15,956 research outputs found
The Stellar Content of Obscured Galactic Giant H II Regions: II. W42
We present near infrared J, H, and K images and K-band spectroscopy in the
giant HII region W42. A massive star cluster is revealed; the color-color plot
and K-band spectroscopic morphology of two of the brighter objects suggest the
presence of young stellar objects. The spectrum of the bright central star is
similar to unobscured stars with MK spectral types of O5-O6.5. If this star is
on the zero age main sequence, then the derived spectrophotometric distance is
considerably smaller than previous estimates. The Lyman continuum luminosity of
the cluster is a few times that of the Trapezium. The slope of the K-band
luminosity function is similar to that for the Trapezium cluster and
significantly steeper than that for the massive star cluster in M17 or the
Arches cluster near the Galactic center.Comment: 30 pages, 11 figures, late
The Refractory-to-Ice Mass Ratio in Comets
We review the complex relationship between the dust-to-gas mass ratio usually estimated in the material lost by comets, and the Refractory-to-Ice mass ratio inside the nucleus, which constrains the origin of comets. Such a relationship is dominated by the mass transfer from the perihelion erosion to fallout over most of the nucleus surface. This makes the Refractory-to-Ice mass ratio inside the nucleus up to ten times larger than the dust-to-gas mass ratio in the lost material, because the lost material is missing most of the refractories which were inside the pristine nucleus before the erosion. We review the Refractory-to-Ice mass ratios available for the comet nuclei visited by space missions, and for the Kuiper Belt Objects with well defined bulk density, finding the 1-σ lower limit of 3. Therefore, comets and KBOs may have less water than CI-chondrites, as predicted by models of comet formation by the gravitational collapse of cm-sized pebbles driven by streaming instabilities in the protoplanetary disc
A Comparative Review of Dimension Reduction Methods in Approximate Bayesian Computation
Approximate Bayesian computation (ABC) methods make use of comparisons
between simulated and observed summary statistics to overcome the problem of
computationally intractable likelihood functions. As the practical
implementation of ABC requires computations based on vectors of summary
statistics, rather than full data sets, a central question is how to derive
low-dimensional summary statistics from the observed data with minimal loss of
information. In this article we provide a comprehensive review and comparison
of the performance of the principal methods of dimension reduction proposed in
the ABC literature. The methods are split into three nonmutually exclusive
classes consisting of best subset selection methods, projection techniques and
regularization. In addition, we introduce two new methods of dimension
reduction. The first is a best subset selection method based on Akaike and
Bayesian information criteria, and the second uses ridge regression as a
regularization procedure. We illustrate the performance of these dimension
reduction techniques through the analysis of three challenging models and data
sets.Comment: Published in at http://dx.doi.org/10.1214/12-STS406 the Statistical
Science (http://www.imstat.org/sts/) by the Institute of Mathematical
Statistics (http://www.imstat.org
The Stellar Content of Obscured Galactic Giant HII Regions. VI: W51A
We present K-band spectra of newly born OB stars in the obscured Galactic
giant H II region W51A and ~ 0.8'' angular resolution images in the J, H and
K_S-bands. Four objects have been spectroscopically classified as O-type stars.
The mean spectroscopic parallax of the four stars gives a distance of 2.0 \pm
0.3 kpc (error in the mean), significantly smaller than the radio recombination
line kinematic value of 5.5 kpc or the values derived from maser propermotion
observations (6--8 kpc). The number of Lyman continuum photons from the
contribution of all massive stars (NLyc ~ 1.5 x 10^{50} s^{-1}) is in good
agreement with that inferred from radio recombination lines (NLyc = 1.3 x
10^{50} s^{-1}) after accounting for the smaller distance derived here.
We present analysis of archival high angular resolution images (NAOS CONICA
at VLT and T-ReCS at Gemini) of the compact region W51 IRS2. The K_S--band
images resolve the infrared source IRS~2 indicating that it is a very young
compact HII region. Sources IRS2E was resolved into compact cluster (within 660
AU of projected distance) of 3 objects, but one of them is just bright extended
emission. W51d1 and W51d2 were identified with compact clusters of 3 objects
(maybe 4 in the case of W51d1) each one. Although IRS~2E is the brightest
source in the K-band and at 12.6 \micron, it is not clearly associated with a
radio continuum source. Our spectrum of IRS~2E shows, similar to previous work,
strong emission in Br and HeI, as well as three forbidden emission
lines of FeIII and emission lines of molecular hydrogen (H_2) marking it as a
massive young stellar object.Comment: 31 pages and 9 figures, submitted to A
Quenched QCD with domain-wall fermions on coarse lattices
We investigate the existence of chiral zero modes at a^{-1} \simeq 1 GeV in
quenched domain-wall QCD. Simulations are carried out for the plaquette and an
RG-improved gauge actions on a 12^3x24xN_s lattice with N_s=10-50. We find that
the pion mass in the chiral limit remains non-vanishing as N_s\to\infty for
both gauge actions. Possible origins of this non-vanishing pion mass are
discussed.Comment: LATTICE99(chiral fermions), 3 pages, 6 ps figures, LaTex, espcrc2.st
- …