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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Far Ultraviolet (FUV, 6 eV< hν <13.6 eV) radiation has been recognized as the main source of heating of the neutral interstellar gas, and, as a consequence, it determines whether the thermal balance of the neutral gas results in cold (T ∼ 50 − 100K) clouds (CNM), warm (T ∼ 10 4 K) clouds (WNM), or a combination of the two. High FUV fields convert the neutral gas to WNM, while low fields result in CNM. The knowledge of how these fractions depend on the FUV sources (i.e. the star formation rate, the IMF, and the size distribution of associations) is a basic step in building any detailed model of the large scale behavior of the ISM and the mutual relation between the ISM and the star formation rate in a galaxy.The sources of FUV radiation are the short-lived massive stars that generally originate in associations that form in Giant Molecular Clouds present in the galactic disk. Using McKee & Williams' (1997) distribution of birthrates for OB associations in the Galaxy, we determine the expected behavior of the time-dependent FUV field for random positions in the local ISM. The FUV field is calculated in two bands (912 − 1100Å and 912 − 2070Å) and at the wavelength 1400Å. In terms of U −17 ≡ U/(10 −17 erg cm −3Å−1 ), where U is the energy density of the radiation field in some band, we find (mean, median) values at the solar circle of U −17 =(15.7, 7.4) and (14.2, 7.2) for the [912-1100Å] and [912-2070Å] bands, respectively. At 1400Å we find (mean, median) values of U −17 =(14.4, 7.5). Our median value for the [912-2070Å] band is G 0 = 1.6 times Habing's (1968) value for the radiation field at the solar circle in this band, and quite close to Draine's (1976) value, G 0 = 1.7. Both the latter values are based on observations of sources of FUV radiation in the solar neighborhood, so all three values are close to observed values. Due to attenuation by dust, only associations within about 500 pc contribute significantly to the energy density at a given point. Large angle scattering produces a diffuse field that is about 10% of the field produced by the sum of direct and small angle (< 5 o ) scattering from discrete sources (the associations), as observed. At a point exposed to the median radiation field, the brightest association typically produces about 20% of the total energy density. At a -2point exposed to an above average radiation field, the brightest association produces most of the energy density. Therefore, the FUV field is asymmetric at a given point, and the asymmetry grows for higher fields.The FUV field fluctuates with a variety of amplitudes, the larger ones being less frequent. The mean field is about twice the median field because of these fluctuations, or spikes, in the radiation field. These spikes, which last ∼ 30 Myr, are caused by the infrequent birth of nearby associations. For spikes that are significantly higher than the mean field, the time interval between spikes is ∼ 2U 3/2 −15 Gyr. We also model shorter duration spikes caused by runaway OB stars. The presence of a fluctuating heating rate cre...

We derive a semi-empirical galactic initial mass function (IMF) from observational constraints. We assume that the star formation rate in a galaxy can be expressed as the product of the IMF, ψ(m), which is a smooth function of mass m (in units of M ⊙ ), and a time-and space-dependent total rate of star formation per unit area of galactic disk, ς * T . The mass dependence of the proposed IMF is determined by five parameters: the low-mass slope γ, the high-mass slope −Γ (taken to be the Salpeter value, -1.35), the characteristic mass m ch (which is close to the mass m peak at which the IMF turns over), and the lower and upper limits on the mass, m ℓ (taken to be 0.004) and m u (taken to be 120). The star formation rate in terms of number of stars per unit area of galactic disk per unit logarithmic mass interval, is proportional to the IMF:where N * is the number of stars, m ℓ < m < m u is the range of stellar masses. The values of γ and m ch are derived from two integral constraints: i) the ratio of the number density of stars in the range m = 0.1 − 0.6 to that in the range m = 0.6 − 0.8 as inferred from the mass distribution of field stars in the local neighborhood, and ii) the ratio of the number of stars in the range m = 0.08 − 1 to the number of brown dwarfs in the range m = 0.03 − 0.08 in young clusters. The IMF satisfying the above constraints is characterized by the parameters γ = 0.51 and m ch = 0.35 (which corresponds to m peak = 0.27). This IMF agrees quite well with the Chabrier (2005) IMF for the entire mass range over which we have compared with data, but predicts significantly more stars with masses < 0.03 M ⊙ ; we also compare with other IMFs in current use. We give a number of important parameters implied by the IMF, such as the fractional number of brown dwarfs and high-mass stars formed at a given time, the average mass of a newly-formed star, and the mass of stars formed per high-mass star.