667 research outputs found
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
Microlens Parallax Asymmetries Toward the LMC
If the microlensing events now being detected toward the Large Magellanic
Cloud (LMC) are due to lenses in the Milky Way halo, then the events should
typically have asymmetries of order 1% due to parallax from the reflex motion
of the Earth. By contrast, if the lenses are in the LMC, the parallax effects
should be negligible. A ground-based search for such parallax asymmetries would
therefore clarify the location of the lenses. A modest effort (2 hours per
night on a 1 m telescope) could measure 15 parallax asymmetries over 5 years
and so marginally discriminate between the halo and the LMC as the source of
the lenses. A dedicated 1 m telescope would approximately double the number of
measurements and would therefore clearly distinguish between the alternatives.
However, compared to satellite parallaxes, the information extracted from
ground-based parallaxes is substantially less useful for understanding the
nature of the halo lenses (if that is what they are). The backgrounds of
asymmetries due to binary-source and binary-lens events are estimated to be
approximately 7% and 12% respectively. These complicate the interpretation of
detected parallax asymmetries, but not critically.Comment: Submitted to ApJ, 17 pages, including 2 embedded figure
Microlens Parallaxes with SIRTF
The Space Infrared Telescope Facility (SIRTF) will drift away from the Earth
at about 0.1 AU/yr. Microlensing events will therefore have different
characteristics as seen from the satellite and the Earth. From the difference,
it is possible in principle to measure v-tilde, the transverse velocity of the
lens projected onto the observer plane. Since v-tilde has very different values
for different populations (disk, halo, Large Magellanic Cloud), such
measurements could help identify the location, and hence the nature, of the
lenses. I show that the method previously developed by Gould for measuring such
satellite parallaxes fails completely in the case of SIRTF: it is overwhelmed
by degeneracies which arise from fact that the Earth and satellite observations
are in different band passes. I develop a new method which allows for
observations in different band passes and yet removes all degeneracies. The
method combines a purely ground-based measurement of the "parallax asymmetry"
with a measurement of the delay between the time the event peaks at the Earth
and satellite. In effect, the parallax asymmetry determines the component of
v-tilde in the Earth-Sun direction, while the delay time measures the component
of v-tilde in the direction of the Earth's orbit.Comment: 21 pages plus 3 figure
Discovery of high proper motion ancient white dwarfs: nearby massive compact halo objects?
We present the discovery and spectroscopic identification of two very high
proper motion ancient white dwarf stars, found in a systematic proper motion
survey. Their kinematics and apparent magnitude clearly indicate that they are
halo members, while their optical spectra are almost identical to the recently
identified cool Halo white dwarf WD0346+246. Canonical stellar halo models
predict a white dwarf volume density of two orders of magnitude less than the
approx 7*10^{-4} Solar masses per pc^{-3} inferred from this survey. With the
caveat that the sample size is very small, it appears that a significant
fraction, about 10%, of the local dark matter halo is in the form of very old,
cool, white dwarfs.Comment: 8 pages, 2 figures, accepted for publication in ApJL; references
adde
Optimal Microlensing Observations
One of the major limitations of microlensing observations toward the Large
Magellanic Cloud (LMC) is the low rate of event detection. What can be done to
improve this rate? Is it better to invest telescope time in more frequent
observations of the inner high surface-brightness fields, or in covering new,
less populated outer fields? How would a factor 2 improvement in CCD
sensitivity affect the detection efficiency? Would a series of major (factor
2--4) upgrades in telescope aperture, seeing, sky brightness, camera size, and
detector efficiency increase the event rate by a huge factor, or only
marginally? I develop a simplified framework to address these questions. With
observational resources fixed at the level of the MACHO and EROS experiments,
the biggest improvement (factor ~2) would come by reducing the time spent on
the inner ~25 deg^2 and applying it to the outer ~100 deg^2. By combining this
change with the characteristics of a good medium-size telescope (2.5 m mirror,
1" point spread function, thinned CCD chips, 1 deg^2 camera, and dark sky), it
should be possible to increase the detection of LMC events to more than 100 per
year (assuming current estimates of the optical depth apply to the entire LMC).Comment: Submitted to ApJ, 13 pages plus 3 figure
- …