507 research outputs found
Semi-relativistic approximation to gravitational radiation from encounters with nonspinning black holes
The capture of compact bodies by black holes in galactic nuclei is an
important prospective source for low frequency gravitational wave detectors,
such as the planned Laser Interferometer Space Antenna. This paper calculates,
using a semirelativistic approximation, the total energy and angular momentum
lost to gravitational radiation by compact bodies on very high eccentricity
orbits passing close to a supermassive, nonspinning black hole; these
quantities determine the characteristics of the orbital evolution necessary to
estimate the capture rate. The semirelativistic approximation improves upon
treatments which use orbits at Newtonian-order and quadrupolar radiation
emission, and matches well onto accurate Teukolsky simulations for low
eccentricity orbits. Formulae are presented for the semirelativistic energy and
angular momentum fluxes as a function of general orbital parameters.Comment: 27 pages, 12 figures; v2: revised manuscript includes small changes
to make paper consistent with published version; v3: a statement about how to
generalise our results to hyperbolic orbits was incorrect, new version
includes published erratum as an appendi
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Admittance measurement for tuning bi-ventricular pacemakers
An apparatus for treating a heart of a patient includes a first lead and at least a second lead for pacing the heart adapted to be in electrical communication with the heart. The apparatus includes a microcontroller in communication with the first and second leads which triggers the first lead at either different times or the same time from when the microcontroller triggers the second lead. Alternatively, the apparatus includes a microcontroller in communication with the first and second leads that determines heart volume, including stroke volume, end-systolic volume, and calculated values including ejection fraction, from admittance from signals from the first and second leads and uses the admittance as feedback to control heart volume ejected, as measured by stroke volume, calculated values such as ejection fraction, and control end-systolic volume, with respect to the first and second leads. A method for treating the heart of a patient.Board of Regents, University of Texas Syste
Adoption and Abandonment of Precision Soil Sampling in Cotton Production
Technology adoption in precision agriculture has received considerable attention, while abandonment has received little. Our objective was to identify factors motivating adoption and abandonment of precision soil sampling in cotton. Results indicate younger producers who farmed more cotton area, owned more of their cropland, planted more non-cotton area, used a computer, or used a Personal Digital Assistant (PDA) were more likely to adopt precision soil sampling. Those with more cotton area or who owned livestock were more likely to abandon, while those who used precision soil sampling longer, used a PDA, or used variable-rate fertilizer application were less likely to abandon.Crop Production/Industries,
Expansion of W 3(OH)
A direct measurement of the expansion of W 3(OH) is made by comparing Very
Large Array images taken about 10 yr apart. The expansion is anisotropic with a
typical speed of 3 to 5 km/s, indicating a dynamical age of only 2300 yr. These
observations are inconsistent with either the freely expanding shell model or a
simple bow shock model. The most favored model is a slowly expanding shell-like
HII region, with either a fast rarefied flow or another less massive diffuse
ionized region moving towards the observer. There is also a rapidly evolving
source near the projected center of emission, perhaps related to the central
star.Comment: LaTeX file, 28 pages, includes 8 figures. To appear in ApJ in
December 10 (1998) issue. Also available at
http://www.submm.caltech.edu/~kawamura/w3oh_pp.p
The Life and Death of Dense Molecular Clumps in the Large Magellanic Cloud
We report the results of a high spatial (parsec) resolution HCO+ (J = 1-0)
and HCN (J = 1-0) emission survey toward the giant molecular clouds of the star
formation regions N105, N113, N159, and N44 in the Large Magellanic Cloud. The
HCO+ and HCN observations at 89.2 and 88.6 GHz, respectively, were conducted in
the compact configuration of the Australia Telescope Compact Array. The
emission is imaged into individual clumps with masses between 10^2 and 10^4
solar masses and radii of <1 pc to ~2 pc. Many of the clumps are coincident
with indicators of current massive star formation, indicating that many of the
clumps are associated with deeply-embedded forming stars and star clusters. We
find that massive YSO-bearing clumps tend to be larger (>1 pc), more massive (M
> 10^3 solar masses), and have higher surface densities (~1 g cm^-2), while
clumps without signs of star formation are smaller (<1 pc), less massive (M <
10^3 solar masses), and have lower surface densities (~0.1 g cm^-2). The dearth
of massive (M >10^3 solar masses) clumps not bearing massive YSOs suggests the
onset of star formation occurs rapidly once the clump has attained physical
properties favorable to massive star formation. Using a large sample of LMC
massive YSO mid-IR spectra, we estimate that ~2/3 of the massive YSOs for which
there are Spitzer mid-IR spectra are no longer located in molecular clumps; we
estimate that these young stars/clusters have destroyed their natal clumps on a
time scale of at least 3 x 10^{5}$ yrs.Comment: Accepted to ApJ 3-19-201
A Case Study of Low-Mass Star Formation
This article synthesizes observational data from an extensive program aimed
toward a comprehensive understanding of star formation in a low-mass
star-forming molecular cloud. New observations and published data spanning from
the centimeter wave band to the near infrared reveal the high and low density
molecular gas, dust, and pre-main sequence stars in L1551.Comment: 24 pages, 21 figures, ApJS accepte
Turbulent Gas Flows in the Rosette and G216-2.5 Molecular Clouds: Assessing Turbulent Fragmentation Descriptions of Star Formation
The role of turbulent fragmentation in regulating the efficiency of star
formation in interstellar clouds is examined from new wide field imaging of
12CO and 13CO J=1-0 emission from the Rosette and G216-2.5 molecular clouds.
The Rosette molecular cloud is a typical star forming giant molecular cloud and
G215-2.5 is a massive molecular cloud with no OB stars and very little low mass
star formation. The properties of the turbulent gas flow are derived from the
set of eigenvectors and eigenimages generated by Principal Component Analysis
of the spectroscopic data cubes. While the two clouds represent quite divergent
states of star formation activity, the velocity structure functions for both
clouds are similar. The sonic scale, lambda_S, defined as the spatial scale at
which turbulent velocity fluctuations are equivalent to the local sound speed,
and the turbulent Mach number evaluated at 1 pc, M_{1pc}, are derived for an
ensemble of clouds including the Rosette and, G216-2.5 regions that span a
large range in star formation activity. We find no evidence for the positive
correlations between these quantities and the star formation efficiency, that
are predicted by turbulent fragmentation models. A correlation does exist
between the star formation efficiency and the sonic scale for a subset of
clouds with L_{FIR}/M(H_2) > 1 that are generating young stellar clusters.
Turbulent fragmentation must play a limited and non-exclusive role in
determining the yield of stellar masses within interstellar clouds.Comment: Accepted by ApJ, 22 pages, 7 figure
The Formation of Massive Stars from Turbulent Cores
Observations indicate that massive stars form in regions of very high surface
density, ~1 g cm^-2. Clusters containing massive stars and globular clusters
have a comparable column density. The total pressure in clouds of such a column
density is P/k~10^8-10^9 K cm^-3, far greater than that in the diffuse ISM or
the average in GMCs. Observations show that massive star-forming regions are
supersonically turbulent, and we show that the molecular cores out of which
individual massive stars form are as well. The protostellar accretion rate in
such a core is approximately equal to the instantaneous mass of the star
divided by the free-fall time of the gas that is accreting onto the star
(Stahler, Shu, & Taam 1980). The star-formation time in this Turbulent Core
model for massive star formation is several mean free-fall timesscales of the
core, but is about equal to that of the region in which the core is embedded.
The typical time for a massive star to form is about 10^5 yr and the accretion
rate is high enough to overcome radiation pressure due to the luminosity of the
star. For the typical case we consider, in which the cores out of which the
stars form have a density structure varying as r^{-1.5}, the protostellar
accretion rate grows linearly with time. We calculate the evolution of the
radius of a protostar and determine the accretion luminosity. At the high
accretion rates that are typical in regions of massive star formation,
protostars join the main sequence at about 20 solar masses. We apply these
results to predict the properties of protostars thought to be powering several
observed hot molecular cores, including the Orion hot core and W3(H2O). In the
Appendixes, we discuss the pressure in molecular clouds and we argue that
``logatropic'' models for molecular clouds are incompatible with observation.Comment: ApJ accepted; 28 pages, some clarification of the text, results
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