378 research outputs found
The effect of photo-ionization on the cooling rates of enriched, astrophysical plasmas
Radiative cooling is central to a wide range of astrophysical problems.
Despite its importance, cooling rates are generally computed using very
restrictive assumptions, such as collisional ionization equilibrium and solar
relative abundances. We simultaneously relax both assumptions and investigate
the effects of photo-ionization of heavy elements by the meta-galactic UV/X-ray
background and of variations in relative abundances on the cooling rates of
optically thin gas in ionization equilibrium. We find that photo-ionization by
the meta-galactic background radiation reduces the net cooling rates by up to
an order of magnitude for gas densities and temperatures typical of the
shock-heated intergalactic medium and proto-galaxies. In addition,
photo-ionization changes the relative contributions of different elements to
the cooling rates. We conclude that photo-ionization by the ionizing background
and heavy elements both need to be taken into account in order for the cooling
rates to be correct to order of magnitude. Moreover, if the rates need to be
known to better than a factor of a few, then departures of the relative
abundances from solar need to be taken into account. We propose a method to
compute cooling rates on an element-by-element basis by interpolating
pre-computed tables that take photo-ionization into account. We provide such
tables for a popular model of the evolving UV/X-ray background radiation,
computed using the photo-ionization package CLOUDY.Comment: 10 pages, 6 figures, Accepted for publication in MNRAS. One figure
added and minor textual changes made to first version. Downloadable tables
and videos available at http://www.strw.leidenuniv.nl/WSS08
Gravitational Heating Helps Make Massive Galaxies Red and Dead
We study the thermal formation history of four simulated galaxies that were
shown in Naab et al. (2007) to reproduce a number of observed properties of
elliptical galaxies. The temperature of the gas in the galaxies is steadily
increasing with decreasing redshift, although much of the gas has a cooling
time shorter than the Hubble time. The gas is being heated and kept hot by
gravitational heating processes through the release of potential energy from
infalling stellar clumps. The energy is dissipated in supersonic collisions of
infalling gas lumps with the ambient gas and through the dynamical capturing of
satellite systems causing gravitational wakes that transfer energy to the
surrounding gas. Furthermore dynamical friction from the infalling clumps
pushes out dark matter, lowering the central dark matter density by up to a
factor of two from z=3 to z=0. In galaxies in which the late formation history
(z<2) is dominated by minor merging and accretion the energy released
(E~5x10^{59} ergs) from gravitational feedback is sufficient to form red and
dead elliptical galaxies by z~1 even in the absence of supernova and AGN
feedback.Comment: 6 pages, 4 figures, accepted to ApJL (minor revisions to match
accepted version
Density profiles of dark matter haloes: diversity and dependence on environment
(Abridged) We study the outer density profiles of dark matter haloes
predicted by a generalized secondary infall model and observed in a N-body
cosmological simulation of a \Lambda CDM model. We find substantial systematic
variations in shapes and concentrations of the halo profiles as well as a
strong correlation of the profiles with the environment. In the N-body
simulation, the average outer slope of the density profiles, \beta (\rho\propto
r^{-\beta}), of isolated haloes is \approx 2.9; 68% of these haloes have values
of \beta between 2.5 and 3.8. Haloes in dense environments of clusters are more
concentrated and exhibit a broad distribution of \beta with values larger than
for isolated haloes . Contrary to what one may expect, the haloes contained
within groups and galaxy systems are less concentrated and have flatter outer
density profiles than the isolated haloes. The concentration decreases with
M_h, but its scatter for a given mass is substantial. The mass and circular
velocity of the haloes are strongly correlated: M_h \propto V_m^{\alpha} with
\alpha ~ 3.3 (isolated) and ~3.5 (haloes in clusters). For M_h=10^12M_sun the
rms deviations from these relations are \Delta logM_h=0.12 and 0.18,
respectively. Approximately 30% of the haloes are contained within larger
haloes or have massive companions (larger than ~0.3 the mass of the current
halo) within 3 virial radii. The remaining 70% of the haloes are isolated
objects. The distribution of \beta as well as the concentration-mass and
M_h-V_m relations for the isolated haloes agree very well with the predictions
of our seminumerical approach which is based on a generalization of the
secondary infall model and on the extended Press-Schechter formalism.Comment: 14 pages, 11 figures included, uses mn.sty, accepted by MNRAS. Minor
modifications, new and updated reference
Interpreting the Evolution of the Size - Luminosity Relation for Disk Galaxies from Redshift 1 to the Present
A sample of very high resolution cosmological disk galaxy simulations is used
to investigate the evolution of galaxy disk sizes back to redshift 1 within the
Lambda CDM cosmology. Artificial images in the rest frame B band are generated,
allowing for a measurement of disk scale lengths using surface brightness
profiles as observations would, and avoiding any assumption that light must
follow mass as previous models have assumed. We demonstrate that these
simulated disks are an excellent match to the observed magnitude - size
relation for both local disks, and for disks at z=1 in the magnitude/mass range
of overlap. We disentangle the evolution seen in the population as a whole from
the evolution of individual disk galaxies. In agreement with observations, our
simulated disks undergo roughly 1.5 magnitudes/arcsec^2 of surface brightness
dimming since z=1. We find evidence that evolution in the magnitude - size
plane varies by mass, such that galaxies with M* > 10^9 M_sun undergo more
evolution in size than luminosity, while dwarf galaxies tend to evolve
potentially more in luminosity. The disks grow in such a way as to stay on
roughly the same stellar mass - size relation with time. Finally, due to an
evolving stellar mass - SFR relation, a galaxy at a given stellar mass (or
size) at z=1 will reside in a more massive halo and have a higher SFR, and thus
a higher luminosity, than a counterpart of the same stellar mass at z=0.Comment: Version resubmitted to ApJ, after referee's comment
Effective-Range Expansion of the Neutron-Deuteron Scattering Studied by a Quark-Model Nonlocal Gaussian Potential
The S-wave effective range parameters of the neutron-deuteron (nd) scattering
are derived in the Faddeev formalism, using a nonlocal Gaussian potential based
on the quark-model baryon-baryon interaction fss2. The spin-doublet low-energy
eigenphase shift is sufficiently attractive to reproduce predictions by the
AV18 plus Urbana three-nucleon force, yielding the observed value of the
doublet scattering length and the correct differential cross sections below the
deuteron breakup threshold. This conclusion is consistent with the previous
result for the triton binding energy, which is nearly reproduced by fss2
without reinforcing it with the three-nucleon force.Comment: 21 pages, 6 figures and 6 tables, submitted to Prog. Theor. Phy
Assessment of a continuous blood gas monitoring system in animals during circulatory stress
<p>Abstract</p> <p>Background</p> <p>The study was aimed to determine the measurement accuracy of The CDI™ blood parameter monitoring system 500 (Terumo Cardiovascular Systems Corporation, Ann Arbor MI) in the real-time continuous measurement of arterial blood gases under different cardiocirculatory stress conditions</p> <p>Methods</p> <p>Inotropic stimulation (Dobutamine 2.5 and 5 μg/kg/min), vasoconstriction (Arginine-vasopressin 4, 8 and 16 IU/h), hemorrhage (-10%, -20%, -35%, and -50% of the theoretical volemia), and volume resuscitation were induced in ten swine (57.4 ± 10.7 Kg).Intermittent blood gas assessments were carried out using a routine gas analyzer at any experimental phase and compared with values obtained at the same time settings during continuous monitoring with CDI™ 500 system. The Bland-Altman analysis was employed.</p> <p>Results</p> <p>Bias and precision for pO<sub>2 </sub>were - 0.06 kPa and 0.22 kPa, respectively (r<sup>2 </sup>= 0.96); pCO<sub>2 </sub>- 0.02 kPa and 0.15 kPa, respectively; pH -0.001 and 0.01 units, respectively ( r<sup>2 </sup>= 0.96). The analysis showed very good agreement for SO<sub>2 </sub>(bias 0.04,precision 0.33, r<sup>2 </sup>= 0.95), Base excess (bias 0.04,precision 0.28, r<sup>2 </sup>= 0.98), HCO<sub>3 </sub>(bias 0.05,precision 0.62, r<sup>2 </sup>= 0.92),hemoglobin (bias 0.02,precision 0.23, r<sup>2 </sup>= 0.96) and K<sup>+ </sup>(bias 0.02, precision 0.27, r<sup>2 </sup>= 0.93). The sensor was reliable throughout the experiment during hemodynamic variations.</p> <p>Conclusions</p> <p>Continuous blood gas analysis with the CDI™ 500 system was reliable and it might represent a new useful tool to accurately and timely monitor gas exchange in critically ill patients. Nonetheless, our findings need to be confirmed by larger studies to prove its reliability in the clinical setting.</p
Clinical anticancer drug development: targeting the cyclin-dependent kinases
Cell division involves a cyclical biochemical process composed of several step-wise reactions that have to occur once per cell cycle. Dysregulation of cell division is a hallmark of all cancers. Genetic and epigenetic mechanisms frequently result in deranged expression and/or activity of cell-cycle proteins including the cyclins, cyclin-dependent kinases (Cdks), Cdk inhibitors and checkpoint control proteins. The critical nature of these proteins in cell cycling raises hope that targeting them may result in selective cytotoxicity and valuable anticancer activity
Reionisation, chemical enrichment and seed black holes from the first stars: is Population III important?
We investigate the effects of a top-heavy stellar initial mass function on
the reionisation history of the intergalactic medium (IGM). We use cosmological
simulations that include self-consistently the feedback from ionising
radiation, H_2 dissociating radiation and supernova (SN) explosions. We find
that it is difficult to reionise the IGM at z_rei>10 with stellar sources even
after making extreme assumptions. If star formation in 10^9 M_\odot galaxies is
not suppressed by SN explosions, the optical depth to Thomson scattering is
tau_e< 0.13. If we allow for the normal energy input from SNe or if
pair-instability SNe are dominant, we find taue<0.09. Assuming normal yields
for the first stars (popIII), the mean metallicity of the IGM is already
Z/Z_\odot=2x10^-3 (10^-3<Z/Z_\odot<1 in overdense regions) when the IGM mean
ionisation fraction is less than 10%. For these reasons popIII stars cannot
contribute significantly to reionisation unless the mechanical energy input
from SNe is greatly reduced and either the metal yield or the mixing efficiency
is reduced by a factor of 1000. Both problems have a solution if popIII stars
collapse to black holes. This can happen if, having masses M_*< 130 M_\odot,
they are characterised by heavy element fall-back or if, having masses M_*>260
M_\odot, they collapse directly onto black holes without exploding as SNe.If
metal-poor stars are initially important and collapse to black holes is the
typical outcome, then the secondary emission of ionising radiation from
accretion on SN induced seed black holes, might be more important than the
primary emission. Surprisingly, including feedback effects, we estimate that a
warm dark matter scenario (with particle mass of 1.25 keV) reduces tau_e by
only approximately 10%.[abridged]Comment: Accepted for publication in MNRAS. 13 pages, including 9 figures and
2 table
The Evolution of Compact Binary Star Systems
We review the formation and evolution of compact binary stars consisting of
white dwarfs (WDs), neutron stars (NSs), and black holes (BHs). Binary NSs and
BHs are thought to be the primary astrophysical sources of gravitational waves
(GWs) within the frequency band of ground-based detectors, while compact
binaries of WDs are important sources of GWs at lower frequencies to be covered
by space interferometers (LISA). Major uncertainties in the current
understanding of properties of NSs and BHs most relevant to the GW studies are
discussed, including the treatment of the natal kicks which compact stellar
remnants acquire during the core collapse of massive stars and the common
envelope phase of binary evolution. We discuss the coalescence rates of binary
NSs and BHs and prospects for their detections, the formation and evolution of
binary WDs and their observational manifestations. Special attention is given
to AM CVn-stars -- compact binaries in which the Roche lobe is filled by
another WD or a low-mass partially degenerate helium-star, as these stars are
thought to be the best LISA verification binary GW sources.Comment: 105 pages, 18 figure
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