10,570 research outputs found
Evidence for the Strong Effect of Gas Removal on the Internal Dynamics of Young Stellar Clusters
We present detailed luminosity profiles of the young massive clusters M82-F,
NGC 1569-A, and NGC 1705-1 which show significant departures from equilibrium
(King and EFF) profiles. We compare these profiles with those from N-body
simulations of clusters which have undergone the rapid removal of a significant
fraction of their mass due to gas expulsion. We show that the observations and
simulations agree very well with each other suggesting that these young
clusters are undergoing violent relaxation and are also losing a significant
fraction of their stellar mass. That these clusters are not in equilibrium can
explain the discrepant mass-to-light ratios observed in many young clusters
with respect to simple stellar population models without resorting to
non-standard initial stellar mass functions as claimed for M82-F and NGC
1705-1. We also discuss the effect of rapid gas removal on the complete
disruption of a large fraction of young massive clusters (``infant
mortality''). Finally we note that even bound clusters may lose >50% of their
initial stellar mass due to rapid gas loss (``infant weight-loss'').Comment: 6 pages, 3 figures, MNRAS letters, accepte
Star Cluster Survival in Star Cluster Complexes under Extreme Residual Gas Expulsion
After the stars of a new, embedded star cluster have formed they blow the
remaining gas out of the cluster. Especially winds of massive stars and
definitely the on-set of the first supernovae can remove the residual gas from
a cluster. This leads to a very violent mass-loss and leaves the cluster out of
dynamical equilibrium. Standard models predict that within the cluster volume
the star formation efficiency (SFE) has to be about 33 per cent for sudden
(within one crossing-time of the cluster) gas expulsion to retain some of the
stars in a bound cluster. If the efficiency is lower the stars of the cluster
disperse mostly. Recent observations reveal that in strong star bursts star
clusters do not form in isolation but in complexes containing dozens and up to
several hundred star clusters, i.e. in super-clusters. By carrying out
numerical experiments for such objects placed at distances >= 10 kpc from the
centre of the galaxy we demonstrate that under these conditions (i.e. the
deeper potential of the star cluster complex and the merging process of the
star clusters within these super-clusters) the SFEs can be as low as 20 per
cent and still leave a gravitationally bound stellar population. Such an object
resembles the outer Milky Way globular clusters and the faint fuzzy star
clusters recently discovered in NGC 1023.Comment: 21 pages, 8 figures, accepted by Ap
A robust method for measuring the Hubble parameter
We obtain a robust, non-parametric, estimate of the Hubble constant from
galaxy linear diameters calibrated using HST Cepheid distances. Our method is
independent of the parametric form of the diameter function and the spatial
distribution of galaxies and is insensitive to Malmquist bias. We include
information on the galaxy rotation velocities; unlike Tully-Fisher, however, we
retain a fully non-parametric treatment. We find km/s/Mpc,
somewhat larger than previous results using galaxy diameters.Comment: 4 pages, 1 figure, Cosmic Flows Workshop, Victoria B.C. Canada, July
1999, ed. S. Courteau, M. Strauss & J. Willick, ASP conf. serie
Simulating star formation in molecular cloud cores IV. The role of turbulence and thermodynamics
We perform SPH simulations of the collapse and fragmentation of low-mass
cores having different initial levels of turbulence
(alpha_turb=0.05,0.10,0.25). We use a new treatment of the energy equation
which captures the transport of cooling radiation against opacity due to both
dust and gas (including the effects of dust sublimation, molecules, and H^-
ions). We also perform comparison simulations using a standard barotropic
equation of state. We find that -- when compared with the barotropic equation
of state -- our more realistic treatment of the energy equation results in more
protostellar objects being formed, and a higher proportion of brown dwarfs; the
multiplicity frequency is essentially unchanged, but the multiple systems tend
to have shorter periods (by a factor ~3), higher eccentricities, and higher
mass ratios. The reason for this is that small fragments are able to cool more
effectively with the new treatment, as compared with the barotropic equation of
state. We find that the process of fragmentation is often bimodal. The first
protostar to form is usually, at the end, the most massive, i.e. the primary.
However, frequently a disc-like structure subsequently forms round this
primary, and then, once it has accumulated sufficient mass, quickly fragments
to produce several secondaries. We believe that this delayed fragmentation of a
disc-like structure is likely to be an important source of very low-mass
hydrogen-burning stars and brown dwarfs.Comment: 14 pages, 8 figures. Accepted for publication by A&
Deep space network
Background, current status, and sites of Deep Space Network stations are briefly discussed
How to identify the youngest protostars
We study the transition from a prestellar core to a Class 0 protostar, using
SPH to simulate the dynamical evolution, and a Monte Carlo radiative transfer
code to generate the SED and isophotal maps. For a prestellar core illuminated
by the standard interstellar radiation field, the luminosity is low and the SED
peaks at ~190 micron. Once a protostar has formed, the luminosity rises (due to
a growing contribution from accretion onto the protostar) and the peak of the
SED shifts to shorter wavelengths (~80-100 micron). However, by the end of the
Class 0 phase, the accretion rate is falling, the luminosity has decreased, and
the peak of the SED shifts back towards longer wavelengths (90-150 micron). In
our simulations, the density of material around the protostar remains
sufficiently high well into the Class 0 phase that the protostar only becomes
visible in the NIR if it is displaced from the centre dynamically. Raw submm/mm
maps of Class 0 protostars tend to be much more centrally condensed than those
of prestellar cores. However, when convolved with a typical telescope beam, the
difference in central concentration is less marked, although the Class 0
protostars appear more circular. Our results suggest that, if a core is deemed
to be prestellar on the basis of having no associated IRAS source, no cm radio
emission, and no outflow, but it has a circular appearance and an SED which
peaks at wavelengths below ~170 micron, it may well contain a very young Class
0 protostar.Comment: Accepted by A&A (avaliable with high-res images at
http://carina.astro.cf.ac.uk/pub/Dimitrios.Stamatellos/publications
Tracking and data systems support for the Helios project. Volume 3: DSN support of Project Helios May 1976 - June 1977
Spacecraft extended mission coverage does not generally carry a high priority, but Helios was fortunate in that a combination of separated viewperiods and unique utilization of the STDN Goldstone antenna have provided a considerable amount of additional science data return, particularly at key times such a perihelion and/or solar occultation
Smarter choices ?changing the way we travel. Case study reports
This report accompanies the following volume:Cairns S, Sloman L, Newson C, Anable J, Kirkbride A and Goodwin P (2004)Smarter Choices ? Changing the Way We Travel. Report published by theDepartment for Transport, London, available via the ?Sustainable Travel? section ofwww.dft.gov.uk, and from http://eprints.ucl.ac.uk/archive/00001224/
Evaporation of Compact Young Clusters near the Galactic Center
We investigate the dynamical evolution of compact young clusters (CYCs) near
the Galactic center (GC) using Fokker-Planck models. CYCs are very young (< 5
Myr), compact (< 1 pc), and only a few tens of pc away from the GC, while they
appear to be as massive as the smallest Galactic globular clusters (~10^4
Msun). A survey of cluster lifetimes for various initial mass functions,
cluster masses, and galactocentric radii is presented. Short relaxation times
due to the compactness of CYCs, and the strong tidal fields near the GC make
clusters evaporate fairly quickly. Depending on cluster parameters, mass
segregation may occur on a time scale shorter than the lifetimes of most
massive stars, which accelerates the cluster's dynamical evolution even more.
When the difference between the upper and lower mass boundaries of the initial
mass function is large enough, strongly selective ejection of lighter stars
makes massive stars dominate even in the outer regions of the cluster, so the
dynamical evolution of those clusters is weakly dependent on the lower mass
boundary. The mass bins for Fokker-Planck simulations were carefully chosen to
properly account for a relatively small number of the most massive stars. We
find that clusters with a mass <~ 2x10^4 Msun evaporate in <~ 10 Myr. A simple
calculation based on the total masses in observed CYCs and the lifetimes
obtained here indicates that the massive CYCs comprise only a fraction of the
star formation rate (SFR) in the inner bulge estimated from Lyman continuum
photons and far-IR observations.Comment: 20 pages in two-column format, accepted for publication in Ap
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