279 research outputs found
Coreshine in L1506C - Evidence for a primitive big-grain component or indication for a turbulent core history?
The recently discovered coreshine effect can aid in exploring the core
properties and in probing the large grain population of the ISM. We discuss the
implications of the coreshine detected from the molecular cloud core L1506C in
the Taurus filament for the history of the core and the existence of a
primitive ISM component of large grains becoming visible in cores. The
coreshine surface brightness of L1506C is determined from IRAC Spitzer images
at 3.6 micron. We perform grain growth calculations to estimate the grain size
distribution in model cores similar in gas density, radius, and turbulent
velocity to L1506C. Scattered light intensities at 3.6 micron are calculated
for a variety of MRN and grain growth distributions to compare with the
observed coreshine. For a core with the overall physical properties of L1506C,
no detectable coreshine is predicted for an MRN size distribution. Extending
the distribution to grain radii of about 0.65 m allows to reproduce the
observed surface brightness level in scattered light. Assuming the properties
of L1506C to be preserved, models for the growth of grains in cores do not
yield sufficient scattered light to account for the coreshine within the
lifetime of the Taurus complex. Only increasing the core density and the
turbulence amplifies the scattered light intensity to a level consistent with
the observed coreshine brightness. The grains could be part of primitive
omni-present large grain population becoming visible in the densest part of the
ISM, could grow under the turbulent dense conditions of former cores, or in
L1506C itself. In the later case, L1506C must have passed through a period of
larger density and stronger turbulence. This would be consistent with the
surprisingly strong depletion usually attributed to high column densities, and
with the large-scale outward motion of the core envelope observed today.Comment: 6 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
Three-dimensional Continuum Radiative Transfer Images of a Molecular Cloud Core Evolution
We analyze a three-dimensional smoothed particle hydrodynamics simulation of
an evolving and later collapsing pre-stellar core. Using a three-dimensional
continuum radiative transfer program, we generate images at 7 micron, 15
micron, 175 micron, and 1.3 mm for different evolutionary times and viewing
angles. We discuss the observability of the properties of pre-stellar cores for
the different wavelengths. For examples of non-symmetric fragments, it is shown
that, misleadingly, the density profiles derived from a one-dimensional
analysis of the corresponding images are consistent with one-dimensional core
evolution models. We conclude that one-dimensional modeling based on column
density interpretation of images does not produce reliable structural
information and that multidimensional modeling is required.Comment: accepted by ApJL, 4 pages, 4 figure
Oscillating Starless Cores: The Nonlinear Regime
In a previous paper, we modeled the oscillations of a thermally-supported
(Bonnor-Ebert) sphere as non-radial, linear perturbations following a standard
analysis developed for stellar pulsations. The predicted column density
variations and molecular spectral line profiles are similar to those observed
in the Bok globule B68 suggesting that the motions in some starless cores may
be oscillating perturbations on a thermally supported equilibrium structure.
However, the linear analysis is unable to address several questions, among them
the stability, and lifetime of the perturbations. In this paper we simulate the
oscillations using a three-dimensional numerical hydrodynamic code. We find
that the oscillations are damped predominantly by non-linear mode-coupling, and
the damping time scale is typically many oscillation periods, corresponding to
a few million years, and persisting over the inferred lifetime of gobules.Comment: 7 pages, 7 figures, accepted by Ap
Mid-infrared observations of the SGR 1900+14 error box
We report on mid-infrared observations of the compact stellar cluster located
in the proximity of SGR 1900+14, and the radio/X-ray position of this
soft-gamma repeater. Observations were performed in May and June of 2001 when
the bursting source was in an active state. At the known radio and X-ray
position of the SGR we did not detect transient mid-IR activity, although the
observations were performed only hours before and after an outburst in the
high-energy band.Comment: 4 pages, 3 figures, to appear in "Gamma-Ray Burst and Afterglow
Astronomy 2001", Woods Hole; 5-9 Nov, 200
Ray-tracing for complex astrophysical high-opacity structures
We present a ray-tracing technique for radiative transfer modeling of complex
three-dimensional (3D) structures which include dense regions of high optical
depth like in dense molecular clouds, circumstellar disks, envelopes of evolved
stars, and dust tori around active galactic nuclei. The corresponding continuum
radiative transfer problem is described and the numerical requirements for
inverse 3D density and temperature modeling are defined. We introduce a
relative intensity and transform the radiative transfer equation along the rays
to solve machine precision problems and to relax strong gradients in the source
term. For the optically thick regions where common ray-tracers are forced to
perform small trace steps, we give two criteria for making use of a simple
approximative solver crossing the optically thick region quickly. Using an
example of a density structure with optical depth changes of 6 orders of
magnitude and sharp temperature variations, we demonstrate the accuracy of the
proposed scheme using a common 5th-order Runge-Kutta ray-tracer with adaptive
step size control. In our test case, the gain in computational speed is about a
factor of 870. The method is applied to calculate the temperature distribution
within a massive molecular cloud core for different boundary conditions for the
radiation field.Comment: 21 pages, 5 figures to appear in Astrophysical Journa
Sensitive Limits on the Water Abundance in Cold Low Mass Molecular Cores
We present SWAS observations of water vapor in two cold star-less clouds, B68
and Core D in rho Ophiuchus. Sensitive non-detections of the 1(10)-1(01)
transition of o-H2O are reported for each source. Both molecular cores have
been previously examined by detailed observations that have characterized the
physical structure. Using these rather well defined physical properties and a
Monte-Carlo radiation transfer model we have removed one of the largest
uncertainties from the abundance calculation and set the lowest water abundance
limit to date in cold low-mass molecular cores. These limits are < 3 x 10^{-8}
(relative to H2) and < 8 x 10^{-9} in B68 and rho Oph D, respectively. Such low
abundances confirm the general lack of ortho-water vapor in cold (T < 20 K)
cores. Provided that the ortho/para ratio of water is not near zero, these
limits are well below theoretical predictions and appear to support the
suggestion that most of the water in dense low-mass cores is frozen onto the
surfaces of cold dust grains.Comment: 12 pages, 3 figures, accepted by Astrophysical Journal Letter
The Intrinsic Shapes of Molecular Cloud Fragments over a Range of Length Scales
We decipher intrinsic three-dimensional shape distributions of molecular
clouds, cloud cores, Bok globules, and condensations using recently compiled
catalogues of observed axis ratios for these objects mapped in carbon monoxide,
ammonia, through optical selection, or in continuum dust emission. We apply
statistical techniques to compare assumed intrinsic axis ratio distributions
with observed projected axis ratio distributions. Intrinsically triaxial shapes
produce projected distributions which agree with observations. Molecular clouds
mapped in CO are intrinsically triaxial but more nearly prolate than
oblate, while the smaller cloud cores, Bok globules, and condensations are also
intrinsically triaxial but more nearly oblate than prolate.Comment: 12 pages, 11 figures. Version with color figures can be found at
http://www.astro.uwo.ca/~cjones/ or http://www.astro.uwo.ca/~basu/. To appear
in ApJ, 10 April 2002, v. 569, no.
Detecting scattered light from low-mass molecular cores at 3.6 m - Impact of global effects on the observation of coreshine
Recently discovered scattered light at 3-5 m from low-mass cores
(so-called "coreshine") reveals the presence of grains around 1 m, which
is larger than the grains found in the low-density interstellar medium. But
only about half of the 100+ cores investigated so far show the effect. This
prompts further studies on the origin of this detection rate. From the 3D
continuum radiative transfer equation, we derive the expected scattered light
intensity from a core placed in an arbitrary direction seen from Earth. We use
the approximation of single scattering, consider extinction up to 2nd-order
Taylor approximation, and neglect spatial gradients in the dust size
distribution. The impact of the directional characteristics of the scattering
on the detection of scattered light from cores is calculated for a given grain
size distribution, and local effects like additional radiation field components
are discussed. The surface brightness profiles of a core with a 1D density
profile are calculated for various Galactic locations, and the results are
compared to the approximate detection limits. We find that for optically thin
radiation and a constant size distribution, a simple limit for detecting
scattered light from a low-mass core can be derived that holds for grains with
sizes smaller than 0.5 m. The extinction by the core prohibits detection
in bright parts of the Galactic plane, especially near the Galactic center. For
scattered light received from low-mass cores with grain sizes beyond 0.5
m, the directional characteristics of the scattering favors the detection
of scattered light above and below the Galactic center, and to some extent near
the Galactic anti-center. We identify the local incident radiation field as the
major unknown causing deviations from this simple scheme.Comment: 10 pages, 10 figures, accepted by Astronomy & Astrophysic
Upper limit for the D2H+ ortho-to-para ratio in the prestellar core 16293E (CHESS)
The H3+ ion plays a key role in the chemistry of dense interstellar gas
clouds where stars and planets are forming. The low temperatures and high
extinctions of such clouds make direct observations of H3+ impossible, but lead
to large abundances of H2D+ and D2H+, which are very useful probes of the early
stages of star and planet formation. The ground-state rotational ortho-D2H+
111-000 transition at 1476.6 GHz in the prestellar core 16293E has been
searched for with the Herschel/HIFI instrument, within the CHESS (Chemical
HErschel Surveys of Star forming regions) Key Program. The line has not been
detected at the 21 mK km/s level (3 sigma integrated line intensity). We used
the ortho-H2D+ 110-111 transition and para-D2H+ 110-101 transition detected in
this source to determine an upper limit on the ortho-to-para D2H+ ratio as well
as the para-D2H+/ortho-H2D+ ratio from a non-LTE analysis. The comparison
between our chemical modeling and the observations suggests that the CO
depletion must be high (larger than 100), with a density between 5e5 and 1e6
cm-3. Also the upper limit on the ortho-D2H+ line is consistent with a low gas
temperature (~ 11 K) with a ortho-to-para ratio of 6 to 9, i.e. 2 to 3 times
higher than the value estimated from the chemical modeling, making it
impossible to detect this high frequency transition with the present state of
the art receivers.Comment: Accepted in A&
Morphological analysis of the cm-wave continuum in the dark cloud LDN1622
The spectral energy distribution of the dark cloud LDN1622, as measured by
Finkbeiner using WMAP data, drops above 30GHz and is suggestive of a Boltzmann
cutoff in grain rotation frequencies, characteristic of spinning dust emission.
LDN1622 is conspicuous in the 31 GHz image we obtained with the Cosmic
Background Imager, which is the first cm-wave resolved image of a dark cloud.
The 31GHz emission follows the emission traced by the four IRAS bands. The
normalised cross-correlation of the 31 GHz image with the IRAS images is higher
by 6.6sigma for the 12um and 25um bands than for the 60um and 100um bands:
C(12+25) = 0.76+/-0.02 and C(60+100) = 0.64+/-0.01.
The mid-IR -- cm-wave correlation in LDN 1622 is evidence for very small
grain (VSG) or continuum emission at 26-36GHz from a hot molecular phase. In
dark clouds and their photon-dominated regions (PDRs) the 12um and 25um
emission is attributed to stochastic heating of the VSGs. The mid-IR and
cm-wave dust emissions arise in a limb-brightened shell coincident with the PDR
of LDN1622, where the incident UV radiation from the Ori OB1b association heats
and charges the grains, as required for spinning dust.Comment: accepted for publication in ApJ - the complete article with
uncompressed figures may be downloaded from
http://www.das.uchile.cl/~simon/ftp/l1622.pd
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