796 research outputs found
Intracluster medium of the merging cluster Abell 3395
We present a detailed imaging and spectral analysis of the merging
environment of the bimodal cluster A3395 using X-ray and radio observations.
X-ray images of the cluster show five main constituents of diffuse emission :
A3395 NE, A3395 SW, A3395 NW, A3395 W, and a filament connecting NE to W. X-ray
surface-brightness profiles of the cluster did not show any shock fronts in the
cluster. Temperature and entropy maps show high temperature and high entropy
regions in the W, the NW, the filament and between the NE and SW subclusters.
The NE, SW and W components have X-ray bolometric luminosities similar to those
of rich clusters of galaxies but have relatively higher temperatures.
Similarly, the NW component has X-ray bolometric luminosity similar to that of
isolated groups but with much higher temperature. It is, therefore, possible
that all the components of the cluster have been heated by the ongoing mergers.
The NE subcluster is the most massive and luminous constituent and other
subclusters are found to be gravitationally bound to it. The W component is
most probably either a clump of gas stripped off the SW due to ram pressure or
a separate subcluster that has merged or is merging with the SW. No X-ray
cavities are seen associated with the Wide Angle Tailed (WAT) radio source near
the centre of the SW subcluster. Minimum energy pressure in the radio
emission-peaks of the WAT galaxy is comparable with the external thermal
pressure. The radio spectrum of the WAT suggests a spectral age of ~10Myr
The Quantum McKay Correspondence for polyhedral singularities
Let G be a polyhedral group, namely a finite subgroup of SO(3). Nakamura's
G-Hilbert scheme provides a preferred Calabi-Yau resolution Y of the polyhedral
singularity C^3/G. The classical McKay correspondence describes the classical
geometry of Y in terms of the representation theory of G. In this paper we
describe the quantum geometry of Y in terms of R, an ADE root system associated
to G. Namely, we give an explicit formula for the Gromov-Witten partition
function of Y as a product over the positive roots of R. In terms of counts of
BPS states (Gopakumar-Vafa invariants), our result can be stated as a
correspondence: each positive root of R corresponds to one half of a genus zero
BPS state. As an application, we use the crepant resolution conjecture to
provide a full prediction for the orbifold Gromov-Witten invariants of [C^3/G].Comment: Introduction rewritten. Issue regarding non-uniqueness of conifold
resolution clarified. Version to appear in Inventione
Microlensing optical depth towards the Galactic bulge from MOA observations during 2000 with Difference Image Analysis
We analyze the data of the gravitational microlensing survey carried out by
by the MOA group during 2000 towards the Galactic Bulge (GB). Our observations
are designed to detect efficiently high magnification events with faint source
stars and short timescale events, by increasing the the sampling rate up to 6
times per night and using Difference Image Analysis (DIA). We detect 28
microlensing candidates in 12 GB fields corresponding to 16 deg^2. We use Monte
Carlo simulations to estimate our microlensing event detection efficiency,
where we construct the I-band extinction map of our GB fields in order to find
dereddened magnitudes. We find a systematic bias and large uncertainty in the
measured value of the timescale in our simulations. They are
associated with blending and unresolved sources, and are allowed for in our
measurements. We compute an optical depth tau = 2.59_{-0.64}^{+0.84} \times
10^{-6} towards the GB for events with timescales 0.3<t_E<200 days. We consider
disk-disk lensing, and obtain an optical depth tau_{bulge} =
3.36_{-0.81}^{+1.11} \times 10^{-6}[0.77/(1-f_{disk})] for the bulge component
assuming a 23% stellar contribution from disk stars. These observed optical
depths are consistent with previous measurements by the MACHO and OGLE groups,
and still higher than those predicted by existing Galactic models. We present
the timescale distribution of the observed events, and find there are no
significant short events of a few days, in spite of our high detection
efficiency for short timescale events down to t_E = 0.3 days. We find that half
of all our detected events have high magnification (>10). These events are
useful for studies of extra-solar planets.Comment: 65 pages and 30 figures, accepted for publication in ApJ. A
systematic bias and uncertainty in the optical depth measurement has been
quantified by simulation
Three-pion exchange: a gap in the nucleon-nucleon potential
The leading contribution to the three-pion exchange nucleon-nucleon potential
is calculated in the framework of chiral symmetry. It has pseudoscalar and
axial components and is dominated by the former, which has a range of about 1.5
fm and tends to enhance the OPEP. The strength of this force does not depend on
the pion mass and hence it survives in the chiral limit.Comment: 20 pages, 5 figures, 1 tabl
Discovery of the optical counterpart and early optical observations of GRB990712
We present the discovery observations of the optical counterpart of the
gamma-ray burster GRB990712 taken 4.16 hours after the outburst and discuss its
light curve observed in the V, R and I bands during the first ~35 days after
the outburst. The observed light curves were fitted with a power-law decay for
the optical transient (OT), plus an additional component which was treated in
two different ways. First, the additional component was assumed to be an
underlying galaxy of constant brightness. The resulting slope of the decay is
0.97+/-0.05 and the magnitudes of the underlying galaxy are: V = 22.3 +/- 0.05,
R = 21.75 +/- 0.05 and I = 21.35 +/- 0.05. Second, the additional component was
assumed to be a galaxy plus an underlying supernova with a time-variable
brightness identical to that of GRB980425, appropriately scaled to the redshift
of GRB990712. The resulting slope of the decay is similar, but the
goodness-of-fit is worse which would imply that either this GRB is not
associated with an underlying supernova or the underlying supernova is much
fainter than the supernova associated with GRB980425. The galaxy in this case
is fainter: V = 22.7 +/- 0.05, R = 22.25 +/- 0.05 and I = 22.15 +/- 0.05; and
the OT plus the underlying supernova at a given time is brighter. Measurements
of the brightnesses of the OT and the galaxy by late-time HST observation and
ground-based observations can thus assess the presence of an underlying
supernova.Comment: To appear in Ap
Study of variable stars in the MOA data base: long-period red variables in the Large Magellanic Cloud
One hundred and forty six long-period red variable stars in the Large
Magellanic Cloud (LMC) from the three year MOA project database were analysed.
A careful periodic analysis was performed on these stars and a catalogue of
their magnitudes, colours, periods and amplitudes is presented. We convert our
blue and red magnitudes to band values using 19 oxygen-rich stars. A group
of red short-period stars separated from the Mira sequence has been found on a
(log P, K) diagram. They are located at the short period side of the Mira
sequence consistent with the work of Wood and Sebo (1996). There are two
interpretations for such stars; a difference in pulsation mode or a difference
in chemical composition. We investigated the properties of these stars together
with their colour, amplitude and periodicity. We conclude that they have small
amplitudes and less regular variability. They are likely to be higher mode
pulsators. A large scatter has been also found on the long period side of the
(log P, K) diagram. This is possibly a systematic spread given that the blue
band of our photometric system covers both standard B and V bands and affects
carbon-rich stars.Comment: 19 pages, 19 figures, accepted for publication in MNRA
On Planetary Companions to the MACHO-98-BLG-35 Microlens Star
We present observations of microlensing event MACHO-98-BLG-35 which reached a
peak magnification factor of almost 80. These observations by the Microlensing
Planet Search (MPS) and the MOA Collaborations place strong constraints on the
possible planetary system of the lens star and show intriguing evidence for a
low mass planet with a mass fraction . A giant planet with is excluded from 95%
of the region between 0.4 and 2.5 from the lens star, where is the
Einstein ring radius of the lens. This exclusion region is more extensive than
the generic "lensing zone" which is . For smaller mass planets,
we can exclude 57% of the "lensing zone" for and 14% of
the lensing zone for . The mass fraction corresponds to an Earth mass planet for a lensing star of mass \sim
0.3 \msun. A number of similar events will provide statistically significant
constraints on the prevalence of Earth mass planets. In order to put our limits
in more familiar terms, we have compared our results to those expected for a
Solar System clone averaging over possible lens system distances and
orientations. We find that such a system is ruled out at the 90% confidence
level. A copy of the Solar System with Jupiter replaced by a second Saturn mass
planet can be ruled out at 70% confidence. Our low mass planetary signal (few
Earth masses to Neptune mass) is significant at the confidence
level. If this planetary interpretation is correct, the MACHO-98-BLG-35 lens
system constitutes the first detection of a low mass planet orbiting an
ordinary star without gas giant planets.Comment: ApJ, April 1, 2000; 27 pages including 8 color postscript figure
Proton--induced deuteron breakup at GeV energies with forward emission of a fast proton pair
A study of the deuteron breakup reaction with forward emission
of a fast proton pair with small excitation energy 3 MeV has been
performed at the ANKE spectrometer at COSY--J\"ulich. An exclusive measurement
was carried out at six proton--beam energies ~0.6,~0.7,~0.8,~0.95,~1.35,
and 1.9 GeV by reconstructing the momenta of the two protons. The differential
cross section of the breakup reaction, averaged up to over the cm
polar angle of the total momentum of the pairs, has been obtained. Since
the kinematics of this process is quite similar to that of backward elastic scattering, the results are compared to calculations based on a
theoretical model previously applied to the process.Comment: 17 pages including 6 figures and 1 table v2: minor changes; v3: minor
change of author list; v4: changes in accordance with referee remark
The Age, Stellar Content and Star Formation Timescale of the B59 Dense Core
We have used moderate resolution, near-infrared spectra from the SpeX
spectrograph on the NASA Infrared Telescope facility to characterize the
stellar content of Barnard 59 (B59), the most active star-forming core in the
Pipe Nebula. Measuring luminosity and temperature sensitive features in the
spectra of 20 candidate YSOs, we identified likely background giant stars and
measured each star's spectral type, extinction, and NIR continuum excess. We
find that B59 is composed of late type (K4-M6) low-mass (0.9--0.1 M_sun) YSOs
whose median stellar age is comparable to, if not slightly older than, that of
YSOs within the Rho Oph, Taurus, and Chameleon star forming regions. Deriving
absolute age estimates from pre-main sequence models computed by D'Antona et
al., and accounting only for statistical uncertainties, we measure B59's median
stellar age to be 2.6+/-0.8 Myrs. Including potential systematic effects
increases the error budget for B59's median (DM98) stellar age to 2.6+4.1/-2.6
Myrs. We also find that the relative age orderings implied by pre-main sequence
evolutionary tracks depend on the range of stellar masses sampled, as model
isochrones possess significantly different mass dependencies. The maximum
likelihood median stellar age we measure for B59, and the region's observed gas
properties, suggest that the B59 dense core has been stable against global
collapse for roughly 6 dynamical timescales, and is actively forming stars with
a star formation efficiency per dynamical time of ~6%. This maximum likelihood
value agrees well with recent star formation simulations that incorporate
various forms of support against collapse, such as sub-critical magnetic
fields, outflows, and radiative feedback from protostellar heating. [abridged]Comment: 20 pages, 12 figures, accepted for publication in the Astrophysical
Journal; updated to amend acknowledgment
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