937 research outputs found
On the feedback from super stellar clusters. I. The structure of giant HII regions and HII galaxies
We review the structural properties of giant extragalactic HII regions and
HII galaxies based on 2D hydrodynamic calculations, and propose an evolutionary
sequence that accounts for their observed detailed structure. The model assumes
a massive and young stellar cluster surrounded by a large collection of clouds.
These are thus exposed to the most important star-formation feedback
mechanisms: photoionization and the cluster wind. The models show how the two
feedback mechanisms compete in the disruption of clouds and lead to two
different hydrodynamic solutions: The storage of clouds into a long lasting
ragged shell that inhibits the expansion of the thermalized wind, and the
steady filtering of the shocked wind gas through channels carved within the
cloud stratum. Both solutions are claimed to be concurrently at work in giant
HII regions and HII galaxies, causing their detailed inner structure. This
includes multiple large-scale shells, filled with an X-ray emitting gas, that
evolve to finally merge with each other, giving the appearance of shells within
shells. The models also show how the inner filamentary structure of the giant
superbubbles is largely enhanced with matter ablated from clouds and how cloud
ablation proceeds within the original cloud stratum. The calculations point at
the initial contrast density between the cloud and the intercloud media as the
factor that defines which of the two feedback mechanisms becomes dominant
throughout the evolution. Animated version of the models can be found at
http://www.iaa.csic.es/\~{}eperez/ssc/ssc.html.Comment: 28 pages, 10 figures, accepted for publication in the ApJ. Animated
version of the models can be found at
http://www.iaa.csic.es/\~{}eperez/ssc/ssc.htm
The Dipole Observed in the COBE DMR Four-Year Data
The largest anisotropy in the cosmic microwave background (CMB) is the
mK dipole assumed to be due to our velocity with respect to the
CMB. Using the four year data set from all six channels of the COBE
Differential Microwave Radiometers (DMR), we obtain a best-fit dipole amplitude
mK in the direction , where the first
uncertainties are statistical and the second include calibration and combined
systematic uncertainties. This measurement is consistent with previous DMR and
FIRAS resultsComment: New and improved version; to be published in ApJ next mont
Two-Dimensional Hydrodynamic Models of Super Star Clusters with a Positive Star Formation Feedback
Using the hydrodynamic code ZEUS, we perform 2D simulations to determine the
fate of the gas ejected by massive stars within super star clusters. It turns
out that the outcome depends mainly on the mass and radius of the cluster. In
the case of less massive clusters, a hot high velocity ( km
s) stationary wind develops and the metals injected by supernovae are
dispersed to large distances from the cluster. On the other hand, the density
of the thermalized ejecta within massive and compact clusters is sufficiently
large as to immediately provoke the onset of thermal instabilities. These
deplete, particularly in the central densest regions, the pressure and the
pressure gradient required to establish a stationary wind, and instead the
thermally unstable parcels of gas are rapidly compressed, by a plethora of
re-pressurizing shocks, into compact high density condensations. Most of these
are unable to leave the cluster volume and thus accumulate to eventually feed
further generations of star formation.
The simulations cover an important fraction of the parameter-space, which
allows us to estimate the fraction of the reinserted gas which accumulates
within the cluster and the fraction that leaves the cluster as a function of
the cluster mechanical luminosity, the cluster size and heating efficiency.Comment: Accepted for publication in ApJ; 27 pages, 9 figures, 1 tabl
Two Large HI Shells in the Outer Galaxy near l=279 degrees
As part of a survey of HI 21-cm emission in the Southern Milky Way, we have
detected two large shells in the interstellar neutral hydrogen near l=279 deg.
The center velocities are +36 and +59 km/s, which puts the shells at kinematic
distances of 7 and 10 kpc. The larger shell is about 610 pc in diameter and
very empty, with density contrast of at least 15 between the middle and the
shell walls. It has expansion velocity of about 20 km/s and swept up mass of
several million solar masses. The energy indicated by the expansion may be as
high as 2.4 X 10^53 ergs. We estimate its age to be 15 to 20 million years. The
smaller shell has diameter of about 400 pc, expansion velocity about 10 km/s
and swept up mass of about 10^6 solar masses.
Morphologically both regions appear to be shells, with high density regions
mostly surrounding the voids, although the first appears to have channels of
low density which connect with the halo above and below the HI layer. They lie
on the edge of the Carina arm, which suggests that they may be expanding
horizontally into the interarm region as well as vertically out of the disk. If
this interpretation is correct, this is the first detection of an HI chimney
which has blown out of both sides of the disk.Comment: 21 pages, 14 jpeg figures, accepted for publication in A
Papillomavirus binding factor (PBF) is an intrinsically disordered protein with potential participation in osteosarcoma genesis, in silico evidence
Supernova Remnants Associated with Molecular Clouds in the Large Magellanic Cloud
We used the Swedish-ESO Submillimeter Telescope (SEST) to search for CO
emission associated with three supernova remnants (SNRs) in the Large
Magellanic Cloud: N49, N132D, and N23. Observations were carried out in the
J=2-1 rotational transition of CO (230.5 GHz) where the half power beamwidth of
the SEST is 23". Molecular clouds were discovered near N49 and N132D; no CO
emission was discovered in the region we mapped near N23. The N49 cloud has a
peak line temperature of 0.75 K, spatial scale of ~7 pc and virial mass of
~30,000 solar masses. The N132D cloud is brighter with a peak temperature of 5
K; it is also larger ~22 pc and considerably more massive 200,000 solar masses.
The velocities derived for the clouds near N49 and N132D, +286.0 km/s and
+264.0 km/s, agree well with the previously known velocities of the associated
SNRs: +286 km/s and +268 km/s, respectively. ROSAT X-ray images show that the
ambient density into which the remnants are expanding appears to be
significantly increased in the direction of the clouds. Taken together these
observations indicate a physical association between the remnants and their
respective, presumably natal, molecular clouds. The association of N49 and
N132D with dense regions of molecular material means that both were likely
products of short-lived progenitors that exploded as core-collapse supernovae.Comment: 26 pages, including 9 postscript figs, LaTeX (includes aaspp.sty),
accepted by Ap.J. for vol 480, May 10, 199
Tests of Gaussianity
We review two powerful methods to test the Gaussianity of the cosmic
microwave background (CMB): one based on the distribution of spherical wavelet
coefficients and the other on smooth tests of goodness-of-fit. The spherical
wavelet families proposed to analyse the CMB are the Haar and the Mexican Hat
ones. The latter is preferred for detecting non-Gaussian homogeneous and
isotropic primordial models containing some amount of skewness or kurtosis.
Smooth tests of goodness-of-fit have recently been introduced in the field
showing some interesting properties. We will discuss the smooth tests of
goodness-of-fit developed by Rayner and Best for the univariate as well as for
the multivariate analysis.Comment: Proceedings of "The Cosmic Microwave Background and its
Polarization", New Astronomy Reviews, (eds. S. Hanany and K.A. Olive), in
pres
Statistics and Topology of the COBE DMR First Year Maps
We use statistical and topological quantities to test the COBE-DMR first year
sky maps against the hypothesis that the observed temperature fluctuations
reflect Gaussian initial density perturbations with random phases. Recent
papers discuss specific quantities as discriminators between Gaussian and
non-Gaussian behavior, but the treatment of instrumental noise on the data is
largely ignored. The presence of noise in the data biases many statistical
quantities in a manner dependent on both the noise properties and the unknown
CMB temperature field. Appropriate weighting schemes can minimize this effect,
but it cannot be completely eliminated. Analytic expressions are presented for
these biases, and Monte Carlo simulations used to assess the best strategy for
determining cosmologically interesting information from noisy data. The genus
is a robust discriminator that can be used to estimate the power law
quadrupole-normalized amplitude independently of the 2-point correlation
function. The genus of the DMR data are consistent with Gaussian initial
fluctuations with Q_rms = 15.7 +/- 2.2 - (6.6 +/- 0.3)(n - 1) uK where n is the
power law index. Fitting the rms temperature variations at various smoothing
angles gives Q_rms = 13.2 +/- 2.5 uK and n = 1.7 +0.3 -0.6. While consistent
with Gaussian fluctuations, the first year data are only sufficient to rule out
strongly non-Gaussian distributions of fluctuations.Comment: 17 pages post scrip
Affine convex body semigroups
In this paper we present a new kind of semigroups called convex body
semigroups which are generated by convex bodies of R^k. They generalize to
arbitrary dimension the concept of proportionally modular numerical semigroup
of [7]. Several properties of these semigroups are proven. Affine convex body
semigroups obtained from circles and polygons of R^2 are characterized. The
algorithms for computing minimal system of generators of these semigroups are
given. We provide the implementation of some of them
Hydrodynamical Models of Outflow Collimation in YSOs
We explore the physics of time-dependent hydrodynamic collimation of jets
from Young Stellar Objects (YSOs). Using parameters appropriate to YSOs we have
carried out high resolution hydrodynamic simulations modeling the interaction
of a central wind with an environment characterized by a moderate opening angle
toroidal density distribution. The results show that the the wind/environment
interaction produces strongly collimated supersonic jets. The jet is composed
of shocked wind gas. Using analytical models of wind blown bubble evolution we
show that the scenario studied here should be applicable to YSOs and can, in
principle, initiate collimation on the correct scales (R ~ 100 AU). The
simulations reveal a number of time-dependent non-linear features not
anticipated in previous analytical studies including: a prolate wind shock; a
chimney of cold swept-up ambient material dragged into the bubble cavity; a
plug of dense material between the jet and bow shocks. We find that the
collimation of the jet occurs through both de Laval nozzles and focusing of the
wind via the prolate wind shock. Using an analytical model for shock focusing
we demonstrate that a prolate wind shock can, by itself, produce highly
collimated supersonic jets.Comment: Accepted by ApJ, 31 pages with 12 figures (3 JPEG's) now included,
using aasms.sty, Also available in postscript via a gzipped tar file at
ftp://s1.msi.umn.edu/pub/afrank/SFIC1/SFIC.tar.g
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