1,155 research outputs found
Physical conditions in photodissociation regions: Application to galactic nuclei
Infrared and sub-millimeter observations are used in a simple procedure to determine average physical properties of the neutral interstellar medium in Galactic photodissociation regions as well as in ensembles of clouds which exist in the nuclei of luminous infrared galaxies. The relevant observations include the Infrared Astronomy Satellite (IRAS) infrared continuum measurements, infrared spectroscopy of the fine-structure lines of SiII 35 microns, OI 63 microns, and CII 158 microns, and the 2.6 mm CO (J=1-0) rotational transition. The diagnostic capabilities of the OI 145 microns line is also addressed. Researchers attribute these emission lines as well as the continuum to the atomic/molecular photodissociation region on the surfaces of molecular clouds which are illuminated by strong ultraviolet fields. They use the theoretical photodissociation region models of Tielens and Hollenbach (1985, Ap. J., 291, 722) to construct simple diagrams which utilize line ratios and line to continuum ratios to determine the average gas density n, the average incident far-ultraviolet flux G sub o, and the temperature of the atomic gas T
Studies of low-mass star formation with the large deployable reflector
Estimates are made of the far-infrared and submillimeter continuum and line emission from regions of low mass star formation. The intensity of this emission is compared with the sensitivity of the large deployable reflector (LDR), a large space telescope designed for this wavelength range. The proposed LDR is designed to probe the temperature, density, chemical structure, and the velocity field of the collapsing envelopes of these protostars. The LDR is also designed to study the accretion shocks on the cores and circumstellar disks of low-mass protostars, and to detect shock waves driven by protostellar winds
Infrared emission associated with chemical reactions on Shuttle and SIRTF surfaces
The infrared intensities which would be observed by the Shuttle Infrared Telescope Facility (SIRTF), and which are produced by surface chemistry following atmospheric impact on SIRTF and the shuttle are estimated. Three possible sources of reactants are analyzed: (1) direct atmospheric and scattered contaminant fluxes onto the shuttle's surface; (2) direct atmospheric and scattered contaminant fluxes onto the SIRTF sunshade; and (3) scattered fluxes onto the cold SIRTF mirror. The chemical reactions are primarily initiated by the dominent flux of reactive atomic oxygen on the surfaces. Using observations of the optical glow to constrain theoretical parameters, it is estimated for source (1) that the infrared glow on the SIRTF mirror will be comparable to the zodiacal background between 1 and 10 micron wavelengths. It is speculated that oxygen reacts with the atoms and the radicals bound in the organic molecules that reside on the shuttle and the Explorer surfaces. It is concluded that for source (2) that with suitable construction, a warm sunshade will produce insignificant infrared glow. It is noted that the atomic oxygen flux on the cold SIRTF mirror (3) is insufficient to produce significant infrared glow. Infrared absorption by the ice buildup on the mirror is also small
Refit to numerically problematic UMIST reaction rate coefficients
Aims. Chemical databases such as the UMIST Database for Astrochemistry (UDFA)
are indispensable in the numerical modeling of astrochemical networks. Several
of the listed reactions in the UDFA have properties that are problematic in
numerical computations: Some are parametrized in a way that leads to extremely
divergent behavior for low kinetic temperatures. Other reactions possess
multiple entries that are each valid in a different temperature regime, but
have no smooth transition when switching from one to another. Numerically, this
introduces many difficulties.We present corrected parametrizations for these
sets of reactions in the UDFA06 database.
Methods. From the tabulated parametrization in UDFA, we created artificial
data points and used a Levenberg-Marquardt algorithm to find a set of improved
fit parameters without divergent behavior for low temperatures. For reactions
with multiple entries in the database that each possess a different temperature
regime, we present one joint parametrization that is designed to be valid over
the whole cumulative temperature range of all individual reactions.
Results. We show that it is possible to parametrize numerically problematic
reactions from UDFA in a form that avoids low temperature divergence.
Additionally, we demonstrate that it is possible to give a collective
parametrization for reaction rate coefficients of reactions with multiple
entries in UDFA. We present these new fitted values in tabulated form.Comment: accepted by A&
Far-IR spectroscopy of the galactic center: Neutral and ionized gas in the central 10 pc of the galaxy
The 3P1 - 3P2 fine structure line emission from neutral atomic oxygen at 63 microns in the vicinity of the galactic center was mapped. The emission is extended over more than 4' (12 pc) along the galactic plane, centered on the position of Sgr A West. The line center velocities show that the O I gas is rotating around the galactic center with an axis close to that of the general galactic rotation, but there appear also to be noncircular motions. The rotational velocity at R is approximately 1 pc corresponds to a mass within the central pc of about 3 x 10(6) solar mass. Between 1 and 6 pc from the center the mass is approximately proportional to radius. The (O I) line probability arises in a predominantly neutral, atomic region immediately outside of the ionized central parsec of out galaxy. Hydrogen densities in the (O I) emitting region are 10(3) to 10(6) cm(-3) and gas temperatures are or = 100 K. The total integrated luminosity radiated in the line is about 10(5) solar luminosity, and is a substantial contribution to the cooling of the gas. Photoelectric heating or heating by ultraviolet excitation of H2 at high densities (10(5) cm(-3)) are promising mechanisms for heating of the gas, but heating due to dissipation of noncircular motions of the gas may be an alternative possibility. The 3P1 - 3P0 fine structure line of (O III) at 88 microns toward Sgr A West was also detected. The (O III) emission comes from high density ionized gas (n 10(4) cm(-3)), and there is no evidence for a medium density region (n 10(3) cm(-3)), such as the ionized halo in Sgr A West deduced from radio observations. This radio halo may be nonthermal, or may consist of many compact, dense clumps of filaments on the inner edges of neutral condensations at R or = 2 pc
Detection of shocked atomic gas in the Kleinmann-Low nebula
The 63 micrometer (3)p(1)-(3)P(2) fine structure line emission of neutral atomic oxygen at the center of the Orion nebula with a resolution of 30" is presented. There are three main emission peaks. One is associated with the region of strongest thermal radio continuum radiation close to the Trapezium cluster, and probably arises at the interface between the HII region and the dense Orion molecular cloud. The other two line emission peaks, associated with the Kleinmann Low nebula, are similar in both distribution and velocity to those of the 2 micrometer S(1) line of molecular hydrogen and of the high velocity wings of rotational CO emission. The OI emission from the KL nebula can be produced in the shocked gas associated with the mass outflows in this region and is an important coolant of the shocked gas
Far Field Monitoring of Rogue Nuclear Activity with an Array of Large anti-neutrino Detectors
The result of a study on the use of an array of large anti-neutrino detectors
for the purpose of monitoring rogue nuclear activity is presented. Targeted
regional monitoring of a nation bordering large bodies of water with no
pre-existing legal nuclear activity may be possible at a cost of about several
billion dollars, assuming several as-yet-untested schemes pan out in the next
two decades. These are: (1) the enabling of a water-based detector to detect
reactor anti-neutrinos by doping with GdCl; (2) the deployment of a
KamLAND-like detector in a deep-sea environment; and (3) the scaling of a
Super-Kamiokande-like detector to a size of one or more megatons. The first may
well prove feasible, and should be tested by phase-III Super-Kamiokande in the
next few years. The second is more of a challenge, but may well be tested by
the Hanohano collaboration in the coming decade. The third is perhaps the least
certain, with no schedule for construction of any such device in the
foreseeable future. In addition to the regional monitoring scheme, several
global, untargeted monitoring schemes were considered. All schemes were found
to fail benchmark sensitivity levels by a wide margin, and to cost at least
several trillion dollars.Comment: 17 pages, 8 figures, proceedings for Neutrino Sciences 2005,
submitted to Earth, Moon, and Planet
The Propagation and Survival of Interstellar Grains
In this paper we discuss the propagation of dust through the interstellar
medium (ISM), and describe the destructive effects of stellar winds, jets, and
supernova shock waves on interstellar dust. We review the probability that
grains formed in stellar outflows or supernovae survive processing in and
propagation through the ISM, and incorporate themselves relatively unprocessed
into meteoritic bodies in the solar system. We show that very large (radii >= 5
micron) and very small grains (radii <= 100 Angstrom) with sizes similar to the
pre-solar SiC and diamond grains extracted from meteorites, can survive the
passage through 100\kms shock waves relatively unscathed. High velocity (>= 250
km/s) shocks destroy dust efficiently. However, a small (~10%) fraction of the
stardust never encountered such fast shocks before incorporation into the solar
system. All grains should therefore retain traces of their passage through
interstellar shocks during their propagation through the ISM. The grain
surfaces should show evidence of processing due to sputtering and pitting due
to small grain cratering collisions on the micron-sized grains. This conclusion
seems to be in conflict with the evidence from the large grains recovered from
meteorites which seem to show little interstellar processing.Comment: 19 pages, 5 figures (.eps), LaTeX, to appear in "Astrophysical
Implications of the Laboratory Study of Presolar Materials" AIP Conference
Proceedings, 1997 T.J. Bernatowicz and E. Zinner (eds.
Gas phase water in the surface layer of protoplanetary disks
Recent observations of the ground state transition of HDO at 464 GHz towards
the protoplanetary disk of DM Tau have detected the presence of water vapor in
the regions just above the outer disk midplane (Ceccarelli et al 2005). In the
absence of non-thermal desorption processes, water should be almost entirely
frozen onto the grain mantles and HDO undetectable. In this Letter we present a
chemical model that explores the possibility that the icy mantles are
photo-desorbed by FUV (6eV < h nu < 13.6eV) photons. We show that the average
Interstellar FUV field is enough to create a layer of water vapor above the
disk midplane over the entire disk. Assuming a photo-desorption yield of
10^{-3}, the water abundance in this layer is predicted to be ~ 3 x 10^{-7} and
the average H2O column density is ~ 1.6x 10^{15} cm^{-2}. The predictions are
very weakly dependent on the details of the model, like the incident FUV
radiation field, and the gas density in the disk. Based on this model, we
predict a gaseous HDO/H2O ratio in DM Tau of ~1%. In addition, we predict the
ground state transition of water at 557 GHz to be undetectable with ODIN and/or
HSO-HIFI.Comment: 13 pages, 4 figures. Astrophysical Journal Letters, in pres
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