Mdwarfs, microlensing, and the mass budget of the Galaxy
Mdwarfs, microlensing, and the mass budget of the Galaxy
, as a function
of luminosity LV, d /d ln LV 
LV-
. For
spheroid stars, < 0.32 over
the range 6 < MV < 17, with
98% confidence. The disk luminosity function falls, < 0, for
15 
MV
19. Faint red stars
in the disk or thick disk, and
stars with MV < 16 in the spheroid
contribute 
< 10-8 to the
optical depth to microlensing toward the Large Magellanic Cloud.
The spheroid luminosity and mass functions from Hubble Space Telescope
star counts
level
with the local spheroid LF of Dahn et al. even when the normalization of
the latter is corrected to take account of the latest data on spheroid
kinematics. The difference may reflect systematic errors in one of the two
studies or features of the spheroid spatial distribution that are not
included in the simplest models.
The mass function, which shows no obvious structure, can be represented by
a power law,
d N/d ln M M
, with = 0.25 ±
0.32 over the mass
range 0.71 M
M > 0.09 M. The spheroid therefore does not
contribute significantly to microlensing unless the mass function changes
slope dramatically in the substellar range. The total local mass density of
spheroid stars (including remnants and unseen binary companions) is
~ 6.4 ×
10-5 M pc-3,
with an uncertainty of about
50%.
The power-law indices = 0.25 for the spheroid and
= 0.44 for
the disk (both uncorrected for binaries)
are similar to those of
globular clusters of moderate to high metallicity.
M dwarfs from Hubble
Space Telescope star counts III: the Groth strip
Groth Strip), increase the total sample to 337 stars, and more than double the number of late M dwarfs (MV > 13.5) from 23 to 47. The mass function changes slope at M ~ 0.6 M
, from a
near-Salpeter power-law index of = -1.21 to
= 0.44. In both regimes the mass function at the Galactic plane
is given
by d 3 N / d log M d MV dV = 8.1 x
10-2 pc-3 (M / 0.59
M). The correction for secondaries in binaries
changes the
low-mass index from = 0.44 to ~ 0.1. If the Salpeter
slope
continued to the hydrogen-burning limit, we would expect 500 stars in
the
last four bins (14.5< MV < 18.5), instead of the 25 actually
detected. The
explanation of the observed microlensing rate towards the Galactic
bulge requires either the disk and bulge mass functions are very different for stars
with M 0.5 M or that a substantial population of bulge brown dwarfs.
Hubble
deep field (HDF) constraint on
baryonic dark matter
The magnitudes and positions of the stars in the HDF reported in this paper are available in a tabular form. See http://astro.utu.fi/~cflynn/projects4.html for further information on this project.
Disk M dwarf luminosity
function from HST star counts
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