505 research outputs found
Oxygen in dense interstellar gas - the oxygen abundance of the star forming core rho Oph A
Oxygen is the third most abundant element in the universe, but its chemistry
in the interstellar medium is still not well understood. In order to critically
examine the entire oxygen budget, we attempt here initially to estimate the
abundance of atomic oxygen, O, in the only one region, where molecular oxygen,
O2, has been detected to date. We analyse ISOCAM-CVF spectral image data toward
rho Oph A to derive the temperatures and column densities of H2 at the
locations of ISO-LWS observations of two [OI] 3P_J lines. The intensity ratios
of the (J=1-2) 63um to (J=0-1) 145um lines largely exceed ten, attesting to the
fact that these lines are optically thin. This is confirmed by radiative
transfer calculations, making these lines suitable for abundance
determinations. For that purpose, we calculate line strengths and compare them
to the LWS observations. Excess [OI] emission is observed to be associated with
the molecular outflow from VLA 1623. For this region, we determine the physical
parameters, T and N(H2), from the CAM observations and the gas density, n(H2),
is determined from the flux ratio of the [O I]63um and [O I]145um lines. For
the oxygen abundance, our analysis leads to essentially three possibilities:
(1) Extended low density gas with standard ISM O-abundance, (2) Compact high
density gas with standard ISM O-abundance and (3) Extended high density gas
with reduced oxygen abundance, [O/H] ~ 2E-5. As option (1) disregards valid [O
I] 145um data, we do not find it very compelling; we favour option (3), as
lower abundances are expected as a result of chemical cloud evolution, but we
are not able to dismiss option (2) entirely. Observations at higher angular
resolution than offered by the LWS are required to decide between these
possibilities.Comment: Accepted for publication in A&
VLA Observations of the Infrared Dark Cloud G19.30+0.07
We present Very Large Array observations of ammonia (NH3) (1,1), (2,2), and
CCS (2_1-1_0) emission toward the Infrared Dark Cloud (IRDC) G19.30+0.07 at
~22GHz. The NH3 emission closely follows the 8 micron extinction. The NH3 (1,1)
and (2,2) lines provide diagnostics of the temperature and density structure
within the IRDC, with typical rotation temperatures of ~10 to 20K and NH3
column densities of ~10^15 cm^-2. The estimated total mass of G19.30+0.07 is
~1130 Msun. The cloud comprises four compact NH3 clumps of mass ~30 to 160
Msun. Two coincide with 24 micron emission, indicating heating by protostars,
and show evidence of outflow in the NH3 emission. We report a water maser
associated with a third clump; the fourth clump is apparently starless. A
non-detection of 8.4GHz emission suggests that the IRDC contains no bright HII
regions, and places a limit on the spectral type of an embedded ZAMS star to
early-B or later. From the NH3 emission we find G19.30+0.07 is composed of
three distinct velocity components, or "subclouds." One velocity component
contains the two 24 micron sources and the starless clump, another contains the
clump with the water maser, while the third velocity component is diffuse, with
no significant high-density peaks. The spatial distribution of NH3 and CCS
emission from G19.30+0.07 is highly anti-correlated, with the NH3 predominantly
in the high-density clumps, and the CCS tracing lower-density envelopes around
those clumps. This spatial distribution is consistent with theories of
evolution for chemically young low-mass cores, in which CCS has not yet been
processed to other species and/or depleted in high-density regions.Comment: 29 pages, 9 figures, accepted for publication by ApJ. Please contact
the authors for higher resolution versions of the figure
VLA Observations of the Infrared Dark Cloud G19.30+0.07
We present Very Large Array observations of ammonia (NH3) (1,1), (2,2), and
CCS (2_1-1_0) emission toward the Infrared Dark Cloud (IRDC) G19.30+0.07 at
~22GHz. The NH3 emission closely follows the 8 micron extinction. The NH3 (1,1)
and (2,2) lines provide diagnostics of the temperature and density structure
within the IRDC, with typical rotation temperatures of ~10 to 20K and NH3
column densities of ~10^15 cm^-2. The estimated total mass of G19.30+0.07 is
~1130 Msun. The cloud comprises four compact NH3 clumps of mass ~30 to 160
Msun. Two coincide with 24 micron emission, indicating heating by protostars,
and show evidence of outflow in the NH3 emission. We report a water maser
associated with a third clump; the fourth clump is apparently starless. A
non-detection of 8.4GHz emission suggests that the IRDC contains no bright HII
regions, and places a limit on the spectral type of an embedded ZAMS star to
early-B or later. From the NH3 emission we find G19.30+0.07 is composed of
three distinct velocity components, or "subclouds." One velocity component
contains the two 24 micron sources and the starless clump, another contains the
clump with the water maser, while the third velocity component is diffuse, with
no significant high-density peaks. The spatial distribution of NH3 and CCS
emission from G19.30+0.07 is highly anti-correlated, with the NH3 predominantly
in the high-density clumps, and the CCS tracing lower-density envelopes around
those clumps. This spatial distribution is consistent with theories of
evolution for chemically young low-mass cores, in which CCS has not yet been
processed to other species and/or depleted in high-density regions.Comment: 29 pages, 9 figures, accepted for publication by ApJ. Please contact
the authors for higher resolution versions of the figure
A self-consistent model of Galactic stellar and dust infrared emission and the abundance of polycyclic aromatic hydrocarbons
We present a self-consistent three-dimensional Monte-Carlo radiative transfer
model of the stellar and dust emission in the Milky-Way, and have computed
synthetic observations of the 3.6 to 100 microns emission in the Galactic
mid-plane. In order to compare the model to observations, we use the GLIMPSE,
MIPSGAL, and IRAS surveys to construct total emission spectra, as well as
longitude and latitude profiles for the emission. The distribution of stars and
dust is taken from the SKY model, and the dust emissivities includes an
approximation of the emission from polycyclic aromatic hydrocarbons in addition
to thermal emission. The model emission is in broad agreement with the
observations, but a few modifications are needed to obtain a good fit. Firstly,
by adjusting the model to include two major and two minor spiral arms rather
than four equal spiral arms, the fit to the longitude profiles for |l|>30
degrees can be improved. Secondly, introducing a deficit in the dust
distribution in the inner Galaxy results in a better fit to the shape of the
IRAS longitude profiles at 60 and 100 microns. With these modifications, the
model fits the observed profiles well, although it systematically
under-estimates the 5.8 and 8.0 microns fluxes. One way to resolve this
discrepancy is to increase the abundance of PAH molecules by 50% compared to
the original model, although we note that changes to the dust distribution or
radiation field may provide alternative solutions. Finally, we use the model to
quantify which stellar populations contribute the most to the heating of
different dust types, and which stellar populations and dust types contribute
the most to the emission at different wavelengths.Comment: Published in A&A. This version has been revised (compared to the
published version) to include additional references to previous work. Scripts
to reproduce the results in this paper can be found as supplementary material
on the A&A site, or at https://github.com/hyperion-rt/paper-galaxy-rt-mode
The Milky Way Project: A statistical study of massive star formation associated with infrared bubbles
The Milky Way Project citizen science initiative recently increased the
number of known infrared bubbles in the inner Galactic plane by an order of
magnitude compared to previous studies. We present a detailed statistical
analysis of this dataset with the Red MSX Source catalog of massive young
stellar sources to investigate the association of these bubbles with massive
star formation. We particularly address the question of massive triggered star
formation near infrared bubbles. We find a strong positional correlation of
massive young stellar objects (MYSOs) and H II regions with Milky Way Project
bubbles at separations of < 2 bubble radii. As bubble sizes increase, a
statistically significant overdensity of massive young sources emerges in the
region of the bubble rims, possibly indicating the occurrence of triggered star
formation. Based on numbers of bubble-associated RMS sources we find that
67+/-3% of MYSOs and (ultra)compact H II regions appear associated with a
bubble. We estimate that approximately 22+/-2% of massive young stars may have
formed as a result of feedback from expanding H II regions. Using MYSO-bubble
correlations, we serendipitously recovered the location of the recently
discovered massive cluster Mercer 81, suggesting the potential of such analyses
for discovery of heavily extincted distant clusters.Comment: 16 pages, 17 figures. Accepted for publication in ApJ, comments
welcome. Milky Way Project public data release available at
http://www.milkywayproject.org/dat
Revealing the environs of the remarkable southern hot core G327.3-0.6
We present a submm study of the massive hot core G327.3-0.6 that constrains
its physical parameters and environment. The APEX telescope was used to image
CO and N2H+ emission, to observe lines from other molecules toward a hot and a
cold molecular core, and to measure the continuum flux density of the hot core.
In the C18O J=3-2 line, two clumps were found, one associated with the HII
region G327.3-0.5 and the other associated with the hot core. An additional
cold clump is found 30 arcsec (0.4 pc) northeast of the hot core in bright N2H+
emission. From the the continuum data, we calculate a mass of 420 Msol and a
size of 0.1 pc for the hot core. A new, more accurate position of the hot core
is reported, which allows the association of the core with a bright
mid-infrared source. The luminosity of the hot core is estimated to be between
5 and 15 10^4 Lsol. This study revealed several different evolutionary stages
of massive star formation in the G327.3-0.6 region.Comment: APEX A&A special issue, accepte
High Mass Starless Cores
We report the identification of a sample of potential High-Mass Starless
Cores (HMSCs). The cores were discovered by comparing images of the fields
containing candidate High-Mass Protostellar Objects (HMPOs) at 1.2mm and
mid-infrared (8.3um; MIR) wavelengths. While the HMPOs are detected at both
wavelengths, several cores emitting at 1.2mm in the same fields show absorption
or no emission at the MIR wavelength. We argue that the absorption is caused by
cold dust. The estimated masses of a few 10^2Msun - 10^3 Msun and the lack of
IR emission suggests that they may be massive cold cores in a pre-stellar
phase, which could presumably form massive stars eventually. Ammonia (1,1) and
(2,2) observations of the cores indicate smaller velocity dispersions and lower
rotation temperatures compared to HMPOs and UCHII regions suggesting a
quiescent pre-stellar stage. We propose that these newly discovered cores are
good candidates for the HMSC stage in high-mass star-formation. This sample of
cores will allow us to study the high-mass star and cluster formation processes
at the earliest evolutionary stages.Comment: 7 pages, 3 figures, 1 table, to be published in ApJL, author names
replaced with comma separatio
Disks around Hot Stars in the Trifid Nebula
We report on mid-IR observations of the central region in the Trifid nebula,
carried out with ISOCAM in several broad-band infrared filters and in the low
resolution spectroscopic mode provided by the circular variable filter.
Analysis of the emission indicates the presence of a hot dust component (500 to
1000 K) and a warm dust component at lower temperatures (150-200 K) around
several members of the cluster exciting the HII region, and other stars
undetected at optical wavelengths. Complementary VLA observations suggest that
the mid-IR emission could arise from a dust cocoon or a circumstellar disk,
evaporated under the ionization of the central source and the exciting star of
the nebula. In several sources the silicate band is seen in
emission. One young stellar source shows indications of crystalline silicates
in the circumstellar dust.Comment: 4 pages with 1 figur
Embarking as Captain of the Ship for the Curriculum Committee
This commentary examines the curriculum chair’s responsibilities and discusses considerations when assuming this role, using a captain of the ship metaphor. From knowing the crew to managing a diverse set of responsibilities, the path to becoming an effective chair is challenging and each captain’s stripe must be earned. Advice is provided to assist with understanding the curriculum and governance processes, as well as the chair’s various roles and professional development. The need for both leadership and management is also emphasized
Effects of the magnetic moment interaction between nucleons on observables in the 3N continuum
The influence of the magnetic moment interaction of nucleons on
nucleon-deuteron elastic scattering and breakup cross sections and on elastic
scattering polarization observables has been studied. Among the numerous
elastic scattering observables only the vector analyzing powers were found to
show a significant effect, and of opposite sign for the proton-deuteron and
neutron-deuteron systems. This finding results in an even larger discrepancy
than the one previously established between neutron-deuteron data and
theoretical calculations. For the breakup reaction the largest effect was found
for the final-state-interaction cross sections. The consequences of this
observation on previous determinations of the ^1S_0 scattering lengths from
breakup data are discussed.Comment: 24 pages, 6 ps figures, 1 png figur
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