901 research outputs found
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
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
On the origin of ionising photons emitted by T Tauri stars
We address the issue of the production of Lyman continuum photons by T Tauri
stars, in an attempt to provide constraints on theoretical models of disc
photoionisation. By treating the accretion shock as a hotspot on the stellar
surface we show that Lyman continuum photons are produced at a rate
approximately three orders of magnitude lower than that produced by a
corresponding black body, and that a strong Lyman continuum is only emitted for
high mass accretion rates. When our models are extended to include a column of
material accreting on to the hotspot we find that the accretion column is
extremely optically thick to Lyman continuum photons. Further, we find that
radiative recombination of hydrogen atoms within the column is not an efficient
means of producing photons with energies greater than 13.6eV, and find that an
accretion column of any conceivable height suppresses the emission of Lyman
continuum photons to a level below or comparable to that expected from the
stellar photosphere. The photospheric Lyman continuum is itself much too weak
to affect disc evolution significantly, and we find that the Lyman continuum
emitted by an accretion shock is similarly unable to influence disc evolution
significantly. This result has important consequences for models which use
photoionisation as a mechanism to drive the dispersal of circumstellar discs,
essentially proving that an additional source of Lyman continuum photons must
exist if disc photoionisation is to be significant.Comment: 6 pages, 4 figures. Accepted for publication in MNRA
Photoionization of Galactic Halo Gas by Old Supernova Remnants
We present new calculations on the contribution from cooling hot gas to the
photoionization of warm ionized gas in the Galaxy. We show that hot gas in
cooling supernova remnants (SNRs) is an important source of photoionization,
particularly for gas in the halo. We find that in many regions at high latitude
this source is adequate to account for the observed ionization so there is no
need to find ways to transport stellar photons from the disk. The flux from
cooling SNRs sets a floor on the ionization along any line of sight. Our model
flux is also shown to be consistent with the diffuse soft X-ray background and
with soft X-ray observations of external galaxies.
We consider the ionization of the clouds observed towards the halo star HD
93521, for which there are no O stars close to the line of sight. We show that
the observed ionization can be explained successfully by our model EUV/soft
X-ray flux from cooling hot gas. In particular, we can match the H alpha
intensity, the S++/S+ ratio, and the C+* column. From observations of the
ratios of columns of C+* and either S+ or H0, we are able to estimate the
thermal pressure in the clouds. The slow clouds require high (~10^4 cm^-3 K)
thermal pressures to match the N(C+*)/N(S+) ratio. Additional heating sources
are required for the slow clouds to maintain their ~7000 K temperatures at
these pressures, as found by Reynolds, Hausen & Tufte (1999).Comment: AASTeX 5.01; 34 pages, 2 figures; submitted to Astrophysical Journa
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
The Chemistry of Interstellar OH+, H2O+, and H3O+: Inferring the Cosmic Ray Ionization Rates from Observations of Molecular Ions
We model the production of OH+, H2O+, and H3O+ in interstellar clouds, using
a steady state photodissociation region code that treats the freeze-out of gas
species, grain surface chemistry, and desorption of ices from grains. The code
includes PAHs, which have important effects on the chemistry. All three ions
generally have two peaks in abundance as a function of depth into the cloud,
one at A_V<~1 and one at A_V~3-8, the exact values depending on the ratio of
incident ultraviolet flux to gas density. For relatively low values of the
incident far ultraviolet flux on the cloud ({\chi}<~ 1000; {\chi}= 1= local
interstellar value), the columns of OH+ and H2O+ scale roughly as the cosmic
ray primary ionization rate {\zeta}(crp) divided by the hydrogen nucleus
density n. The H3O+ column is dominated by the second peak, and we show that if
PAHs are present, N(H3O+) ~ 4x10^{13} cm^{-2} independent of {\zeta}(crp) or n.
If there are no PAHs or very small grains at the second peak, N(H3O+) can
attain such columns only if low ionization potential metals are heavily
depleted. We also model diffuse and translucent clouds in the interstellar
medium, and show how observations of N(OH+)/N(H) and N(OH+)/N(H2O+) can be used
to estimate {\zeta}(crp)/n, {\chi}/n and A_V in them. We compare our models to
Herschel observations of these two ions, and estimate {\zeta}(crp) ~ 4-6 x
10^-16 (n/100 cm^-3) s^-1 and \chi/n = 0.03 cm^3 for diffuse foreground clouds
towards W49N
Detection of Far-Infrared Water Vapor, Hydroxyl, and Carbon Monoxide Emissions from the Supernova Remnant 3C 391
We report the detection of shock-excited far-infrared emission of H2O, OH,
and CO from the supernova remnant 3C 391, using the ISO Long-Wavelength
Spectrometer. This is the first detection of thermal H2O and OH emission from a
supernova remnant. For two other remnants, W~28 and W~44, CO emission was
detected but OH was only detected in absorption. The observed H2O and OH
emission lines arise from levels within ~400 K of the ground state, consistent
with collisional excitation in warm, dense gas created after the passage of the
shock front through the dense clumps in the pre-shock cloud. The post-shock gas
we observe has a density ~2x10^5 cm^{-3} and temperature 100-1000 K, and the
relative abundances of CO:OH:H2O in the emitting region are 100:1:7 for a
temperature of 200 K. The presence of a significant column of warm H2O suggests
that the chemistry has been significantly changed by the shock. The existence
of significant column densities of both OH and H2O, which is at odds with
models for non-dissociative shocks into dense gas, could be due to
photodissociation of H2O or a mix of fast and slow shocks through regions with
different pre-shock density.Comment: AASTeX manuscript and 4 postscript figure
Using The Barton Libraries Dataset As An RDF benchmark
This report describes the Barton Libraries RDF dataset and Longwell querybenchmark that we use for our recent VLDB paper on Scalable Semantic WebData Management Using Vertical Partitioning
The Photoevaporative Wind from the Disk of TW Hya
Photoevaporation driven by the central star is expected to be a ubiquitous
and important mechanism to disperse the circumstellar dust and gas from which
planets form. Here, we present a detailed study of the circumstellar disk
surrounding the nearby star TW Hya and provide observational constraints to its
photoevaporative wind. Our new high-resolution (R ~ 30,000) mid-infrared
spectroscopy in the [Ne II] 12.81 {\mu}m line confirms that this gas diagnostic
traces the unbound wind component within 10AU from the star. From the blueshift
and asymmetry in the line profile, we estimate that most (>80%) of the [Ne II]
emission arises from disk radii where the midplane is optically thick to the
redshifted outflowing gas, meaning beyond the 1 or 4AU dust rim inferred from
other observations. We re-analyze high-resolution (R ~ 48, 000) archival
optical spectra searching for additional transitions that may trace the
photoevaporative flow. Unlike the [Ne II] line, optical forbidden lines from
OI, SII, and MgI are centered at the stellar velocity and have symmetric
profiles. The only way these lines could trace the photoevaporative flow is if
they arise from a disk region physically distinct from that traced by the [Ne
II] line, specifically from within the optically thin dust gap. However, the
small (~10 km/s) FWHM of these lines suggest that most of the emitting gas
traced at optical wavelengths is bound to the system rather than unbound. We
discuss the implications of our results for a planet-induced versus a
photoevaporation-induced gap.Comment: Accepted for publication in Ap
Photodissociation and the Morphology of HI in Galaxies
Young massive stars produce Far-UV photons which dissociate the molecular gas
on the surfaces of their parent molecular clouds. Of the many dissociation
products which result from this ``back-reaction'', atomic hydrogen \HI is one
of the easiest to observe through its radio 21-cm hyperfine line emission. In
this paper I first review the physics of this process and describe a simplified
model which has been developed to permit an approximate computation of the
column density of photodissociated \HI which appears on the surfaces of
molecular clouds. I then review several features of the \HI morphology of
galaxies on a variety of length scales and describe how photodissociation might
account for some of these observations. Finally, I discuss several consequences
which follow if this view of the origin of HI in galaxies continues to be
successful.Comment: 18 pages, 7 figures in 8 files, invited review paper for the
conference "Penetrating Bars Through Masks of Cosmic Dust: The Hubble Tuning
Fork Strikes a New Note", South Africa, June 2004. Proceedings to be
published by Kluwer, eds. D.L. Block, K.C. Freeman, I. Puerari, R. Groess, &
E.K. Bloc
- …