1,946 research outputs found
Asymptotics of self-similar solutions to coagulation equations with product kernel
We consider mass-conserving self-similar solutions for Smoluchowski's
coagulation equation with kernel with
. It is known that such self-similar solutions
satisfy that is bounded above and below as . In
this paper we describe in detail via formal asymptotics the qualitative
behavior of a suitably rescaled function in the limit . It turns out that as . As becomes larger
develops peaks of height that are separated by large regions
where is small. Finally, converges to zero exponentially fast as . Our analysis is based on different approximations of a nonlocal
operator, that reduces the original equation in certain regimes to a system of
ODE
Chasing the identification of ASCA Galactic Objects (ChIcAGO): An X-ray survey of unidentified sources in the galactic plane. I : Source sample and initial results
We present the Chasing the Identification of ASCA Galactic Objects (ChIcAGO) survey, which is designed to identify the unknown X-ray sources discovered during the ASCA Galactic Plane Survey (AGPS). Little is known about most of the AGPS sources, especially those that emit primarily in hard X-rays (2-10 keV) within the Fx 10-13 to 10-11 erg cm -2 s-1 X-ray flux range. In ChIcAGO, the subarcsecond localization capabilities of Chandra have been combined with a detailed multiwavelength follow-up program, with the ultimate goal of classifying the >100 unidentified sources in the AGPS. Overall to date, 93 unidentified AGPS sources have been observed with Chandra as part of the ChIcAGO survey. A total of 253 X-ray point sources have been detected in these Chandra observations within 3âČ of the original ASCA positions. We have identified infrared and optical counterparts to the majority of these sources, using both new observations and catalogs from existing Galactic plane surveys. X-ray and infrared population statistics for the X-ray point sources detected in the Chandra observations reveal that the primary populations of Galactic plane X-ray sources that emit in the Fx 10-13 to 10-11 erg cm -2 s-1 flux range are active stellar coronae, massive stars with strong stellar winds that are possibly in colliding wind binaries, X-ray binaries, and magnetars. There is also another primary population that is still unidentified but, on the basis of its X-ray and infrared properties, likely comprises partly Galactic sources and partly active galactic nuclei.Peer reviewedSubmitted Versio
Diamagnetic Suppression of Component Magnetic Reconnection at the Magnetopause
We present particle-in-cell simulations of collisionless magnetic
reconnection in a system (like the magnetopause) with a large density asymmetry
across the current layer. In the presence of an ambient component of the
magnetic field perpendicular to the reconnection plane the gradient creates a
diamagnetic drift that advects the X-line with the electron diamagnetic
velocity. When the relative drift between the ions and electrons is of the
order the Alfven speed the large scale outflows from the X-line necessary for
fast reconnection cannot develop and the reconnection is suppressed. We discuss
how these effects vary with both the plasma beta and the shear angle of the
reconnecting field and discuss observational evidence for diamagnetic
stabilization at the magnetopause.Comment: 10 pages, 10 figures; accepted by JGR; agu2001.cls and agu.bst
include
Grain Dynamics in a Two-dimensional Granular Flow
We have used particle tracking methods to study the dynamics of individual
balls comprising a granular flow in a small-angle two-dimensional funnel. We
statistically analyze many ball trajectories to examine the mechanisms of shock
propagation. In particular, we study the creation of, and interactions between,
shock waves. We also investigate the role of granular temperature and draw
parallels to traffic flow dynamics.Comment: 17 pages, 24 figures. To appear in Phys.Rev.E. High res./color
figures etc. on http://www.nbi.dk/CATS/Granular/GrainDyn.htm
X-ray view of IC348 in the light of an updated cluster census
We study the properties of the coronae of the low-mass stars in the young
(~2-3Myr), nearby (~310pc) open cluster IC348 combining X-ray and
optical/infrared data. The four existing Chandra observations of IC348 are
merged, thus providing a deeper and spatially more complete X-ray view than
previous X-ray studies of the cluster. We have compiled a comprehensive catalog
of IC348 members taking into account recent updates to the cluster census. Our
data collection comprises fundamental stellar parameters, infrared excess
indicating the presence of disks, Halpha emission as a tracer of chromospheric
emission or accretion and mass accretion rates. We have detected 290 X-ray
sources in four merged Chandra exposures, of which 187 are associated with
known cluster members. Only four of the X-ray sources are brown dwarfs
(spectral type M6 and later). The detection rate is highest for diskless Class
III stars and increases with stellar mass. This may be explained with higher
X-ray luminosities for higher mass and later evolutionary stage that is evident
in the X-ray luminosity functions. In particular, we find that for the lowest
examined masses (0.1-0.25 Msun) there is a difference between the X-ray
luminosity functions of accreting and non-accreting stars (classified on the
basis of their Halpha emission strength) as well as those of disk-bearing and
diskless stars (classified on the basis of the slope of the spectral energy
distribution). These differences disappear for higher masses. This is related
to our finding that the L_x/L_bol ratio is non-constant across the
mass/luminosity sequence of IC348 with a decrease towards lower luminosity
stars. Our analysis of an analogous stellar sample in the Orion Nebula Cluster
suggests that the decline of L_x/L_ bol for young stars at the low-mass end of
the stellar sequence is likely universal.Comment: Accepted for publication in Astronomy & Astrophysic
Single-epoch VLBI imaging study of bright active galactic nuclei at 2 and 8 GHz
We investigate statistical and individual astrophysical properties of active
galactic nuclei (AGNs), such as parsec-scale flux density, core dominance,
angular and linear sizes, maximum observed brightness temperatures of VLBI core
components, spectral index distributions for core and jet components, and
evolution of brightness temperature along the jets. Furthermore, we
statistically compare core flux densities and brightness temperature as well as
jet spectral indices of gamma-ray bright and weak sources. We used 19 very long
baseline interferometry (VLBI) observing sessions carried out simultaneously at
2.3 and 8.6 GHz with the participation of 10 Very Long Baseline Array (VLBA)
stations and up to 10 additional geodetic telescopes. The observations span the
period 1998-2003. We present here single-epoch results from high-resolution
radio observations of 370 AGNs. Our VLBI images at 2.3 and 8.6 GHz as well as
Gaussian models are presented and analyzed. At least one-fourth of the cores
are completely unresolved on the longest baselines of the global VLBI
observations. The VLBI core components are partially opaque with the median
value of spectral index of alpha_core=0.3, while the jet features are usually
optically thin alpha_jet=-0.7. The spectral index typically decreases along the
jet ridge line owing to the spectral aging, with a median value of -0.05
mas^-1. Brightness temperatures are found to be affected by Doppler boosting
and reaching up to \sim10^13 K with a median of \sim2.5x10^11 K at both
frequencies. The brightness temperature gradients along the jets typically
follow a power law T_b\simr^-2.2 at both frequencies. 147 sources (40%)
positionally associated with gamma-ray detections from the Fermi LAT Second
Source Catalog have higher core flux densities and brightness temperatures, and
are characterized by the less steep radio spectrum of the optically thin jet
emission.Comment: 15 pages, 21 figures, accepted by A&A on May 22, 2012, machine
readable tables are available from the source of the pape
Keck Observations of the Young Metal-Poor Host Galaxy of the Super-Chandrasekhar-Mass Type Ia Supernova SN 2007if
We present Keck LRIS spectroscopy and -band photometry of the metal-poor,
low-luminosity host galaxy of the super-Chandrasekhar mass Type Ia supernova SN
2007if. Deep imaging of the host reveals its apparent magnitude to be
, which at the spectroscopically-measured redshift of
corresponds to an absolute magnitude of
. Galaxy color constrains the mass-to-light ratio,
giving a host stellar mass estimate of . Balmer
absorption in the stellar continuum, along with the strength of the 4000\AA\
break, constrain the age of the dominant starburst in the galaxy to be
Myr, corresponding to a main-sequence
turn-off mass of . Using the R method of
calculating metallicity from the fluxes of strong emission lines, we determine
the host oxygen abundance to be ,
significantly lower than any previously reported spectroscopically-measured
Type Ia supernova host galaxy metallicity. Our data show that SN 2007if is very
likely to have originated from a young, metal-poor progenitor.Comment: 15 pages, 9 figures; accepted for publication in Ap
Caltech Core-Collapse Project (CCCP) observations of type IIn supernovae: typical properties and implications for their progenitor stars
Type IIn Supernovae (SNe IIn) are rare events, constituting only a few
percent of all core-collapse SNe, and the current sample of well observed SNe
IIn is small. Here, we study the four SNe IIn observed by the Caltech
Core-Collapse Project (CCCP). The CCCP SN sample is unbiased to the extent that
object selection was not influenced by target SN properties. Therefore, these
events are representative of the observed population of SNe IIn. We find that a
narrow P-Cygni profile in the hydrogen Balmer lines appears to be a ubiquitous
feature of SNe IIn. Our light curves show a relatively long rise time (>20
days) followed by a slow decline stage (0.01 to 0.15 mag/day), and a typical
V-band peak magnitude of M_V=-18.4 +/- 1.0 mag. We measure the progenitor star
wind velocities (600 - 1400 km/s) for the SNe in our sample and derive
pre-explosion mass loss rates (0.026 - 0.12 solar masses per year). We compile
similar data for SNe IIn from the literature, and discuss our results in the
context of this larger sample. Our results indicate that typical SNe IIn arise
from progenitor stars that undergo LBV-like mass-loss shortly before they
explode.Comment: ApJ, submitte
A Featureless Infrared Transmission Spectrum for the Super-puff Planet Kepler-79d
Extremely low-density planets ("super-puffs") are a small but intriguing subset of the transiting planet population. With masses in the super-Earth range (1 â 10 M_â) and radii akin to those of giant planets (> 4 R_â), their large envelopes may have been accreted beyond the water snow line and many appear to be susceptible to catastrophic mass loss. Both the presence of water and the importance of mass loss can be explored using transmission spectroscopy. Here, we present new Hubble space telescope WFC3 spectroscopy and updated Kepler transit depth measurements for the super-puff Kepler-79d. We do not detect any molecular absorption features in the 1.1 â 1.7 ÎŒm WFC3 bandpass, and the combined Kepler and WFC3 data are consistent with a flat-line model, indicating the presence of aerosols in the atmosphere. We compare the shape of Kepler-79d's transmission spectrum to predictions from a microphysical haze model that incorporates an outward particle flux due to ongoing mass loss. We find that photochemical hazes offer an attractive explanation for the observed properties of super-puffs like Kepler-79d, as they simultaneously render the near-infrared spectrum featureless and reduce the inferred envelope mass-loss rate by moving the measured radius (optical depth unity surface during transit) to lower pressures. We revisit the broader question of mass-loss rates for super-puffs and find that the age estimates and mass-loss rates for the majority of super-puffs can be reconciled if hazes move the photosphere from the typically assumed pressure of ~10 mbar to ~10 ”bar
Profile shape stability and phase jitter analyses of millisecond pulsars
Millisecond pulsars (MSPs) have been studied in detail since their discovery
in 1982. The integrated pulse profiles of MSPs appear to be stable, which
enables precision monitoring of the pulse times of arrival (TOAs). However, for
individual pulses the shape and arrival phase can vary dramatically, which is
known as pulse jitter. In this paper, we investigate the stability of
integrated pulse profiles for 5 MSPs, and estimate the amount of jitter for PSR
J0437-4715. We do not detect intrinsic profile shape variation based on
integration times from ~10 to ~100 s with the provided instrumental
sensitivity. For PSR J0437-4715 we calculate the jitter parameter to be
f_J=0.067+-0.002, and demonstrate that the result is not significantly affected
by instrumental TOA uncertainties. Jitter noise is also found to be independent
of observing frequency and bandwidth around 1.4 GHz on frequency scales of <100
MHz, which supports the idea that pulses within narrow frequency scale are
equally jittered. In addition, we point out that pulse jitter would limit TOA
calculation for the timing observations with future telescopes like the Square
Kilometre Array and the Five hundred metre Aperture Spherical Telescope. A
quantitative understanding of pulse profile stability and the contribution of
jitter would enable improved TOA calculations, which are essential for the
ongoing endeavours in pulsar timing, such as the detection of the stochastic
gravitational wave background.Comment: 9 pages, 9 figures, 3 Tables, accepted by MNRA
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