429 research outputs found
On the mass function of star clusters
Clusters that form in total 10^3 < N < 10^5 stars (type II clusters) lose
their gas within a dynamical time as a result of the photo-ionising flux from O
stars. Sparser (type I) clusters get rid of their residual gas on a timescale
longer or comparable to the nominal crossing time and thus evolve approximately
adiabatically. This is also true for massive embedded clusters (type III) for
which the velocity dispersion is larger than the sound speed of the ionised
gas. On expelling their residual gas, type I and III clusters are therefore
expected to lose a smaller fraction of their stellar component than type II
clusters. We outline the effect this has on the transformation of the mass
function of embedded clusters (ECMF), which is directly related to the mass
function of star-cluster-forming molecular cloud cores, to the ``initial'' MF
of bound gas-free star clusters (ICMF). The resulting ICMF has, for a
featureless power-law ECMF, a turnover near 10^{4.5} Msun and a peak near 10^3
Msun. The peak lies around the initial masses of the Hyades, Praesepe and
Pleiades clusters. We also find that the entire Galactic population II stellar
spheroid can be generated if star formation proceeded via embedded clusters
distributed like a power-law MF with exponent 0.9 < beta < 2.6.Comment: 10 pages, 4 figures, accepted by MNRAS, small adjustments for
consistency with published versio
The impact of mass loss on star cluster formation. I. Analytic results
We study analytically the disruptive effect of instantaneous gas removal from
a cluster containing O stars.
We setup an iterative calculation based on the stellar velocity distribution
function to compute the fraction of stars that remain bound once the cluster
has ejected the gas and is out of equilibrium. We show that the stellar bound
fraction is a function of the initial cluster distribution function as well as
the star formation efficiency, , taken constant throughout the
cluster.
The case of the Plummer sphere is dealt with in greater details. We find for
this case that up to ~ 50% of the stars may remain bound when
assumes values < 1/2, contrary to expectations derived from the virial theorem.
The fraction of bound stars is expressed algebraically for polytropic
distribution functions.Comment: to appear in M
The impact of mass loss on star cluster formation. II. Numerical N-body integration & further applications
We subject to an N-body numerical investigation our analysis of Paper I on
the survival of stellar clusters undergoing rapid mass loss. We compare
analytical tracks of bound mass-fraction {\it vs} star formation efficiency
to those obtained with N-body integration. We use these to argue
that stellar clusters must develop massive cores of high-binding energy if they
are to remain bound despite a star formation efficiency as low as 30% or lower
suggested by observations. The average local virial ratio
is introduced to classify bound clusters as function of their critical
for dissolution. Clusters dissolving at lower achieve the
lowest ratio. We applied this classification parameter successfully to
Michie-King and Hernquist-type distribution functions. The Plummer sphere is
exceptional in that it defies this and other classification parameters we
tried.
The reasons for the discrepancy include less effective energy redistribution
during the expansion phase for this case.Comment: MN, in the pres
A comprehensive set of simulations studying the influence of gas expulsion on star cluster evolution
We have carried out a large set of N-body simulations studying the effect of
residual-gas expulsion on the survival rate and final properties of star
clusters.
We have varied the star formation efficiency, gas expulsion timescale and
strength of the external tidal field, obtaining a three-dimensional grid of
models which can be used to predict the evolution of individual star clusters
or whole star cluster systems by interpolating between our runs. The complete
data of these simulations is made available on the Internet.
Our simulations show that cluster sizes, bound mass fraction and velocity
profile are strongly influenced by the details of the gas expulsion. Although
star clusters can survive star formation efficiencies as low as 10% if the
tidal field is weak and the gas is removed only slowly, our simulations
indicate that most star clusters are destroyed or suffer dramatic loss of stars
during the gas removal phase. Surviving clusters have typically expanded by a
factor 3 or 4 due to gas removal, implying that star clusters formed more
concentrated than as we see them today. Maximum expansion factors seen in our
runs are around 10. If gas is removed on timescales smaller than the initial
crossing time, star clusters acquire strongly radially anisotropic velocity
dispersions outside their half-mass radii. Observed velocity profiles of star
clusters can therefore be used as a constraint on the physics of cluster
formation.Comment: 12 pages, 9 figures, MNRAS accepte
Global and regional estimates of the contribution of herpes simplex virus Type 2 infection to HIV incidence: a population attributable fraction analysis using published epidemiological data
Background A 2017 systematic review and meta-analysis of 55 prospective studies found the adjusted risk of HIV acquisition to be at least tripled in individuals with herpes simplex virus type 2 (HSV-2) infection. We aimed to assess the potential contribution of HSV-2 infection to HIV incidence, given an effect of HSV-2 on HIV acquisition. Methods We used a classic epidemiological formula to estimate the global and regional (WHO regional) population attributable fraction (PAF) and number of incident HIV infections attributable to HSV-2 infection by age (15â24 years, 25â49 years, and 15â49 years), sex, and timing of HSV-2 infection (established vs recently acquired). Estimates were calculated by incorporating HSV-2 and HIV infection data with pooled relative risk (RR) estimates for the effect of HSV-2 infection on HIV acquisition from a systematic review and meta-analysis. Because HSV-2 and HIV have shared sexual and other risk factors, in addition to HSV-related biological factors that increase HIV risk, we only used RR estimates that were adjusted for potential confounders. Findings An estimated 420â000 (95% uncertainty interval 317â000â546â000; PAF 29·6% [22·9â37·1]) of 1·4 million sexually acquired incident HIV infections in individuals aged 15â49 years in 2016 were attributable to HSV-2 infection. The contribution of HSV-2 to HIV was largest for the WHO African region (PAF 37·1% [28·7â46·3]), women (34·8% [23·5â45·0]), individuals aged 25â49 years (32·4% [25·4â40·2]), and established HSV-2 infection (26·8% [19·7â34·5]). Interpretation A large burden of HIV is likely to be attributable to HSV-2 infection, even if the effect of HSV-2 infection on HIV had been imperfectly measured in studies providing adjusted RR estimates, potentially because of residual confounding. The contribution is likely to be greatest in areas where HSV-2 is highly prevalent, particularly Africa. New preventive interventions against HSV-2 infection could not only improve the quality of life of millions of people by reducing the prevalence of herpetic genital ulcer disease, but could also have an additional, indirect effect on HIV transmission
A Bayesian approach to estimate changes in condom use from limited human immunodeficiency virus prevalence data.
Evaluation of large-scale intervention programmes against human immunodeficiency virus (HIV) is becoming increasingly important, but impact estimates frequently hinge on knowledge of changes in behaviour such as the frequency of condom use over time, or other self-reported behaviour changes, for which we generally have limited or potentially biased data. We employ a Bayesian inference methodology that incorporates an HIV transmission dynamics model to estimate condom use time trends from HIV prevalence data. Estimation is implemented via particle Markov chain Monte Carlo methods, applied for the first time in this context. The preliminary choice of the formulation for the time varying parameter reflecting the proportion of condom use is critical in the context studied, because of the very limited amount of condom use and HIV data available. We consider various novel formulations to explore the trajectory of condom use over time, based on diffusion-driven trajectories and smooth sigmoid curves. Numerical simulations indicate that informative results can be obtained regarding the amplitude of the increase in condom use during an intervention, with good levels of sensitivity and specificity performance in effectively detecting changes. The application of this method to a real life problem demonstrates how it can help in evaluating HIV interventions based on a small number of prevalence estimates, and it opens the way to similar applications in different contexts
Surviving infant mortality in the hierarchical merging scenario
We examine the effects of gas expulsion on initially sub-structured and
out-of-equilibrium star clusters. We perform -body simulations of the
evolution of star clusters in a static background potential before removing
that potential to model gas expulsion. We find that the initial star formation
efficiency is not a good measure of the survivability of star clusters. This is
because the stellar distribution can change significantly, causing a large
change in the relative importance of the stellar and gas potentials. We find
that the initial stellar distribution and velocity dispersion are far more
important parameters than the initial star formation efficiency, and that
clusters with very low star formation efficiencies can survive gas expulsion.
We suggest that it is variations in cluster initial conditions rather than in
their star formation efficiencies that cause some clusters to be destroyed
while a few survive.Comment: 9 pages, 10 figures, 1 tabl
The M/L ratio of massive young clusters
We point out a strong time-evolution of the mass-to-light conversion factor
\eta commonly used to estimate masses of dense star clusters from observed
cluster radii and stellar velocity dispersions. We use a gas-dynamical model
coupled with the Cambridge stellar evolution tracks to compute line-of-sight
velocity dispersions and half-light radii weighted by the luminosity. Stars at
birth are assumed to follow the Salpeter mass function in the range [0.15--17
M_\sun]. We find that , and hence the estimated cluster mass, increases
by factors as large as 3 over time-scales of 20 million years. Increasing the
upper mass limit to 50 M_\sun leads to a sharp rise of similar amplitude but
in as little as 10 million years.
Fitting truncated isothermal (Michie-King) models to the projected light
profile leads to over-estimates of the concentration par ameter c of compared to the same functional fit applied to the proj ected
mass density.Comment: Draft version of an ApJ lette
The effect of the dynamical state of clusters on gas expulsion and infant mortality
The star formation efficiency (SFE) of a star cluster is thought to be the
critical factor in determining if the cluster can survive for a significant
(>50 Myr) time. There is an often quoted critical SFE of ~30 per cent for a
cluster to survive gas expulsion. I reiterate that the SFE is not the critical
factor, rather it is the dynamical state of the stars (as measured by their
virial ratio) immediately before gas expulsion that is the critical factor. If
the stars in a star cluster are born in an even slightly cold dynamical state
then the survivability of a cluster can be greatly increased.Comment: 6 pages, 2 figures. Review talk given at the meeting on "Young
massive star clusters - Initial conditions and environments", E. Perez, R. de
Grijs, R. M. Gonzalez Delgado, eds., Granada (Spain), September 2007,
Springer: Dordrecht. Replacement to correct mistake in a referenc
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