956 research outputs found
Non-equilibrium photodissociation regions
We discuss the theory of coupled ionization--dissociation fronts produced when molecular clouds are exposed to \lambda < 1110\AA\ radiation from hot stars. A steady, composite structure is developed, which generally includes an ionized outflow away from the cloud, an ionization front, a layer of photodissociated gas, a photodissociation front, and a shock wave preceding the photodissociation front. We show that the properties of the structure are determined by two dimensionless parameters, \psi and \delta, and by the Alfv\'en speed in the preshock gas. For a broad range of parameters of interest, the ionization front and the hydrogen photodissociation front do not separate, the H_2 photodissociation and photoionization take place together, and a classical hydrogen ``photodissociation region'' (PDR) does not exist. We also show that even when a distinct photodissociation region exists, in many cases the dissociation front propagates too rapidly for the usual stationary models of PDRs to be applicable. We discuss several famous PDRs, e.g., in M17 and Orion and conclude that they cannot be described by equilibrium PDR models
Spitzer Infrared Spectrograph Detection of Molecular Hydrogen Rotational Emission towards Translucent Clouds
Using the Infrared Spectrograph on board the Spitzer Space Telescope, we have detected emission in the S(0), S(1), and S(2) pure-rotational (v = 0-0) transitions of molecular hydrogen (H_2) toward six positions in two translucent high Galactic latitude clouds, DCld 300.2–16.9 and LDN 1780. The detection of these lines raises important questions regarding the physical conditions inside low-extinction clouds that are far from ultraviolet radiation sources. The ratio between the S(2) flux and the flux from polycyclic aromatic hydrocarbons (PAHs) at 7.9 μm averages 0.007 for these six positions. This is a factor of about four higher than the same ratio measured toward the central regions of non-active Galaxies in the Spitzer Infrared Nearby Galaxies Survey. Thus, the environment of these translucent clouds is more efficient at producing rotationally excited H_2 per PAH-exciting photon than the disks of entire galaxies. Excitation analysis finds that the S(1) and S(2) emitting regions are warm (T ≳ 300 K), but comprise no more than 2% of the gas mass. We find that UV photons cannot be the sole source of excitation in these regions and suggest mechanical heating via shocks or turbulent dissipation as the dominant cause of the emission. The clouds are located on the outskirts of the Scorpius-Centaurus OB association and may be dissipating recent bursts of mechanical energy input from supernova explosions. We suggest that pockets of warm gas in diffuse or translucent clouds, integrated over the disks of galaxies, may represent a major source of all non-active galaxy H_2 emission
Non-LTE dust nucleation in sub-saturated vapors
We use the kinetic theory of nucleation to explore the properties of dust
nucleation in sub-saturated vapors. Due to radiation losses, the sub-critical
clusters have a smaller temperature compared to their vapor. This alters the
dynamical balance between attachment and detachment of monomers, allowing for
stable nucleation of grains in vapors that are sub-saturated for their
temperature. We find this effect particularly important at low densities and in
the absence of a strong background radiation field. We find new conditions for
stable nucleation in the n-T phase diagram. The nucleation in the non-LTE
regions is likely to be at much slower rate than in the super-saturated vapors.
We evaluate the nucleation rate, warning the reader that it does depend on
poorly substantiated properties of the macro-molecules assumed in the
computation. On the other hand, the conditions for nucleation depend only on
the properties of the large stable grains and are more robust. We finally point
out that this mechanism may be relevant in the early universe as an initial
dust pollution mechanism, since once the interstellar medium is polluted with
dust, mantle growth is likely to be dominant over non-LTE nucleation in the
diffuse medium.Comment: 8 pages, 8 figures, accepted for publication in MNRA
Constraints on Cold H_2 Clouds from Gravitational Microlensing Searches
It has been proposed that the Galaxy might contain a population of cold
clouds in numbers sufficient to account for a substantial fraction of the total
mass of the Galaxy. These clouds would have masses of the order of 10^{-3}
Solar mass and sizes of the order of 10 AU. We consider here the lensing
effects of such clouds on the light from background stars. A semianalytical
formalism for calculation of the magnification event rate produced by such
gaseous lensing is developed, taking into account the spatial distribution of
the dark matter in the Galaxy, the velocity distribution of the lensing clouds
and source stars, and motion of the observer. Event rates are calculated for
the case of gaseous lensing of stars in the Large Magellanic Cloud and results
are directly compared with the results of the search for gravitational
microlensing events undertaken by the MACHO collaboration. The MACHO experiment
strongly constrains the properties of the proposed molecular clouds, but does
not completely rule them out. Future monitoring programs will either detect or
more strongly constrain this proposed population.Comment: 36 pages, 9 figures, 1 table, typos corrected, minor change
Fragmentation Instability of Molecular Clouds: Numerical Simulations
We simulate fragmentation and gravitational collapse of cold, magnetized
molecular clouds. We explore the nonlinear development of an instability
mediated by ambipolar diffusion, in which the collapse rate is intermediate to
fast gravitational collapse and slow quasistatic collapse. Initially uniform
stable clouds fragment into elongated clumps with masses largely determined by
the cloud temperature, but substantially larger than the thermal Jeans mass.
The clumps are asymmetric, with significant rotation and vorticity, and lose
magnetic flux as they collapse. The clump shapes, intermediate collapse rates,
and infall profiles may help explain observations not easily fit by
contemporary slow or rapid collapse models.Comment: 25pp, 20 small eps figures, in press ApJ, April 1, 200
Determining the Parameters of Massive Protostellar Clouds via Radiative Transfer Modeling
A one-dimensional method for reconstructing the structure of prestellar and
protostellar clouds is presented. The method is based on radiative transfer
computations and a comparison of theoretical and observed intensity
distributions at both millimeter and infrared wavelengths. The radiative
transfer of dust emission is modeled for specified parameters of the density
distribution, central star, and external background, and the theoretical
distribution of the dust temperature inside the cloud is determined. The
intensity distributions at millimeter and IR wavelengths are computed and
quantitatively compared with observational data. The best-fit model parameters
are determined using a genetic minimization algorithm, which makes it possible
to reveal the ranges of parameter degeneracy as well. The method is illustrated
by modeling the structure of the two infrared dark clouds IRDC-320.27+029 (P2)
and IRDC-321.73+005 (P2). The derived density and temperature distributions can
be used to model the chemical structure and spectral maps in molecular lines.Comment: Accepted for publication in Astronomy Report
Spatially Resolved Spitzer-IRS Spectral Maps of the Superwind in M82
We have mapped the superwind/halo region of the nearby starburst galaxy M82
in the mid-infrared with . The spectral regions covered include
the H, [NeII], [NeIII] emission lines and PAH features. We
estimate the total warm H mass and the kinetic energy of the outflowing
warm molecular gas to be between M and
erg. Using the ratios of the 6.2, 7.7 and 11.3
micron PAH features in the IRS spectra, we are able to estimate the average
size and ionization state of the small grains in the superwind. There are large
variations in the PAH flux ratios throughout the outflow. The 11.3/7.7 and the
6.2/7.7 PAH ratios both vary by more than a factor of five across the wind
region. The Northern part of the wind has a significant population of PAH's
with smaller 6.2/7.7 ratios than either the starburst disk or the Southern
wind, indicating that on average, PAH emitters are larger and more ionized. The
warm molecular gas to PAH flux ratios (H) are enhanced in the outflow
by factors of 10-100 as compared to the starburst disk. This enhancement in the
H ratio does not seem to follow the ionization of the atomic gas (as
measured with the [NeIII]/[NeII] line flux ratio) in the outflow. This suggests
that much of the warm H in the outflow is excited by shocks. The observed
H line intensities can be reproduced with low velocity shocks ( km
s) driven into moderately dense molecular gas (
cm) entrained in the outflow.Comment: 19 pages and 12 figures; accepted in MNRA
Modeling Dust and Starlight in Galaxies Observed by Spitzer and Herschel: NGC 628 and NGC 6946
We characterize the dust in NGC628 and NGC6946, two nearby spiral galaxies in
the KINGFISH sample. With data from 3.6um to 500um, dust models are strongly
constrained. Using the Draine & Li (2007) dust model, (amorphous silicate and
carbonaceous grains), for each pixel in each galaxy we estimate (1) dust mass
surface density, (2) dust mass fraction contributed by polycyclic aromatic
hydrocarbons (PAH)s, (3) distribution of starlight intensities heating the
dust, (4) total infrared (IR) luminosity emitted by the dust, and (5) IR
luminosity originating in regions with high starlight intensity. We obtain maps
for the dust properties, which trace the spiral structure of the galaxies. The
dust models successfully reproduce the observed global and resolved spectral
energy distributions (SEDs). The overall dust/H mass ratio is estimated to be
0.0082+/-0.0017 for NGC628, and 0.0063+/-0.0009 for NGC6946, consistent with
what is expected for galaxies of near-solar metallicity. Our derived dust
masses are larger (by up to a factor 3) than estimates based on
single-temperature modified blackbody fits. We show that the SED fits are
significantly improved if the starlight intensity distribution includes a
(single intensity) "delta function" component. We find no evidence for
significant masses of cold dust T<12K. Discrepancies between PACS and MIPS
photometry in both low and high surface brightness areas result in large
uncertainties when the modeling is done at PACS resolutions, in which case
SPIRE, MIPS70 and MIPS160 data cannot be used. We recommend against attempting
to model dust at the angular resolution of PACS.Comment: To be published in Apj, September 2012. See the full version at
http://www.astro.princeton.edu/~ganiano/Papers
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