Sulphur abundances from the S i near-infrared triplet at 1045 nm ^*,**

¹ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, Place Jules Janssen, 92190 Meudon, France e-mail: Elisabetta.Caffau@obspm.fr
² Università degli Studi di Trieste, Dipartimento di Astronomia, Via Tiepolo 11, 34131 Trieste, Italy
³ CIFIST Marie Curie Excellence Team
⁴ Istituto Nazionale di Astrofisica - Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34143 Trieste, Italy
⁵ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany

Received: 27 January 2007
Accepted: 11 April 2007

Abstract

Context.Unlike silicon and calcium, sulphur is an α-element that does not form dust. Some of the available observations of the evolution of sulphur with metallicity indicate an increased scatter of sulphur-to-iron ratios at low metallicities or even a bimodal distribution, with some stars showing constant S/Fe at all metallicities and others showing an increasing S/Fe ratio with decreasing metallicity. In metal-poor stars S i lines of Multiplet 1 at 920 nm are not yet too weak to permit the measurement of the sulphur abundance A(S); however, in ground-based observations they are severely affected by telluric lines.

Aims.We investigate the possibility of measuring sulphur abundances from S iMult. 3 at 1045 nm lines. These lie in the near infrared and are slightly weaker than those of Mult. 1, but lie in a range not affected by telluric lines.

Methods.We investigated the lines of Mult. 3 in the Sun (G2V), Procyon (F5V), HD 33256 (F5V), HD 25069 (G9V), and ϵ Eri (HD 22049, K2V). For the Sun and Procyon the analysis was performed with CO⁵BOLD 3D hydrodynamical model atmospheres, while the three other stars, for which hydrodynamical simulations are not available, were analysed using 1D model atmospheres.

Results.For our sample of stars we find a global agreement between A(S) from lines of different multiplets.

Conclusions.Our results suggest that the infrared lines of Mult. 3 are a viable indicator of the sulphur abundance that, because of the intrinsic strength of this multiplet, should be suitable for studying the trend of [S/Fe] at low metallicities.