289 research outputs found
Microlensing Halo Models with Abundant Brown Dwarfs
All previous attempts to understand the microlensing results towards the
Large Magellanic Cloud (LMC) have assumed homogeneous present day mass
functions (PDMFs) for the lensing populations. Here, we present an
investigation into the microlensing characteristics of haloes with spatially
varying PDMFs and anisotropic velocity dispersion tensors. One attractive
possibility -- suggested by baryonic dark cluster formation in pregalactic and
protogalactic cooling flows -- is that the inner halo is dominated by stellar
mass objects, whereas low mass brown dwarfs become more prevalent on moving
outwards. The contribution to the microlensing rate must be dominated by dark
remnants (of about 0.5 solar masses) to recover the observed timescales of the
microlensing experiments. But, even though stellar remnants control the rate,
they do not dominate the mass of the baryonic halo, and so the well-known
enrichment and mass budget problems are much less severe. Using a simple ansatz
for the spatial variation of the PDMF, models are constructed in which the
contribution of brown dwarfs to the mass of the baryonic halo is 55 % and to
the total halo is 30 %. An unusual property of the models is that they predict
that the average timescale of events towards M31 is shorter than the average
timescale towards the LMC. This is because the longer line of sight towards M31
probes more of the far halo where brown dwarfs are the most common constituent.Comment: 17 pages, 1 figure, in press at The Astrophysical Journal (Letters
Gamma ray astronomy and baryonic dark matter
Recently, Dixon et al. have re-analyzed the EGRET data, finding a
statistically significant diffuse -ray emission from the galactic halo.
We show that this emission can naturally be explained within a
previously-proposed model for baryonic dark matter, in which -rays are
produced through the interaction of high-energy cosmic-ray protons with cold
clouds clumped into dark clusters - these dark clusters supposedly
populate the outer galactic halo and can show up in microlensing observations.
Our estimate for the halo -ray flux turns out to be in remarkably good
agreement with the discovery by Dixon et al. We also address future prospects
to test our predictions.Comment: 9 pages, 1 figure included, to appear in ApJ 510, L103 (1999
Novae In External Galaxies From The POINT-AGAPE Survey And The Liverpool Telescope
We have recently begun a search for Classical Novae in M31 using three years
of multicolour data taken by the POINT-AGAPE microlensing collaboration with
the 2.5m Isaac Newton Telescope (INT) on La Palma. This is a pilot program
leading to the use of the Liverpool Telescope (LT) to systematically search for
and follow novae of all speed classes in external galaxies to distances up to
around 5Mpc.Comment: 5 pages, 4 figures, To appear in the Proceedings of the Conference
``Classical Nova Explosions'', M. Hernanz and J. Jose eds., American Inst. of
Physics, 200
Monte Carlo analysis of the MEGA microlensing events towards M31
We perform an analytical study and a Monte Carlo (MC) analysis of the main
features for microlensing events in pixel lensing observations towards M31. Our
main aim is to investigate the lens nature and location of the 14 candidate
events found by the MEGA collaboration. Assuming a reference model for the mass
distribution in M31 and the standard model for our galaxy, we estimate the
MACHO-to-self lensing probability and the event time duration towards M31.
Reproducing the MEGA observing conditions, as a result we get the MC event
number density distribution as a function of the event full-width half-maximum
duration and the magnitude at maximum . For a
MACHO mass of we find typical values of day
and , for both MACHO-lensing and self-lensing
events occurring beyond about 10 arcminutes from the M31 center. A comparison
of the observed features ( and ) with our MC
results shows that for a MACHO mass the four innermost MEGA
events are most likely self-lensing events, whereas the six outermost events
must be genuine MACHO-lensing events.Comment: in press on A&
Gravitomagnetic corrections to the lensing deflection angle for spiral galaxy models
We investigate the effects of the gravitomagnetic corrections to the usual
gravitational lens quantities for a specific lensing mass distribution modelled
after spiral galaxies. An exponential disk is embedded into two different
spherical halo models where disk and haloes parameters are fixed according to
the observed mass to light ratios, galaxy magnitudes and rotation curves. The
general expressions for the lensing deflection angle are given also taking into
account the orientation of the galaxy disk plane with respect to the lens
plane. It is found that the gravitomagnetic term changes the deflection angle
by a typical amount of the order of ten microarcseconds.Comment: 7 pages, 2 figures, accepted for publication on MNRA
Classical novae from the POINT-AGAPE microlensing survey of M31 -- I. The nova catalogue
The POINT-AGAPE survey is an optical search for gravitational microlensing
events towards the Andromeda Galaxy (M31). As well as microlensing, the survey
is sensitive to many different classes of variable stars and transients. Here
we describe the automated detection and selection pipeline used to identify M31
classical novae (CNe) and we present the resulting catalogue of 20 CN
candidates observed over three seasons. CNe are observed both in the bulge
region as well as over a wide area of the M31 disk. Nine of the CNe are caught
during the final rise phase and all are well sampled in at least two colours.
The excellent light-curve coverage has allowed us to detect and classify CNe
over a wide range of speed class, from very fast to very slow. Among the
light-curves is a moderately fast CN exhibiting entry into a deep transition
minimum, followed by its final decline. We have also observed in detail a very
slow CN which faded by only 0.01 mag day over a 150 day period. We
detect other interesting variable objects, including one of the longest period
and most luminous Mira variables. The CN catalogue constitutes a uniquely
well-sampled and objectively-selected data set with which to study the
statistical properties of classical novae in M31, such as the global nova rate,
the reliability of novae as standard-candle distance indicators and the
dependence of the nova population on stellar environment. The findings of this
statistical study will be reported in a follow-up paper.Comment: 21 pages, 13 figures, re-submitted for publication in MNRAS, typos
corrected, references updated, figures 5-9 made cleare
Transiting Exoplanet Yields for the Roman Galactic Bulge Time Domain Survey Predicted from Pixel-Level Simulations
The Nancy Grace Roman Space Telescope (Roman) is NASA's next astrophysics
flagship mission, expected to launch in late 2026. As one of Roman's core
community science surveys, the Galactic Bulge Time Domain Survey (GBTDS) will
collect photometric and astrometric data for over 100 million stars in the
Galactic bulge to search for microlensing planets. To assess the potential with
which Roman can detect exoplanets via transit, we developed and conducted
pixel-level simulations of transiting planets in the GBTDS. From these
simulations, we predict that Roman will find between 60,000 and
200,000 transiting planets, over an order of magnitude more planets than
are currently known. While the majority of these planets will be giants
() on close-in orbits ( au), the yield also includes
between 7,000 and 12,000 small planets (). The
yield for small planets depends sensitively on the observing cadence and season
duration, with variations on the order of 10-20% for modest changes in
either parameter, but is generally insensitive to the trade between surveyed
area and cadence given constant slew/settle times. These predictions depend
sensitively on the Milky Way's metallicity distribution function, highlighting
an opportunity to significantly advance our understanding of exoplanet
demographics, particularly across stellar populations and Galactic
environments.Comment: Accepted to ApJS; 64 pages, 18 figure
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