436 research outputs found
Basic Tools for Studies on the Formation and Disruption of Star Clusters: the Luminosity Function
The luminosity function (LF) of young star clusters is often approximated by
a power law function. For clusters in a wide range of galactic environments
this has resulted in fit indices near -2, but on average slightly steeper. A
fundamental property of the -2 power law function is that the luminosity of the
brightest object (L_max) scales linearly with the total number of clusters,
which is close to what is observed. This suggests that the formation of Young
Massive Clusters (YMCs) is a result of the size of the sample, i.e. when the
SFR is high it is statistically more likely to form YMCs, but no particular
physical conditions are required. In this contribution we provide evidence that
the LF of young clusters is not a -2 power law, but instead is curved, showing
a systematic decrease of the (logarithmic) slope from roughly -1.8 at low
luminosities to roughly -2.8 at high luminosities. The empirical LFs can be
reproduced by model LFs using an underlying cluster IMF with a Schechter type
truncation around M*=2x10^5 M_sun. This value of M* can not be universal since
YMCs well in excess of this M* are known in merging galaxies and merger
remnants. Therefore, forming super massive clusters (>10^6 M_sun) probably
requires conditions different from those in (quiescent) spiral galaxies and
hence is not only the result of a size-of-sample effect. From the vertical
offset a cluster formation efficiency of ~10% is derived. We find indications
for this efficiency to be higher when the SFR is higher.Comment: 6 pages, 4 figures. To appear in the proceedings of "Galaxy Wars:
Stellar Populations and Star Formation in Interacting Galaxies" (Tennessee
July 2009
Conference summary: Mass loss from stellar clusters
This conference dealt with the mass loss from stars and from stellar
clusters. In this summary of the cluster section of the conference, I highlight
some of the results on the formation and the fundamental properties of star
clusters (Sect. 2), the early stages of their evolution (Sect. 3) and go into
more detail on the subsequent mass evolution of clusters (Sect. 4). A
discussion on how this may, or may not, depend on mass is given in Sect. 5.
Obviously, there will be a bias towards the topics where Henny Lamers has
contributed. Some of the contributions to these proceedings have already
reviewed extensively the topics of clusters mass loss and disruption, so I will
try to fit these in a general framework as much as possible.Comment: 6 pages, To appear in "Mass loss from stars and the evolution of
stellar clusters". Proc. of a workshop held in honour of H.J.G.L.M. Lamers,
Lunteren, The Netherlands. Eds. A. de Koter, L. Smith and R. Waters (San
Francisco: ASP
The effect of giant molecular clouds on star clusters
We study the encounters between stars clusters and giant molecular clouds
(GMCs). The effect of these encounters has previously been studied analytically
for two cases: 1) head-on encounters, for which the cluster moves through the
centre of the GMC and 2) distant encounters, where the encounter distance p >
3*R_n, with p the encounter parameter and R_n the radius of the GMC. We
introduce an expression for the energy gain of the cluster due to GMC
encounters valid for all values of p and R_n. This analytical result is
confronted with results from N-body simulations and excellent agreement is
found. From the simulations we find that the fractional mass loss is only 25%
of the fractional energy gain. This is because stars escape with velocities
much higher than the escape velocity. Based on the mass loss, we derive a
disruption time for star clusters due to encounters with GMCs of the form t_dis
[Gyr] = 2.0*S*(M_c/10^4 M_sun)^gamma, with S=1 for the solar neighbourhood and
inversely proportional with the global GMC density and gamma=1-3lambda, with
lambda the index that relates the cluster half-mass radius to the cluster mass
(r_h ~ M_c^lambda). The observed shallow relation between cluster radius and
mass (e.g. lambda=0.1), makes the index (gamma=0.7) similar to the index found
both from observations and from simulations of clusters dissolving in tidal
fields (gamma=0.62). The constant of 2.0 Gyr, which is the disruption time of a
10^4 M_sun cluster in the solar neighbourhood, is close to the value of 1.3 Gyr
which was empirically determined from the age distribution of open clusters.
This suggests that the combined effect of GMC encounters, stellar evolution and
galactic tidal field can explain the lack of old open clusters in the solar
neighbourhood.Comment: 2 pages, 2 figures, contribution to "Globular Clusters: Guides to
Galaxies", March 6th-10th, 200
Integrated properties of mass segregated star clusters
In this contribution we study integrated properties of dynamically segregated
star clusters. The observed core radii of segregated clusters can be 50%
smaller than the ``true'' core radius. In addition, the measured radius in the
red filters is smaller than those measured in blue filters. However, these
difference are small (), making it observationally challenging to
detect mass segregation in extra-galactic clusters based on such a comparison.
Our results follow naturally from the fact that in nearly all filters most of
the light comes from the most massive stars. Therefore, the observed surface
brightness profile is dominated by stars of similar mass, which are centrally
concentrated and have a similar spatial distribution.Comment: 2 pages, 2 figures. To appear in proceedings of the 246th IAU
symposium on "Dynamical evolution of dense stellar systems"; acknowledgements
include
The role of tidal forces in star cluster disruption
Star clusters are subject to density irregularities in their host galaxy,
such as giant molecular clouds (GMCs), the galactic disc and spiral arms, which
are largely ignored in present day (N-body) simulations of cluster evolution.
Time dependent external potentials give rise to tidal forces that accelerate
stars leading to an expansion and more rapid dissolution of the cluster. I
explain the basic principles of this tidal heating in the impulse approximation
and show how related disruption time-scales depend on properties of the
cluster.Comment: 2 pages, To appear in "Mass loss from stars and the evolution of
stellar clusters". Proc. of a workshop held in honour of H.J.G.L.M. Lamers,
Lunteren, The Netherlands. Eds. A. de Koter, L. Smith and R. Waters (San
Francisco: ASP
What determines the mass of the most massive star cluster in a galaxy: statistics, physics or disruption?
In many different galactic environments the cluster initial mass function
(CIMF) is well described by a power-law with index -2. This implies a linear
relation between the mass of the most massive cluster (M_max) and the number of
clusters. Assuming a constant cluster formation rate and no disruption of the
most massive clusters it also means that M_max increases linearly with age when
determining M_max in logarithmic age bins. We observe this increase in five out
of the seven galaxies in our sample, suggesting that M_max is determined by the
size of the sample. It also means that massive clusters are very stable against
disruption, in disagreement with the mass independent disruption (MID) model
presented at this conference. For the clusters in M51 and the Antennae galaxies
the size-of-sample prediction breaks down around 10^6 M_sun, suggesting that
this is a physical upper limit to the masses of star clusters in these
galaxies. In this method there is a degeneracy between MID and a CIMF
truncation. We show how the cluster luminosity function can serve as a tool to
distinguish between the two.Comment: 6 pages, 3 figures, to appear in ``Young Massive Star Clusters -
Initial Conditions and Environments'', 2008, Astrophysics & Space Science,
eds. E. Perez, R. de Grijs, R. M. Gonzalez Delgad
The Star Cluster Population of M51
We present the age and mass distribution of star clusters in M51. The
structural parameters are found by fitting cluster evolution models to the
spectral energy distribution consisting of 8 HST-WFPC2 pass bands. There is
evidence for a burst of cluster formation at the moment of the second encounter
with the companion NGC5195 (50-100 Myr ago) and a hint for an earlier burst
(400-500 Myr ago). The cluster
IMF has a power law slope of -2.1. The disruption time of clusters is
extremely short (< 100 Myr for a 10^4 Msun cluster).Comment: 2 pages, to appear in "The Formation and Evolution of Massive Young
Star Clusters", 17-21 November 2003, Cancun (Mexico
Theoretical and Observational Agreement on Mass Dependence of Cluster Life Times
Observations and N-body simulations both support a simple relation for the
disruption time of a cluster as a function of its mass of the form: t_dis = t_4
* (M/10^4 Msun)^gamma. The scaling factor t_4 seems to depend strongly on the
environment. Predictions and observations show that gamma ~ 0.64 +/- 0.06.
Assuming that t_dis ~ M^0.64 is caused by evaporation and shocking implies a
relation between the radius and the mass of a cluster of the form: r_h ~
M^0.07, which has been observed in a few galaxies. The suggested relation for
the disruption time implies that the lower mass end of the cluster initial mass
function will be disrupted faster than the higher mass end, which is needed to
evolve a young power law shaped mass function into the log-normal mass function
of old (globular) clusters.Comment: 2 pages, to appear in "The Formation and Evolution of Massive Young
Star Clusters", 17-21 November 2003, Cancun (Mexico
The Maximum Mass of Star Clusters
When an universal untruncated star cluster initial mass function (CIMF)
described by a power-law distribution is assumed, the mass of the most massive
star cluster in a galaxy (M_max) is the result of the size-of-sample (SoS)
effect. This implies a dependence of M_max on the total number of star clusters
(N). The SoS effect also implies that M_max within a cluster population
increases with equal logarithmic intervals of age. This is because the number
of clusters formed in logarithmic age intervals increases (assuming a constant
cluster formation rate). This effect has been observed in the SMC and LMC.
Based on the maximum pressure (P_int) inside molecular clouds, it has been
suggested that a physical maximum mass (M_max[phys]) should exist. The theory
predicts that M_max[phys] should be observable, i.e. lower than M_max that
follows from statistical arguments, in big galaxies with a high star formation
rate. We compare the SoS relations in the SMC and LMC with the ones in M51 and
model the integrated cluster luminosity function (CLF) for two cases: 1) M_max
is determined by the SoS effect and 2) M_max=M_max[phys]=constant. The observed
CLF of M51 and the comparison of the SoS relations with the SMC and LMC both
suggest that there exists a M_max[phys] of 5*10^5 M_sun in M51. The CLF of M51
looks very similar to the one observed in the ``Antennae'' galaxies. A direct
comparison with our model suggests that there M_max[phys]=2*10^6 M_sun.Comment: 4 pages, contribution to "Globular Clusters: Guides to Galaxies",
March 6th-10th, 200
Spectroscopic Constraints on the Form of the Stellar Cluster Mass Function
This contribution addresses the question of whether the initial cluster mass
function (ICMF) has a fundamental limit (or truncation) at high masses. The
shape of the ICMF at high masses can be studied using the most massive young
(<10 Myr) clusters, however this has proven difficult due to low-number
statistics. In this contribution we use an alternative method based on the
luminosities of the brightest clusters, combined with their ages. If a
truncation is present, a generic prediction (nearly independent of the cluster
disruption law adopted) is that the median age of bright clusters should be
younger than that of fainter clusters. In the case of an non-truncated ICMF,
the median age should be independent of cluster luminosity. Here, we present
optical spectroscopy of twelve young stellar clusters in the face-on spiral
galaxy NGC 2997. The spectra are used to estimate the age of each cluster, and
the brightness of the clusters is taken from the literature. The observations
are compared with the model expectations of Larsen (2009) for various ICMF
forms and both mass dependent and mass independent cluster disruption. While
there exists some degeneracy between the truncation mass and the amount of mass
independent disruption, the observations favour a truncated ICMF. For low or
modest amounts of mass independent disruption, a truncation mass of 5-6*10^5
Msun is estimated, consistent with previous determinations. Additionally, we
investigate possible truncations in the ICMF in the spiral galaxy M83, the
interacting Antennae galaxies, and the collection of spiral and dwarf galaxies
present in Larsen (2009) based on photometric catalogues taken from the
literature, and find that all catalogues are consistent with having a
(environmentally dependent) truncation in the cluster mass functions.Comment: 6 pages, 5 figures, in press, A&A Research Note
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