108 research outputs found
Growth and migration of solids in evolving protostellar disks I: Methods and Analytical tests
This series of papers investigates the early stages of planet formation by
modeling the evolution of the gas and solid content of protostellar disks from
the early T Tauri phase until complete dispersal of the gas. In this first
paper, I present a new set of simplified equations modeling the growth and
migration of various species of grains in a gaseous protostellar disk evolving
as a result of the combined effects of viscous accretion and photo-evaporation
from the central star. Using the assumption that the grain size distribution
function always maintains a power-law structure approximating the average
outcome of the exact coagulation/shattering equation, the model focuses on the
calculation of the growth rate of the largest grains only. The coupled
evolution equations for the maximum grain size, the surface density of the gas
and the surface density of solids are then presented and solved
self-consistently using a standard 1+1 dimensional formalism. I show that the
global evolution of solids is controlled by a leaky reservoir of small grains
at large radii, and propose an empirically derived evolution equation for the
total mass of solids, which can be used to estimate the total heavy element
retention efficiency in the planet formation paradigm. Consistency with
observation of the total mass of solids in the Minimum Solar Nebula augmented
with the mass of the Oort cloud sets strong upper limit on the initial grain
size distribution, as well as on the turbulent parameter \alphat. Detailed
comparisons with SED observations are presented in a following paper.Comment: Submitted to ApJ. 23 pages and 13 figure
Gamma Rays from Star Formation in Clusters of Galaxies
Star formation in galaxies is observed to be associated with gamma-ray
emission. The detection of gamma rays from star-forming galaxies by the Fermi
Large Area Telescope (LAT) has allowed the determination of a functional
relationship between star formation rate and gamma-ray luminosity (Ackermann
et. al. 2012). Since star formation is known to scale with total infrared
(8-1000 micrometers) and radio (1.4 GHz) luminosity, the observed infrared and
radio emission from a star-forming galaxy can be used to quantitatively infer
the galaxy's gamma-ray luminosity. Similarly, star forming galaxies within
galaxy clusters allow us to derive lower limits on the gamma-ray emission from
clusters, which have not yet been conclusively detected in gamma rays. In this
study we apply the relationships between gamma-ray luminosity and radio and IR
luminosities derived in Ackermann et. al. 2012 to a sample of galaxy clusters
from Ackermann et. al. 2010 in order to place lower limits on the gamma-ray
emission associated with star formation in galaxy clusters. We find that
several clusters have predicted lower limits on gamma-ray emission that are
within an order of magnitude of the upper limits derived in Ackermann et. al.
2010 based on non-detection by Fermi-LAT. Given the current gamma-ray limits,
star formation likely plays a significant role in the gamma-ray emission in
some clusters, especially those with cool cores. We predict that both Fermi-LAT
over the course of its lifetime and the future Cherenkov Telescope Array will
be able to detect gamma-ray emission from star-forming galaxies in clusters.Comment: 17 pages, 2 figures, 2 tables. Minor revisions made to match version
accepted to Ap
Low Risk Technique for Sample Acquisition from Remote and Hazardous Sites on a Comet
This paper describes a mission comet sampling strategy, known as CORSAIR (COmet Rendezvous, Sample Acquisition, Investigation, and Return), which was proposed for NASA New Frontiers 2017. The proposal was led by Applied Physics Lab (APL) with partners Goddard Space Flight Center (GSFC) and Deutsches Zentrum fr Luft- und Raumfahrt (DLR). The mission concept is to launch a projectile from a satellite that is capable of gathering a 300 cc sample. The projectile is tethered and is reeled back to the spacecraft after gathering the sample. Once back at the spacecraft, a robotic manipulator extracts the sample cartridge and places the cartridge into an earth return vehicle (ERV). This method has the following favorable characteristics: 1. Places the mission at minimal risk by isolating the spacecraft from the comet 2. Allows access to remote and otherwise inaccessible locations 3. Permits deep penetration into the surfac
Feedback Heating by Cosmic Rays in Clusters of Galaxies
Recent observations show that the cooling flows in the central regions of
galaxy clusters are highly suppressed. Observed AGN-induced cavities/bubbles
are a leading candidate for suppressing cooling, usually via some form of
mechanical heating. At the same time, observed X-ray cavities and synchrotron
emission point toward a significant non-thermal particle population. Previous
studies have focused on the dynamical effects of cosmic-ray pressure support,
but none have built successful models in which cosmic-ray heating is
significant. Here we investigate a new model of AGN heating, in which the
intracluster medium is efficiently heated by cosmic-rays, which are injected
into the ICM through diffusion or the shredding of the bubbles by
Rayleigh-Taylor or Kelvin-Helmholtz instabilities. We include thermal
conduction as well. Using numerical simulations, we show that the cooling
catastrophe is efficiently suppressed. The cluster quickly relaxes to a
quasi-equilibrium state with a highly reduced accretion rate and temperature
and density profiles which match observations. Unlike the conduction-only case,
no fine-tuning of the Spitzer conduction suppression factor f is needed. The
cosmic ray pressure, P_c/P_g <~ 0.1 and dP_c/dr <~ 0.1 \rho g, is well within
observational bounds. Cosmic ray heating is a very attractive alternative to
mechanical heating, and may become particularly compelling if GLAST detects the
gamma-ray signature of cosmic-rays in clusters.Comment: Revised version accepted for publication in MNRAS. Significantly
expanded discussion and new simulations exploring parameter space/model
robustness; conclusions unchange
Detection of diffuse TeV gamma-ray emission from the nearby starburst galaxy NGC 253
We report the TeV gamma-ray observations of the nearby normal spiral galaxy
NGC 253. At a distance of 2.5 Mpc, NGC 253 is one of the nearest
starburst galaxies. This relative closeness, coupled with the high star
formation rate in the galaxy, make it a good candidate TeV gamma-ray source.
Observations were carried out in 2000 and 2001 with the CANGAROO-II 10 m
imaging atmospheric Cerenkov telescope. TeV gamma-ray emission is detected at
the level with a flux of at energies 0.5 TeV. The data indicate that the
emission region is broader than the point spread function of our telescope.Comment: 4 pages, double colomn, 3 figures, aa.cl
Characterizing the gamma-ray long-term variability of PKS 2155-304 with H.E.S.S. and Fermi-LAT
Studying the temporal variability of BL Lac objects at the highest energies
provides unique insights into the extreme physical processes occurring in
relativistic jets and in the vicinity of super-massive black holes. To this
end, the long-term variability of the BL Lac object PKS 2155-304 is analyzed in
the high (HE, 100 MeV 200 GeV)
gamma-ray domain. Over the course of ~9 yr of H.E.S.S observations the VHE
light curve in the quiescent state is consistent with a log-normal behavior.
The VHE variability in this state is well described by flicker noise
(power-spectral-density index {\ss}_VHE = 1.10 +0.10 -0.13) on time scales
larger than one day. An analysis of 5.5 yr of HE Fermi LAT data gives
consistent results ({\ss}_HE = 1.20 +0.21 -0.23, on time scales larger than 10
days) compatible with the VHE findings. The HE and VHE power spectral densities
show a scale invariance across the probed time ranges. A direct linear
correlation between the VHE and HE fluxes could neither be excluded nor firmly
established. These long-term-variability properties are discussed and compared
to the red noise behavior ({\ss} ~ 2) seen on shorter time scales during
VHE-flaring states. The difference in power spectral noise behavior at VHE
energies during quiescent and flaring states provides evidence that these
states are influenced by different physical processes, while the compatibility
of the HE and VHE long-term results is suggestive of a common physical link as
it might be introduced by an underlying jet-disk connection.Comment: 11 pages, 16 figure
Three-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Temperature Analysis
We present new full-sky temperature maps in five frequency bands from 23 to
94 GHz, based on the first three years of the WMAP sky survey. The new maps,
which are consistent with the first-year maps and more sensitive, incorporate
improvements in data processing made possible by the additional years of data
and by a more complete analysis of the polarization signal. These include
refinements in the gain calibration and beam response models. We employ two
forms of multi-frequency analysis to separate astrophysical foreground signals
from the CMB, each of which improves on our first-year analyses. First, we form
an improved 'Internal Linear Combination' map, based solely on WMAP data, by
adding a bias correction step and by quantifying residual uncertainties in the
resulting map. Second, we fit and subtract new spatial templates that trace
Galactic emission; in particular, we now use low-frequency WMAP data to trace
synchrotron emission. The WMAP point source catalog is updated to include 115
new sources. We derive the angular power spectrum of the temperature anisotropy
using a hybrid approach that combines a maximum likelihood estimate at low l
(large angular scales) with a quadratic cross-power estimate for l>30. Our best
estimate of the CMB power spectrum is derived by averaging cross-power spectra
from 153 statistically independent channel pairs. The combined spectrum is
cosmic variance limited to l=400, and the signal-to-noise ratio per l-mode
exceeds unity up to l=850. The first two acoustic peaks are seen at l=220.8 +-
0.7 and l=530.9 +- 3.8, respectively, while the first two troughs are seen at
l=412.4 +- 1.9 and l=675.1 +- 11.1, respectively. The rise to the third peak is
unambiguous; when the WMAP data are combined with higher resolution CMB
measurements, the existence of a third acoustic peak is well established.Comment: 116 pgs, 24 figs. Accepted version of the 3-year paper as posted to
http://lambda.gsfc.nasa.gov/product/map/dr2/map_bibliography.cfm in January
200
A Millimetre Survey of Starburst Dominated Ultraluminous Infrared Galaxies at z~2
We present millimetre observations of a sample of 12 high redshift
ultraluminous infrared galaxies (ULIRGs) in the Extended Growth Strip (EGS).
These objects were initially selected on the basis of their observed mid--IR
colours (0.0 < [3.6]-[4.5] < 0.4 and -0.7 < [3.6]-[8.0] < 0.5) to lie at high
redshift 1.5 < z < 3, and subsequent 20-38 micron mid-IR spectroscopy confirms
that they lie in a narrow redshift window centered on z=2. We detect 9/12 of
the objects in our sample at high significance (>3 sigma) with a mean
1200\micron flux of = 1.6+/-0.1 mJy. Our millimetre photometry,
combined with existing far-IR photometry from the Far-IR Deep Extragalactic
Legacy (FIDEL) Survey and accurate spectroscopic redshifts, places constraints
both sides of the thermal dust peak. This allows us to estimate the dust
properties, including the far--IR luminosity, dust temperature, and dust mass.
We find that our sample is similar to other high-z and intermediate-z ULIRGs,
and local systems, but has a different dust selection function than
submillimeter-selected galaxies. Finally, we use existing 20cm radio continuum
imaging to test the far-IR/radio correlation at high redshift. We find that our
sample is consistent with the local relation, implying little evolution.
Furthermore, this suggests that our sample selection method is efficient at
identifying ultraluminous, starburst--dominated systems within a very narrow
redshift range centered at z~2.Comment: 11 pages, 10 figures, resubmitted to MNRAS with minor revision
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