614 research outputs found
Gravitational Waves from Axisymmetric, Rotational Stellar Core Collapse
We have carried out an extensive set of two-dimensional, axisymmetric,
purely-hydrodynamic calculations of rotational stellar core collapse with a
realistic, finite-temperature nuclear equation of state and realistic massive
star progenitor models. For each of the total number of 72 different
simulations we performed, the gravitational wave signature was extracted via
the quadrupole formula in the slow-motion, weak-field approximation. We
investigate the consequences of variation in the initial ratio of rotational
kinetic energy to gravitational potential energy and in the initial degree of
differential rotation. Furthermore, we include in our model suite progenitors
from recent evolutionary calculations that take into account the effects of
rotation and magnetic torques. For each model, we calculate gravitational
radiation wave forms, characteristic wave strain spectra, energy spectra, final
rotational profiles, and total radiated energy. In addition, we compare our
model signals with the anticipated sensitivities of the 1st- and 2nd-generation
LIGO detectors coming on line. We find that most of our models are detectable
by LIGO from anywhere in the Milky Way.Comment: 13 pages, 22 figures, accepted for publication in ApJ (v600, Jan.
2004). Revised version: Corrected typos and minor mistakes in text and
references. Minor additions to the text according to the referee's
suggestions, conclusions unchange
The density, the cosmic microwave background and the proton-to-electron mass ratio in a cloud at redshift 0.9
Based on measurements with the Effelsberg 100-m telescope, a multi-line study of molecular species is presented toward the gravitational lens system PKS 1830–211, which is by far the best known target to study dense cool gas in absorption at intermediate redshift. Determining average radial velocities and performing Large Velocity Gradient radiative transfer calculations, the aims of this study are (1) to determine the density of the gas, (2) to constrain the temperature of the cosmic microwave background (CMB), and (3) to evaluate the proton-to-electron mass ratio at redshift z ∼ 0.89. Analyzing data from six rotational HC_3N transitions (this includes the J = 7 ← 6 line, which is likely detected for the first time in the interstellar medium) we obtain n(H_2) ∼ 2600 cm^(−3) for the gas density of the south-western absorption component, assuming a background source covering factor, which is independent of frequency. With a possibly more realistic frequency dependence proportional to ν^(0.5) (the maximal exponent permitted by observational boundary conditions), n(H2) ∼ 1700 cm^(−3). Again toward the south-western source, excitation temperatures of molecular species with optically thin lines and higher rotational constants are, on average, consistent with the expected temperature of the cosmic microwave background, T^(CMB) = 5.14 K. However, individually, there is a surprisingly large scatter which far surpasses expected uncertainties. A comparison of CS J = 1 ← 0 and 4 ← 3 optical depths toward the weaker north-western absorption component results in T_(ex) = 11 K and a 1-σ error of 3 K. For the main component, a comparison of velocities determined from ten optically thin NH_3 inversion lines with those from five optically thin rotational transitions of HC_3N, observed at similar frequencies, constrains potential variations of the proton-to-electron mass ratio μ to Δμ/μ < 1.4 × 10^(−6) with 3-σ confidence. Also including optically thin rotational lines from other molecular species, it is emphasized that systematic errors are ΔV < 1 kms^(−1), corresponding to Δμ/μ < 1.0 × 10^(−6)
Redshifted formaldehyde from the gravitational lens B0218+357
The gravitational lens toward B0218+357 offers the unique possibility to
study cool moderately dense gas with high sensitivity and angular resolution in
a cloud that existed half a Hubble time ago. Observations of the radio
continuum and six formaldehyde (H2CO) lines were carried out with the VLA, the
Plateau de Bure interferometer, and the Effelsberg 100-m telescope. Three radio
continuum maps indicate a flux density ratio between the two main images, A and
B, of ~ 3.4 +/- 0.2. Within the errors the ratio is the same at 8.6, 14.1, and
43 GHz. The 1_{01}-0_{00} line of para-H2CO is shown to absorb the continuum of
image A. Large Velocity Gradient radiative transfer calculations are performed
to reproduce the optical depths of the observed two cm-wave "K-doublet" and
four mm-wave rotational lines. These calculations also account for a likely
frequency-dependent continuum cloud coverage. Confirming the diffuse nature of
the cloud, an n(H2) density of < 1000 cm^{-3} is derived, with the best fit
suggesting n(H2) ~ 200 cm^{-3}. The H2CO column density of the main velocity
component is ~5 * 10^{13} cm^{-2}, to which about 7.5 * 10^{12} cm^{-2} has to
be added to also account for a weaker feature on the blue side, 13 km/s apart.
N(H2CO)/N(NH3) ~ 0.6, which is four times less than the average ratio obtained
from a small number of local diffuse (galactic) clouds seen in absorption. The
ortho-to-para H2CO abundance ratio is 2.0 - 3.0, which is consistent with the
kinetic temperature of the molecular gas associated with the lens of B0218+357.
With the gas kinetic temperature and density known, it is found that optically
thin transitions of CS, HCN, HNC, HCO+, and N2H+ (but not CO) will provide
excellent probes of the cosmic microwave background at redshift z=0.68.Comment: Accepted for A&A, 8 Pages, 3 Figures, 5 Table
An ALMA Early Science survey of molecular absorption lines toward PKS1830-211 -- Analysis of the absorption profiles
We present the first results of an ALMA spectral survey of strong absorption
lines for common interstellar species in the z=0.89 molecular absorber toward
the lensed blazar PKS1830-211. The dataset brings essential information on the
structure and composition of the absorbing gas in the foreground galaxy. In
particular, we find absorption over large velocity intervals (gtrsim 100 km/s)
toward both lensed images of the blazar. This suggests either that the galaxy
inclination is intermediate and that we sample velocity gradients or streaming
motions in the disk plane, that the molecular gas has a large vertical
distribution or extraplanar components, or that the absorber is not a simple
spiral galaxy but might be a merger system. The number of detected species is
now reaching a total of 42 different species plus 14 different rare
isotopologues toward the SW image, and 14 species toward the NE line-of-sight.
The abundances of CH, H2O, HCO+, HCN, and NH3 relative to H2 are found to be
comparable to those in the Galactic diffuse medium. Of all the lines detected
so far toward PKS1830-211, the ground-state line of ortho-water has the deepest
absorption. We argue that ground-state lines of water have the best potential
for detecting diffuse molecular gas in absorption at high redshift.Comment: Accepted for publication in A&
Sub-millimeter Observations of Giant Molecular Clouds in the Large Magellanic Cloud: Temperature and Density as Determined from J=3-2 and J=1-0 transitions of CO
We have carried out sub-mm 12CO(J=3-2) observations of 6 giant molecular
clouds (GMCs) in the Large Magellanic Cloud (LMC) with the ASTE 10m sub-mm
telescope at a spatial resolution of 5 pc and very high sensitivity. We have
identified 32 molecular clumps in the GMCs and revealed significant details of
the warm and dense molecular gas with n(H2) 10 cm and
Tkin 60 K. These data are combined with 12CO(J=1-0) and 13CO(J=1-0)
results and compared with LVG calculations. We found that the ratio of
12CO(J=3-2) to 12CO(J=1-0) emission is sensitive to and is well correlated with
the local Halpha flux. We interpret that differences of clump propeties
represent an evolutionary sequence of GMCs in terms of density increase leading
to star formation.Type I and II GMCs (starless GMCs and GMCs with HII regions
only, respectively) are at the young phase of star formation where density does
not yet become high enough to show active star formation and Type III GMCs
(GMCs with HII regions and young star clusters) represents the later phase
where the average density is increased and the GMCs are forming massive stars.
The high kinetic temperature correlated with \Halpha flux suggests that FUV
heating is dominant in the molecular gas of the LMC.Comment: 74 pages, including 41 figures, accepted for publication in ApJ
Detection of HCN Direct l-Type Transitions Probing Hot Molecular Gas in the Proto-Planetary Nebula CRL 618
We report the detection of direct l-type transitions towards the
proto-planetary nebula CRL 618 during a study of vibrationally excited carbon
chains. The J=8,10,11,12,13,14 DJ=0 transitions of HCN in its first excited
bending mode v2=1 were detected in absorption against the continuum of the
central HII region making use of the Effelsberg 100m telescope and the Very
Large Array. Additionally, the J=9 direct l-type transition was detected in
emission presumably indicating a weak maser. All lines are blueshifted with
respect to the systemic velocity of CRL 618 indicating that the lines originate
from a hot, expanding circumstellar envelope. The HCN column density along the
line of sight in front of the continuum is 2E18 cm^-2.Comment: 14 pages, 4 figures, to appear in Ap
Atomic diffraction from nanostructured optical potentials
We develop a versatile theoretical approach to the study of cold-atom
diffractive scattering from light-field gratings by combining calculations of
the optical near-field, generated by evanescent waves close to the surface of
periodic nanostructured arrays, together with advanced atom wavepacket
propagation on this optical potential.Comment: 8 figures, 10 pages, submitted to Phys. Rev.
An Overview of the 2014 ALMA Long Baseline Campaign
A major goal of the Atacama Large Millimeter/submillimeter Array (ALMA) is to
make accurate images with resolutions of tens of milliarcseconds, which at
submillimeter (submm) wavelengths requires baselines up to ~15 km. To develop
and test this capability, a Long Baseline Campaign (LBC) was carried out from
September to late November 2014, culminating in end-to-end observations,
calibrations, and imaging of selected Science Verification (SV) targets. This
paper presents an overview of the campaign and its main results, including an
investigation of the short-term coherence properties and systematic phase
errors over the long baselines at the ALMA site, a summary of the SV targets
and observations, and recommendations for science observing strategies at long
baselines. Deep ALMA images of the quasar 3C138 at 97 and 241 GHz are also
compared to VLA 43 GHz results, demonstrating an agreement at a level of a few
percent. As a result of the extensive program of LBC testing, the highly
successful SV imaging at long baselines achieved angular resolutions as fine as
19 mas at ~350 GHz. Observing with ALMA on baselines of up to 15 km is now
possible, and opens up new parameter space for submm astronomy.Comment: 11 pages, 7 figures, 2 tables; accepted for publication in the
Astrophysical Journal Letters; this version with small changes to
affiliation
Implications For The Origin Of GRB 051103 From LIGO Observations
We present the results of a LIGO search for gravitational waves (GWs)
associated with GRB 051103, a short-duration hard-spectrum gamma-ray burst
(GRB) whose electromagnetically determined sky position is coincident with the
spiral galaxy M81, which is 3.6 Mpc from Earth. Possible progenitors for
short-hard GRBs include compact object mergers and soft gamma repeater (SGR)
giant flares. A merger progenitor would produce a characteristic GW signal that
should be detectable at the distance of M81, while GW emission from an SGR is
not expected to be detectable at that distance. We found no evidence of a GW
signal associated with GRB 051103. Assuming weakly beamed gamma-ray emission
with a jet semi-angle of 30 deg we exclude a binary neutron star merger in M81
as the progenitor with a confidence of 98%. Neutron star-black hole mergers are
excluded with > 99% confidence. If the event occurred in M81 our findings
support the the hypothesis that GRB 051103 was due to an SGR giant flare,
making it the most distant extragalactic magnetar observed to date.Comment: 8 pages, 3 figures. For a repository of data used in the publication,
go to: https://dcc.ligo.org/cgi-bin/DocDB/ShowDocument?docid=15166 . Also see
the announcement for this paper on ligo.org at:
http://www.ligo.org/science/Publication-GRB051103/index.ph
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