26,062 research outputs found
Out of equilibrium quantum field dynamics of an initial thermal state after a change in the external field
The effects of the initial temperature in the out of equilibrium quantum
field dynamics in the presence of an homogeneous external field are
investigated. We consider an initial thermal state of temperature T for a
constant external field J. A subsequent sign flip of the external field, J to
-J, gives rise to an out of equilibrium nonperturbative quantum field dynamics.
The dynamics is studied here for the symmetry broken lambda(Phi^2)^2 scalar N
component field theory in the large N limit. We find a dynamical effective
potential for the expectation value that helps to understand the dynamics. The
dynamics presents two regimes defined by the presence or absence of a temporal
trapping close to the metastable equilibrium position of the potential. The two
regimes are separated by a critical value of the external field that depends on
the initial temperature. The temporal trapping is shorter for larger initial
temperatures or larger external fields. Parametric resonances and spinodal
instabilities amplify the quantum fluctuations in the field components
transverse to the external field. When there is a temporal trapping this is the
main mechanism that allows the system to escape from the metastable state for
large N. Subsequently backreaction stops the growth of the quantum fluctuations
and the system enters a quasiperiodic regime.Comment: LaTeX, 19 pages, 12 .eps figures, improved version to appear in Phys
Rev
Vector meson spectral function and dilepton rate in an effective mean field model
We have studied the vector meson spectral function (VMSF) in a hot and dense
medium within an effective QCD model namely the Nambu-Jona-Lasinio (NJL) and
its Polyakov Loop extended version (PNJL) with and without the effect of
isoscalar vector interaction (IVI). The effect of the IVI has been taken into
account using the ring approximation. We obtained the dilepton production rate
(DPR) using the VMSF and observed that at moderate temperature it is enhanced
in the PNJL model as compared to the NJL and Born rate due to the suppression
of color degrees of freedom.Comment: 5 pages, 7 figures, conference proceedings of the XXI DAE-BRNS HEP
Symposium, IIT Guwahati, December 2014; to appear in 'Springer Proceedings in
Physics Series
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Morphological evidence for a sea-ice origin for Elysium Planitia platy terrain
Abstract not available
Infrared Search for Young Stars in HI High-velocity Clouds
We have searched the IRAS Point Source Catalog and HIRES maps for young
stellar objects (YSOs) in the direction of five \HI high-velocity clouds
(HVCs). In agreement with optical searches in the halo, no evidence was found
for extensive star-forming activity inside the high-latitude HVCs.
Specifically, we have found no signs of star formation or YSOs in the direction
of the A IV cloud or in the very-high-velocity clouds HVC~110-7-465 and
HVC~114-10-440. We have identified only one young star in the direction of the
M~I.1 cloud, which shows almost perfect alignment with a knot of \HI emission.
Because of the small number of early-type stars observed in the halo, the
probability for such a positional coincidence is low; thus, this young star
appears to be physically associated with the M~I.1 cloud. We have also
identified a good YSO candidate in the \HI shell-like structure observed in the
core region of the low-latitude cloud complex H (HVC~131+1-200). This region
could be a supernova remnant with several other YSO candidates formed along the
shock front produced by the explosion. In agreement with recent theoretical
estimates, these results point to a low but significant star-formation rate in
intermediate and high Galactic latitude HVCs. For M~I.1 in particular, we
estimate that the efficiency of the star-formation process is M(YSO)/M(\HI)\ga
10^{-4}-10^{-3} by mass. Such efficiency is sufficient to account for (a) the
existence of the few young blue stars whose ages imply that they were born in
the Galactic halo, and (b) the nonprimordial metallicities inferred for some
HVCs if their metal content proves to be low.Comment: 9 pages, 4 JPEG figures. PostScript figures available from author
High Ratio of 44Ti/56Ni in Cas A and Axisymmetric Collapse-Driven Supernova Explosion
The large abundance ratio of in Cas A is puzzling. In fact,
the ratio seems to be larger than the theoretical constraint derived by Woosley
& Hoffman (1991). However, this constraint is obtained on the assumption that
the explosion is spherically symmetric, whereas Cas A is famous for the
asymmetric form of the remnant. Recently, Nagataki et al. (1997) calculated the
explosive nucleosynthesis of axisymmetrically deformed collapse-driven
supernova. They reported that the ratio of was enhanced by
the stronger alpha-rich freezeout in the polar region. In this paper, we apply
these results to Cas A and examine whether this effect can explain the large
amount of and the large ratio of . We demonstrate
that the conventional spherically symmetric explosion model can not explain the
Ti mass produced in Cas A if its lifetime is shorter than 80
years and the intervening space is transparent to the gamma-ray line from the
decay of Ti. On the other hand, we show the axisymmetric explosion
models can solve the problem. We expect the same effect from a three
dimensionally asymmetric explosion, since the stronger alpha-rich freezeout
will also occur in that case in the region where the larger energy is
deposited.Comment: 10 pages, LaTeX text and 3 postscript figure
The Arecibo L-band Feed Array Zone of Avoidance Survey I: Precursor Observations through the Inner and Outer Galaxy
The Arecibo L-band Feed Array (ALFA) is being used to conduct a low-Galactic
latitude survey, to map the distribution of galaxies and large-scale structures
behind the Milky Way through detection of galaxies' neutral hydrogen (HI) 21-cm
emission. This Zone of Avoidance (ZOA) survey finds new HI galaxies which lie
hidden behind the Milky Way, and also provides redshifts for partially-obscured
galaxies known at other wavelengths. Before the commencement of the full
survey, two low-latitude precursor regions were observed, totalling 138 square
degrees, with 72 HI galaxies detected. Detections through the inner Galaxy
generally have no cataloged counterparts in any other waveband, due to the
heavy extinction and stellar confusion. Detections through the outer Galaxy are
more likely to have 2MASS counterparts. We present the results of these
precursor observations, including a catalog of the detected galaxies, with
their HI parameters. The survey sensitivity is well described by a flux- and
linewidth-dependent signal-to-noise ratio of 6.5. ALFA ZOA galaxies which also
have HI measurements in the literature show good agreement between our
measurements and previous work. The inner Galaxy precursor region was chosen to
overlap the HI Parkes Zone of Avoidance Survey so ALFA performance could be
quickly assessed. The outer Galaxy precursor region lies north of the Parkes
sky. Low-latitude large-scale structure in this region is revealed, including
an overdensity of galaxies near l = 183 deg and between 5000 - 6000 km/s in the
ZOA. The full ALFA ZOA survey will be conducted in two phases: a shallow survey
using the observing techniques of the precursor observations, and also a deep
phase with much longer integration time, with thousands of galaxies predicted
for the final catalog.Comment: 26 pages, 7 figures, 2 tables, Astronomical Journal accepte
Numerical Simulations of Equatorially-Asymmetric Magnetized Supernovae: Formation of Magnetars and Their Kicks
A series of numerical simulations on magnetorotational core-collapse
supernovae are carried out. Dipole-like configurations which are offset
northward are assumed for the initially strong magnetic fields together with
rapid differential rotations. Aims of our study are to investigate effects of
the offset magnetic field on magnetar kicks and on supernova dynamics. Note
that we study a regime where the proto-neutron star formed after collapse has a
large magnetic field strength approaching that of a ``magnetar'', a highly
magnetized slowly rotating neutron star. As a result, equatorially-asymmetric
explosions occur with a formation of the bipolar jets. Resultant magnetar's
kick velocities are km s. We find that the acceleration
is mainly due to the magnetic pressure while the somewhat weaker magnetic
tension works toward the opposite direction, which is due to stronger magnetic
field in the northern hemisphere. Noted that observations of magnetar's proper
motions are very scarce, our results supply a prediction for future
observations. Namely, magnetars possibly have large kick velocities, several
hundred km s, as ordinary neutron stars do, and in an extreme case they
could have those up to 1000 km s.Comment: 36 pages, 9 figures, accepted by the Astrophysical Journa
Simian immunodeficiency virus infection in wild-caught chimpanzees from Cameroon
Simian immunodeficiency viruses (SIVcpz) infecting chimpanzees (Pan troglodytes) in west central Africa are the closest relatives to all major variants of human immunodeficiency virus type 1 ([HIV-1]; groups M, N and O), and have thus been implicated as the source of the human infections; however, information concerning the prevalence, geographic distribution, and subspecies association of SIVcpz still remains limited. In this study, we tested 71 wild-caught chimpanzees from Cameroon for evidence of SIVcpz infection. Thirty-nine of these were of the central subspecies (Pan troglodytes troglodytes), and 32 were of the Nigerian subspecies (Pan troglodytes vellerosus), as determined by mitochondrial DNA analysis. Serological analysis determined that one P. t. troglodytes ape (CAM13) harbored serum antibodies that cross-reacted strongly with HIV-1 antigens; all other apes were seronegative. To characterize the newly identified virus, 14 partially overlapping viral fragments were amplified from fecal virion RNA and concatenated to yield a complete SIVcpz genome (9,284 bp). Phylogenetic analyses revealed that SIVcpzCAM13 fell well within the radiation of the SIVcpzPtt group of viruses, as part of a clade including all other SIVcpzPtt strains as well as HIV-1 groups M and N. However, SIVcpzCAM13 clustered most closely with SIVcpzGAB1 from Gabon rather than with SIVcpzCAM3 and SIVcpzCAM5 from Cameroon, indicating the existence of divergent SIVcpzPtt lineages within the same geographic region. These data, together with evidence of recombination among ancestral SIVcpzPtt lineages, indicate long-standing endemic infection of central chimpanzees and reaffirm a west central African origin of HIV-1. Whether P. t. vellerosus apes are naturally infected with SIVcpz requires further study
Phylogenetic inferences of Atelinae (Platyrrhini) based on multi-directional chromosome painting in Brachyteles arachnoides, Ateles paniscus paniscus and Ateles b. marginatus
We performed multi-directional chromosome painting in a comparative cytogenetic study of the three Atelinae species Brachyteles arachnoides, Ateles paniscus paniscus and Ateles belzebuth marginatus, in order to reconstruct phylogenetic relationships within this Platyrrhini subfamily. Comparative chromosome maps between these species were established by multi-color fluorescence in situ hybridization ( FISH) employing human, Saguinus oedipus and Lagothrix lagothricha chromosome-specific probes. The three species included in this study and four previously analyzed species from all four Atelinae genera were subjected to a phylogenetic analysis on the basis of a data matrix comprised of 82 discrete chromosome characters. The results confirmed that Atelinae represent a monophyletic clade with a putative ancestral karyotype of 2n = 62 chromosomes. Phylogenetic analysis revealed an evolutionary branching sequence \{Alouatta \{Brachyteles \{Lagothrix and Ateles\}\}\} in Atelinae and \{Ateles belzebuth marginatus \{Ateles paniscus paniscus \{Ateles belzebuth hybridus and Ateles geoffroyi\}\}\} in genus Ateles. The chromosomal data support a re-evaluation of the taxonomic status of Ateles b. hybridus. Copyright (C) 2005 S. Karger AG, Basel
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