Numerical Evidence for Dark Star Formation: A Comment on "Weakly Interacting Massive Particle Dark Matter and First Stars: Suppression of Fragmentation in Primordial Star Formation" by Smith et al. 2012, ApJ 761, 154 (1304.7415v1)

Abstract: (abridged) This comment is intended to show that simulations by Smith et al. (S12) support the Dark Star (DS) scenario and even remove some potential obstacles. Our previous work illustrated that the initial hydrogen densities of the first equilibrium DSs are high, ~10^{17}/cm^3 for the case of 100 GeV WIMPs, with a stellar radius of ~2-3 AU. Subsequent authors have somehow missed the fact that equilibrium DSs have the high densities they do. S12 have numerically simulated the effect of dark matter annihilation on the contraction of a protostellar gas cloud en route to forming the first stars. They show results at a density ~5 10^{14}/cm^3, slightly higher than the value at which annihilation heating prevails over cooling. However, they are apparently unable to reach the ~10^{17}/cm^3 density of our hydrostatic DS solutions. We are in complete agreement with their physical result that the gas keeps collapsing to densities > 5 10^{14}/cm^3, as it must before equilibrium DSs can form. However we are in disagreement with some of the words in their paper which imply that DSs never come to exist. It seems to us that S12 supports the DS scenario. They use the sink particle approach to treat the gas that collapses to scales smaller than their resolution limit. We argue that their sink is effectively a DS, or contains one. An accretion disk forms as more mass falls onto the sink, and the DS grows. S12 not only confirm our predictions about DS in the range where the simulations apply, but also solve a potential obstruction to DS formation by showing that dark matter annihilation prevents the fragmentation of the collapsing gas. Whereas fragmentation might perturb the dark matter away from the DS and remove its power source, instead S12 show that further sinks, if any, form only far enough away as to leave the DS undisturbed in the comfort of its dark matter surroundings.