7,958 research outputs found
Physical properties of outflows: Comparing CO and H2O based parameters in Class 0 sources
Context. The observed physical properties of outflows from low-mass sources
put constraints on possible ejection mechanisms. Historically, these quantities
have been derived from CO using ground-based observations. It is thus important
to investigate whether parameters such as momentum rate (thrust) and mechanical
luminosity (power) are the same when different molecular tracers are used.
Aims. We aim at determining the outflow momentum, dynamical time-scale, thrust,
energy and power using CO and H2O as tracers of outflow activity. Methods.
Within the framework of the WISH key program, three molecular outflows from
Class 0 sources have been mapped using the HIFI instrument aboard Herschel. We
use these observations together with previously published H2 data to infer the
physical properties of the outflows. We compare the physical properties derived
here with previous estimates based on CO observations. Results. Inspection of
the spatial distribution of H2O and H2 confirms that these molecules are
co-spatial. The most prominent emission peaks in H2 coincide with strong H2O
emission peaks and the estimated widths of the flows when using the two tracers
are comparable. Conclusions. For the momentum rate and the mechanical
luminosity, inferred values are independent of which tracer that is used, i.e.,
the values agree to within a factor of 4 and 3 respectively.Comment: Accepted for publication in A&A, 5 pages, 2 figure
Magnetoresistance of Three-Constituent Composites: Percolation Near a Critical Line
Scaling theory, duality symmetry, and numerical simulations of a random
network model are used to study the magnetoresistance of a
metal/insulator/perfect conductor composite with a disordered columnar
microstructure. The phase diagram is found to have a critical line which
separates regions of saturating and non-saturating magnetoresistance. The
percolation problem which describes this line is a generalization of
anisotropic percolation. We locate the percolation threshold and determine the
t = s = 1.30 +- 0.02, nu = 4/3 +- 0.02, which are the same as in
two-constituent 2D isotropic percolation. We also determine the exponents which
characterize the critical dependence on magnetic field, and confirm numerically
that nu is independent of anisotropy. We propose and test a complete scaling
description of the magnetoresistance in the vicinity of the critical line.Comment: Substantially revised version; description of behavior in finite
magnetic fields added. 7 pages, 7 figures, submitted to PR
Detailed modelling of the circumstellar molecular line emission of the S-type AGB star W Aquilae
S-type AGB stars have a C/O ratio which suggests that they are transition
objects between oxygen-rich M-type stars and carbon-rich C-type stars. As such,
their circumstellar compositions of gas and dust are thought to be sensitive to
their precise C/O ratio, and it is therefore of particular interest to examine
their circumstellar properties.
We present new Herschel HIFI and PACS sub-millimetre and far-infrared line
observations of several molecular species towards the S-type AGB star W Aql. We
use these observations, which probe a wide range of gas temperatures, to
constrain the circumstellar properties of W Aql, including mass-loss rate and
molecular abundances. We used radiative transfer codes to model the
circumstellar dust and molecular line emission to determine circumstellar
properties and molecular abundances. We assumed a spherically symmetric
envelope formed by a constant mass-loss rate driven by an accelerating wind.
Our model includes fully integrated H2O line cooling as part of the solution of
the energy balance. We detect circumstellar molecular lines from CO, H2O, SiO,
HCN, and, for the first time in an S-type AGB star, NH3. The radiative transfer
calculations result in an estimated mass-loss rate for W Aql of 4.0e-6 Msol
yr-1 based on the 12CO lines. The estimated 12CO/13CO ratio is 29, which is in
line with ratios previously derived for S-type AGB stars. We find an H2O
abundance of 1.5e-5, which is intermediate to the abundances expected for M and
C stars, and an ortho/para ratio for H2O that is consistent with formation at
warm temperatures. We find an HCN abundance of 3e-6, and, although no CN lines
are detected using HIFI, we are able to put some constraints on the abundance,
6e-6, and distribution of CN in W Aql's circumstellar envelope using
ground-based data. We find an SiO abundance of 3e-6, and an NH3 abundance of
1.7e-5, confined to a small envelope.Comment: 17 pages, 15 figure
Preliminary results on SiO v=3 J=1-0 maser emission from AGB stars
We present the results of SiO maser observations at 43GHz toward two AGB
stars using the VLBA. Our preliminary results on the relative positions of the
different J=1-0 SiO masers (v=1,2 and 3) indicate that the current ideas on SiO
maser pumping could be wrong at some fundamental level. A deep revision of the
SiO pumping models could be necessary.Comment: poster, 2 pages, 2 figures, Proc. IAU Symp. 287 "Cosmic Masers: from
OH to H0", R.S. Booth, E.M.L. Humphreys and W.H.T. Vlemmings, ed
SiO masers from AGB stars in the vibrationally excited v=1,v=2, and v=3 states
The v=1 and v=2 J=1-0 (43 GHz), and v=1 J=2-1 (86 GHz) SiO masers are intense
in AGB stars and have been mapped using VLBI showing ring-like distributions.
Those of the v=1, v=2 J=1-0 masers are similar, but the spots are rarely
coincident, while the v=1 J=2-1 maser arises from a well separated region
farther out. These relative locations can be explained by models tools that
include the overlap of two IR lines of SiO and H2O. The v=3 J=1-0 line is not
directly affected by any line overlap and its spot structure and position,
relative to the other lines, is a good test to the standard pumping models. We
present single-dish and simultaneous VLBI observations of the v=1, v=2, and v=3
J=1-0 maser transitions of 28SiO in several AGB stars. The spatial distribution
of the SiO maser emission in the v=3 J=1-0 transition from AGB stars is
systematically composed of a series of spots that occupy a ring-like structure.
The overall ring structure is extremely similar to that found in the other 43
GHz transitions and is very different from the structure of the v=1 J=2-1
maser. The positions of the individual spots of the different 43 GHz lines are,
however, very rarely coincident, which in general is separated by about 0.3 AU
(between 1 and 5 mas). These results are very difficult to reconcile with
standard pumping models, which predict that the masers of rotational
transitions within a given vibrational state require very similar excitation
conditions, while the transitions of different vibrational states should appear
in different positions. However, models including line overlap tend to predict
v=1, v=2, v=3 J=1-0 population inversion to occur under very similar
conditions, while the requirements for v=1 J=2-1 appear clearly different, and
are compatible with the observational results.Comment: 9 pages, 4 figures accepted by A&
Semi-classical twists for sl(3) and sl(4) boundary r-matrices of Cremmer-Gervais type
We obtain explicit formulas for the semi-classical twists deforming the
coalgebraic structure of U(sl(3)) and U(sl(4)). In rank 2 and 3 the
corresponding universal R-matrices quantize the boundary r-matrices of
Cremmer-Gervais type defining Lie Frobenius structures on the maximal parabolic
subalgebras in sl(n)
Fiber Optic Tactical Local Network (FOTLAN)
A 100 Mbit/s FDDI (fiber distributed data interface) network interface unit is described that supports real-time data, voice and video. Its high-speed interrupt-driven hardware architecture efficiently manages stream and packet data transfer to the FDDI network. Other enhancements include modular single-mode laser-diode fiber optic links to maximize node spacing, optic bypass switches for increased fault tolerance, and a hardware performance monitor to gather real-time network diagnostics
Lectures on Non-BPS Dirichlet branes
A comprehensive introduction to the boundary state approach to Dirichlet
branes is given. Various examples of BPS and non-BPS Dirichlet branes are
discussed. In particular, the non-BPS states in the duality of Type IIA on K3
and the heterotic string on T4 are analysed in detail.Comment: 46 pages, 5 figures, LaTeX; lectures given at the TMR network school
on `Quantum aspects of gauge theories, supersymmetry and quantum gravity',
Torino, 26 January - 2 February 2000, and at the `Spring workshop on
Superstrings and related matters', Trieste, 27 March - 4 April 2000;
references adde
Detection of interstellar hydrogen peroxide
The molecular species hydrogen peroxide, HOOH, is likely to be a key
ingredient in the oxygen and water chemistry in the interstellar medium. Our
aim with this investigation is to determine how abundant HOOH is in the cloud
core {\rho} Oph A. By observing several transitions of HOOH in the
(sub)millimeter regime we seek to identify the molecule and also to determine
the excitation conditions through a multilevel excitation analysis. We have
detected three spectral lines toward the SM1 position of {\rho} Oph A at
velocity-corrected frequencies that coincide very closely with those measured
from laboratory spectroscopy of HOOH. A fourth line was detected at the
4{\sigma} level. We also found through mapping observations that the HOOH
emission extends (about 0.05 pc) over the densest part of the {\rho} Oph A
cloud core. We derive an abundance of HOOH relative to that of H_2 in the SM1
core of about 1\times10^(-10). To our knowledge, this is the first reported
detection of HOOH in the interstellar medium.Comment: 5 pages, 4 figures, accepted for publication in Astronomy &
Astrophysics, new version corrects a typo in Table 1 (and consequently in Fig
4
Revealing the environs of the remarkable southern hot core G327.3-0.6
We present a submm study of the massive hot core G327.3-0.6 that constrains
its physical parameters and environment. The APEX telescope was used to image
CO and N2H+ emission, to observe lines from other molecules toward a hot and a
cold molecular core, and to measure the continuum flux density of the hot core.
In the C18O J=3-2 line, two clumps were found, one associated with the HII
region G327.3-0.5 and the other associated with the hot core. An additional
cold clump is found 30 arcsec (0.4 pc) northeast of the hot core in bright N2H+
emission. From the the continuum data, we calculate a mass of 420 Msol and a
size of 0.1 pc for the hot core. A new, more accurate position of the hot core
is reported, which allows the association of the core with a bright
mid-infrared source. The luminosity of the hot core is estimated to be between
5 and 15 10^4 Lsol. This study revealed several different evolutionary stages
of massive star formation in the G327.3-0.6 region.Comment: APEX A&A special issue, accepte
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