Death of Stellar Baryonic Dark Matter Candidates

Abstract

The nature of the dark matter in the Universe is one of the outstanding questions in astrophysics. In this talk, I address possible stellar baryonic contributions to the 50-90% of our Galaxy that is made of unknown dark matter. First I show that faint stars and brown dwarfs constitute only a few percent of the mass of the Galaxy. Next, I show that stellar remnants, including white dwarfs and neutron stars, are also insufficient in abundance to explain all the dark matter of the Galaxy. High energy gamma-rays observed in HEGRA data place the most robust constraints, $\hat{I}{\text{c◯}}_{WD}<3\times 10^{-3}{h}^{-1}$, where $h$ is the Hubble constant in units of 100 km s${}^{-1}$ Mpc${}^{-1}$. Overproduction of chemical abundances (carbon, nitrogen, and helium) provide the most stringent constraints, $\hat{I}{\text{c◯}}_{WD}<2\times 10^{-4}{h}^{-1}$. Comparison with recent updates of microlensing data are also made. According to the gamma-ray limit, all Massive Compact Halo Objects seen by the experiments (Machos) can be white dwarfs if one takes the extreme numbers; however, from chemical overproduction limits, NOT all Machos can be white dwarfs. Comments on recent observations of the infrared background and of white dwarfs are also made. In conclusion, a nonbaryonic component in the Halo seems to be required.

contribution to: Sources and Detection of Dark Matter in the Universe, Marina del Rey, CA, February 2000