Evolution of Protostars Accreting Mass at Very High Rates: Is Orion IRc2 a Huge Protostar?

Abstract

The recent near-infrared spectroscopy of scattered light from Orion IRc2 suggests that the illuminating source at the K' band is either a protostar with a radius 300 R☉ or a disk with an accretion rate ≈10-2 M☉ yr-1. To test the former interpretation, we present a simplified stellar model accreting mass at a very high rate, ≈10-2 M☉ yr-1. We find that the protostar is fully convective at almost all stages of the stellar mass M 15 M☉, and thus a polytrope of index 1.5 is a good approximation of the stellar structure. The maximum radius 30 R☉ is attained at M ~ 7 M☉. The shell deuterium burning, which would occur afterward, cannot greatly blow up the protostar because the energy released by deuterium burning is small and also because the protostar is already shrinking rapidly. The only remaining possibility to make a huge protostar resides in the rotation of the surface layer almost at its breakup velocity. On the other hand, we find no difficulty in the alternative interpretation that the illuminating source is the accretion disk. In this case we predict that the 2.3 μm CO absorption lines should be observed with a width ~50 km s-1 due to the Keplerian motion in the disk. The accretion rate as high as 10-2 M☉ yr-1 is compatible with the velocity dispersion in the Orion KL molecular cloud core. Because the luminosity of IRc2 is dominated by accretion, the protostellar mass is overestimated if the observed luminosity is regarded as intrinsic. Because the K'-band luminosity is emitted in the disk region far from the protostellar surface, the total accretion luminosity must be significantly higher than the observed K'-band luminosity.