38200 La Laguna, Tenerife, Spain
³ Centro Astronómico Hispano Alemán, Calar Alto (CSIC-MPG), C/Jesús Durbán Remón 2-2, 04004 Almería, Spain
⁴ Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 Av Groningen, The Netherlands
⁵ Centre for Astrophysics, University of Central Lancashire, Preston PR1 2HE, UK

Received: 24 April 2008
Accepted: 30 May 2008

Abstract

Context. The analysis of unresolved stellar populations demands evolutionary synthesis models with realistic physical ingredients and extended wavelength coverage.

Aims. We quantitatively describe the first CO bandhead at 2.3 μm to allow stellar population models to provide improved predictions in this wavelength range.

Methods. We observed a new stellar library with a better coverage of the stellar atmospheric parameter space than in earlier works. We performed a detailed analysis of the robustness of previous CO index definitions with spectral resolution, wavelength calibration, signal-to-noise ratio, and flux calibration.

Results. We define a new line-strength index for the first CO bandhead at 2.3 μm, D_CO, better suited for stellar population studies than previous index definitions. We derive empirical fitting functions for the CO feature as a function of the stellar parameters (T_eff, and [Fe/H]), showing a detailed quantitative metallicity dependence.