VLT observations of GRS 1915+105

We present near infrared spectroscopy of the super-luminal microquasar GRS 1915+105 obtained with the first unit of the VLT¹ and the ISAAC spectro-imager. The emission features detected in the VLT data have been identified as He I, Br γ, He II and Na I. The detection of Na I is reported here for the first time, while our confirmation of weak He II emission provides support to previous marginal detections of this feature. By comparing the observed spectra with those of massive stars, we find that our results are very consistent with GRS 1915+105 being a high mass X-ray system with an early type primary, as previously proposed by Chaty et al. (1996) and Mirabel et al. (1997). The VLT spectra also provide evidence of P Cygni profiles, that turn into blue emission wings when the system is in outburst. This observed line profile evolution implies that GRS 1915+105 must be surrounded by an expanding envelope, that is partially blown out during the X-ray outbursts. The presence of such circumstellar gaseous material around GRS 1915+105 is more naturally understood in the context of a massive luminous star than if the system was a low-mass X-ray binary.