1,170 research outputs found
OH 1720 MHz Masers in Supernova Remnants --- C-Shock Indicators
Recent observations show that the OH 1720 MHz maser is a powerful probe of
the shocked region where a supernova remnant strikes a molecular cloud. We
perform a thorough study of the pumping of this maser and find tight
constraints on the physical conditions needed for its production. The presence
of the maser implies moderate temperatures (50 -- 125 K) and densities (), and OH column densities of order . We show
that these conditions can exist only if the shocks are of C-type. J-shocks fail
by such a wide margin that the presence of this maser could become the most
powerful indicator of C-shocks. These conditions also mean that the 1720 MHz
maser will be inherently weak compared to the other ground state OH masers. All
the model predictions are in good agreement with the observations.Comment: 16 pages, 5 Postscript figures (included), uses aaspp4.sty. To appear
in the Astrophysical Journa
Polarization of astronomical maser radiation; 3, arbitrary Zeeman splitting and anisotropic pumping
General solutions of the maser polarization problem are presented for arbitrary absorption coefficients. The results are used to calculate polarization for masers permeated by magnetic fields with arbitrary values of \xB, the ratio of Zeeman splitting to Doppler linewidth, and for anisotropic pumping. The \xb \to 0 limit of the magnetic solution reproduces the linear polarization derived in previous studies, which were always conducted at this unphysical limit. While terms of higher order in \xb\ have a negligible effect on the magnitude of q, they produce some major new results. In particular, the linear polarization is accompanied by circular polarization, proportional to \xb. Because \xb\ is proportional to the transition wavelength, the circular polarization of SiO masers should decrease with rotation quantum number, as observed. In the absence of theory for \xb < 1, previous estimates of magnetic fields from detected maser circular polarization had to rely on conjectures in this case and generally need to be revised downward. The fields in SiO masers are \about\ 2--10 G and were overestimated by a factor of 8. The OH maser regions around supergiants have fields of \about\ 0.1--0.5 mG, which were overestimated by factors of 10--100. The fields were properly estimated for OH/IR masers (\la 0.1 mG) and \H2O masers in star-forming regions (\about\ 15--50 mG). Spurious solutions that required stability analysis for their removal in all previous studies are never reproduced here; in particular, there are no stationary physical solutions for propagation at \sin^2\theta < \third, where \theta is the angle from the direction of the magnetic field, so such radiation is unpolarized. These spurious solutions can be identified as the \xb\ = 0 limits of non-physical solutions and they never arise at finit
Observational evidence for the shrinking of bright maser spots
The nature of maser emission means that the apparent angular size of an
individual maser spot is determined by the amplification process as well as by
the instrinsic size of the emitting cloud. Highly sensitive MERLIN radio
interferometry images spatially and spectrally resolve water maser clouds
around evolved stars. We measured the properties of clouds around the red
supergiant S Per and the AGB stars IK Tau, RT Vir, U Her and U Ori, to test
maser beaming theory. Spherical clouds are expected to produce an inverse
relationship between maser intensity and apparent size, which would not be seen
from cylindrical or slab-like regions. We analysed the maser properties, in
order to estimate the saturation state, and investigated the variation of
observed spot size with intensity and across the spectral line profiles.
Circumstellar masers emanate from discrete clouds from about one to 20 AU in
diameter depending on the star. Most of the maser features have negative
excitation temperatures close to zero and modest optical depths, showing that
they are mainly unsaturated. Around S Per and (at most epochs) RT Vir and IK
Tau, the maser component size shrinks with increasing intensity. In contrast,
the masers around U Ori and U Her tend to increase in size, with a larger
scatter. The water masers from S Per, RT Vir and IK Tau are mainly beamed into
spots with an observed angular size much smaller than the emitting clouds and
smallest of all at the line peaks. This suggests that the masers are
amplification-bounded, emanating from approximately spherical clouds. Many of
the masers around U Her and U Ori have apparent sizes which are more similar to
the emitting clouds and have less or no dependence on intensity, suggesting
that these masers are matter-bounded. This is consistent with an origin in
flattened clouds and these two stars have shown other behaviour indicating the
presence of shocks.Comment: 17 pages, 26 figure files, accepted by A&A 2010 Oct 2
On canonical quantization of the gauged WZW model with permutation branes
In this paper we perform canonical quantization of the product of the gauged
WZW models on a strip with boundary conditions specified by permutation branes.
We show that the phase space of the -fold product of the gauged WZW model
on a strip with boundary conditions given by permutation branes is
symplectomorphic to the phase space of the double Chern-Simons theory on a
sphere with holes times the time-line with and gauge fields both
coupled to two Wilson lines. For the special case of the topological coset
we arrive at the conclusion that the phase space of the -fold product
of the topological coset on a strip with boundary conditions given by
permutation branes is symplectomorphic to the phase space of Chern-Simons
theory on a Riemann surface of the genus times the time-line with four
Wilson lines.Comment: 18 page
Evidence for bipolar jets in late stages of AGB winds
Bipolar expansion at various stages of evolution has been recently observed
in a number of AGB stars. The expansion is driven by bipolar jets that emerge
late in the evolution of AGB winds. The wind traps the jets, resulting in an
expanding, elongated cocoon. Eventually the jets break-out from the confining
spherical wind, as recently observed in W43A. This source displays the most
advanced evolutionary stage of jets in AGB winds. The earliest example is
IRC+10011, where the asymmetry is revealed in high-resolution near-IR imaging.
In this source the jets turned on only ~200 years ago, while the spherical wind
is ~4000 years old.Comment: 6 pages, to appear in "Asymmetrical Planetary Nebulae III" editors M.
Meixner, J. Kastner, N. Soker, & B. Balick (ASP Conf. Series
Nonlocal Effects of Partial Measurements and Quantum Erasure
Partial measurement turns the initial superposition not into a definite
outcome but into a greater probability for it. The probability can approach
100%, yet the measurement can undergo complete quantum erasure. In the EPR
setting, we prove that i) every partial measurement nonlocally creates the same
partial change in the distant particle; and ii) every erasure inflicts the same
erasure on the distant particle's state. This enables an EPR experiment where
the nonlocal effect does not vanish after a single measurement but keeps
"traveling" back and forth between particles. We study an experiment in which
two distant particles are subjected to interferometry with a partial "which
path" measurement. Such a measurement causes a variable amount of correlation
between the particles. A new inequality is formulated for same-angle
polarizations, extending Bell's inequality for different angles. The resulting
nonlocality proof is highly visualizable, as it rests entirely on the
interference effect. Partial measurement also gives rise to a new form of
entanglement, where the particles manifest correlations of multiple
polarization directions. Another novelty in that the measurement to be erased
is fully observable, in contrast to prevailing erasure techniques where it can
never be observed. Some profound conceptual implications of our experiment are
briefly pointed out.Comment: To be published in Phys. Rev. A 63 (2001). 19 pages, 12 figures,
RevTeX 3.
Detections of water ice, hydrocarbons, and 3.3um PAH in z~2 ULIRGs
We present the first detections of the 3um water ice and 3.4um amorphous
hydrocarbon (HAC) absorption features in z~2 ULIRGs. These are based on deep
rest-frame 2-8um Spitzer IRS spectra of 11 sources selected for their
appreciable silicate absorption. The HAC-to-silicate ratio for our z~2 sources
is typically higher by a factor of 2-5 than that observed in the Milky Way.
This HAC `excess' suggests compact nuclei with steep temperature gradients as
opposed to predominantly host obscuration. Beside the above molecular
absorption features, we detect the 3.3um PAH emission feature in one of our
sources with three more individual spectra showing evidence for it. Stacking
analysis suggests that water ice, hydrocarbons, and PAH are likely present in
the bulk of this sample even when not individually detected. The most
unexpected result of our study is the lack of clear detections of the 4.67um CO
gas absorption feature. Only three of the sources show tentative signs of this
feature and at significantly lower levels than has been observed in local
ULIRGs. Overall, we find that the closest local analogs to our sources, in
terms of 3-4um color, HAC-to-silicate and ice-to-silicate ratios, as well as
low PAH equivalent widths are sources dominated by deeply obscured nuclei. Such
sources form only a small fraction of ULIRGs locally and are commonly believed
to be dominated by buried AGN. Our sample suggests that, in absolute number,
such buried AGN are at least an order of magnitude more common at z~2 than
today. The presence of PAH suggests that significant levels of star-formation
are present even if the obscured AGN typically dominate the power budget.Comment: 39 pages, 14 figures, accepted for publication in Ap
The central molecular gas structure in LINERs with low luminosity AGN: evidence for gradual disappearance of the torus
We present observations of the molecular gas in the nuclear environment of
three prototypical low luminosity AGN (LLAGN), based on VLT/SINFONI AO-assisted
integral-field spectroscopy of H2 1-0 S(1) emission at angular resolutions of
~0.17". On scales of 50-150 pc the spatial distribution and kinematics of the
molecular gas are consistent with a rotating thin disk, where the ratio of
rotation (V) to dispersion (sigma) exceeds unity. However, in the central 50
pc, the observations reveal a geometrically and optically thick structure of
molecular gas (V/sigma10^{23} cm^{-2}) that is likely to be
associated with the outer extent of any smaller scale obscuring structure. In
contrast to Seyfert galaxies, the molecular gas in LLAGN has a V/sigma<1 over
an area that is ~9 times smaller and column densities that are in average ~3
times smaller. We interpret these results as evidence for a gradual
disappearance of the nuclear obscuring structure. While a disk wind may not be
able to maintain a thick rotating structure at these luminosities, inflow of
material into the nuclear region could provide sufficient energy to sustain it.
In this context, LLAGN may represent the final phase of accretion in current
theories of torus evolution. While the inflow rate is considerable during the
Seyfert phase, it is slowly decreasing, and the collisional disk is gradually
transitioning to become geometrically thin. Furthermore, the nuclear region of
these LLAGN is dominated by intermediate-age/old stellar populations (with
little or no on-going star formation), consistent with a late stage of
evolution.Comment: 15 pages, including 4 figures and 1 table, Accepted for publication
in ApJ Letter
An all-order discontinuity at the electroweak phase transition
We define a non-local gauge-invariant Green's function which can distinguish
between the symmetric (confinement) and broken (Higgs) phases of the hot
SU(2)xU(1) electroweak theory to all orders in the perturbative expansion. It
is related to the coupling of the Chern-Simons number to a massless Abelian
gauge field. The result implies either that there is a way to distinguish
between the phases, even though the macroscopic thermodynamical properties of
the system have been observed to be smoothly connected, or that the
perturbative Coleman-Hill theorem on which the argument is based, is
circumvented by non-perturbative effects. We point out that this question could
in principle be studied with three-dimensional lattice simulations.Comment: 9 pages; misprint corrected, reference and small clarifications
added; to appear in Phys.Lett.
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