2,068 research outputs found
APEX/SABOCA observations of small-scale structure of infrared-dark clouds I. Early evolutionary stages of star-forming cores
Infrared-dark clouds (IRDCs) harbor the early phases of cluster and high-mass
star formation and are comprised of cold (~20 K), dense (n > 10 cm)
gas. The spectral energy distribution (SED) of IRDCs is dominated by the
far-infrared and millimeter wavelength regime, and our initial Herschel study
examined IRDCs at the peak of the SED with high angular resolution. Here we
present a follow-up study using the SABOCA instrument on APEX which delivers
7.8" angular resolution at 350 micron, matching the resolution we achieved with
Herschel/PACS, and allowing us to characterize substructure on ~0.1pc scales.
Our sample of 11 nearby IRDCs are a mix of filamentary and clumpy morphologies,
and the filamentary clouds show significant hierarchical structure, while the
clumpy IRDCs exhibit little hierarchical structure. All IRDCs, regardless of
morphology, have about 14% of their total mass in small scale core-like
structures which roughly follow a trend of constant volume density over all
size scales. Out of the 89 protostellar cores we identified in this sample with
Herschel, we recover 40 of the brightest and re-fit their SEDs and find their
properties agree fairly well with our previous estimates ( ~ 19K). We detect
a new population of "cold cores" which have no 70 micron counterpart, but are
100 and 160 micron-bright, with colder temperatures ( ~ 16K). This latter
population, along with SABOCA-only detections, are predominantly low-mass
objects, but their evolutionary diagnostics are consistent with the earliest
starless or prestellar phase of cores in IRDCs.Comment: accepted to A&A. 28 pages, 27 figures. For full-resolution image
gallery, see http://www.mpia.de/~ragan/saboca.html (v2 includes only minor
typographical corrections, changed to agree with published version
Detection of Structure in Infrared-Dark Clouds with Spitzer: Characterizing Star Formation in the Molecular Ring
We have conducted a survey of a sample of infrared-dark clouds (IRDCs) with
the Spitzer Space Telescope in order to explore their mass distribution. We
present a method for tracing mass using dust absorption against the bright
Galactic background at 8 microns. The IRDCs in this sample are comprised of
tens of clumps, ranging in sizes from 0.02 to 0.3 pc in diameter and masses
from 0.5 to a few 10 Msun, the broadest dynamic range in any clump mass
spectrum study to date. Structure with this range in scales confirms that IRDCs
are the the precursors to stellar clusters in an early phase of fragmentation.
Young stars are distributed in the vicinity of the IRDCs, but the clumps are
typically not associated with stars and appear pre-stellar in nature. We find
an IRDC clump mass spectrum with a slope of 1.76 +/- 0.05 for masses from 30 to
3000 Msun. This slope is consistent with numerous studies, culled from a
variety of observational techniques, of massive star formation regions and is
close to the mass function of Galactic stellar clusters and star clusters in
other galaxies. We assert that the shape of the mass function is an intrinsic
and universal feature of massive star formation regions, that are the birth
sites of stellar clusters. As these clouds evolve and their constituent clumps
fragment, the mass spectrum will steepen and eventually assume the form of the
core mass function that is observed locally.Comment: Accepted to ApJ. 37 pages, 24 figures. Full-resolution versions of
the figures are available at
http://www.astro.lsa.umich.edu/~seragan/ftp/irdc_figs
High-fidelity view of the structure and fragmentation of the high-mass, filamentary IRDC G11.11-0.12
Star formation in molecular clouds is intimately linked to their internal
mass distribution. We present an unprecedentedly detailed analysis of the
column density structure of a high-mass, filamentary molecular cloud, namely
IRDC G11.11-0.12 (G11). We use two novel column density mapping techniques:
high-resolution (FWHM=2", or ~0.035 pc) dust extinction mapping in near- and
mid-infrared, and dust emission mapping with the Herschel satellite. These two
completely independent techniques yield a strikingly good agreement,
highlighting their complementarity and robustness. We first analyze the dense
gas mass fraction and linear mass density of G11. We show that G11 has a top
heavy mass distribution and has a linear mass density (M_l ~ 600 Msun pc^{-1})
that greatly exceeds the critical value of a self-gravitating, non-turbulent
cylinder. These properties make G11 analogous to the Orion A cloud, despite its
low star-forming activity. This suggests that the amount of dense gas in
molecular clouds is more closely connected to environmental parameters or
global processes than to the star-forming efficiency of the cloud. We then
examine hierarchical fragmentation in G11 over a wide range of size-scales and
densities. We show that at scales 0.5 pc > l > 8 pc, the fragmentation of G11
is in agreement with that of a self-gravitating cylinder. At scales smaller
than l < 0.5 pc, the results agree better with spherical Jeans' fragmentation.
One possible explanation for the change in fragmentation characteristics is the
size-scale-dependent collapse time-scale that results from the finite size of
real molecular clouds: at scales l < 0.5 pc, fragmentation becomes sufficiently
rapid to be unaffected by global instabilities.Comment: 8 pages, 8 figures, accepted to A&
Very Large Array Observations of Ammonia in Infrared-Dark Clouds II: Internal Kinematics
Infrared-dark clouds (IRDCs) are believed to be the birthplaces of rich
clusters and thus contain the earliest phases of high-mass star formation. We
use the Green Bank Telescope (GBT) and Very Large Array (VLA) maps of ammonia
(NH3) in six IRDCs to measure their column density and temperature structure
(Paper 1), and here, we investigate the kinematic structure and energy content.
We find that IRDCs overall display organized velocity fields, with only
localized disruptions due to embedded star formation. The local effects seen in
NH3 emission are not high velocity outflows but rather moderate (few km/s)
increases in the line width that exhibit maxima near or coincident with the
mid-infrared emission tracing protostars. These line width enhancements could
be the result of infall or (hidden in NH3 emission) outflow. Not only is the
kinetic energy content insufficient to support the IRDCs against collapse, but
also the spatial energy distribution is inconsistent with a scenario of
turbulent cloud support. We conclude that the velocity signatures of the IRDCs
in our sample are due to active collapse and fragmentation, in some cases
augmented by local feedback from stars.Comment: 15 pages, 12 figures, accepted for publication in Ap
Evidence that widespread star formation may be underway in G0.253+016, "The Brick"
Image cubes of differential column density as a function of dust temperature
are constructed for Galactic Centre molecular cloud G0.253+0.016 ("The Brick")
using the recently described PPMAP procedure. The input data consist of
continuum images from the Herschel Space Telescope in the wavelength range
70-500 m, supplemented by previously published interferometric data at 1.3
mm wavelength. While the bulk of the dust in the molecular cloud is consistent
with being heated externally by the local interstellar radiation field, our
image cube shows the presence, near one edge of the cloud, of a filamentary
structure whose temperature profile suggests internal heating. The structure
appears as a cool ( K) tadpole-like feature, pc in length, in
which is embedded a thin spine of much hotter ( 40-50 K) material. We
interpret these findings in terms of a cool filament whose hot central region
is undergoing gravitational collapse and fragmentation to form a line of
protostars. If confirmed, this would represent the first evidence of widespread
star formation having started within this cloud.Comment: 5 pages, 4 figures; accepted for publication in MNRAS Letter
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Kinetics governing phase separation of nanostructured Sn_xGe_(1–x) alloys
We have studied the dynamic phenomenon of Sn_xGe_(1–x)/Ge phase separation during deposition by molecular beam epitaxy on Ge(001) substrates. Phase separation leads to the formation of direct band gap semiconductor nanowire arrays embedded in Ge oriented along the [001] growth direction. The effect of strain and composition on the periodicity were decoupled by growth on Ge(001) and partially relaxed Si_yGe_(1–y)/Ge(001) virtual substrates. The experimental results are compared with three linear instability models of strained film growth and find good agreement with only one of the models for phase separation during dynamic growth
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