DISK M DWARF LUMINOSITY FUNCTION FROM HUBBLE SPACE TELESCOPE STAR COUNTS

摘要

We study a sample of 257 Galactic disk M dwarfs (8 ≤ M_v ≤ 18.5) found in images obtained using the Hubble Space Telescope (HST). These include 192 stars in 22 fields imaged with the repaired Wide Field Camera (WFC2) with mean limiting mag I = 23.7 and 65 stars in 162 fields imaged with the prerepair Planetary Camera (PC1) with mean limiting mag V = 21.3. We find that the disk luminosity function (LF) drops sharply for M_v > 12 (M < 0.25 solar mass), decreasing by a factor approx > 3 by M_v ～ 14 (M ～ 0.14 solar mass). This decrease in the LF is in good agreement with the ground-based photometric study of nearby stars by Stobie, Ishida, & Peacock, and in mild conflict with the most recent LF measurements based on local parallax stars by Reid, Hawley, & Gizis. The local LF of the faint Galactic disk stars can be transformed into a local mass function using an empirical mass-M_v relation. The mass function can be represented analytically over the mass range 0.1 solar mass < M < 1.6 solar mass by log (φ) = -1.35 -1.33 log (M/ solar mass) -1.82[log (M/ solar mass)]~2, where φ is the number density per logarithmic unit of mass. The total column density of M stars is only Σ_M = 12.4 ± 1.9 solar mass pc~(-2), implying a total "observed" disk column density of Σ_(obs) approx = 40 solar mass pc~(-2), lower than previously believed, and also lower than all estimates with which we are familiar of the dynamically inferred mass of the disk. The measured scale length for the M-star disk is 3.0 ± 0.4 kpc. The optical depth to microlensing toward the Large Magellanic Cloud (LMC) by the observed stars in the Milky Way disk is % approx < 1 x 10~(-8), compared to the observed optical depth found in ongoing experiments τ_(obs) ～ 10~(-7). The M-stars show evidence for a population with characteristics intermediate between thin disk and spheroid populations. Approximating what may be a continuum of populations by two separate components, we find a vertical density profile ν(z) ∝ 0.80 sech~2 (z/323 pc) + 0.20 exp (- ╚Oz ∣/656 pc). If we combine the HST data with ground-based measurements of the local density of M dwarfs, then a traditional double-exponential vertical density profile is strongly excluded.