237 research outputs found
Tracing the Conversion of Gas into Stars in Young Massive Cluster Progenitors
Whilst young massive clusters (YMCs; 10 M, age
100 Myr) have been identified in significant numbers, their
progenitor gas clouds have eluded detection. Recently, four extreme molecular
clouds residing within 200 pc of the Galactic centre have been identified as
having the properties thought necessary to form YMCs. Here we utilise far-IR
continuum data from the Herschel Infrared Galactic Plane Survey (HiGAL) and
millimetre spectral line data from the Millimetre Astronomy Legacy Team 90 GHz
Survey (MALT90) to determine their global physical and kinematic structure. We
derive their masses, dust temperatures and radii and use virial analysis to
conclude that they are all likely gravitationally bound -- confirming that they
are likely YMC progenitors. We then compare the density profiles of these
clouds to those of the gas and stellar components of the Sagittarius B2 Main
and North proto-clusters and the stellar distribution of the Arches YMC. We
find that even in these clouds -- the most massive and dense quiescent clouds
in the Galaxy -- the gas is not compact enough to form an Arches-like ( =
2x10 M, R = 0.4 pc) stellar distribution. Further
dynamical processes would be required to condense the resultant population,
indicating that the mass becomes more centrally concentrated as the
(proto)-cluster evolves. These results suggest that YMC formation may proceed
hierarchically rather than through monolithic collapse.Comment: 12 pages, 8 figures, 1 table. Accepted by MNRA
Feedback from massive stars at low metallicities : MUSE observations of N44 and N180 in the Large Magellanic Cloud
Accepted for publication in MNRAS, 27 pages, 21 figuresWe present MUSE integral field data of two HII region complexes in the Large Magellanic Cloud (LMC), N44 and N180. Both regions consist of a main superbubble and a number of smaller, more compact HII regions that formed on the edge of the superbubble. For a total of 11 HII regions, we systematically analyse the radiative and mechanical feedback from the massive O-type stars on the surrounding gas. We exploit the integral field property of the data and the coverage of the HeII5412 line to identify and classify the feedback-driving massive stars, and from the estimated spectral types and luminosity classes we determine the stellar radiative output in terms of the ionising photon flux . We characterise the HII regions in terms of their sizes, morphologies, ionisation structure, luminosity and kinematics, and derive oxygen abundances via emission line ratios. We analyse the role of different stellar feedback mechanisms for each region by measuring the direct radiation pressure, the pressure of the ionised gas, and the pressure of the shock-heated winds. We find that stellar winds and ionised gas are the main drivers of HII region expansion in our sample, while the direct radiation pressure is up to three orders of magnitude lower than the other terms. We relate the total pressure to the star formation rate per unit area, , for each region and find that stellar feedback has a negative effect on star formation, and sets an upper limit to as a function of increasing pressure.Peer reviewe
Mass loss rates and the mass evolution of star clusters
We describe the interplay between stellar evolution and dynamical mass loss
of evolving star clusters, based on the principles of stellar evolution and
cluster dynamics and on a grid of N-body simulations of cluster models. The
cluster models have different initial masses, different orbits, including
elliptical ones, and different initial density profiles. We use two sets of
cluster models: initially Roche-lobe filling and Roche-lobe underfilling. We
identify four distinct mass loss effects: (1) mass loss by stellar evolution,
(2) loss of stars induced by stellar evolution and (3) relaxation-driven mass
loss before and (4) after core collapse. Both the evolution-induced loss of
stars and the relaxation-driven mass loss need time to build up. This is
described by a delay-function of a few crossing times for Roche-lobe filling
clusters and a few half mass relaxation times for Roche-lobe underfilling
clusters. The relaxation-driven mass loss can be described by a simple power
law dependence of the mass dM/dt =-M^{1-gamma}/t0, (with M in Msun) where t0
depends on the orbit and environment of the cluster. Gamma is 0.65 for clusters
with a King-parameter W0=5 and 0.80 for more concentrated clusters with W0=7.
For initially Roche-lobe underfilling clusters the dissolution is described by
the same gamma=0.80. The values of the constant t0 are described by simple
formulae that depend on the orbit of the cluster. The mass loss rate increases
by about a factor two at core collapse and the mass dependence of the
relaxation-driven mass loss changes to gamma=0.70 after core collapse. We also
present a simple recipe for predicting the mass evolution of individual star
clusters with various metallicities and in different environments, with an
accuracy of a few percent in most cases. This can be used to predict the mass
evolution of cluster systems.Comment: 25 pages, 17 figures, 4 tables, 2 appendices; accepted for
publication in MNRA
VLASSICK: The VLA Sky Survey in the Central Kiloparsec
At a distance of 8 kpc, the center of our Galaxy is the nearest galactic
nucleus, and has been the subject of numerous key projects undertaken by great
observatories such as Chandra, Spitzer, and Herschel. However, there are still
no surveys of molecular gas properties in the Galactic center with less than
30" (1 pc) resolution. There is also no sensitive polarization survey of this
region, despite numerous nonthermal magnetic features apparently unique to the
central 300 parsecs. In this paper, we outline the potential the VLASS has to
fill this gap. We assess multiple considerations in observing the Galactic
center, and recommend a C-band survey with 10 micro-Jy continuum RMS and
sensitive to molecular gas with densities greater than 10^4 cm^{-3}, covering
17 square degrees in both DnC and CnB configurations ( resolution ~5"),
totaling 750 hours of observing time. Ultimately, we wish to note that the
upgraded VLA is not just optimized for fast continuum surveys, but has a
powerful correlator capable of simultaneously observing continuum emission and
dozens of molecular and recombination lines. This is an enormous strength that
should be fully exploited and highlighted by the VLASS, and which is ideally
suited for surveying the center of our Galaxy.Comment: 13 pages, 3 figures, a White Paper submitted to provide input in
planning the Very Large Array Sky Surve
Loss and waste in fish value chains: A review of the evidence from low and middle-income countries
Abstract This paper reviews the literature assessing fish waste and loss in low- and middle-income countries. We find significant variation in estimates of loss in different parts of the value chain, due in part to the diversity in approaches used to measure it. Studies of physical and nutritional loss are more common than those of quality or market force loss although nutritional loss has largely been studied with experimental rather than field-based approaches. Research gaps include the need for robust impact assessments of interventions to reduce fish loss and waste for consumers and actors and studies assessing the extent of loss affecting men and women differently. Standardized approaches are needed to accurately quantify loss in its various forms
A novel peanut allergoid is safe and effective in immunotherapy in a peanut allergy mouse model
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