956 research outputs found
Death of Stellar Baryonic Dark Matter
The nature of the dark matter in the haloes of galaxies is one of the
outstanding questions in astrophysics. All stellar candidates, until recently
thought to be likely baryonic contributions to the Halo of our Galaxy, are
shown to be ruled out. Faint stars and brown dwarfs are found to constitute
only a few percent of the mass of the Galaxy. Stellar remnants, including white
dwarfs and neutron stars, are shown to be very constrained as well. High energy
gamma-rays observed in HEGRA data place the strongest constraints, , where is the Hubble constant in units of 100 km
s Mpc. Hence one is left with several unanswered questions: 1)
What are MACHOs seen in microlensing surveys? 2) What is the dark matter in our
Galaxy? Indeed a nonbaryonic component in the Halo seems to be required.Comment: 6 pages ps fil
Massive Compact Halo Objects Viewed from a Cosmological Perspective: Contribution to the Baryonic Mass Density of the Universe
[Abridged] We estimate the contribution of Massive Compact Halo Objects
(Machos) and their stellar progenitors to the mass density of the Universe. If
the Machos that have been detected reside in the Halo of our Galaxy, then a
simple extrapolation of the Galactic population (out to 50 kpc) of Machos to
cosmic scales gives a cosmic density \rho_{Macho} = (1-5) \times 10^9 h \msun
\Mpc^{-3}, which in terms of the critical density corresponds to
. Such a mass density is comparable to
the baryon density implied by Big Bang Nucleosynthesis. If we take the central
values of the estimates, then Machos dominate the baryonic content of the
Universe today, with . However, the
cumulative uncertainties in the density determinations only require that
, where the fraction of
galaxies that contain Machos , and is the Hubble constant
in units of 100 km s Mpc. Our best estimate for
is hard to reconcile with the current best estimates of the baryonic content of
the intergalactic medium indicated by measurements of the Lyman-
forest. We explore the addition constraints that arise if the Machos are white
dwarfs as suggested by the present microlensing data. We discuss the challenges
this scenario presents at both the local and cosmic scales, emphasizing in
particular the constraints on the required mass budget and nucleosynthesis
products (particularly carbon).Comment: 18 pages, LaTeX, uses AASTeX macros. In press, New Astronomy
(submitted Jan. 20, 1998
No nearby counterparts to the moving objects in the Hubble Deep Field
Ibata et al (1999) have recently discovered faint, moving objects in the
Hubble Deep Field. The quantity, magnitudes and proper motions of these objects
are consistent with old white dwarfs making up the Galactic dark halo. We
review a number of ground-based proper motion surveys in which nearby dark halo
white dwarfs might be present, if they have the colours and absolute magnitudes
proposed. No such objects have been found, whereas we argue here that several
times more would be expected than in the Hubble Deep Field. We conclude it is
unlikely that hydrogen atmosphere white dwarfs make up a significant fraction
of the halo dark matter. No limits can be placed yet on helium atmosphere
dwarfs from optical searches.Comment: 7 pages, 4 figures, MNRAS LaTeX forma
Analysis of a Hubble Space Telescope Search for Red Dwarfs: Limits on Baryonic Matter in the Galactic Halo
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
mass function. Then the total mass of red dwarfs and brown dwarfs
is 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 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 mass function, this suggests
a total density of red dwarfs and brown dwarfs of 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).Comment: 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
reference
Magnitude bias of microlensed sources towards the Large Magellanic Cloud
There are lines of evidence suggesting that some of the observed microlensing
events in the direction of the Large Magellanic Cloud (LMC) are caused by
ordinary star lenses as opposed to dark Machos in the Galactic halo. Efficient
lensing by ordinary stars generally requires the presence of one or more
additional concentrations of stars along the line of sight to the LMC disk. If
such a population behind the LMC disk exists, then the source stars (for
lensing by LMC disk objects) will be drawn preferentially from the background
population and will show systematic differences from LMC field stars. One such
difference is that the (lensed) source stars will be farther away than the
average LMC field stars, and this should be reflected in their apparent
baseline magnitudes. We focus on red clump stars: these should appear in the
color-magnitude diagram at a few tenths of a magnitude fainter than the field
red clump. Suggestively, one of the two near-clump confirmed events,
MACHO-LMC-1, is a few tenths of magnitude fainter than the clump.Comment: To appear in ApJ Letters. Shortened to match the accepted version, 8
pages plus 1 ps figur
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