Letter to the Editor

On the abundances of GRO J1655-40^*

¹ European Southern Observatory, 3107 Alonso de Cordova, Casilla 19001, Vitacura, Santiago, Chile e-mail: cfoellmi@eso.org
² Laboratoire d'Astrophysique, Observatoire de Grenoble, BP 53, 38041 Grenoble Cedex 9, France
³ Gemini Observatory, 670 N. A'ohoku Place, Hilo, HI 96720, USA
⁴ Departamento de Astronomía y Astrofísica, Pontificia Universidad Católica de Chile, Campus San Joaquín, Vicuña Mackenna 4860, Casilla 306, Santiago 22, Chile

Received: 16 January 2007
Accepted: 1 February 2007

Abstract

Context.The detection of overabundances of α-elements and lithium in the secondary star of a black-hole binary provides important insights about the formation of a stellar-mass black-hole. α-enhancement might theoretically also be the result of pollution by the nucleosynthesis occurring during an outburst, or through spallation by the jet.

Aims.We study the abundances, and their possible variations with time, in the secondary star of the runaway black-hole binary GRO J1655–40, in order to understand their origin.

Methods.We present a detailed comparison between a Keck spectrum obtained in 1998 found in the literature, archival VLT-UVES data taken in 2004 and new VLT-UVES spectra obtained early 2006. We carefully determine the equivalent widths of different α-elements (Mg, O, Ti, S and Si) with their associated uncertainty. We use the well-studied comparison star HD 156098 as well as synthetic spectra to match the spectrum of GRO J1655–40 in order to determine the abundances of these elements.

Results.We see no significant variations of equivalent widths with time. Our fit using HD 156098 reveals that there is significant overabundance of oxygen in all our spectra, but no overabundances of any of the other α-elements. Finally, we do not detect the lithium line at 6707 Å.

Conclusions.We show that there is no detected pollution in GRO J1655–40 after the burst in 2005. Moreover, we argue that uncertainties in the equivalent widths were previously underestimated by a factor of ~3. Consequently, our results challenge the existence of general overabundances of α-elements observed in this galactic black-hole binary, and thus the accepted interpretation that they are of supernova origin. The physical cause of the overabundance of oxygen remains unclear.