Hubble Space Telescope (HST) star counts: dark matter, M dwarfs, spheroid stars

Mdwarfs, microlensing, and the mass budget of the Galaxy
Author(s):John N. Bahcall, Chris Flynn, Andrew Gould, and Sofia Kirhakos
Journal: The Astrophysical Journal, 435, L51-L54 (November 1, 1994); astro-ph/9406019.

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Abstract: We show that faint red stars do not contribute significantly to the mass budget of the Galaxy or to microlensing statistics. Our results are obtained by analyzing two long exposures of a high-latitude field taken with the Wide Field Camera on the newly repaired Hubble Space Telescope. Stars are easily distinguished from galaxies essentially to the limiting magnitudes of the images. We find five stars with 2.0 < V <  I < 3.0 and I < 25.3 and no stars with V-I > 3.0. Therefore, main-sequence stars with MI > 10 that are above the hydrogen-burning limit in the dark halo or the spheroid contribute < 6% of the unseen matter. Faint red disk stars, M-dwarfs, contribute at most 15% to the mass of the disk. We parameterize the faint end of the cumulative distribution of stars, , 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
Author(s):Andrew Gould, Chris Flynn, and John N. Bahcall
Journal: The Astrophysical Journal, 503, 798-808 (August 20, 1998) astro-ph/9711263.

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Abstract: We analyze 166 spheroid subdwarfs (6.5 < MV < 14.5) found in 53 fields observed with the Wide Field Camera on the Hubble Space Telescope. The fields cover 221 arcmin² over a wide range of directions. The spheroid luminosity function (LF) is inconsistent at about the 3 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, 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
Author(s):Andrew Gould, John N. Bahcall, Chris Flynn
Journal:The Astrophysical Journal, 482, 913-918 (June 20, 1997); astro-ph/9611157.

Abstract: We analyze the disk M dwarfs found in 31 new fields observed with the Wide Field Camera (WFC2) on the Hubble Space Telescope (HST) together with the sample previously analyzed from 22 WFC2 fields and 162 prerepair Planetary Camera (PC1) fields. The new observations, which include the 28 high-latitude fields comprising the Large Area Multi-Color Survey (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
Author(s): Chris Flynn, Andrew Gould, and John N. Bahcall
Journal: The Astrophysical Journal, 466, L55-L58 (August 1, 1996); astro-ph/9603035.

Abstract: We use a new technique to search for faint red stars in the Hubble Deep Field. We distinguish unambiguously between stars and galaxies to I = 26.3. Our results place strong and general constraints on the I-band luminosity of the constituents of the Galactic dark halo. For exmaple, if the dark halo is made of white dwarfs they must be two magnitudes fainter than the observed disk white dwarf sequence.

The magnitudes and positions of the stars in the HDF reported in this paper are available in a tabular form. See for further information on this project.

Disk M dwarf luminosity function from HST star counts
Author(s): Andrew Gould, John N. Bahcall, and Chris Flynn
Journal: The Astrophysical Journal, 465, 759-768 (July 10, 1996); astro-ph/9505087.

Abstract: We determine the luminosity function of disk M dwarfs using 22 fields imaged by the Hubble Space Telescope and the repaired Wide Field Camera plus 162 fields imaged with the pre-repair Planetary Camera. We find that the disk luminosity function drops sharply for masses less than 0.25 solar mass. An analytic representation of the disk mass function is presented. The total column density of disk M stars is only 12.4 ± 1.9 solar masses per square parsec, less than previously believed. The measured scale length of the disk M stars is 3.0 ± 0.4 kpc.

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