11,169 research outputs found
Measuring Fundamental Parameters of Substellar Objects. II: Masses and Radii
We present mass and radius derivations for a sample of very young, mid- to
late M, low-mass stellar and substellar objects in Upper Sco and Taurus. In a
previous paper, we determined effective temperatures and surface gravities for
these targets, from an analysis of their high-resolution optical spectra and
comparisons to the latest synthetic spectra. We now derive extinctions, radii,
masses and luminosities by combining our previous results with observed
photometry, surface fluxes from the synthetic spectra and the known cluster
distances. These are the first mass and radius estimates for young, very low
mass bodies that are independent of theoretical evolutionary models (though our
estimates do depend on spectral modeling). We find that for most of our sample,
our derived mass-radius and mass-luminosity relationships are in very good
agreement with the theoretical predictions. However, our results diverge from
the evolutionary model values for the coolest, lowest-mass targets: our
inferred radii and luminosities are significantly larger than predicted for
these objects at the likely cluster ages, causing them to appear much younger
than expected. We suggest that uncertainties in the evolutionary models - e.g.,
in the choice of initial conditions and/or treatment of interior convection -
may be responsible for this discrepancy. Finally, two of our late-M objects
(USco 128 and 130) appear to have masses close to the deuterium-fusion boundary
(9--14 Jupiters, within a factor of 2). This conclusion is primarily a
consequence of their considerable faintness compared to other targets with
similar extinction, spectral type and temperature (difference of 1 mag). Our
result suggests that the faintest young late-M or cooler objects may be
significantly lower in mass than the current theoretical tracks indicate.Comment: 54 pages, incl. 5 figs, accepted Ap
How Many Templates for GW Chirp Detection? The Minimal-Match Issue Revisited
In a recent paper dealing with maximum likelihood detection of gravitational
wave chirps from coalescing binaries with unknown parameters we introduced an
accurate representation of the no-signal cumulative distribution of the
supremum of the whole correlator bank. This result can be used to derive a
refined estimate of the number of templates yielding the best tradeoff between
detector's performance (in terms of lost signals among those potentially
detectable) and computational burden.Comment: submitted to Class. Quantum Grav. Typing error in eq. (4.8) fixed;
figure replaced in version
Quantum Friction of Micromechanical Resonators at Low Temperatures
Dissipation of micro- and nano-scale mechanical structures is dominated by
quantum-mechanical tunneling of two-level defects intrinsically present in the
system. We find that at high frequencies--usually, for smaller, micron-scale
structures--a novel mechanism of phonon pumping of two-level defects gives rise
to weakly temperature-dependent internal friction, , concomitant to the
effects observed in recent experiments. Due to their size, comparable to or
shorter than the emitted phonon wavelength, these structures suffer from
superradiance-enhanced dissipation by the collective relaxation of a large
number of two-level defects contained within the wavelength.Comment: To apear in Phys. Rev. Let
Detecting an association between Ray and Gravitational Wave Bursts
If -ray bursts (GRBs) are accompanied by gravitational wave bursts (GWBs) the correlated output of two gravitational wave detectors evaluated in the moments just prior to a GRB will differ from that evaluated at times not associated with a GRB. We can test for this difference independently of any model of the GWB signal waveform. If we invoke a model for the GRB source population and GWB radiation spectral density we can find a confidence interval or upper limit on the root-mean-square GWB signal amplitude in the detector waveband. To illustrate we adopt a simple, physically motivated model and estimate that initial LIGO detector observations coincident with 1000 GRBs could lead us to exclude, with 95% confidence, associated GWBs with $h_{RMS} be Gaussian or that any inter-detector correlated noise be measured or measurable; it does not require advanced or a priori knowledge of the source waveform; and the limits obtained on the wave-strength improve with the number of observed GRBs
Velocity of sound in relativistic heavy-ion collisions
We have studied the rapidity distribution of secondary hadrons produced in
nucleus-nucleus collisions at ultra-relativistic energies within the ambit of
the Landau's hydrodynamical model. A reasonable description of the data can
also be obtained by using the Bjorken's hydrodynamical model if the boost
invariance is restricted to a finite rapidity range. The sensitivity of the
hadronic spectra on the equation of state vis- a -vis the velocity of sound has
been discussed. The correlation between the velocity of sound and the
freeze-out temperature has been indicated. The effects of the non-zero widths
of various mesonic and baryonic degrees of freedom up to the mass value ~ 2.5
GeV is seen to be small.Comment: 9 pages and 11 figures. Major changes. To appear in Physical Review
Data analysis of continuous gravitational wave: All sky search and study of templates
We have studied the problem of all sky search in reference to continuous
gravitational wave particularly for such sources whose wave-form are known in
advance. We have made an analysis of the number of templates required for
matched filter analysis as applicable to these sources. We have employed the
concept of {\it fitting factor} {\it (FF)}; treating the source location as the
parameters of the signal manifold and have studied the matching of the signal
with templates corresponding to different source locations. We have
investigated the variation of FF with source location and have noticed a
symmetry in template parameters, and . It has been found
that the two different template values in source location, each in
and , have same {\it FF}. We have also computed the number of templates
required assuming the noise power spectral density to be flat. It is
observed that higher {\it FF} requires exponentially increasing large number of
templates.Comment: Accepted in MNRAS, 14 pages, 5 figure
Search algorithm for a gravitational wave signal in association with Gamma Ray Burst GRB030329 using the LIGO detectors
One of the brightest Gamma Ray Burst ever recorded, GRB030329, occurred
during the second science run of the LIGO detectors. At that time, both
interferometers at the Hanford, WA LIGO site were in lock and acquiring data.
The data collected from the two Hanford detectors was analyzed for the presence
of a gravitational wave signal associated with this GRB. This paper presents a
detailed description of the search algorithm implemented in the current
analysis.Comment: To appear in the Proceedings of 8th Gravitational Wave Data Analysis
Workshop (Milwaukee, WI) (Class. Quantum Grav.
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