Collisions of giant stars with compact objects: Hydrodynamical calculations

Encounters between a 0.8M_⊙ giant star and a 1.4M⊙ compact object on a parabolic orbit have been calculated using smooth particle hydrodynamics (SPH). Both the stellar core and the compact object are represented by point masses interacting with the gas through gravity only. We find that all encounters with periastron distance r_p ≲ 2.5R_G, where R_G is the stellar radius, lead to significant disruption of the stellar envelope, including some mass loss. When r_p/R_G ≲ 0.5, the envelope is completely destroyed during the collision. About 40% of the gas escapes, while the rest forms a massive disk around the compact object, leaving behind the bare stellar core on a highly eccentric orbit. Subsequent close passages lead to continuous extraction of gas from the disk at a large rate, and the binary orbit may eventually stabilize while still retaining a large eccentricity. For 0.5 ≲ r_p/R_G ≲ 2.5, complete destruction of the envelope is avoided during the first close passage. However, nonlinear effects tend to make the envelope expand by a large factor, so that subsequent close passages will likely destroy it. In particular, we confirm Bailyn's prediction that all captures of a neutron star by a red giant in globular clusters are likely to result in the eventual destruction of the red giant envelope. We discuss the implications of our results in light of recent new detections of millisecond pulsars in globular clusters. Most importantly, we suggest that many globular cluster pulsars may have been spun-up by accretion from a massive disk rather than from a binary companion, thereby providing a possible solution to the birthrate problem. To support our findings, we present various new tests of our SPH treatment. In particular, we calculate tidal encounters between a polytrope and a point mass and find good agreement with semianalytic, linear perturbation calculations. Both head-on and off-axis collisions between a polytrope and a point mass are also examined, and our SPH results are compared to those of recent finite-difference calculations.