1,898 research outputs found
The Structure of a Low-Metallicity Giant Molecular Cloud Complex
To understand the impact of low metallicities on giant molecular cloud (GMC)
structure, we compare far infrared dust emission, CO emission, and dynamics in
the star-forming complex N83 in the Wing of the Small Magellanic Cloud. Dust
emission (measured by Spitzer as part of the S3MC and SAGE-SMC surveys) probes
the total gas column independent of molecular line emission and traces
shielding from photodissociating radiation. We calibrate a method to estimate
the dust column using only the high-resolution Spitzer data and verify that
dust traces the ISM in the HI-dominated region around N83. This allows us to
resolve the relative structures of H2, dust, and CO within a giant molecular
cloud complex, one of the first times such a measurement has been made in a
low-metallicity galaxy. Our results support the hypothesis that CO is
photodissociated while H2 self-shields in the outer parts of low-metallicity
GMCs, so that dust/self shielding is the primary factor determining the
distribution of CO emission. Four pieces of evidence support this view. First,
the CO-to-H2 conversion factor averaged over the whole cloud is very high 4-11
\times 10^21 cm^-2/(K km/s), or 20-55 times the Galactic value. Second, the
CO-to-H2 conversion factor varies across the complex, with its lowest (most
nearly Galactic) values near the CO peaks. Third, bright CO emission is largely
confined to regions of relatively high line-of-sight extinction, A_V >~ 2 mag,
in agreement with PDR models and Galactic observations. Fourth, a simple model
in which CO emerges from a smaller sphere nested inside a larger cloud can
roughly relate the H2 masses measured from CO kinematics and dust.Comment: 17 pages, 10 figures (including appendix), accepted for publication
in the Astrophysical Journa
Socio-Environmental Assessment of Gender Equality, Pastoralism, Agriculture and Climate Information in Rural Communities of Northern Tanzania
Investment in climate services in support of climate change adaptation has increased, especially in Sub-Saharan Africa. As this is a relatively new field of practice, little research is available to inform the design of these interventions. The aim of this research is to contribute to building knowledge around this theme. Given the gender dynamics inherent in decision making on livelihoods in Sub-Saharan Africa, we focus on differences in the use of climate services between men and women. We use quantitative and qualitative methods to critically discuss and review the barriers that exist for the use of climate information in making livelihoods related decisions. The results indicate that a link exists between households accessing productive assets and taking action on the basis of climate information, and revealed a large gender inequality across all the assessed variables. What emerged clearly is the need for interventions to be based on needs assessments to ensure that services provided are usable
Tracing the Bipolar Outflow from Orion Source I
Using CARMA, we imaged the 87 GHz SiO v=0 J=2-1 line toward Orion-KL with
0.45 arcsec angular resolution. The maps indicate that radio source I drives a
bipolar outflow into the surrounding molecular cloud along a NE--SW axis, in
agreement with the model of Greenhill et al. (2004). The extended high velocity
outflow from Orion-KL appears to be a continuation of this compact outflow.
High velocity gas extends farthest along a NW--SE axis, suggesting that the
outflow direction changes on time scales of a few hundred years.Comment: 4 pages, 4 figures; accepted for publication in Ap J Letter
An Adaptive Optics Survey for Close Protostellar Binaries
In order to test the hypothesis that Class I protostellar binary stars are a
product of ejections during the dynamical decay of non-hierarchical multiple
systems, we combined the results of new adaptive optics (AO) observations of
Class I protostars with our previously published AO data to investigate whether
Class I protostars with a widely separated companion (r>200 AU) are more likely
to also have a close companion (r<200 AU). In total, we observed 47 embedded
young stellar objects (YSOs) with either the Subaru natural guide star AO
system or the Keck laser guide star AO system. We found that targets with a
widely separated companion within 5,000 AU are not more likely to have a close
companion. However, targets with another YSO within a projected separation of
25,000 AU are much more likely to have a close companion. Most importantly,
every target with a close companion has another YSO within a projected
separation of 25,000 AU. We came to the same conclusions after considering a
restricted sample of targets within 500 pc and close companions wider than 50
AU to minimize incompleteness effects. The Orion star forming region was found
to have an excess of both close binaries and YSOs within 25,000 AU compared to
other star forming regions. We interpret these observations as strong evidence
that many close Class I binary stars form via ejections and that many of the
ejected stars become unbound during the Class I phase.Comment: 22 pages, 3 figures, 2 table
The EMPIRE Survey: Systematic Variations in the Dense Gas Fraction and Star Formation Efficiency from Full-Disk Mapping of M51
We present the first results from the EMPIRE survey, an IRAM large program
that is mapping tracers of high density molecular gas across the disks of nine
nearby star-forming galaxies. Here, we present new maps of the 3-mm transitions
of HCN, HCO+, and HNC across the whole disk of our pilot target, M51. As
expected, dense gas correlates with tracers of recent star formation, filling
the "luminosity gap" between Galactic cores and whole galaxies. In detail, we
show that both the fraction of gas that is dense, f_dense traced by HCN/CO, and
the rate at which dense gas forms stars, SFE_dense traced by IR/HCN, depend on
environment in the galaxy. The sense of the dependence is that high surface
density, high molecular gas fraction regions of the galaxy show high dense gas
fractions and low dense gas star formation efficiencies. This agrees with
recent results for individual pointings by Usero et al. 2015 but using unbiased
whole-galaxy maps. It also agrees qualitatively with the behavior observed
contrasting our own Solar Neighborhood with the central regions of the Milky
Way. The sense of the trends can be explained if the dense gas fraction tracks
interstellar pressure but star formation occurs only in regions of high density
contrast.Comment: 7 pages, 5 figures, ApJL accepte
Full-disc CO(1-0) mapping across nearby galaxies of the EMPIRE survey and the CO-to-H conversion factor
Carbon monoxide (CO) provides crucial information about the molecular gas
properties of galaxies. While CO has been targeted extensively,
isotopologues such as CO have the advantage of being less optically
thick and observations have recently become accessible across full galaxy
discs. We present a comprehensive new dataset of CO(1-0) observations
with the IRAM 30-m telescope of the full discs of 9 nearby spiral galaxies from
the EMPIRE survey at a spatial resolution of 1.5kpc. CO(1-0) is
mapped out to and detected at high signal-to-noise throughout our
maps. We analyse the CO(1-0)-to-CO(1-0) ratio () as a
function of galactocentric radius and other parameters such as the
CO(2-1)-to-CO(1-0) intensity ratio, the 70-to-160m flux
density ratio, the star-formation rate surface density, the star-formation
efficiency, and the CO-to-H conversion factor. We find that varies by
a factor of 2 at most within and amongst galaxies, with a median value of 11
and larger variations in the galaxy centres than in the discs. We argue that
optical depth effects, most likely due to changes in the mixture of
diffuse/dense gas, are favored explanations for the observed variations,
while abundance changes may also be at play. We calculate a spatially-resolved
CO(1-0)-to-H conversion factor and find an average value of
cm (K.km/s) over our sample with a standard
deviation of a factor of 2. We find that CO(1-0) does not appear to be a
good predictor of the bulk molecular gas mass in normal galaxy discs due to the
presence of a large diffuse phase, but it may be a better tracer of the mass
than CO(1-0) in the galaxy centres where the fraction of dense gas is
larger.Comment: accepted for publication in MNRA
Declining HIV-1 Prevalence and Incidence among Police Officers - A potential Cohort for HIV Vaccine Trials, in Dar es Salaam, Tanzania.
A safe effective and affordable HIV vaccine is the most cost effective way to prevent HIV infection worldwide. Current studies of HIV prevalence and incidence are needed to determine potentially suitable cohorts for vaccine studies. The prevalence and incidence of HIV-1 infection among the police in Dar es Salaam in 1996 were 13.8% and 19.6/1000 PYAR respectively. This study aimed at determining the current prevalence and incidence of HIV in a police cohort 10 years after a similar study was conducted. Police officers in Dar es Salaam, Tanzania were prospectively enrolled into the study from 2005 and followed-up in an incidence study three years later. HIV infection was determined by two sequential enzyme linked immunosorbent assays (ELISAs) in the prevalence study and discordant results between two ELISAs were resolved by a Western blot assay. Rapid HIV assays (SD Bioline and Determine) were used for the incidence study. A total of 1,240 police participated in the HIV prevalence study from August 2005 to November 2008. Of these, 1101 joined the study from August 2005-September 2007 and an additional 139 were recruited between October 2007 to November 2008 while conducting the incidence study. A total of 726 (70%) out of the 1043 eligible police participated in the incidence study.The overall HIV-1 prevalence was 65/1240 (5.2%). Females had a non-statistically significant higher prevalence of HIV infection compared to males 19/253, (7.5%) vs. 46/987 (4.7%) respectively (p = 0.07). The overall incidence of HIV-1 was 8.4 per 1000 PYAR (95% CI 4.68-14.03), and by gender was 8.8 and 6.9 per 1000 PYAR, among males and females respectively, (p = 0.82). The HIV prevalence and incidence among the studied police has declined over the past 10 years, and therefore this cohort is better suited for phase I/II HIV vaccine studies than for efficacy trials
Spitzer Imaging and Spectral Mapping of the Oxygen-Rich Supernova Remnant G292.0+1.8
We present mid-infrared continuum and emission line images of the Galactic
oxygen-rich supernova remnant (SNR) G292.0+1.8, acquired using the MIPS and IRS
instruments on the Spitzer Space Telescope. The MIPS 24 micron and 70 micron
images of G292.0+1.8 are dominated by continuum emission from a network of
filaments encircling the SNR. The morphology of the SNR, as seen in the
mid-infrared, resembles that seen in X-rays with the Chandra X-ray Observatory.
Most of the mid-infrared emission in the MIPS images is produced by
circumstellar dust heated in the non-radiative shocks around G292.0+1.8,
confirming the results of earlier mid-IR observations with AKARI. In addition
to emission from hot dust, we have also mapped atomic line emission between 14
micron and 36 micron using IRS spectral maps. The line emission is primarily
associated with the bright oxygen-rich optical knots, but is also detected from
fast-moving knots of ejecta. We confirm our earlier detection of 15-25 micron
emission characteristic of magnesium silicate dust in spectra of the
radiatively shocked ejecta. We do not detect silicon line emission from any of
the radiatively shocked ejecta in the southeast of the SNR, possibly because
that the reverse shock has not yet penetrated most of the Si-rich ejecta in
that region. This may indicate that G292.0+1.8 is less evolved in the southeast
than the rest of the SNR, and may be further evidence in favor of an asymmetric
SN explosion as proposed in recent X-ray studies of G292.0+1.8.Comment: 16 pages, 1 table, 7 figures, accepted for publication in Ap
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