4,152 research outputs found
NICMOS2 hubble space telescope observations of the embedded cluster associated with Mon R2: Constraining the substellar initial mass function
We have analyzed Hubble Space Telescope NICMOS2 F110W-, F160W-, F165M-, and F207M-band images covering the central 1' Ă 1' region of the cluster associated with Mon R2 in order to constrain the initial mass function (IMF) down to 20M_J. The flux ratio between the F165M and F160W bands was used to measure the strength of the water-band absorption feature and select a sample of 12 out of the total sample of 181 objects that have effective temperatures between 2700 and 3300 K. These objects are placed in the H-R diagram together with sources observed by Carpenter et al. to estimate an age of ~1 Myr for the low-mass cluster population. By constructing extinction-limited samples, we are able to constrain the IMF and the fraction of stars with a circumstellar disk in a sample that is 90% complete for both high- and low-mass objects. For stars with estimated masses between 0.1 and 1.0 M_â for a 1 Myr population with A_V †19 mag, we find that 27% ± 9% have a near-infrared excess indicative of a circumstellar disk. The derived fraction is similar to or slightly lower than the fraction found in other star-forming regions of comparable age. We constrain the number of stars in the mass interval 0.08-1.0 M_â to the number of objects in the mass interval 0.02-0.08 M_â by forming the ratio R^(**) = N(0.08-1 M_â)/N(0.02-0.08 M_â) for objects in an extinction-limited sample complete for A_V †7 mag. The ratio is found to be R^(**) = 2.2 ± 1.3, assuming an age of 1 Myr, consistent with the similar ratio predicted by the system IMF proposed by Chabrier. The ratio is similar to the ratios observed toward the Orion Nebula Cluster and IC 348, as well as the ratio derived in the 28 deg^2 survey of Taurus by Guieu et al
Monitoring SO2 emission at the Soufriere Hills Volcano: implications for changes in erruptive conditions
FLWINinfo:eu-repo/semantics/publishe
Atmospheric, Evolutionary, and Spectral Models of the Brown Dwarf Gliese 229 B
Theoretical spectra and evolutionary models that span the giant planet--brown
dwarf continuum have been computed based on the recent discovery of the brown
dwarf, Gliese 229 B. A flux enhancement in the 4--5 micron window is a
universal feature from Jovian planets to brown dwarfs. We confirm the existence
of methane and water in Gl 229 B's spectrum and find its mass to be 30 to 55
Jovian masses. Although these calculations focus on Gliese 229 B, they are also
meant to guide future searches for extra-solar giant planets and brown dwarfs.Comment: 8 pages, plain TeX, plus four postscript figures, gzipped and
uuencoded, accepted for Scienc
Hubble Space Telescope Imaging of the Expanding Nebular Remnant of the Recurrent Nova RS Ophiuchi (2006)
We report Hubble Space Telescope imaging obtained 155 days after the 2006
outburst of RS Ophiuchi. We detect extended emission in both [O III] and [Ne V]
lines. In both lines, the remnant has a double ring structure. The E-W
orientation and total extent of these structures (580+-50 AU at d=1.6kpc) is
consistent with that expected due to expansion of emitting regions imaged
earlier in the outburst at radio wavelengths. Expansion at high velocity
appears to have been roughly constant in the E-W direction (v_{exp} = 3200+-300
km/s in the plane of the sky), with tentative evidence of deceleration N-S. We
present a bipolar model of the remnant whose inclination is consistent with
that of the central binary. The true expansion velocities of the polar
components are then v = 5600+-1100 km/s. We suggest that the bipolar morphology
of the remnant results from interaction of the outburst ejecta with a
circumstellar medium that is significantly denser in the equatorial regions of
the binary than at the poles. This is also consistent with observations of
shock evolution in the X-ray and the possible presence of dust in the infrared.
Furthermore, it is in line with models of the shaping of planetary nebulae with
close binary central systems, and also with recent observations relating to the
progenitors of Type Ia supernovae, for which recurrent novae are a proposed
candidate. Our observations also reveal more extended structures to the S and E
of the remnant whose possible origin is briefly discussed.Comment: 13 pages, 2 figures, accepted for publication in ApJ
Evolving Einstein's Field Equations with Matter: The ``Hydro without Hydro'' Test
We include matter sources in Einstein's field equations and show that our
recently proposed 3+1 evolution scheme can stably evolve strong-field
solutions. We insert in our code known matter solutions, namely the
Oppenheimer-Volkoff solution for a static star and the Oppenheimer-Snyder
solution for homogeneous dust sphere collapse to a black hole, and evolve the
gravitational field equations. We find that we can evolve stably static,
strong-field stars for arbitrarily long times and can follow dust sphere
collapse accurately well past black hole formation. These tests are useful
diagnostics for fully self-consistent, stable hydrodynamical simulations in 3+1
general relativity. Moreover, they suggest a successive approximation scheme
for determining gravitational waveforms from strong-field sources dominated by
longitudinal fields, like binary neutron stars: approximate quasi-equilibrium
models can serve as sources for the transverse field equations, which can be
evolved without having to re-solve the hydrodynamical equations (``hydro
without hydro'').Comment: 4 postscript figures. Submitted to Phys. Rev. D15 as a Brief Repor
The Large, Oxygen-Rich Halos of Star-Forming Galaxies Are A Major Reservoir of Galactic Metals
The circumgalactic medium (CGM) is fed by galaxy outflows and accretion of
intergalactic gas, but its mass, heavy element enrichment, and relation to
galaxy properties are poorly constrained by observations. In a survey of the
outskirts of 42 galaxies with the Cosmic Origins Spectrograph onboard the
Hubble Space Telescope, we detected ubiquitous, large (150 kiloparsec) halos of
ionized oxygen surrounding star-forming galaxies, but we find much less ionized
oxygen around galaxies with little or no star formation. This ionized CGM
contains a substantial mass of heavy elements and gas, perhaps far exceeding
the reservoirs of gas in the galaxies themselves. It is a basic component of
nearly all star-forming galaxies that is removed or transformed during the
quenching of star formation and the transition to passive evolution.Comment: This paper is part of a set of three papers on circumgalactic gas
observed with the Cosmic Origins Spectrograph on HST, to be published in
Science, together with related papers by Tripp et al. and Lehner & Howk, in
the November 18, 2011 edition. This version has not undergone final
copyediting. Please see Science online for the final printed versio
Swift observations of the 2006 outburst of the recurrent nova RS Ophiuchi: II. 1D hydrodynamical models of wind driven shocks
Following the early Swift X-ray observations of the latest outburst of the
recurrent nova RS Ophiuchi in February 2006 (Paper I), we present new 1D
hydrodynamical models of the system which take into account all three phases of
the remnant evolution. The models suggest a novel way of modelling the system
by treating the outburst as a sudden increase then decrease in wind mass-loss
rate and velocity. The differences between this wind model and previous
Primakoff-type simulations are described. A more complex structure, even in 1D,
is revealed through the presence of both forward and reverse shocks, with a
separating contact discontinuity. The effects of radiative cooling are
investigated and key outburst parameters such as mass-loss rate, ejecta
velocity and mass are varied. The shock velocities as a function of time are
compared to the ones derived in Paper I. We show how the manner in which the
matter is ejected controls the evolution of the shock and that for a
well-cooled remnant, the shock deceleration rate depends on the amount of
energy that is radiated away.Comment: 9 pages, 5 figure
Resolution of Cosmological Singularities
We show that a class of 3+1 dimensional Friedmann-Robertson-Walker
cosmologies can be embedded within a variety of solutions of string theory. In
some realizations the apparent singularities associated with the big bang or
big crunch are resolved at non-singular horizons of higher-dimensional
quasi-black hole solutions (with compactified real time); in others plausibly
they are resolved at D-brane bound states having no conventional space-time
interpretation.Comment: 11 pages, latex. Two references added, one typo correcte
Management of Children With Chronic Wet Cough and Protracted Bacterial Bronchitis CHEST Guideline and Expert Panel Report
BACKGROUND: Wet or productive cough is common in children with chronic cough. We formulated recommendations based on systematic reviews related to the management of chronic wet cough in children (aged METHODS: We used the CHEST expert cough panel\u27s protocol for systematic reviews and the American College of Chest Physicians (CHEST) methodologic guidelines and GRADE framework (the Grading of Recommendations Assessment, Development and Evaluation). Data from the systematic reviews in conjunction with patients\u27 values and preferences and the clinical context were used to form recommendations. Delphi methodology was used to obtain consensus for the recommendations/suggestions made. RESULTS: Combining data from the systematic reviews, we found high-quality evidence in children aged 4 weeks\u27 duration) wet/productive cough that using appropriate antibiotics improves cough resolution, and further investigations (eg, flexible bronchoscopy, chest CT scans, immunity tests) should be undertaken when specific cough pointers (eg, digital clubbing) are present. When the wet cough does not improve following 4 weeks of antibiotic treatment, there is moderate-quality evidence that further investigations should be considered to look for an underlying disease. New recommendations include the recognition of the clinical diagnostic entity of protracted bacterial bronchitis. CONCLUSIONS: Compared with the 2006 Cough Guidelines, there is now high-quality evidence for some, but not all, aspects of the management of chronic wet cough in specialist settings. However, further studies (particularly in primary health) are required
Thermodynamics with long-range interactions: from Ising models to black-holes
New methods are presented which enables one to analyze the thermodynamics of
systems with long-range interactions. Generically, such systems have entropies
which are non-extensive, (do not scale with the size of the system). We show
how to calculate the degree of non-extensivity for such a system. We find that
a system interacting with a heat reservoir is in a probability distribution of
canonical ensembles. The system still possesses a parameter akin to a global
temperature, which is constant throughout the substance. There is also a useful
quantity which acts like a {\it local temperatures} and it varies throughout
the substance. These quantities are closely related to counterparts found in
general relativity. A lattice model with long-range spin-spin coupling is
studied. This is compared with systems such as those encountered in general
relativity, and gravitating systems with Newtonian-type interactions. A
long-range lattice model is presented which can be seen as a black-hole analog.
One finds that the analog's temperature and entropy have many properties which
are found in black-holes. Finally, the entropy scaling behavior of a
gravitating perfect fluid of constant density is calculated. For weak
interactions, the entropy scales like the volume of the system. As the
interactions become stronger, the entropy becomes higher near the surface of
the system, and becomes more area-scaling.Comment: Corrects some typos found in published version. Title changed 22
pages, 2 figure
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