125 research outputs found
Mass segregation in young compact star clusters in the Large Magellanic Cloud: II. Mass Functions
We review the complications involved in the conversion of stellar
luminosities into masses and apply a range of mass-to-luminosity relations to
our Hubble Space Telescope observations of the young LMC star clusters NGC 1805
and NGC 1818.
Both the radial dependence of the mass function (MF) and the dependence of
the cluster core radii on mass indicate clear mass segregation in both clusters
at radii r <= 20-30'', for masses in excess of ~1.6-2.5 Msun. This result does
not depend on the mass range used to fit the slopes or the metallicity assumed.
It is clear that the cluster MFs, at any radius, are not simple power laws.
The global and the annular MFs near the core radii appear to be characterised
by similar slopes in the mass range (-0.15 <= log m/Msun <= 0.85), the MFs
beyond r >= 30'' have significantly steeper slopes.
We estimate that while the NGC 1818 cluster core is between ~5 and ~30
crossing times old, the core of NGC 1805 is likely crossing
times old. However, since strong mass segregation is observed out to ~6 Rcore
and ~3 Rcore in NGC 1805 and NGC 1818, respectively, it is most likely that
significant primordial mass segregation was present in both clusters,
particularly in NGC 1805.Comment: 16 pages, incl. 9 embedded postscript figures, MNRAS, resubmitted
(referee's comments included
Planets around evolved intermediate-mass stars. I. Two substellar companions in the open clusters NGC 2423 and NGC 4349
Context. Many efforts are being made to characterize extrasolar planetary
systems and unveil the fundamental mechanisms of planet formation. An important
aspect of the problem, which remains largely unknown, is to understand how the
planet formation process depends on the mass of the parent star. In particular,
as most planets discovered to date orbit a solar-mass primary, little is known
about planet formation around more massive stars. Aims. To investigate this
point, we present first results from a radial velocity planet search around red
giants in the clump of intermediate-age open clusters. We choose clusters
harbouring red giants with masses between 1.5 and 4 M_sun, using the well-known
cluster parameters to accurately determine the stellar masses. We are therefore
exploring a poorly-known domain of primary masses, which will bring new
insights into the properties of extrasolar planetary systems. Methods. We are
following a sample of about 115 red giants with the Coralie and HARPS
spectrographs to obtain high-precision radial velocity (RV) measurements and
detect giant planets around these stars. We use bisector and activity index
diagnostics to distinguish between planetary-induced RV variations and stellar
photospheric jitter. Results. We present the discoveries of a giant planet and
a brown dwarf in the open clusters NGC 2423 and NGC 4349, orbiting the 2.4
M_sun-star NGC2423 No3 (TYC 5409-2156-1) and the 3.9 M_sun-star NGC4349 No127
(TYC 8975-2606-1). These low-mass companions have orbital periods of 714 and
678 days and minimum masses of 10.6 and 19.8 M_jup, respectively. Combined with
the other known planetary systems, these detections indicate that the frequency
of massive planets is higher around intermediate-mass stars, and therefore
probably scales with the mass of the protoplanetary disk.Comment: 9 pages, 11 figures, accepted for publication in A&
The influence of stellar-dynamical ejections and collisions on the relation between the maximum-star and star-cluster-mass
We perform the largest currently available set of direct N-body calculations
of young star cluster models to study the dynamical influence, especially
through the ejections of the most massive star in the cluster, on the current
relation between the maximum-stellar-mass and the star-cluster-mass. We vary
several initial parameters such as the initial half-mass radius of the cluster,
the initial binary fraction, and the degree of initial mass segregation. Two
different pairing methods are used to construct massive binaries for more
realistic initial conditions of massive binaries. We find that lower mass
clusters (<= 10^2.5 Msun) do not shoot out their heaviest star. In the case of
massive clusters (>= 1000 Msun), no most-massive star escapes the cluster
within 3 Myr regardless of the initial conditions if clusters have initial
half-mass radii, r_0.5, >= 0.8 pc. However, a few of the initially smaller
sized clusters (r_0.5 = 0.3 pc), which have a higher density, eject their most
massive star within 3 Myr. If clusters form with a compact size and their
massive stars are born in a binary system with a mass-ratio biased towards
unity, the probability that the mass of the most massive star in the cluster
changes due to the ejection of the initially most massive star can be as large
as 20 per cent. Stellar collisions increase the maximum-stellar-mass in a large
number of clusters when clusters are relatively dense (M_ecl >= 10^3 Msun and
r_0.5 = 0.3 pc) and binary-rich. Overall, we conclude that dynamical effects
hardly influence the observational maximum-stellar-mass -- cluster mass
relation.Comment: 16 pages, 8 figures, 5 tables, accepted for publication in MNRA
Mass and luminosity evolution of young stellar objects
A model of protostar mass and luminosity evolution in clusters gives new
estimates of cluster age, protostar birthrate, accretion rate and mean
accretion time. The model assumes constant protostar birthrate, core-clump
accretion, and equally likely accretion stopping. Its parameters are set to
reproduce the initial mass function, and to match protostar luminosity
distributions in nearby star-forming regions. It obtains cluster ages and
birthrates from the observed numbers of protostars and pre-main sequence (PMS)
stars, and from the modal value of the protostar luminosity. In 31 embedded
clusters and complexes the global cluster age is 1-3 Myr, matching available
estimates based on optical spectroscopy and evolutionary tracks. This method of
age estimation is simpler than optical spectroscopy, and is more useful for
young embedded clusters where optical spectrocopy is not possible. In the
youngest clusters, the protostar fraction decreases outward from the densest
gas, indicating that the local star-forming age increases outward from a few
0.1 Myr in small protostar-dominated zones to a few Myr in large PMS-dominated
zones.Comment: To appear in The Astrophysical Journal, Part
Mass Segregation in Young Magellanic Clouds Star Clusters: Four Clusters observed with HST
We present the results of our investigation on the phenomenon of mass
segregation in young star clusters in the Magellanic Clouds. HST/WFPC2
observations on NGC 1818, NGC 2004 & NGC 2100 in the Large Magellanic Cloud and
NGC 330 in the Small Magellanic Cloud have been used for the application of
diagnostic tools for mass segregation: i) the radial density profiles of the
clusters for various mass groups and ii) their mass functions (MFs) at various
radii around their centres. All four clusters are found to be mass segregated,
but each one in a different manner. Specifically not all the clusters in the
sample show the same dependence of their density profiles on the selected
magnitude range, with NGC 1818 giving evidence of a strong such relation and
NGC 330 showing only a hint of the phenomenon. NGC 2004 did not also show any
significant signature of mass segregation in its density profiles. The MFs
radial dependence provides clear proof of the phenomenon for NGC 1818, NGC 2100
and NGC 2004, while for NGC 330 it gives only indications. An investigation on
the constraints introduced by the application of both diagnostic tools is
presented. We also discuss the problems related to the construction of a
reliable MF for a cluster and their impact on the investigation of the
phenomenon of mass segregation. We find that the MFs of these clusters as they
were constructed with two methods, are comparable to Salpeter's IMF. A
discussion is given on the dynamical status of the clusters and a test is
applied on the equipartition among several mass groups in them. Both showed
that the observed mass segregation in the clusters is of primordial nature.Comment: A&A Accepted, 20 pages, 9 Figures, Version with language errors and
typos correcte
Quantifying the Universality of the Stellar Initial Mass Function in Old Star Clusters
We present a new technique to quantify cluster-to-cluster variations in the
observed present-day stellar mass functions of a large sample of star clusters.
Our method quantifies these differences as a function of both the stellar mass
and the total cluster mass, and offers the advantage that it is insensitive to
the precise functional form of the mass function. We applied our technique to
data taken from the ACS Survey for Globular Clusters, from which we obtained
completeness-corrected stellar mass functions in the mass range 0.25-0.75
M for a sample of 27 clusters. The results of our observational
analysis were then compared to Monte Carlo simulations for globular cluster
evolution spanning a range of initial mass functions, total numbers of stars,
concentrations, and virial radii.
We show that the present-day mass functions of the clusters in our sample can
be reproduced by assuming an universal initial mass function for all clusters,
and that the cluster-to-cluster differences are consistent with what is
expected from two-body relaxation. A more complete exploration of the initial
cluster conditions will be needed in future studies to better constrain the
precise functional form of the initial mass function. This study is a first
step toward using our technique to constrain the dynamical histories of a large
sample of old Galactic star clusters and, by extension, star formation in the
early Universe.Comment: 11 pages, 4 figures, 4 tables, accepted for publication in MNRAS,
proof corrections made in updated versio
A Minimum Column Density of 1 g cm^-2 for Massive Star Formation
Massive stars are very rare, but their extreme luminosities make them both
the only type of young star we can observe in distant galaxies and the dominant
energy sources in the universe today. They form rarely because efficient
radiative cooling keeps most star-forming gas clouds close to isothermal as
they collapse, and this favors fragmentation into stars <~1 Msun. Heating of a
cloud by accreting low-mass stars within it can prevent fragmentation and allow
formation of massive stars, but what properties a cloud must have to form
massive stars, and thus where massive stars form in a galaxy, has not yet been
determined. Here we show that only clouds with column densities >~ 1 g cm^-2
can avoid fragmentation and form massive stars. This threshold, and the
environmental variation of the stellar initial mass function (IMF) that it
implies, naturally explain the characteristic column densities of massive star
clusters and the difference between the radial profiles of Halpha and UV
emission in galactic disks. The existence of a threshold also implies that
there should be detectable variations in the IMF with environment within the
Galaxy and in the characteristic column densities of massive star clusters
between galaxies, and that star formation rates in some galactic environments
may have been systematically underestimated.Comment: Accepted for publication in Nature; Nature manuscript style; main
text: 14 pages, 3 figures; supplementary text: 8 pages, 1 figur
Rapid star formation and global gravitational collapse
Most young stars in nearby molecular clouds have estimated ages of 1â2âMyr, suggesting that star formation is rapid. However, small numbers of stars in these regions with inferred ages of > rsim 5â10âMyr have been cited to argue that star formation is instead a slow, quasiâstatic process. When considering these alternative pictures it is important to recognize that the age spread in a given starâforming cloud is necessarily an upper limit to the timeâscales of local collapse, as not all spatially distinct regions will start contracting at precisely the same instant. Moreover, starâforming clouds may dynamically evolve on timeâscales of a few Myr; in particular, global gravitational contraction will tend to yield increasing star formation rates with time due to generally increasing local gas densities. We show that two different numerical simulations of dynamic, flowâdriven molecular cloud formation and evolution (1) predict age spreads for the main stellar population roughly consistent with observations and (2) raise the possibility of forming small numbers of stars early in cloud evolution, before global contraction concentrates the gas and the bulk of the stellar population is produced. In general, the existence of a small number of older stars among a generally much younger population is consistent with the picture of dynamic star formation and may even provide clues to the time evolution of starâforming clouds.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90399/1/j.1365-2966.2011.20131.x.pd
The stellar and sub-stellar IMF of simple and composite populations
The current knowledge on the stellar IMF is documented. It appears to become
top-heavy when the star-formation rate density surpasses about 0.1Msun/(yr
pc^3) on a pc scale and it may become increasingly bottom-heavy with increasing
metallicity and in increasingly massive early-type galaxies. It declines quite
steeply below about 0.07Msun with brown dwarfs (BDs) and very low mass stars
having their own IMF. The most massive star of mass mmax formed in an embedded
cluster with stellar mass Mecl correlates strongly with Mecl being a result of
gravitation-driven but resource-limited growth and fragmentation induced
starvation. There is no convincing evidence whatsoever that massive stars do
form in isolation. Various methods of discretising a stellar population are
introduced: optimal sampling leads to a mass distribution that perfectly
represents the exact form of the desired IMF and the mmax-to-Mecl relation,
while random sampling results in statistical variations of the shape of the
IMF. The observed mmax-to-Mecl correlation and the small spread of IMF
power-law indices together suggest that optimally sampling the IMF may be the
more realistic description of star formation than random sampling from a
universal IMF with a constant upper mass limit. Composite populations on galaxy
scales, which are formed from many pc scale star formation events, need to be
described by the integrated galactic IMF. This IGIMF varies systematically from
top-light to top-heavy in dependence of galaxy type and star formation rate,
with dramatic implications for theories of galaxy formation and evolution.Comment: 167 pages, 37 figures, 3 tables, published in Stellar Systems and
Galactic Structure, Vol.5, Springer. This revised version is consistent with
the published version and includes additional references and minor additions
to the text as well as a recomputed Table 1. ISBN 978-90-481-8817-
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