706 research outputs found

### Carbon Fractionation in PDRs

We upgraded the chemical network from the UMIST Database for Astrochemistry
2006 to include isotopes such as ^{13}C and ^{18}O. This includes all
corresponding isotopologues, their chemical reactions and the properly scaled
reaction rate coefficients. We study the fractionation behavior of
astrochemically relevant species over a wide range of model parameters,
relevant for modelling of photo-dissociation regions (PDRs). We separately
analyze the fractionation of the local abundances, fractionation of the total
column densities, and fractionation visible in the emission line ratios. We
find that strong C^+ fractionation is possible in cool C^+ gas. Optical
thickness as well as excitation effects produce intensity ratios between 40 and
400. The fractionation of CO in PDRs is significantly different from the
diffuse interstellar medium. PDR model results never show a fractionation ratio
of the CO column density larger than the elemental ratio. Isotope-selective
photo-dissociation is always dominated by the isotope-selective chemistry in
dense PDR gas. The fractionation of C, CH, CH^+, and HCO^+ is studied in
detail, showing that the fractionation of C, CH and CH^+ is dominated by the
fractionation of their parental species. The light hydrides chemically derive
from C^+, and, consequently, their fractionation state is coupled to that of
C^+. The fractionation of C is a mixed case depending on whether formation from
CO or HCO^+ dominates. Ratios of the emission lines of [C II], [C I], ^{13}CO,
and H^{13}CO^+ provide individual diagnostics to the fractionation status of
C^+, C, and CO.Comment: to be published in A&

### Wavelet-based cross-correlation analysis of structure scaling in turbulent clouds

We propose a statistical tool to compare the scaling behaviour of turbulence
in pairs of molecular cloud maps. Using artificial maps with well defined
spatial properties, we calibrate the method and test its limitations to
ultimately apply it to a set of observed maps. We develop the wavelet-based
weighted cross-correlation (WWCC) method to study the relative contribution of
structures of different sizes and their degree of correlation in two maps as a
function of spatial scale, and the mutual displacement of structures in the
molecular cloud maps. We test the WWCC for circular structures having a single
prominent scale and fractal structures showing a self-similar behavior without
prominent scales. Observational noise and a finite map size limit the scales
where the cross-correlation coefficients and displacement vectors can be
reliably measured. For fractal maps containing many structures on all scales,
the limitation from the observational noise is negligible for signal-to-noise
ratios >5. (abbrev). Application of the WWCC to the observed line maps of the
giant molecular cloud G333 allows to add specific scale information to the
results obtained earlier using the principle component analysis. It confirms
the chemical and excitation similarity of $^{13}$CO and C$^{18}$O on all
scales, but shows a deviation of HCN at scales of up to 7' (~7 pc). This can be
interpreted as a chemical transition scale. The largest structures also show a
systematic offset along the filament, probably due to a large-scale density
gradient. The WWCC can compare correlated structures in different maps of
molecular clouds identifying scales that represent structural changes such as
chemical and phase transitions and prominent or enhanced dimensions.Comment: 26 pages, 41 figures, accepted to A&

### The Turbulence Power Spectrum in Optically Thick Interstellar Clouds

The Fourier power spectrum is one of the most widely used statistical tools
to analyze the nature of magnetohydrodynamic turbulence in the interstellar
medium. Lazarian & Pogosyan (2004) predicted that the spectral slope should
saturate to -3 for an optically thick medium and many observations exist in
support of their prediction. However, there have not been any numerical studies
to-date testing these results. We analyze the spatial power spectrum of MHD
simulations with a wide range of sonic and Alfv\'enic Mach numbers, which
include radiative transfer effects of the $^{13}$CO transition. We confirm
numerically the predictions of Lazarian & Pogosyan (2004) that the spectral
slope of line intensity maps of an optically thick medium saturates to -3.
Furthermore, for very optically thin supersonic CO gas, where the density or CO
abundance values are too low to excite emission in all but the densest shock
compressed gas, we find that the spectral slope is shallower than expected from
the column density. Finally, we find that mixed optically thin/thick CO gas,
which has average optical depths on order of unity, shows mixed behavior: for
super-Alfv\'enic turbulence, the integrated intensity power spectral slopes
generally follow the same trend with sonic Mach number as the true column
density power spectrum slopes. However, for sub-Alfv\'enic turbulence the
spectral slopes are steeper with values near -3 which are similar to the very
optically thick regime.Comment: accepted to Ap

### Modelling clumpy PDRs in 3D - Understanding the Orion Bar stratification

Context. Models of photon-dominated regions (PDRs) still fail to fully
reproduce some of the observed properties, in particular the combination of the
intensities of different PDR cooling lines together with the chemical
stratification, as observed e.g. for the Orion Bar PDR. Aims. We aim to
construct a numerical PDR model, KOSMA-\tau 3D, to simulate full spectral cubes
of line emission from arbitrary PDRs in three dimensions (3D). The model is to
reproduce the intensity of the main cooling lines from the Orion Bar PDR and
the observed layered structure of the different transitions. Methods. We build
up a 3D compound, made of voxels ("3D pixels") that contain a discrete mass
distribution of spherical "clumpy" structures, approximating the fractal ISM.
To analyse each individual clump the new code is combined with the KOSMA-\tau
PDR model. Probabilistic algorithms are used to calculate the local FUV flux
for each voxel as well as the voxel-averaged line emissivities and optical
depths, based on the properties of the individual clumps. Finally, the
computation of the radiative transfer through the compound provides full
spectral cubes. To test the new model we try to simulate the structure of the
Orion Bar PDR and compare the results to observations from HIFI/Herschel and
from the Caltech Submillimetre Observatory (CSO). In this context new Herschel
data from the HEXOS guaranteed-time key program is presented. Results. Our
model is able to reproduce the line integrated intensities within a factor 2.5
and the observed stratification pattern within 0.016 pc for the [Cii] 158 \mu m
and different 12/13 CO and HCO+ transitions, based on the representation of the
Orion Bar PDR by a clumpy edge-on cavity wall. In the cavity wall, a large
fraction of the total mass needs to be contained in clumps. The mass of the
interclump medium is constrained by the FUV penetration. Furthermore, ...Comment: Major changes compared to v1. Also several references have been adde

### Calculating Cross Sections of Composite Interstellar Grains

Interstellar grains may be composite collections of particles of distinct
materials, including voids, agglomerated together. We determine the various
optical cross sections of such composite grains, given the optical properties
of each constituent, using an approximate model of the composite grain. We
assume it consists of many concentric spherical layers of the various
materials, each with a specified volume fraction. In such a case the usual Mie
theory can be generalized and the extinction, scattering, and other cross
sections determined exactly.
We find that the ordering of the materials in the layering makes some
difference to the derived cross sections, but averaging over the various
permutations of the order of the materials provides rapid convergence as the
number of shells (each of which is filled by all of the materials
proportionately to their volume fractions) is increased. Three shells, each
with one layer of a particular constituent material, give a very satisfactory
estimate of the average cross section produced by larger numbers of shells.
We give the formulae for the Rayleigh limit (small size parameter) for
multi-layered spheres and use it to propose an ``Effective Medium Theory''
(EMT), in which an average optical constant is taken to represent the ensemble
of materials.
Multi-layered models are used to compare the accuracies of several EMTs
already in the literature.Comment: 29 pages, 6 figures, accepted for publication in the Astrophysical
Journal (part 1, scheduled in Vol. 526, #1, Nov. 20

### A Tidally-Disrupted Asteroid Around the White Dwarf G29-38

The infrared excess around the white dwarf G29-38 can be explained by
emission from an opaque flat ring of dust with an inner radius 0.14 of the
radius of the Sun and an outer radius approximately equal to the Sun's. This
ring lies within the Roche region of the white dwarf where an asteroid could
have been tidally destroyed, producing a system reminiscent of Saturn's rings.
Accretion onto the white dwarf from this circumstellar dust can explain the
observed calcium abundance in the atmosphere of G29-38. Either as a bombardment
by a series of asteroids or because of one large disruption, the total amount
of matter accreted onto the white dwarf may have been comparable to the total
mass of asteroids in the Solar System, or, equivalently, about 1% of the mass
in the asteroid belt around the main sequence star zeta Lep.Comment: ApJ Letters, in pres

### A High-Mass Protobinary System in the Hot Core W3(H2O)

We have observed a high-mass protobinary system in the hot core W3(H2O) with
the BIMA Array. Our continuum maps at wavelengths of 1.4mm and 2.8mm both
achieve sub-arcsecond angular resolutions and show a double-peaked morphology.
The angular separation of the two sources is 1.19" corresponding to 2.43X10^3
AU at the source distance of 2.04 kpc. The flux densities of the two sources at
1.4mm and 2.8mm have a spectral index of 3, translating to an opacity law of
kappa ~ nu. The small spectral indices suggest that grain growth has begun in
the hot core. We have also observed 5 K components of the CH3CN (12-11)
transitions. A radial velocity difference of 2.81 km/s is found towards the two
continuum peaks. Interpreting these two sources as binary components in orbit
about one another, we find a minimum mass of 22 Msun for the system. Radiative
transfer models are constructed to explain both the continuum and methyl
cyanide line observations of each source. Power-law distributions of both
density and temperature are derived. Density distributions close to the
free-fall value, r^-1.5, are found for both components, suggesting continuing
accretion. The derived luminosities suggest the two sources have equivalent
zero-age main sequence (ZAMS) spectral type B0.5 - B0. The nebular masses
derived from the continuum observations are about 5 Msun for source A and 4
Msun for source C. A velocity gradient previously detected may be explained by
unresolved binary rotation with a small velocity difference.Comment: 38 pages, 9 figures, accepted by The Astrophysical Journa

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