680 research outputs found
Massive envelopes and filaments in the NGC 3603 star forming region
The formation of massive stars and their arrival on the zero-age
main-sequence occurs hidden behind dense clouds of gas and dust. In the giant
Hii region NGC 3603, the radiation of a young cluster of OB stars has dispersed
dust and gas in its vicinity. At a projected distance of 2:5 pc from the
cluster, a bright mid-infrared (mid-IR) source (IRS 9A) had been identified as
a massive young stellar object (MYSO), located on the side of a molecular clump
(MM2) of gas facing the cluster. We investigated the physical conditions in
MM2, based on APEX sub-mm observations using the SABOCA and SHFI instruments,
and archival ATCA 3 mm continuum and CS spectral line data. We resolved MM2
into several compact cores, one of them closely associated with IRS 9A. These
are likely infrared dark clouds as they do not show the typical hot-core
emission lines and are mostly opaque against the mid-IR background. The compact
cores have masses of up to several hundred times the solar mass and gas
temperatures of about 50 K, without evidence of internal ionizing sources. We
speculate that IRS 9A is younger than the cluster stars, but is in an
evolutionary state after that of the compact cores
Molecular CO outflows in the L1641-N cluster: kneading a cloud core
We present results of 1.3mm interferometric and single-dish observations of
the center of the L1641-N cluster in Orion. Single-dish wide-field continuum
and CO(2-1) observations reveal the presence of several molecular outflows
driven by deeply embedded protostellar sources. At higher angular resolution,
the dominant millimeter source in the cluster center is resolved into a pair of
protostars (L1641-N-MM1 and MM3), each driving a collimated outflow, and a more
extended, clumpy core. Low-velocity CO line-wing emission is widely spread over
much of the cluster area. We detect and map the distribution of several other
molecular transitions (13CO, C18O, 13CS, SO, CH3OH, CH3CN, and OCS). CH3CN and
OCS may indicate the presence of a hot corino around L1641-N-MM1. We
tentatively identify a velocity gradient over L1641-N-MM1 in CH3CN and OCS,
oriented roughly perpendicular to the outflow direction, perhaps indicative of
a circumstellar disk. An analysis of the energy and momentum load of the CO
outflows, along with the notion that apparently a large volume fraction is
affected by the multiple outflow activity, suggests that outflows from a
population of low-mass stars might have a significant impact on clustered (and
potentially high-mass) star formation.Comment: 16 pages plus 10 figures accepted by AJ, full resolution version
available at http://www.eso.org/~tstanke/preprints.htm
Unveiling the near-infrared structure of the massive-young stellar object NGC 3603 IRS 9A with sparse aperture masking and spectroastrometry
Contemporary theory holds that massive stars gather mass during their initial
phases via accreting disk-like structures. However, conclusive evidence for
disks has remained elusive for the most massive young objects. This is mainly
due to significant observational challenges. Incisive studies, even targeting
individual objects, are therefore relevant to the progression of the field. NGC
3603 IRS 9A* is a young massive stellar object still surrounded by an envelope
of molecular gas. Previous mid-infrared observations with long-baseline
interferometry provided evidence for a disk of 50 mas diameter at its core.
This work aims at a comprehensive study of the physics and morphology of IRS 9A
at near-infrared wavelengths. New sparse aperture masking interferometry data
taken with NACO/VLT at Ks and Lp filters were obtained and analysed together
with archival CRIRES spectra of the H2 and BrG lines. The calibrated
visibilities recorded at Ks and Lp bands suggest the presence of a partially
resolved compact object of 30 mas at the core of IRS 9A, together with the
presence of over-resolved flux. The spectroastrometric signal of the H2 line
shows that this spectral feature proceeds from the large scale extended
emission (300 mas) of IRS 9A, while the BrG line appears to be formed at the
core of the object (20 mas). This scenario is consistent with the brightness
distribution of the source for near- and mid-infrared wavelengths at various
spatial scales. However, our model suffers from remaining inconsistencies
between SED modelling and the interferometric data. Moreover, the BrG
spectroastrometric signal indicates that the core of IRS 9A exhibits some form
of complexity such as asymmetries in the disk. Future high-resolution
observations are required to confirm the disk/envelope model and to flesh out
the details of the physical form of the inner regions of IRS 9A.Comment: Accepted to be published in Astronomy & Astrophysics, 13 pages, 14
figure
A mid-IR survey of the L 1641-N region with ISOCAM
We present an analysis of the L 1641 outflow region using broad-band and
narrow-band imaging data at mid-infrared wavelengths from ISOCAM. We detect a
total of 34 sources in the x region covered by
the broad-band filters. Four of these sources have no reported detection in
previous studies of the region. We find that the source previously identified
as the near-IR counter-part to the IRAS detected point-source (IRAS 05338-0624)
is not the brightest source in the wavelength region of the IRAS 12 \micron\
filter. We find instead that a nearby object (within the beam of IRAS and not
detected at near-IR wavelengths) outshines all others sources in the area by a
factor of 2. We submit that this source is likely to be the IRAS detected
point source. A comparison of the near-IR (J-H vs H-K) and mid-IR (J-K vs [6.7
um]-[14 um]) color-color plots shows only four sources with excess emission at
near-IR wavelengths, but atleast 85% of all sources show excess emission at
mid-IR wavelengths. The CVF spectra suggest a range of evolutionary status in
the program stars ranging from embedded YSOs to the young disks. When combined
with optical and near-IR age estimates, these results show active current
star-formation in the region that has been on-going for at least 2 Myr.Comment: Submitted to ApJ. Abstracted edited for arXiv submission Replaced by
version accepted by Ap
The thermal state of molecular clouds in the Galactic Center: evidence for non-photon-driven heating
We used the Atacama Pathfinder Experiment (APEX) 12 m telescope to observe
the J_KaKc=3_03-2_02, 3_22-2_21, and 3_21-2_20 transitions of para-H_2CO at 218
GHz simultaneously to determine kinetic temperatures of the dense gas in the
central molecular zone (CMZ) of our Galaxy. The map extends over approximately
40 arcmin x 8 arcmin (~100x20 pc^2) along the Galactic plane with a linear
resolution of 1.2 pc. The strongest of the three lines, the H_2CO (3_03-2_02)
transition, is found to be widespread, and its emission shows a spatial
distribution similar to ammonia. The relative abundance of para-H_2CO is
0.5-1.2 10^{-9}, which is consistent with results from lower frequency H_2CO
absorption lines. Derived gas kinetic temperatures for individual molecular
clouds range from 50 K to values in excess of 100 K. While a systematic trend
toward (decreasing) kinetic temperature versus (increasing) angular distance
from the Galactic center (GC) is not found, the clouds with highest temperature
(T_kin > 100 K) are all located near the nucleus. For the molecular gas outside
the dense clouds, the average kinetic temperature is 65+/-10 K. The high
temperatures of molecular clouds on large scales in the GC region may be driven
by turbulent energy dissipation and/or cosmic-rays instead of photons. Such a
non-photon-driven thermal state of the molecular gas provides an excellent
template for the more distant vigorous starbursts found in ultraluminous
infrared galaxies (ULIRGs).Comment: 23 pages, 11 figures, A&A in pres
Effect of Grain Size and Preferred Crystal Texture on Acoustic Properties of 304 Stainless Steel
Several acoustic properties have been measured in an annealed round rolled bar of type 304 stainless steel. These properties were observed to depend upon both microstructure and the non-random crystal texture of this single phase, polycrystalline, face-centered cubic alloy. All measurements were made with longitudinal waves propagating parallel to the bar axis. The acoustic velocity was observed to be 0.3% higher near the outer surface than along the center of the bar, consistent with a stronger [111] texture in the outer fibers than at the center. The attenuation coefficient at the center was twice as large as at the outer surface. The measured difference in grain size at the center and surface was consistent with the observed attenuation difference. The acousto-elastic coefficient measured for a uniaxial tension stress applied along a bar diameter was at least 20% higher at the bar center than at its outer surface
Improving Taxonomic Delimitation of Fungal Species in the Age of Genomics and Phenomics
Species concepts have long provided a source of debate among biologists. These lively debates have been important for reaching consensus on how to communicate across scientific disciplines and for advancing innovative strategies to study evolution, population biology, ecology, natural history, and disease epidemiology. Species concepts are also important for evaluating variability and diversity among communities, understanding biogeographical distributions, and identifying causal agents of disease across animal and plant hosts. While there have been many attempts to address the concept of species in the fungi, there are several concepts that have made taxonomic delimitation especially challenging. In this review we discuss these major challenges and describe methodological approaches that show promise for resolving ambiguity in fungal taxonomy by improving discrimination of genetic and functional traits. We highlight the relevance of eco-evolutionary theory used in conjunction with integrative taxonomy approaches to improve the understanding of interactions between environment, ecology, and evolution that give rise to distinct species boundaries. Beyond recent advances in genomic and phenomic methods, bioinformatics tools and modeling approaches enable researchers to test hypothesis and expand our knowledge of fungal biodiversity. Looking to the future, the pairing of integrative taxonomy approaches with multi-locus genomic sequencing and phenomic techniques, such as transcriptomics and proteomics, holds great potential to resolve many unknowns in fungal taxonomic classification
Rate coefficients for rovibrational transitions in H_2 due to collisions with He
We present quantum mechanical and quasiclassical trajectory calculations of
cross sections for rovibrational transitions in ortho- and para-H_2 induced by
collisions with He atoms. Cross sections were obtained for kinetic energies
between 10^-4 and 3 eV, and the corresponding rate coefficients were calculated
for the temperature range 100<T<4000 K. Comparisons are made with previous
calculations.Comment: 21 pages, 2 figures, AAS, eps
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