Analysis of a Hubble Space Telescope Search for Red Dwarfs: Limits on Baryonic Matter in the Galactic Halo

Abstract

We re-examine a deep {\it Hubble Space Telescope} pencil-beam search for red dwarfs, stars just massive enough to burn Hydrogen. The authors of this search (Bahcall, Flynn, Gould \& Kirhakos 1994) found that red dwarfs make up less than 6\% of the galactic halo. First, we extrapolate this result to include brown dwarfs, stars not quite massive enough to burn hydrogen; we assume a $1/\mathcal{M}$ mass function. Then the total mass of red dwarfs and brown dwarfs is $\le$18\% of the halo. This result is consistent with microlensing results assuming a popular halo model. However, using new stellar models and parallax observations of low mass, low metallicity stars, we obtain much tighter bounds on low mass stars. We find the halo red dwarf density to be $<1\%$ of the halo, while our best estimate of this value is 0.14-0.37\%. Thus our estimate of the halo mass density of red dwarfs drops to 16-40 times less than the reported result of Bahcall et al (1994). For a $1/\mathcal{M}$ mass function, this suggests a total density of red dwarfs and brown dwarfs of $\sim$0.25-0.67\% of the halo, \ie , $(0.9-2.5)\times 10^9\msun$ out to 50 kpc. Such a low result would conflict with microlensing estimates by the \macho\ group (Alcock \etal 1995a,b).

13 pages, 2 figures. Figure one only available via fax or snail-mail To be published in ApJL. fig. 2 now available in postscript. Some minor changes in dealing with disk forground. Some cosmetic changes. Updated references

References in corpus (3)

• EXPERIMENTAL LIMITS ON THE DARK MATTER HALO OF THE GALAXY FROM GRAVITATIONAL MICROLENSING.
• The Local Halo Density
• UNIFICATION OF THE NEARBY AND PHOTOMETRIC STELLAR LUMINOSITY FUNCTIONS

Cited by in corpus (37)

• Galactic Stellar and Substellar Initial Mass Function
• Theory of Low-Mass Stars and Substellar Objects
• Gravitational Waves from Coalescing Black Hole MACHO Binaries
• Primordial Black Hole Formation during First-Order Phase Transitions
• Black Hole Binary Formation in the Expanding Universe --- Three Body Problem Approximation ---
• The Dawes Review 8: Measuring the Stellar Initial Mass Function
• The Spheroid Luminosity and Mass Functions From HST Star Counts
• Pure hydrogen atmosphere for very cool white dwarfs
• Hubble Deep Field Constraint on Baryonic Dark Matter
• Primordial Black Hole Formation in Supergravity
• Mirror Dark Matter and Core Density of Galaxies
• Could MACHOS be Primordial Black Holes formed during the QCD Epoch?
• The LMC Microlensing Events: Evidence for a Warped and Flaring Milky Way Disk?
• Chemical Abundance Constraints on White Dwarfs as Halo Dark Matter
• Low frequency gravitational waves from black hole MACHO binaries
• The Mass-Function of Low Mass Halo Stars: Limits on Baryonic Halo Dark Matter
• Towards a full general relativistic approach to galaxies
• Further constraints on white dwarf galactic halos
• Low Frequency Gravitational Waves from White Dwarf MACHO Binaries
• No Need for MACHOS in the Halo
• On the Nature and Location of the Microlenses
• MACHOs, White Dwarfs, and the Age of the Universe
• Microlensig Binaries with Candidate Brown Dwarf Companions
• Constraining the cosmic abundance of stellar remnants with multi-TeV gamma-rays
• Halo dark clusters of brown dwarfs and molecular clouds
• The Origin of Primordial Dwarf Stars and Baryonic Dark Matter
• Magellanic Cloud Gravitational Microlensing Results: What Do They Mean?
• Declining Rotation Curve and Brown Dwarf MACHOs
• Low Frequency Gravitational Waves from Black Hole MACHO Binaries
• Microlensing Halo Models with Abundant Brown Dwarfs
• The Minimum Total Mass of MACHOs and Halo Models of the Galaxy
• Searching for Gravitational Radiation from Binary Black Hole MACHOs in the Galactic Halo
• Brown Dwarfs, White Knights and Demons
• Neutron Stars and Black Holes as MACHOs
• Observations from Australasia using the Gravitational Microlensing Technique
• New Models for a Triaxial Milky Way Spheroid and Effect on the Microlensing Optical Depth to the Large Magellanic Cloud
• The Galactic Dark Matter: Predictions and Obsevations