892 research outputs found
Detection of extrasolar planets by the large deployable reflector
The best wavelength for observing Jupiter-size planetary companions to stars other than the Sun is one at which a planet's thermal emission is strongest; typically this would occur in the far-infrared region. It is assumed that the orbiting infrared telescope used is diffraction-limited so that the resolution of the planet from the central star is accomplished in the wings of the star's Airy pattern. Proxima Centauri, Barnard's Star, Wolf 359, and Epsilon Eridani are just a few of the many nearest main-sequence stars that could be studied with the large deployable relfector (LDR). The detectability of a planet improves for warmer planets and less luminous stars; therefore, planets around white dwarfs and those young planets which have sufficient internal gravitational energy release so as to cause a significant increase in their temperatures are considered. If white dwarfs are as old as they are usually assumed to be (5-10 billion yr), then only the nearest white dwarf (Sirius B) is within the range of LDR. The Ursa Major cluster and Perseu cluster are within LDR's detection range mainly because of their proximity and young age, respectively
An extended soft-cube model for the thermal accommodation of gas atoms on solid surfaces
A numerical soft cube model was developed for calculating thermal accommodation coefficients alpha and trapping fractions f sub t for the interaction of gases incident upon solid surfaces. A semiempirical correction factor c which allows the calculation of alpha and f sub t when the collision times are long compared to the surface oscillator period were introduced. The processes of trapping, evaporation, and detailed balancing were discussed. The numerical method was designed to treat economically and with moderate (+ or - 20 percent) accuracy the dependence of alpha and f sub t on finite and different surface and gas temperatures for a large number of gas/surface combinations. Comparison was made with experiments of rare gases on tungsten and on alkalis, as well as one astrophysical case of H2 on graphite. The dependence of alpha on the soft cube dimensionless parameters is presented graphically
Large Deployable Reflector Science and Technology Workshop. Volume 2: Scientific Rationale and Technology Requirements
The scientific rationale for the large deployable reflector (LDR) and the overall technological requirements are discussed. The main scientific objectives include studies of the origins of planets, stars and galaxies, and of the ultimate fate of the universe. The envisioned studies require a telescope with a diameter of at least 20 m, diffraction-limited to wavelengths as short as 30-50 micron. In addition, light-bucket operation with 1 arcsec spatial resolution in the 2-4 microns wavelength region would be useful in studies of high-redshifted galaxies. Such a telescope would provide a large increase in spectroscopic sensitivity and spatial resolving power compared with existing or planned infrared telescopes
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
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
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.
Tree and string analysis of a Copala Trique sentence
From the introduction: This paper is simply a presentation of two analyses of the same Copala Trique sentence. The first analysis is done by David Thomas and represents a string analysis, with a distinction between deep structure and surface structure. The second analysis is done by Bruce Hollenbach and represents a tree analysis done from the point of view of generative semantics. We hope that this presentation may be useful to those who might be interested in comparing and contrasting these two analytical techniques
Line Emission from Gas in Optically Thick Dust Disks around Young Stars
We present self-consistent models of gas in optically-thick dusty disks and
calculate its thermal, density and chemical structure. The models focus on an
accurate treatment of the upper layers where line emission originates, and at
radii AU. We present results of disks around stars where we have varied dust properties, X-ray luminosities and
UV luminosities. We separately treat gas and dust thermal balance, and
calculate line luminosities at infrared and sub-millimeter wavelengths from all
transitions originating in the predominantly neutral gas that lies below the
ionized surface of the disk. We find that the [ArII] 7m, [NeII]
12.8m, [FeI] 24m, [SI] 25m, [FeII] 26m, [SiII] 35 m,
[OI] 63m and pure rotational lines of H, HO and CO can be quite
strong and are good indicators of the presence and distribution of gas in
disks. We apply our models to the disk around the nearby young star, TW Hya,
and find good agreement between our model calculations and observations. We
also predict strong emission lines from the TW Hya disk that are likely to be
detected by future facilities. A comparison of CO observations with our models
suggests that the gas disk around TW Hya may be truncated to AU,
compared to its dust disk of 174 AU. We speculate that photoevaporation due to
the strong stellar FUV field from TW Hya is responsible for the gas disk
truncation.Comment: Accepted to Astrophysical Journa
Photoevaporation of protoplanetary discs I: hydrodynamic models
In this paper we consider the effect of the direct ionizing stellar radiation
field on the evolution of protoplanetary discs subject to photoevaporative
winds. We suggest that models which combine viscous evolution with
photoevaporation of the disc (e.g. Clarke, Gendrin & Sotomayor 2001)
incorrectly neglect the direct field after the inner disc has drained, at late
times in the evolution. We construct models of the photoevaporative wind
produced by the direct field, first using simple analytic arguments and later
using detailed numerical hydrodynamics. We find that the wind produced by the
direct field at late times is much larger than has previously been assumed, and
we show that the mass-loss rate scales as (where is the
radius of the instantaneous inner disc edge). We suggest that this result has
important consequences for theories of disc evolution, and go on to consider
the effects of this result on disc evolution in detail in a companion paper
(Alexander, Clarke & Pringle 2006b).Comment: 13 pages, 9 figures. Accepted for publication in MNRA
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