The Unusual Evolutionary State of GRO J1655 − 40

We have investigated possible models for the evolutionary status of the black hole soft X-ray transient GRO J1655-40. A secondary star evolving across the Hertzsprung gap and filling its Roche lobe can be ruled out, because the expected mass transfer rate is over 1000 times greater than the estimated upper limit based on observations. A secondary star evolving across the Hertzsprung gap, not yet filling its Roche lobe, but transferring mass via an enhanced stellar wind cannot be ruled out, but it may be unsatisfactory, because the deep convective envelope, normally associated with slow red-giant winds, has not really developed at the observed temperature. We propose that the secondary of GRO J1655-40 may be a main-sequence star in which convective overshooting, or some equivalent mixing, occurs at the core-envelope boundary. With such a model, the predicted mass transfer rate can be as low as 16 times the observational estimate, and we indicate how this factor might be accounted for so as to fully reconcile this model with the observations.