COMPTEL limits on ²⁶Al 1.809 MeV line emission from gamma² Velorum

Abstract

The Wolf-Rayet binary system gamma2 Vel (WR 11) is the closest known Wolf-Rayet (WR) star. Recently, its distance has been redetermined by parallax measurements with the HIPPARCOS astrometric satellite yielding 258⁺⁴¹_-31 pc, significantly lower than previous estimates (300 - 450 pc). Wolf-Rayet stars have been proposed as a major source of the Galactic \nuc{26}{Al} observed at 1.809 Me. The gamma-ray telescope COMPTEL has previously reported 1.8 Me emission from the Vela region, yet located closer to the Galactic plane than the position of ensuremath γamma² Vel. We derive an upper 1.8 Me flux limit of 1.1 ; 10^-5 gamma{cm}^{-2} {s}^{-1} (2 sigma ) for the WR star. With the new distance estimate, COMPTEL measurements place a limit of (6.3^+2.1_-1.4right ) ; 10^-5 {M}_{\odot} on the \nuc{26}{Al} yield of gamma² Vel, thus constrains theories of nucleosynthesis in Wolf-Rayet stars. We discuss the implications in the context of the binary nature of gamma² Vel and present a new interpretation of the IRAS Vela shell.