202 research outputs found
The distinction between star clusters and associations
In Galactic studies a distinction is made between (open) star clusters and
associations. For barely resolved objects at a distance of several Mpc this
distinction is not trivial to make. Here we provide an objective definition by
comparing the age of the stars to the crossing time of nearby stellar
agglomerates. We find that a satisfactory separation can be made where this
ratio equals unity. Stellar agglomerates for which the age of the stars exceeds
the crossing time are bound, and are referred to as star clusters.
Alternatively, those for which the crossing time exceeds the stellar age are
unbound and are referred to as associations. This definition is useful whenever
reliable measurements for the mass, radius and age are available.Comment: 2 pages, 2 figures, accepted for MNRAS Letter
The Initial Distribution of Stars
The primary focus of my PhD is to quantify the spatial distribution of star-forming environments from optical to radio wavelengths using data from the Hubble Space Telescope, the Very Large Telescope, the Spitzer Space Telescope, the Herschel Space Observatory, and the Caltech Submillimeter Observatory. Towards the end of my PhD study I have developed theoretical models. With these observational and theoretical avenues I have led a series of research projects to (1) quantify the initial spatial structure of pre-stellar cores and proto-stars, (2) test whether massive stars can form in isolation or not, (3) and develop a theoretical model on how young massive clusters form.
These research projects have been fruitful as my collaborators and I have shown that pre-stellar cores and stars form in a smooth continuum of surface densities from a few to thousands of stars per pc^2. These two works have important implications on our understanding of what a young stellar cluster is and how star forming environments can evolve to form field star populations or gravitationally bound clusters. In my second study my collaborators and I found evidence for isolated massive star formation in the active star forming region 30 Doradus, in the Large Magellanic Cloud. The result impacts the field of the initial mass function and star formation models. Massive stars forming in isolation is consistent with a stochastically sampled initial mass function. Additionally, the result would put constraints on theoretical models on massive star formation. Continuing my work on massive star forming environments my collaborators and I have developed a theoretical model on how young massive clusters form. From the models we argue that feedback energies can be contained by the gravitational potential well of the massive progenitors. Furthermore, we predict the physical properties the massive cluster progenitors in terms of initial gas mass, radii and flux brightness to enable a search for these objects in Galactic plane surveys and upcoming telescopes. Using the common thread of spatial distribution analysis of star formation I describe my future research plans, which entails studies on extragalactic scales in the conclusion
A galactic-scale origin for stellar clustering
We recently presented a model for the cluster formation efficiency (CFE),
i.e. the fraction of star formation occurring in bound stellar clusters. It
utilizes the idea that the formation of stars and stellar clusters occurs
across a continuous spectrum of ISM densities. Bound stellar clusters naturally
arise from the high-density end of this density spectrum. Due to short
free-fall times, these high-density regions can achieve high star formation
efficiencies (SFEs) and can be unaffected by gas expulsion. Lower-density
regions remain gas-rich and substructured, and are unbound upon gas expulsion.
The model enables the CFE to be calculated using galactic-scale observables. I
present a brief summary of the model physics, assumptions and caveats, and show
that it agrees well with observations. Fortran and IDL routines for calculating
the CFE are publicly available at http://www.mpa-garching.mpg.de/cfe.Comment: 4 pages, 1 figure; to appear in The Labyrinth of Star Formation,
(eds.) D. Stamatellos, S. Goodwin, and D. Ward-Thompson, Springer, in pres
Formation and dissolution of leaky clusters
Massive Galactic clusters (> 1000 Msun) exhibit a clear correlation between
cluster density, size and age and can be sorted in two categories, i.e.
starburst and leaky clusters. The reason for the existance of two types of
massive clusters is an open question. However, the answer is probably connected
to a different formation histories of the two types. In this study we
concentrate onleaky clusters only and investigate possible formation scenarios
and gas expulsion phase. This is done by using existing observational data and
numerical results of embedded cluster properties. Assuming that a clear
correlation between cluster density, size and age exists, it is shown that the
density-radius development over time forembedded clusters can be approximated
by rho \approx 100*r ^{-1.3} Msun pc^{-3}. The consequences for the star
formation process in leaky clusters are discussed and found to favour an
inside-out star formation scenario with an initially low but later accelerated
star formation rate. It is shown how the leaky clusters form in a unique
sequential manner and that rapid gas expulsion is responsible for the 80-90\%
mass loss over the next 20 Myr.Comment: 7 pages, 4 figures, accepted by A&
The Variable Reflection Nebula Cepheus A East
We report K'-band imaging observations of the reflection nebula associated
with Cepheus A East covering the time interval from 1990 to 2004. Over this
time the reflection nebula shows variations of flux distribution, which we
interpret as the effect of inhomogeneous and varying extinction in the light
path from the illuminating source HW2 to the reflection nebula. The obscuring
material is located within typical distances of approximately 10 AU from the
illuminating source.Comment: 22 pages, including 6 figures, accepted for publication in The
Astronomical Journa
The VLT-FLAMES Tarantula Survey. VII. A low velocity dispersion for the young massive cluster R136
Detailed studies of resolved young massive star clusters are necessary to
determine their dynamical state and evaluate the importance of gas expulsion
and early cluster evolution. In an effort to gain insight into the dynamical
state of the young massive cluster R136 and obtain the first measurement of its
velocity dispersion, we analyse multi-epoch spectroscopic data of the inner
regions of 30 Doradus in the Large Magellanic Cloud (LMC) obtained as part of
the VLT-FLAMES Tarantula Survey. Following a quantitative assessment of the
variability, we use the radial velocities of non-variable sources to place an
upper limit of 6 km/s on the line-of-sight velocity dispersion of stars within
a projected distance of 5 pc from the centre of the cluster. After accounting
for the contributions of undetected binaries and measurement errors through
Monte Carlo simulations, we conclude that the true velocity dispersion is
likely between 4 and 5 km/s given a range of standard assumptions about the
binary distribution. This result is consistent with what is expected if the
cluster is in virial equilibrium, suggesting that gas expulsion has not altered
its dynamics. We find that the velocity dispersion would be ~25 km/s if
binaries were not identified and rejected, confirming the importance of the
multi-epoch strategy and the risk of interpreting velocity dispersion
measurements of unresolved extragalactic young massive clusters.Comment: 18 pages, 7 figures, accepted by A&
Variations in the Galactic star formation rate and density thresholds for star formation
The conversion of gas into stars is a fundamental process in astrophysics and
cosmology. Stars are known to form from the gravitational collapse of dense
clumps in interstellar molecular clouds, and it has been proposed that the
resulting star formation rate is proportional to either the amount of mass
above a threshold gas surface density, or the gas volume density. These
star-formation prescriptions appear to hold in nearby molecular clouds in our
Milky Way Galaxy's disk as well as in distant galaxies where the star formation
rates are often much larger. The inner 500 pc of our Galaxy, the Central
Molecular Zone (CMZ), contains the largest concentration of dense, high-surface
density molecular gas in the Milky Way, providing an environment where the
validity of star-formation prescriptions can be tested. Here we show that by
several measures, the current star formation rate in the CMZ is an
order-of-magnitude lower than the rates predicted by the currently accepted
prescriptions. In particular, the region 1 deg < l < 3.5 deg, |b| < 0.5 deg
contains ~10^7 Msun of dense molecular gas -- enough to form 1000 Orion-like
clusters -- but the present-day star formation rate within this gas is only
equivalent to that in Orion. In addition to density, another property of
molecular clouds, such as the amplitude of turbulent motions, must be included
in the star-formation prescription to predict the star formation rate in a
given mass of molecular gas.Comment: 17 pages, 6 figures, submitted MNRA
Stellar contents and Star formation in the young cluster Stock 18
We have carried out deep (V21 mag) \ubvri photometric study of the star
cluster Stock 18. These along with archival Infrared data have been used to
derive the basic cluster parameters and also to study the star formation
processes in and around the cluster region. The distance to the cluster is
derived as 2.80.2 kpc while its age is estimated as Myr.
Present study indicates that interstellar reddening is normal in the direction
of the cluster. The mass function slope is found to be -1.370.27 for the
mass range 111.9. There is no evidence found for the effect of
mass segregation in main-sequence stars of the cluster. A young stellar
population with age between 1-2 Myr have been found in and around the cluster
region. The presence of IRAS and AKARI sources with MSX intensity map also show
the youth of the Sh2-170 region.Comment: 25 pages, 11 figures, 3 tables (Accepted for publication in New
Astronomy
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