38,546 research outputs found
The mass function of hydrogen-rich white dwarfs: robust observational evidence for a distinctive high-mass excess near 1Msun
The mass function of hydrogen-rich atmosphere white dwarfs has been
frequently found to reveal a distinctive high-mass excess near 1Msun. However,
a significant excess of massive white dwarfs has not been detected in the mass
function of the largest white dwarf catalogue to date from the Sloan Digital
Sky Survey. Hence, whether a high-mass excess exists or not has remained an
open question. In this work we build the mass function of the latest catalogue
of data release 10 SDSS hydrogen-rich white dwarfs, including the cool and
faint population (i.e. effective temperatures 6,000 <~ Teff <~ 12,000 K,
equivalent to 12 mag <~ Mbol <~ 13 mag). We show that the high-mass excess is
clearly present in our mass function, and that it disappears only if the
hottest (brightest) white dwarfs (those with Teff >~ 12,000 K, Mbol <~ 12 mag)
are considered. This naturally explains why previous SDSS mass functions failed
at detecting a significant excess of high-mass white dwarfs. Thus, our results
provide additional and robust observational evidence for the existence of a
distinctive high-mass excess near 1Msun. We investigate possible origins of
this feature and argue that the most plausible scenario that may lead to an
observed excess of massive white dwarfs is the merger of the degenerate core of
a giant star with a main sequence or a white dwarf companion during or shortly
after a common envelope event.Comment: Accepted for publication by MNRA
Measurement of the Probability Distribution of Total Transmission in Random Waveguides
Measurements have been made of the probability distribution of total
transmission of microwave radiation in waveguides filled with randomly
positioned scatterers which would have values of the dimensionless conductance
g near unity. The distributions are markedly non-Gaussian and have exponential
tails. The measured distributions are accurately described by diagrammatic and
random matrix calculations carried out for nonabsorbing samples in the limit g
>> 1 when g is expressed in terms of the variance of the distribution, which
equals the degree of long-range intensity correlation across the output face of
the sample.Comment: 5 pages, 5 post script figures, RevTe
Monitoring luminous yellow massive stars in M33: new yellow hypergiant candidates
The evolution of massive stars surviving the red supergiant (RSG) stage
remains unexplored due to the rarity of such objects. The yellow hypergiants
(YHGs) appear to be the warm counterparts of post-RSG classes located near the
Humphreys-Davidson upper luminosity limit, which are characterized by
atmospheric instability and high mass-loss rates. We aim to increase the number
of YHGs in M33 and thus to contribute to a better understanding of the
pre-supernova evolution of massive stars. Optical spectroscopy of five
dust-enshrouded YSGs selected from mid-IR criteria was obtained with the goal
of detecting evidence of extensive atmospheres. We also analyzed BVI photometry
for 21 of the most luminous YSGs in M33 to identify changes in the spectral
type. To explore the properties of circumstellar dust, we performed SED-fitting
of multi-band photometry of the 21 YSGs. We find three luminous YSGs in our
sample to be YHG candidates, as they are surrounded by hot dust and are
enshrouded within extended, cold dusty envelopes. Our spectroscopy of star 2
shows emission of more than one H component, as well as emission of
CaII, implying an extended atmospheric structure. In addition, the long-term
monitoring of the star reveals a dimming in the visual light curve of amplitude
larger than 0.5 mag that caused an apparent drop in the temperature that
exceeded 500 K. We suggest the observed variability to be analogous to that of
the Galactic YHG Cas. Five less luminous YSGs are suggested as post-RSG
candidates showing evidence of hot or/and cool dust emission. We demonstrate
that mid-IR photometry, combined with optical spectroscopy and time-series
photometry, provide a robust method for identifying candidate YHGs. Future
discovery of YHGs in Local Group galaxies is critical for the study of the late
evolution of intermediate-mass massive stars.Comment: 24 pages, 12 figures, 7 Tables. A&A in pres
Echo Emission From Dust Scattering and X-Ray Afterglows of Gamma-Ray Bursts
We investigate the effect of X-ray echo emission in gamma-ray bursts (GRBs).
We find that the echo emission can provide an alternative way of understanding
X-ray shallow decays and jet breaks. In particular, a shallow decay followed by
a "normal" decay and a further rapid decay of X-ray afterglows can be together
explained as being due to the echo from prompt X-ray emission scattered by dust
grains in a massive wind bubble around a GRB progenitor. We also introduce an
extra temporal break in the X-ray echo emission. By fitting the afterglow light
curves, we can measure the locations of the massive wind bubbles, which will
bring us closer to finding the mass loss rate, wind velocity, and the age of
the progenitors prior to the GRB explosions.Comment: 25 pages, 3 figures, 2 tables. Accepted for publication in Ap
Rotationally invariant family of L\'evy like random matrix ensembles
We introduce a family of rotationally invariant random matrix ensembles
characterized by a parameter . While corresponds to
well-known critical ensembles, we show that describes "L\'evy
like" ensembles, characterized by power law eigenvalue densities. For the density is bounded, as in Gaussian ensembles, but
describes ensembles characterized by densities with long tails. In particular,
the model allows us to evaluate, in terms of a novel family of orthogonal
polynomials, the eigenvalue correlations for L\'evy like ensembles. These
correlations differ qualitatively from those in either the Gaussian or the
critical ensembles.Comment: 9 pages, 5 figure
Field and intensity correlations in random media
Measurements of the microwave field transmitted through a random medium
allows direct access to the field correlation function, whose complex square is
the short range or C1 contribution to the intensity correlation function C. The
frequency and spatial correlation function are compared to their Fourier pairs,
the time of flight distribution and the specific intensity, respectively. The
longer range contribution to intensity correlation is obtained directly by
subtracting C1 from C and is in good agreement with theory.Comment: 9 pages, 5 figures, submitted to Phys.Rev.
Semantic Privacy Policies for Service Description and Discovery in Service-Oriented Architecture
Privacy preservation in Service-Oriented Architecture (SOA) is an open problem. This paper focuses on the areas of service description and discovery. The problems in these areas are that currently it is not possible to describe how a service provider deals with information received from a service consumer as well as discover a service that satisfies the privacy preferences of a consumer. There is currently no framework which offers a solution that supports a rich description of privacy policies and their integration in the process of service discovery. Thus, the main goal of this paper is to propose a privacy preservation framework for the areas of service description and discovery in SOA. The framework enhances service description and discovery with the specification and intersection of privacy policies using a base and domain-specific privacy ontologies. Moreover, the framework extends SOA to include roles responsible for implementing a privacy registry as well as mediating the interactions between service consumers and providers and the privacy preservation component
Supersymmetric exact sequence, heat kernel and super KdV hierarchy
We introduce the free N=1 supersymmetric derivation ring and prove the
existence of an exact sequence of supersymmetric rings and linear
transformations. We apply necessary and sufficient conditions arising from this
exact supersymmetric sequence to obtain the essential relations between
conserved quantities, gradients and the N=1 super KdV hierarchy. We combine
this algebraic approach with an analytic analysis of the super heat operator.We
obtain the explicit expression for the Green's function of the super heat
operator in terms of a series expansion and discuss its properties. The
expansion is convergent under the assumption of bounded bosonic and fermionic
potentials. We show that the asymptotic expansion when of the Green's
function for the super heat operator evaluated over its diagonal generates all
the members of the N=1 super KdV hierarchy.Comment: 20 pages, to be published in JM
Recent Technological Developments on LGAD and iLGAD Detectors for Tracking and Timing Applications
This paper reports the last technological development on the Low Gain
Avalanche Detector (LGAD) and introduces a new architecture of these detectors
called inverse-LGAD (iLGAD). Both approaches are based on the standard
Avalanche Photo Diodes (APD) concept, commonly used in optical and X-ray
detection applications, including an internal multiplication of the charge
generated by radiation. The multiplication is inherent to the basic n++-p+-p
structure, where the doping profile of the p+ layer is optimized to achieve
high field and high impact ionization at the junction. The LGAD structures are
optimized for applications such as tracking or timing detectors for high energy
physics experiments or medical applications where time resolution lower than 30
ps is required. Detailed TCAD device simulations together with the electrical
and charge collection measurements are presented through this work.Comment: Keywords: silicon detectors, avalanche multiplication, timing
detectors, tracking detectors. 8 pages. 8 Figure
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