1,552 research outputs found
A multi-scale, multi-wavelength source extraction method: getsources
We present a multi-scale, multi-wavelength source extraction algorithm called
getsources. Although it has been designed primarily for use in the far-infrared
surveys of Galactic star-forming regions with Herschel, the method can be
applied to many other astronomical images. Instead of the traditional approach
of extracting sources in the observed images, the new method analyzes fine
spatial decompositions of original images across a wide range of scales and
across all wavebands. It cleans those single-scale images of noise and
background, and constructs wavelength-independent single-scale detection images
that preserve information in both spatial and wavelength dimensions. Sources
are detected in the combined detection images by following the evolution of
their segmentation masks across all spatial scales. Measurements of the source
properties are done in the original background-subtracted images at each
wavelength; the background is estimated by interpolation under the source
footprints and overlapping sources are deblended in an iterative procedure. In
addition to the main catalog of sources, various catalogs and images are
produced that aid scientific exploitation of the extraction results. We
illustrate the performance of getsources on Herschel images by extracting
sources in sub-fields of the Aquila and Rosette star-forming regions. The
source extraction code and validation images with a reference extraction
catalog are freely available.Comment: 31 pages, 27 figures, to be published in Astronomy & Astrophysic
Massive Infrared-Quiet Dense Cores: Unveiling the Initial Conditions of High-Mass Star Formation
As Pr. Th. Henning said at the conference, cold precursors of high-mass stars
are now "hot topics". We here propose some observational criteria to identify
massive infrared-quiet dense cores which can host the high-mass analogs of
Class 0 protostars and pre-stellar condensations. We also show how far-infrared
to millimeter imaging surveys of entire complexes forming OB stars are starting
to unveil the initial conditions of high-mass star formation
Ammonia from cold high-mass clumps discovered in the inner Galactic disk by the ATLASGAL survey
The APEX Telescope Large Area Survey: The Galaxy (ATLASGAL) is an unbiased
continuum survey of the inner Galactic disk at 870 \mu m. It covers +/- 60 deg
in Galactic longitude and aims to find all massive clumps at various stages of
high-mass star formation in the inner Galaxy, particularly the earliest
evolutionary phases. We aim to determine properties such as the gas kinetic
temperature and dynamics of new massive cold clumps found by ATLASGAL. Most
importantly, we derived their kinematical distances from the measured line
velocities. We observed the ammonia (J,K) = (1,1) to (3,3) inversion
transitions toward 862 clumps of a flux-limited sample of submm clumps detected
by ATLASGAL and extracted 13CO (1-0) spectra from the Galactic Ring Survey
(GRS). We determined distances for a subsample located at the tangential points
(71 sources) and for 277 clumps whose near/far distance ambiguity is resolved.
Most ATLASGAL clumps are cold with rotational temperatures from 10-30 K. They
have a wide range of NH3 linewidths, which by far exceeds the thermal
linewidth, as well as a broad distribution of high column densities with an NH3
abundance in the range of 5 to 30 * 10^{-8}. We found an enhancement of clumps
at Galactocentric radii of 4.5 and 6 kpc. The high detection rate (87%)
confirms ammonia as an excellent probe of the molecular content of the massive,
cold clumps revealed by ATLASGAL. A clear trend of increasing rotational
temperatures and linewidths with evolutionary stage is seen for source samples
ranging from 24 \mu m dark clumps to clumps with embedded HII regions. The
survey provides the largest ammonia sample of high-mass star forming clumps and
thus presents an important repository for the characterization of statistical
properties of the clumps and the selection of subsamples for detailed,
high-resolution follow-up studies
Dust emission from young outflows: the case of L1157
We present new high-sensitivity 1.3 mm bolometer observations of the young
outflow L1157. These data show that the continuum emission arises from four
distinct components: a circumstellar disk, a protostellar envelope, an extended
flattened envelope --the dense remnant of the molecular cloud in which the
protostar was formed--, and the outflow itself, which represents ~20% of the
total flux. The outflow emission exhibits two peaks that are coincident with
the two strong shocks in the southern lobe of L1157. We show that the mm
continuum is dominated by thermal dust emission arising in the high velocity
material. The spectral index derived from the new 1.3 mm data and 850 mu
observations from Shirley et al. (2000), is ~5 in the outflow, significantly
higher than in the protostellar envelope (~3.5). This can be explained by an
important line contamination of the 850 mu map, and/or by different dust
characteristics in the two regions, possibly smaller grains in the post-shocks
regions of the outflow. Our observations show that bipolar outflows can present
compact emission peaks which must not be misinterpreted as protostellar
condensations when mapping star forming regions
Molecular Tracers of Embedded Star Formation in Ophiuchus
In this paper we analyze nine SCUBA cores in Ophiuchus using the
second-lowest rotational transitions of four molecular species (12CO, 13CO,
C18O, and C17O) to search for clues to the evolutionary state and
star-formation activity within each core. Specifically, we look for evidence of
outflows, infall, and CO depletion. The line wings in the CO spectra are used
to detect outflows, spectral asymmetries in 13CO are used to determine infall
characteristics, and a comparison of the dust emission (from SCUBA
observations) and gas emission (from C18O) is used to determine the fractional
CO freeze-out.
Through comparison with Spitzer observations of protostellar sources in
Ophiuchus, we discuss the usefulness of CO and its isotopologues as the sole
indicators of the evolutionary state of each core. This study is an important
pilot project for the JCMT Legacy Survey of the Gould Belt (GBS) and the
Galactic Plane (JPS), which intend to complement the SCUBA-2 dust continuum
observations with HARP observations of 12CO, 13CO, C18O, and C17O J = 3 - 2 in
order to determine whether or not the cold dust clumps detected by SCUBA-2 are
protostellar or starless objects.
Our classification of the evolutionary state of the cores (based on molecular
line maps and SCUBA observations) is in agreement with the Spitzer designation
for six or seven of the nine SCUBA cores. However, several important caveats
exist in the interpretation of these results, many of which large mapping
surveys like the GBS may be able to overcome to provide a clearer picture of
activity in crowded fields.Comment: 43 pages including 19 postscript figures. Accepted for publication in
the PAS
Size and polydispersity effect on the magnetization of densely packed magnetic nanoparticles
The magnetic properties of densely packed magnetic nanoparticles (MNP)
assemblies are investigated from Monte Carlo simulations. The case of iron
oxide nanoparticles is considered as a typical example of MNP. The main focus
is put on particle size and size polydispersity influences on the magnetization
curve. The particles are modeled as uniformly magnetized spheres isolated one
from each other by a non magnetic layer representing the organic coating. A
comparison with recent experimental results on FeO powder
samples differing by their size is given.Comment: To be published in the Journal of Applied Physics, to be found at
http://jap.aip.org
Three-dimensional electron microscopy of ribosomal chromatin in two higher plants: a cytochemical, immunocytochemical, and in situ hybridization approach.
We report the 3-D arrangement of DNA within the nucleolar subcomponents from two evolutionary distant higher plants, Zea mays and Sinapis alba. These species are particularly convenient to study the spatial organization of plant intranucleolar DNA, since their nucleoli have been previously reconstructed in 3-D from serial ultra-thin sections. We used the osmium ammine-B complex (a specific DNA stain) on thick sections of Lowicryl-embedded root fragments. Immunocytochemical techniques using anti-DNA antibodies and rDNA/rDNA in situ hybridization were also applied on ultra-thin sections. We showed on tilted images that the OA-B stains DNA throughout the whole thickness of the section. In addition, very low quantities of cytoplasmic DNA were stained by this complex, which is now the best DNA stain used in electron microscopy. Within the nucleoli the DNA was localized in the fibrillar centers, where large clumps of dense chromatin were also visible. In the two plant species intranucleolar chromatin forms a complex network with strands partially linked to chromosomal nucleolar-organizing regions identified by in situ hybridization. This study describes for the first time the spatial arrangement of the intranucleolar chromatin in nucleoli of higher plants using high-resolution techniques
On the fidelity of the core mass functions derived from dust column density data
Aims: We examine the recoverability and completeness limits of the dense core
mass functions (CMFs) derived for a molecular cloud using extinction data and a
core identification scheme based on two-dimensional thresholding.
Methods: We performed simulations where a population of artificial cores was
embedded into the variable background extinction field of the Pipe nebula. We
extracted the cores from the simulated extinction maps, constructed the CMFs,
and compared them to the input CMFs. The simulations were repeated using a
variety of extraction parameters and several core populations with differing
input mass functions and differing degrees of crowding.
Results: The fidelity of the observed CMF depends on the parameters selected
for the core extraction algorithm for our background. More importantly, it
depends on how crowded the core population is. We find that the observed CMF
recovers the true CMF reliably when the mean separation of cores is larger than
their mean diameter (f>1). If this condition holds, the derived CMF is accurate
and complete above M > 0.8-1.5 Msun, depending on the parameters used for the
core extraction. In the simulations, the best fidelity was achieved with the
detection threshold of 1 or 2 times the rms-noise of the extinction data, and
with the contour level spacings of 3 times the rms-noise. Choosing larger
threshold and wider level spacings increases the limiting mass. The simulations
show that when f>1.5, the masses of individual cores are recovered with a
typical uncertainty of 25-30 %. When f=1 the uncertainty is ~60 %. In very
crowded cases where f<1 the core identification algorithm is unable to recover
the masses of the cores adequately. For the cores of the Pipe nebula f~2.0 and
therefore the use of the method in that region is justified.Comment: 9 pages, 6 figures, accepted for publication in A&
Massive Protoplanetary Disks in the Trapezium Region
(abridged) We determine the disk mass distribution around 336 stars in the
young Orion Nebula cluster by imaging a 2.5' x 2.5' region in 3 mm continuum
emission with the Owens Valley Millimeter Array. For this sample of 336 stars,
we observe 3 mm emission above the 3-sigma noise level toward ten sources, six
of which have also been detected optically in silhouette against the bright
nebular background. In addition, we detect 20 objects that do not correspond to
known near-IR cluster members. Comparisons of our measured fluxes with longer
wavelength observations enable rough separation of dust emission from thermal
free-free emission, and we find substantial dust emission toward most objects.
For the ten objects detected at both 3 mm and near-IR wavelengths, eight
exhibit substantial dust emission. Excluding the high-mass stars and assuming a
gas-to-dust ratio of 100, we estimate circumstellar masses ranging from 0.13 to
0.39 Msun. For the cluster members not detected at 3 mm, images of individual
objects are stacked to constrain the mean 3 mm flux of the ensemble. The
average flux is detected at the 3-sigma confidence level, and implies an
average disk mass of 0.005 Msun, comparable to the minimum mass solar nebula.
The percentage of stars in Orion surrounded by disks more massive than ~0.1
Msun is consistent with the disk mass distribution in Taurus, and we argue that
massive disks in Orion do not appear to be truncated through close encounters
with high-mass stars. Comparison of the average disk mass and number of massive
dusty structures in Orion with similar surveys of the NGC 2024 and IC 348
clusters constrains the evolutionary timescales of massive circumstellar disks
in clustered environments.Comment: 27 pages, including 7 figures. Accepted by Ap
High mass star formation in the infrared dark cloud G11.11-0.12
We report detection of moderate to high-mass star formation in an infrared
dark cloud (G11.11-0.12) where we discovered class II methanol and water maser
emissions at 6.7 GHz and 22.2 GHz, respectively. We also observed the object in
ammonia inversion transitions. Strong emission from the (3,3) line indicates a
hot (~60 K) compact component associated with the maser emission. The line
width of the hot component (4 km/s), as well as the methanol maser detection,
are indicative of high mass star formation. To further constrain the physical
parameters of the source, we derived the spectral energy distribution (SED) of
the dust continuum by analysing data from the 2MASS survey, HIRAS, MSX, the
Spitzer Space Telescope, and interferometric 3mm observations. The SED was
modelled in a radiative transfer program: a) the stellar luminosity equals 1200
L_sun corresponding to a ZAMS star of 8 M_sun; b) the bulk of the envelope has
a temperature of 19 K; c) the mass of the remnant protostellar cloud in an area
8x10^17 cm or 15 arcsec across amounts to 500M_sun, if assuming standard dust
of the diffuse medium, and to about 60 M_sun, should the grains be fluffy and
have ice mantles; d) the corresponding visual extinction towards the star is a
few hundred magnitudes. The near IR data can be explained by scattering from
tenuous material above a hypothetical disk. The class II methanol maser lines
are spread out in velocity over 11 km/s. To explain the kinematics of the
masing spots, we propose that they are located in a Kepler disk at a distance
of about 250 AU. The dust temperatures there are around 150 K, high enough to
evaporate methanol--containing ice mantles.Comment: 10 pages, 6 figures, Accepted for publication in Astronomy &
Astrophysics Journa
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