233 research outputs found
Discovery of Enhanced Germanium Abundances in Planetary Nebulae with FUSE
We report the discovery of Ge III 1088.46 in the planetary nebulae
(PNe) SwSt 1, BD+303639, NGC 3132, and IC 4593, observed with the Far
Ultraviolet Spectroscopic Explorer. This is the first astronomical detection of
this line and the first measurement of Ge (Z = 32) in PNe. We estimate Ge
abundances using S and Fe as reference elements, for a range of assumptions
about gas-phase depletions. The results indicate that Ge, which is synthesized
in the initial steps of the s-process and therefore can be self-enriched in
PNe, is enhanced by factors of > 3-10. The strongest evidence for enrichment is
seen for PNe with Wolf-Rayet central stars, which are likely to contain heavily
processed material.Comment: 11 pages, 1 figure, accepted for publication in ApJ Letter
Ultrafast effective multi-level atom method for primordial hydrogen recombination
Cosmological hydrogen recombination has recently been the subject of renewed
attention because of its importance for predicting the power spectrum of cosmic
microwave background anisotropies. It has become clear that it is necessary to
account for a large number n >~ 100 of energy shells of the hydrogen atom,
separately following the angular momentum substates in order to obtain
sufficiently accurate recombination histories. However, the multi-level atom
codes that follow the populations of all these levels are computationally
expensive, limiting recent analyses to only a few points in parameter space. In
this paper, we present a new method for solving the multi-level atom
recombination problem, which splits the problem into a computationally
expensive atomic physics component that is independent of the cosmology, and an
ultrafast cosmological evolution component. The atomic physics component
follows the network of bound-bound and bound-free transitions among excited
states and computes the resulting effective transition rates for the small set
of "interface" states radiatively connected to the ground state. The
cosmological evolution component only follows the populations of the interface
states. By pre-tabulating the effective rates, we can reduce the recurring cost
of multi-level atom calculations by more than 5 orders of magnitude. The
resulting code is fast enough for inclusion in Markov Chain Monte Carlo
parameter estimation algorithms. It does not yet include the radiative transfer
or high-n two-photon processes considered in some recent papers. Further work
on analytic treatments for these effects will be required in order to produce a
recombination code usable for Planck data analysis.Comment: Version accepted by Phys. Rev. D. Proof of equivalence of effective
and standard MLA methods moved to the main text. Some rewording
A model for atomic and molecular interstellar gas: The Meudon PDR code
We present the revised ``Meudon'' model of Photon Dominated Region (PDR
code), presently available on the web under the Gnu Public Licence at:
http://aristote.obspm.fr/MIS. General organisation of the code is described
down to a level that should allow most observers to use it as an interpretation
tool with minimal help from our part. Two grids of models, one for low
excitation diffuse clouds and one for dense highly illuminated clouds, are
discussed, and some new results on PDR modelisation highlighted.Comment: accepted in ApJ sup
A simple test for the existence of two accretion modes in Active Galactic Nuclei
By analogy to the different accretion states observed in black-hole X-ray
binaries (BHXBs), it appears plausible that accretion disks in active galactic
nuclei (AGN) undergo a state transition between a radiatively efficient and
inefficient accretion flow. If the radiative efficiency changes at some
critical accretion rate, there will be a change in the distribution of black
hole masses and bolometric luminosities at the corresponding transition
luminosity. To test this prediction, I consider the joint distribution of AGN
black hole masses and bolometric luminosities for a sample taken from the
literature. The small number of objects with low Eddington-scaled accretion
rates mdot < 0.01 and black hole masses Mbh < 10^9 Msun constitutes tentative
evidence for the existence of such a transition in AGN. Selection effects, in
particular those associated with flux-limited samples, systematically exclude
objects in particular regions of the black hole mass-luminosity plane.
Therefore, they require particular attention in the analysis of distributions
of black hole mass, bolometric luminosity, and derived quantities like the
accretion rate. I suggest further observational tests of the BHXB-AGN
unification scheme which are based on the jet domination of the energy output
of BHXBs in the hard state, and on the possible equivalence of BHXB in the very
high (or "steep power-law") state showing ejections and efficiently accreting
quasars and radio galaxies with powerful radio jets.Comment: Accepted by ApJ; 14 pages, 4 figures, uses emulateap
Transgenic avian-derived recombinant human interferon-alpha2b (AVI-005) in healthy subjects: an open-label, single-dose, controlled study.
BACKGROUND/AIMS: This study characterized the safety and pharmacological properties of AVI-005, a novel glycosylated recombinant human interferon-alpha2b produced from the egg whites of chickens transfected with human cDNA.
METHODS: 18 healthy volunteers received single subcutaneous rising doses (0.5, 1.66 or 5 million international units, MIU) of AVI-005. A randomized parallel comparator group of 10 subjects received 5 MIU of unglycosylated IFN-alpha2b (Intron A). The pharmacokinetic parameters t1/2, tmax, Cmax, AUC0-24h, Vd, and clearance were compared between AVI-005 and unglycosylated IFN-alpa2b.
RESULTS: At equipotent doses, AVI-005 had a larger AUC0-24h than the control interferon. Pharmacodynamic markers ofneopterin and beta2-microglobulin for the two treatments were similar. These markers were increased by AVI-005 in a dose-dependent manner. Pharmacodynamic responses to treatment with AVI-005 were shown by the change in mRNA expression for interferon inducible protein kinase and 2\u275\u27-oligoadenylate synthetase. Adverse events in the two groups were qualitatively and quantitatively similar.
CONCLUSION: AVI-005 demonstrates biological activity and pharmaco-kinetic properties in humans that support further development
From Local Velocities to Microwave Background
The mass density field as extracted from peculiar velocities in our
cosmological neighborhood is mapped back in time to the CMB in two ways. First,
the density power spectrum () is translated into a temperature angular
power spectrum of sub-degree resolution () and compared to observations.
Second, the local density field is translated into a temperature map in a patch
on the last-scattering surface of a distant observer. A likelihood analysis of
the Mark III peculiar velocity data have constrained the range of parameters
for within the family of COBE-normalized CDM models (Zaroubi et al 1996),
favoring a slight tilt in the initial spectrum, . The corresponding range
of 's is plotted against current observations, indicating that the CMB
data can tighten the constraints further: only models with ``small'' tilt
() and ``high'' baryonic content () could survive
the two data sets simultaneously. The local mass density field that has been
recovered from the velocities via a Wiener method is convolved with a Boltzmann
calculation to recover resolution temperature maps as viewed from
different directions. The extent of the CMB patch and the amplitude of
fluctuations depend on the choice of cosmological parameters, e.g., the local
100\hmpc sphere corresponds to to at the CMB for between
1 and 0 respectively. The phases of the temperature map are correlated with
those of the density field, contrary to the contribution of the Sachs-Wolfe
effect alone. This correlation suggests the possibility of an inverse
reconstruction of the underlying density field from CMB data with interesting
theoretical implications.Comment: 16 pages, 6 figures. Submitted to Ap.
Toward the Evidence of the Accretion Disk Emission in the Symbiotic Star RR Tel
In this paper, we argue that in the symbiotic star RR Tel the existence of an
accretion disk around the hot companion is strongly implied by the
characteristic features exhibited by the Raman-scattered O VI lines around 6830
\AA and 7088 \AA. High degrees of polarization and double-peaked profiles in
the Raman-scattered lines and single-peak profiles for other emission lines are
interpreted as line-of-sight effects, where the H I scatterers near the giant
see an incident double-peaked profile and an observer with a low inclination
sees single-peak profiles. It is predicted that different mass concentrations
around the accretion disk formed by a dusty wind may lead to the disparate
ratios of the blue peak strength to the red counterpart observed in the 6830
and 7088 features. We discuss the evolutionary links between symbiotic stars
and bipolar protoplanetary nebulae and conclude that the Raman scattering
processes may play an important role in investigation of the physical
properties of these objects.Comment: 11 pages, 3 figures, accepted for publication in the ApJ Letter
Energy Dissipation in Interstellar Cloud Collisions
We present a study of the kinetic energy dissipation in interstellar cloud
collisions. The main aim is to understand the dependence of the elasticity
(defined as the ratio of the final to the initial kinetic energy of the clouds)
on the velocity and mass ratio of the colliding clouds, magnetic field
strength, and gas metallicity for head-on collisions. The problem has been
studied both analytically and via numerical simulations. We have derived handy
analytical relationships that well approximate the analogous numerical results.
The main findings of this work are: (i) the kinetic energy dissipation in cloud
collisions is minimum (i.e. the collision elasticity is maximum) for a cloud
relative velocity ; (ii) the above minimum value is
proportional , where is the metallicity and is the cloud
size: the larger is the more dissipative (i.e. inelastic) the
collision will be; (iii) in general, we find that the energy dissipation
decreases when the magnetic field strength, and mass ratio of the clouds are
increased and the metallicity is decreased, respectively. We briefly discuss
the relevance of this study to the global structure of the interstellar medium
and to galaxy formation and evolution.Comment: 16 pages, aasms LaTeX, 7 figures. ApJ, accepte
Molecular Cloud Formation Behind Shock Waves
We examine the formation of molecular gas behind shocks in atomic gas using a
chemical/dynamical model, particular emphasis is given to constraints the
chemistry places on the dynamical evolution. The most important result of this
study is to stress the importance of shielding the molecular gas from the
destructive effects of UV radiation. For shock ram pressures comparable to or
exceeding typical local ISM pressures, self-shielding controls the formation
time of H2 but CO formation requires shielding of the interstellar radiation
field by dust grains. We find that the molecular hydrogen fractional abundance
can become significant well before CO forms. The timescale for (CO) molecular
cloud formation is not set by H2 formation, but rather by the timescale for
accumulating a sufficient column density or extinction, A_V > 0.7. The local
ratio of atomic to molecular gas (4:1), coupled with short estimates for cloud
lifetimes (3-5 Myr), suggests that the timescales for accumulating molecular
clouds from atomic material typically must be no longer than about 12-20 Myr.
Based on the shielding requirement, this implies that the typical product of
pre-shock density and velocity must be n*v > 20 cm^-3 km s^-1. Based on these
results we find that flow-driven formation of molecular clouds in the local
interstellar medium can occur sufficiently rapidly to account for observations.
We also provide detailed predictions of atomic and molecular emission and
absorption that track molecular cloud formation, with a view toward helping to
verify cloud formation by shock waves. Finally, we provide an analytic solution
for time-dependent H2 formation which may be of use in numerical hydrodynamic
calculations.Comment: 43 pages, 13 figures, accepted by ApJ main journa
Raman Scattered He II 6545 Line in the Symbiotic Star V1016 Cygni
We present a spectrum of the symbiotic star V1016 Cyg observed with the 3.6 m
Canada-France-Hawaii Telescope, in order to illustrate a method to measure the
covering factor of the neutral scattering region around the giant component
with respect to the hot emission region around the white dwarf component. In
the spectrum, we find broad wings around H and a broad emission feature
around 6545 that is blended with the [N II] 6548 line.
These two features are proposed to be formed by Raman scattering by atomic
hydrogen, where the incident radiation is proposed to be UV continuum radiation
around Ly in the former case and He II 1025 emission line
arising from transitions for the latter feature. We remove the
H wings by a template Raman scattering wing profile and subtract the [N
II] 6548 line using the 3 times stronger [N II] 6583
feature in order to isolate the He II Raman scattered 6545 \AA line. We obtain
the flux ratio of the He II 6560 emission
line and the 6545 \AA feature for V1016 Cyg. Under the assumption that the He
II emission from this object is isotropic, this ratio is converted to the ratio
of the number of the incident photons and that
of the scattered photons. This implies that the scattering region with H I
column density covers 17 per cent of the
emission region. By combining the presumed binary period yrs of this
system we infer that a significant fraction of the slow stellar wind from the
Mira component is ionized and that the scattering region around the Mira
extends a few tens of AU, which is closely associated with the mass loss
process of the Mira component.Comment: 12 pages, 6 figures, accepted for publication in Ap
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