135 research outputs found
Large-Scale Gravitational Instability and Star Formation in the Large Magellanic Cloud
Large-scale star formation in disk galaxies is hypothesized to be driven by
global gravitational instability. The observed gas surface density is commonly
used to compute the strength of gravitational instability, but according to
this criterion star formation often appears to occur in gravitationally stable
regions. One possible reason is that the stellar contribution to the
instability has been neglected. We have examined the gravitational instability
of the Large Magellanic Cloud (LMC) considering the gas alone, and considering
the combination of collisional gas and collisionless stars. We compare the
gravitationally unstable regions with the on-going star formation revealed by
Spitzer observations of young stellar objects. Although only 62% of the massive
young stellar object candidates are in regions where the gas alone is unstable,
some 85% lie in regions unstable due to the combination of gas and stars. The
combined stability analysis better describes where star formation occurs. In
agreement with other observations and numerical models, a small fraction of the
star formation occurs in regions with gravitational stability parameter Q > 1.
We further measure the dependence of the star formation timescale on the
strength of gravitational instability, and quantitatively compare it to the
exponential dependence expected from numerical simulations.Comment: Accepted for publication in ApJ, 10 pages, 5 figure
Global Health Needs Modernized Containment Strategies to Prepare for the Next Pandemic
COVID-19 continues to be a public health crisis, while severely impacting global financial markets causing significant economic and social hardship. As with any emerging disease, pharmaceutical interventions required time, emphasizing the initial and continuing need for non-pharmaceutical interventions. We highlight the role of anthropological and historical perspectives to inform approaches to non-pharmaceutical interventions for future preparedness. The National Academy of Medicine, a not-for-profit, non-governmental US-based medical watchdog organization, published a key document early in the COVID-19 pandemic which points to inadequate quarantine and containment infrastructure as a significant obstacle to an effective pandemic response. In considering how to implement effective quarantine policies and infrastructure, we argue that it is essential to take a longitudinal approach to assess interventions that have been effective in past pandemics while simultaneously addressing and eliminating the negative socio-historical legacies of ineffective quarantine practices. Our overview reinforces the need for social equity and compassion when implementing containment
Star Formation in Isolated Disk Galaxies. I. Models and Characteristics of Nonlinear Gravitational Collapse
We model gravitational collapse leading to star formation in a wide range of
isolated disk galaxies using a three-dimensional, smoothed particle
hydrodynamics code. The model galaxies include a dark matter halo and a disk of
stars and isothermal gas. Absorbing sink particles are used to directly measure
the mass of gravitationally collapsing gas. They reach masses characteristic of
stellar clusters. In this paper, we describe our galaxy models and numerical
methods, followed by an investigation of the gravitational instability in these
galaxies. Gravitational collapse forms star clusters with correlated positions
and ages, as observed, for example, in the Large Magellanic Cloud.
Gravitational instability alone acting in unperturbed galaxies appears
sufficient to produce flocculent spiral arms, though not more organized
patterns. Unstable galaxies show collapse in thin layers in the galactic plane;
associated dust will form thin dust lanes in those galaxies, in agreement with
observations. (abridged)Comment: 49 pages, 22 figures, to appear in ApJ (July, 2005), version with
high quality color images can be fond in
http://research.amnh.org/~yuexing/astro-ph/0501022.pd
An Interstellar Conduction Front Within a Wolf-Rayet Ring Nebula Observed with the GHRS
With the High Resolution Spectrograph aboard the Hubble Space Telescope we
obtained high signal-to-noise (S/N > 200 - 600 per 17 km/s resolution element)
spectra of narrow absorption lines toward the Wolf-Rayet star HD 50896. The
ring nebula S308 that surrounds this star is thought to be caused by a
pressure-driven bubble bounded by circumstellar gas (most likely from a red
supergiant or luminous blue variable progenitor) pushed aside by a strong
stellar wind. Our observation has shown for the first time that blueshifted
(approximately 70 km/s relative to the star) absorption components of C IV and
N V arise in a conduction front between the hot interior of the bubble and the
cold shell of swept-up material. These lines set limits on models of the
conduction front. Nitrogen in the shell appears to be overabundant by a factor
~10. The P Cygni profiles of N V and C IV are variable, possibly due to a
suspected binary companion to HD 50896.Comment: 32 pages, Latex, to appear in the Astrophysical Journal, April, 199
Mushroom-Shaped Structures as Tracers of Buoyant Flow in the Galactic Disk
Recent HI emission observations of the Southern Galactic hemisphere have
revealed a mushroom-like structure extending from z=-70 to -450 pc, composed of
a stem and a cap. Similar structures occur in three-dimensional simulations of
a dynamic galactic disk driven by isolated and clustered supernovae. Using
these simulations, we show that hot gas in the Galactic disk that is not
evacuated through chimneys expands into the cooler gas of the thick disk,
forming mushroom-shaped structures. This new class of objects traces buoyant
flow of hot gas into the thick disk.Comment: Accepted for publication in ApJ Letters. Latex manuscript, 3 figures
(4 postsript files
A systematic review of the data, methods and environmental covariates used to map Aedes-borne arbovirus transmission risk
BACKGROUND: Aedes (Stegomyia)-borne diseases are an expanding global threat, but gaps in surveillance make comprehensive and comparable risk assessments challenging. Geostatistical models combine data from multiple locations and use links with environmental and socioeconomic factors to make predictive risk maps. Here we systematically review past approaches to map risk for different Aedes-borne arboviruses from local to global scales, identifying differences and similarities in the data types, covariates, and modelling approaches used. METHODS: We searched on-line databases for predictive risk mapping studies for dengue, Zika, chikungunya, and yellow fever with no geographical or date restrictions. We included studies that needed to parameterise or fit their model to real-world epidemiological data and make predictions to new spatial locations of some measure of population-level risk of viral transmission (e.g. incidence, occurrence, suitability, etc.). RESULTS: We found a growing number of arbovirus risk mapping studies across all endemic regions and arboviral diseases, with a total of 176 papers published 2002-2022 with the largest increases shortly following major epidemics. Three dominant use cases emerged: (i) global maps to identify limits of transmission, estimate burden and assess impacts of future global change, (ii) regional models used to predict the spread of major epidemics between countries and (iii) national and sub-national models that use local datasets to better understand transmission dynamics to improve outbreak detection and response. Temperature and rainfall were the most popular choice of covariates (included in 50% and 40% of studies respectively) but variables such as human mobility are increasingly being included. Surprisingly, few studies (22%, 31/144) robustly tested combinations of covariates from different domains (e.g. climatic, sociodemographic, ecological, etc.) and only 49% of studies assessed predictive performance via out-of-sample validation procedures. CONCLUSIONS: Here we show that approaches to map risk for different arboviruses have diversified in response to changing use cases, epidemiology and data availability. We identify key differences in mapping approaches between different arboviral diseases, discuss future research needs and outline specific recommendations for future arbovirus mapping
The Distribution of Pressures in a Supernova-Driven Interstellar Medium. I. Magnetized Medium
Observations have suggested substantial departures from pressure equilibrium
in the interstellar medium (ISM) in the plane of the Galaxy, even on scales
under 50 pc. Nevertheless, multi-phase models of the ISM assume at least
locally isobaric gas. The pressure then determines the density reached by gas
cooling to stable thermal equilibrium. We use numerical models of the
magnetized ISM to examine the consequences of supernova driving for
interstellar pressures. In this paper we examine a (200 pc)^3 periodic domain
threaded by magnetic fields. Individual parcels of gas at different pressures
reach widely varying points on the thermal equilibrium curve: no unique set of
phases is found, but rather a dynamically-determined continuum of densities and
temperatures. A substantial fraction of the gas remains entirely out of thermal
equilibrium. Our results appear consistent with observations of interstellar
pressures. They also suggest that the high pressures observed in molecular
clouds may be due to ram pressures in addition to gravitational forces. Much of
the gas in our model lies far from equipartition between thermal and magnetic
pressures, with ratios ranging from 0.1 to and ratios of uniform to
fluctuating magnetic field of 0.5--1. Our models show broad pressure
probability distribution functions with log-normal functional forms produced by
both shocks and rarefaction waves, rather than power-law distributions produced
by isolated supernova remnants. The width of the distribution can be described
quantitatively by a formula derived from the work of Padoan, Nordlund, & Jones
(1997).Comment: Revised version submitted to ApJ, 10 figures, 6 color. Minor
revisions onl
Simulating the formation of molecular clouds. I. Slow formation by gravitational collapse from static initial conditions
We study the formation of H2 in the ISM, using a modified version of the
astrophysical magnetohydrodynamical code ZEUS-MP that includes a
non-equilibrium treatment of the formation and destruction of H2. We examine
two different approximations to treat the shielding of H2 against
photodissociation: a local approximation, which gives us a solid lower bound on
the amount of shielding, and a method based on ray-tracing that is considerably
more accurate in some circumstances but that produces results that are harder
to clearly interpret. Either approximation allows one to perform
three-dimensional high-resolution simulations of cloud formation with only
modest computational resources. We also include a detailed treatment of the
thermal behaviour of the gas.
In this paper, we focus on the problem of molecular cloud formation in
gravitationally unstable, initially static gas. We show that in these
conditions, and for initial densities consistent with those observed in the
cold, neutral atomic phase of the interstellar medium, H2 formation occurs on a
timescale t > 10 Myr, comparable to or longer than the gravitational free-fall
timescale of the cloud. We also show that the collapsing gas very quickly
reaches thermal equilibrium and that the equation of state of the gas is
generally softer than isothermal.
Finally, we demonstrate that although these results show little sensitivity
to variations in most of our simulation parameters, they are highly sensitive
to the assumed initial density n_i. Reducing n_i significantly increases the
cloud formation timescale and decreases the amount of hydrogen ultimately
converted to H2. (Abridged).Comment: 89 pages, 40 figures, AASTex. Results section significantly revised
and extended. Includes results from a large number of new simulations
performed using a treatment of H2 photodissociation based on ray-tracing.
This version matches that accepted by ApJ
- …