404 research outputs found
The Effects of UV Continuum and Lyman alpha Radiation on the Chemical Equilibrium of T Tauri Disks
We show in this Letter that the spectral details of the FUV radiation fields
have a large impact on the chemistry of protoplanetary disks surrounding T
Tauri stars. We show that the strength of a realistic stellar FUV field is
significantly lower than typically assumed in chemical calculations and that
the radiation field is dominated by strong line emission, most notably Lyman
alpha radiation. The effects of the strong Lyman alpha emission on the chemical
equilibrium in protoplanetary disks has previously been unrecognized. We
discuss the impact of this radiation on molecular observations in the context
of a radiative transfer model that includes both direct attenuation and
scattering. In particular, Lyman alpha radiation will directly dissociate water
vapor and may contribute to the observed enhancements of CN/HCN in disks.Comment: 14 pages, 4 figures, accepted by ApJ Letter
Rate coefficients for rovibrational transitions in H_2 due to collisions with He
We present quantum mechanical and quasiclassical trajectory calculations of
cross sections for rovibrational transitions in ortho- and para-H_2 induced by
collisions with He atoms. Cross sections were obtained for kinetic energies
between 10^-4 and 3 eV, and the corresponding rate coefficients were calculated
for the temperature range 100<T<4000 K. Comparisons are made with previous
calculations.Comment: 21 pages, 2 figures, AAS, eps
Molecules in the Circumstellar Disk Orbiting BP Piscium
BP Psc is a puzzling late-type, emission-line field star with large infrared
excess. The star is encircled and enshrouded by a nearly edge-on, dust
circumstellar disk, and displays an extensive jet system similar to those
associated with pre-main sequence (pre-MS) stars. We conducted a mm-wave
molecular line survey of BP Psc with the 30 m telescope of the Institut de
Radio Astronomie Millimetrique (IRAM). We detected lines of 12CO and 13CO and,
possibly, very weak emission from HCO+ and CN; HCN, H2CO, and SiO are not
detected. The CO line profiles of BP Psc are well fit by a model invoking a
disk in Keplerian rotation. The mimumum disk gas mass, inferred from the 12CO
line intensity and 13CO/12CO line ratio, is ~0.1 Jupiter masses. The weakness
of HCO+ and CN (relative to 13CO) stands in sharp contrast to the strong HCO+
and CN emission that characterizes most low-mass, pre-main sequence stars that
have been the subjects of molecular emission-line surveys, and is suggestive of
a very low level of X-ray-induced molecular ionization within the BP Psc disk.
These results lend some support to the notion that BP Psc is an evolved star
whose circumstellar disk has its origins in a catastrophic interaction with a
close companion.Comment: 6 pages, 4 figures; to appear in Astronomy & Astrophysic
Molecular Tracers of Filamentary CO Emission Regions Surrounding the Central Galaxies of Clusters
Optical emission is detected from filaments around the central galaxies of
clusters of galaxies. These filaments have lengths of tens of kiloparsecs. The
emission is possibly due to heating caused by the dissipation of mechanical
energy and by cosmic ray induced ionisation. CO millimeter and submillimeter
line emissions as well as H infrared emission originating in such
filaments surrounding NGC~1275, the central galaxy of the Perseus cluster, have
been detected. Our aim is to identify those molecular species, other than CO,
that may emit detectable millimeter and submillimeter line features arising in
these filaments, and to determine which of those species will produce emissions
that might serve as diagnostics of the dissipation and cosmic ray induced
ionisation. The time-dependent UCL photon-dominated region modelling code was
used in the construction of steady-state models of molecular filamentary
emission regions at appropriate pressures, for a range of dissipation and
cosmic ray induced ionisation rates and incident radiation fields.HCO and
CH emissions will potentially provide information about the cosmic ray
induced ionisation rates in the filaments. HCN and, in particular, CN are
species with millimeter and submillimeter lines that remain abundant in the
warmest regions containing molecules. Detections of the galaxy cluster
filaments in HCO, CH, and CN emissions and further detections of
them in HCN emissions would provide significant constraints on the dissipation
and cosmic ray induced ionisation rates.Comment: 11 pages, 3 figures, 3 tables, accepted in A&
HCN J=5-4 Emission in APM08279+5255 at z=3.91
We detect HCN J=5-4 emission from the ultraluminous quasar APM08279+5255 at
z=3.911 using the IRAM Plateau de Bure interferometer. This object is strongly
gravitationally lensed, yet still thought to be one of the most intrinsically
luminous objects in the Universe. The new data imply a line luminosity
L'_HCN(J=5-4) = 4.0+/-0.5 x 10^(10) K km/s pc^2. The ~440 km/s full width half
maximum of the HCN J=5-4 line matches that of the previously observed high-J CO
lines in this object and suggests that the emission from both species emerges
from the same region: a warm, dense circumnuclear disk. Simple radiative
transfer models suggest an enhanced abundance of HCN relative to CO in the
nuclear region of APM08279+5255, perhaps due to increased ionization, or
possibly the selective depletion of oxygen. The ratio of far-infrared
luminosity to HCN luminosity is at the high end of the range found for nearby
star forming galaxies, but comparable to that observed in the few high redshift
objects detected in the HCN J=1-0 line. This is the first clear detection of
high-J HCN emission redshifted into the 3-millimeter atmospheric window.Comment: Accepted for publication in ApJ
Fragmentation and the formation of primordial protostars: the possible role of Collision Induced Emission
The mechanisms which could lead to chemo-thermal instabilities and
fragmentation during the formation of primordial protostars are investigated
analytically. We introduce approximations for H2 cooling rates bridging the
optically thin and thick regimes. These allow us to discuss instabilities up to
densities when protostars become optically thick to continuum radiation
(n~10^16 cm^-3). During the collapse, instability arises at two different
stages: at low density (n~10^8-10^11 cm^-3), it is due to fast 3-body reactions
converting H into H2; at high density (n>10^13 cm^-3), it is due to Collisional
Induced Emission (CIE). In agreement with the 3D simulations, we find that the
instability at low densities cannot lead to fragmentation, because fluctuations
do not survive turbulent mixing, and because their growth is slow. The
situation at high density is similar. The CIE-induced instability is as weak as
the low density one, with similar ratios of growth and dynamical time scales.
Fluctuation growth time is longer than free fall time, and fragmentation seems
unlikely. One then expects the first stars to be massive, not to form binaries
nor harbour planets. Nevertheless, full 3D simulations are required. They could
become possible using simplified estimates of radiative transfer effects, which
we show to work very well in the 1D case. This indicates that the effects of
radiative transfer during the initial stages of formation of primordial
protostars can be treated as local corrections to cooling. (Abridged)Comment: 17 pages, 9 figures; accepted for publication in MNRA
Influence heat-reflective coating on the decrease of heat losses of window constructions
Developed theoretical and methodological foundations of the optimal choice of space-planning and constructive decisions of low-rise buildings blocked type, aimed at improving efficiency of investment, energy and resource saving, creation of comfortable conditions for the population, ensure sustainable development of low-rise construction in the context of socio-economic priorities in the climatic zoning of the area of construction
H-alpha and Free-Free Emission from the WIM
Recent observations have found the ratio of H-alpha to free-free radio
continuum to be surprisingly high in the diffuse ionized ISM (the so-called
WIM), corresponding to an electron temperature of only ~3000K. Such low
temperatures were unexpected in gas that was presumed to be photoionized. We
consider a 3-component model for the observed diffuse emission, consisting of a
mix of (1) photoionized gas, (2) gas that is recombining and cooling, and (3)
cool H I gas. This model can successfully reproduce the observed intensities of
free-free continuum, H-alpha, and collisionally-excited lines such as NII 6583.
To reproduce the low observed value of free-free to H-alpha, the PAH abundance
in the photoionized regions must be lowered by a factor ~3, and ~20% of the
diffuse H-alpha must be reflected from dust grains, as suggested by Wood &
Reynolds (1999).Comment: 25 pages, 7 figures, 4 tables, single column, details of the
calculation and atomic physics added, accepted by Ap
Search for Interstellar Water in the Translucent Molecular Cloud toward HD 154368
We report an upper limit of 9 x 10^{12} cm-2 on the column density of water
in the translucent cloud along the line of sight toward HD 154368. This result
is based upon a search for the C-X band of water near 1240 \AA carried out
using the Goddard High Resolution Spectrograph of the Hubble Space Telescope.
Our observational limit on the water abundance together with detailed chemical
models of translucent clouds and previous measurements of OH along the line of
sight constrain the branching ratio in the dissociative recombination of H_3O+
to form water. We find at the level that no more than 30% of
dissociative recombinations of H_3O+ can lead to H_2O. The observed spectrum
also yielded high-resolution observations of the Mg II doublet at 1239.9 \AA
and 1240.4 \AA, allowing the velocity structure of the dominant ionization
state of magnesium to be studied along the line of sight. The Mg II spectrum is
consistent with GHRS observations at lower spectral resolution that were
obtained previously but allow an additional velocity component to be
identified.Comment: Accepted by ApJ, uses aasp
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