Stars, Gas, and Dark Matter in the Solar Neighborhood

Abstract

The surface density and vertical distribution of stars, stellar remnants, and gas in the solar vicinity form important ingredients for understanding the star formation history of the Galaxy as well as for inferring the local density of dark matter by using stellar kinematics to probe the gravitational potential. In this paper we review the literature for these baryonic components, reanalyze data, and provide tables of the surface densities and exponential scale heights of main sequence stars, giants, brown dwarfs, and stellar remnants. We also review three components of gas (H 2 , HI, and HII), give their surface densities at the solar circle, and discuss their vertical distribution. We find a local total surface density of M dwarfs of 17.3 ± 2.3 M ⊙ pc −2 , significantly higher than previous values. Our result for the total local surface density of visible stars (main sequence stars and giants), 27.0 ± 2.7 M ⊙ pc −2 , is close to previous estimates due to a cancellation of opposing effects: more mass in M dwarfs, less mass in the others. The total local surface density in white dwarfs is 4.9 ± 0.6 M ⊙ pc −2 ; in brown dwarfs, it is ∼ 1.2 M ⊙ pc −2 , but with considerable uncertainty. We find that the total local surface density of stars and stellar remnants is 33.4 ± 3 M ⊙ pc −2 , somewhat less than previous estimates but within the errors of many of them. We analyze data on 21 cm emission and absorption and obtain good agreement with recent results on the local amount of neutral atomic hydrogen obtained with the Planck satellite. The local surface density of gas is 13.7 ± 1.6 M ⊙ pc −2 . The total baryonic mass surface density that we derive for the solar neighborhood is 47.1 ± 3.4 M ⊙ pc −2 (43.8 M ⊙ pc −2 within 1.1 kpc of the midplane). Combining these results with others' measurements of the total surface density of matter within 1-1.1 kpc of the plane, we find that the local density of dark matter is ρ DM = 0.013 ± 0.003 M ⊙ pc −3 = 0.49 ± 0.13 GeV cm −3 . The local density of all matter is 0.097 ± 0.013 M ⊙ pc −3 . We discuss limitations on the properties of a possible thin disk of dark matter.

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