110 research outputs found
The photometric evolution of dissolving star clusters: II. Realistic models. Colours and M/L ratios
Evolutionary synthesis models are the prime method to construct models of
stellar populations, and to derive physical parameters from observations. One
of the assumptions for such models so far has been the time-independence of the
stellar mass function. However, dynamical simulations of star clusters in tidal
fields have shown the mass function to change due to the preferential removal
of low-mass stars from clusters. Here we combine the results from dynamical
simulations of star clusters in tidal fields with our evolutionary synthesis
code GALEV to extend the models by a new dimension: the total cluster
disruption time. We reanalyse the mass function evolution found in N-body
simulations of star clusters in tidal fields, parametrise it as a function of
age and total cluster disruption time and use this parametrisation to compute
GALEV models as a function of age, metallicity and the total cluster disruption
time. We study the impact of cluster dissolution on the colour (generally, they
become redder) and magnitude (they become fainter) evolution of star clusters,
their mass-to-light ratios (off by a factor of ~2 -- 4 from standard
predictions), and quantify the effect on the cluster age determination from
integrated photometry (in most cases, clusters appear to be older than they
are, between 20 and 200%). By comparing our model results with observed M/L
ratios for old compact objects in the mass range 10^4.5 -- 10^8 Msun, we find a
strong discrepancy for objects more massive than 10^7 Msun (higher M/L). This
could be either caused by differences in the underlying stellar mass function
or be an indication for the presence of dark matter in these objects. Less
massive objects are well represented by the models. The models for a range of
total cluster disruption times are available online. (shortened)Comment: MNRAS, in press, data are available at
http://www.phys.uu.nl/~anders/data/SSP_varMF/, http://data.galev.org and soon
also from CDS ... sorry for the sometimes strange layout, that's LaTe
The Star Cluster Population of M51: II. Age distribution and relations among the derived parameters
We use archival {\it Hubble Space Telescope} observations of broad-band
images from the ultraviolet (F255W-filter) through the near infrared (NICMOS
F160W-filter) to study the star cluster population of the interacting spiral
galaxy M51. We obtain age, mass, extinction, and effective radius estimates for
1152 star clusters in a region of kpc centered on the
nucleus and extending into the outer spiral arms. In this paper we present the
data set and exploit it to determine the age distribution and relationships
among the fundamental parameters (i.e. age, mass, effective radius). Using this
dataset we find: {\it i}) that the cluster formation rate seems to have had a
large increase 50-70 Myr ago, which is coincident with the suggested
{\it second passage} of its companion, NGC 5195, {\it ii}) a large number of
extremely young ( 10 Myr) star clusters, which we interpret as a population
of unbound clusters of which a large majority will disrupt within the next
10 Myr, and {\it iii)} that the distribution of cluster sizescan be well
approximated by a power-law with exponent, , which is
very similar to that of Galactic globular clusters, indicating that cluster
disruption is largely independent of cluster radius. In addition, we have used
this dataset to search for correlations among the derived parameters. In
particular, we do not find any strong trends between the age and mass, mass and
effective radius, nor between the galactocentric distance and effective radius.
There is, however, a strong correlation between the age of a cluster and its
extinction, with younger clusters being more heavily reddened than older
clusters.Comment: 21 pages, 20 figures, accepted A&
Low energy onset of nuclear shadowing in photoabsorption
The early onset of nuclear shadowing in photoabsorption at low photon
energies has recently been interpreted as a possible signature of a decrease of
the rho meson mass in nuclei. We show that one can understand this early onset
within simple Glauber theory if one takes the negative real part of the rho N
scattering amplitudes into account, corresponding to a higher effective mass of
the rho meson in nuclear medium.Comment: REVTEX, 9 pages, including 4 eps figure
An analytical description of the disruption of star clusters in tidal fields with an application to Galactic open clusters
We present a simple analytical description of the disruption of star clusters
in a tidal field, which agrees excellently with detailed N-body simulations.
The analytic expression can be used to predict the mass and age histograms of
surviving clusters for any cluster initial mass function and any cluster
formation history. The method is applied to open clusters in the solar
neighbourhood, based on the new cluster sample of Kharchenko et al. From a
comparison between the observed and predicted age distributions in the age
range between 10 Myr to 3 Gyr we find the following results: (1) The disruption
time of a 10^4 M_sun cluster in the solar neighbourhood is about 1.3+/-0.5 Gyr.
This is a factor 5 shorter than derived from N-body simulations of clusters in
the tidal field of the galaxy. (2) The present starformation rate in bound
clusters within 600 pc from the Sun is 5.9+/-0.8 * 10^2 M_sun / Myr, which
corresponds to a surface star formation rate in bound clusters of 5.2+/-0.7
10^(-10) M_sun/yr/pc^2. (3) The age distribution of open clusters shows a bump
between 0.26 and 0.6 Gyr when the cluster formation rate was 2.5 times higher
than before and after. (4) The present star formation rate in bound clusters is
half as small as that derived from the study of embedded clusters. The
difference suggests that half of the clusters in the solar neighbourhood become
unbound within 10 Myr. (5) The most massive clusters within 600 pc had an
initial mass of 3*10^4 M_sun. This is in agreement with the statistically
expected value based on a cluster initial mass function with a slope of -2,
even if the physical upper mass limit is as high as 10^6 M_sun.Comment: 14 pages, 15 figures, to appear in Astronomy & Astrophysic
The photometric evolution of dissolving star clusters I: First predictions
We calculated the broad-band photometric evolution of unresolved star
clusters, including the preferential loss of low-mass stars due to mass
segregation. The stellar mass function of a cluster evolves due to three
effects: (a) the evolution of massive stars; (b) early tidal effects reduce the
mass function independently of the stellar mass; (c) after mass segregation has
completed, tidal effects preferentially remove the lowest-mass stars from the
cluster. Results: (1) During the first ~40% of the lifetime of a cluster the
cluster simply gets fainter due to the loss of stars by tidal effects. (2)
Between ~40 and ~80% of its lifetime the cluster gets bluer due to the loss of
low-mass stars. This will result in an underestimate of the age of clusters if
standard cluster evolution models are used (0.15 -- 0.5 dex). (3) After ~80% of
the total lifetime of a cluster it will rapidly get redder. This is because
stars at the low-mass end of the main sequence, which are preferentially lost,
are bluer than the AGB stars that dominate the light at long wavelengths,
resulting in an age overestimate. (4) Clusters with mass segregation and the
preferential loss of low-mass stars evolve along almost the same tracks in
colour-colour diagrams as clusters without mass segregation. Therefore it will
be difficult to distinguish this effect from that due to the cluster age for
unresolved clusters, unless the total lifetime of the clusters can be
estimated. (5) The changes in the colour evolution of unresolved clusters due
to the preferential loss of low-mass stars will affect the determination of the
SFHs. (6) The preferential loss of low-mass stars might explain the presence of
old (~13 Gyr) clusters in NGC 4365 which are photometrically disguised as
intermediate-age clusters (2 - 5 Gyr). [Abridged]Comment: accepted for publication in A&
The Star Cluster Population of M51: III. Cluster disruption and formation history
In this work we concentrate on the evolution of the cluster population of the
interacting galaxy M51 (NGC 5194), namely the timescale of cluster disruption
and possible variations in the cluster formation rate. We present a method to
compare observed age vs. mass number density diagrams with predicted
populations including various physical input parameters like the cluster
initial mass function, cluster disruption, cluster formation rate and star
bursts. If we assume that the cluster formation rate increases at the moments
of the encounters with NGC 5195, we find an increase in the cluster formation
rate of a factor of 3, combined with a disruption timescale which is slightly
higher then when assuming a constant formation rate (t_4 = 200 Myr vs. 100
Myr). The measured cluster disruption time is a factor of 5 shorter than
expected on theoretical grounds. This implies that the disk of M51 is not a
preferred location for survival of young globular clusters, since even clusters
with masses of the order of 10^6 M_sun will be destroyed within a few Gyr.Comment: 13 pages, A&A, accepte
Clusters in the solar neighbourhood: how are they destroyed?
We predict the survival time of initially bound star clusters in the solar
neighbourhood taking into account: (1) stellar evolution, (2) tidal stripping,
(3) shocking by spiral arms and (4) encounters with giant molecular clouds. We
find that the predicted dissolution time is t_dis= 1.7 (M_i/10^4 M_sun)^0.67
Gyr for clusters in the mass range of 10^2 < M_i < 10^5 M_sun, where
M_i is the initial mass of the cluster.. The resulting predicted shape of the
logarithmic age distribution agrees very well with the empirical one, derived
from a complete sample of clusters in the solar neighbourhood within 600 pc.
The required scaling factor implies a star formation rate of 400 M_sun/Myr
within 600 pc from the Sun or a surface formation rate of 3.5 10^-10 M_sun/(yr
pc^2) for stars in bound clusters with an initial mass in the range of 10^2 to
3 10^4 M_sun.Comment: Accepted for A&A Letters, 5 pages, 3 figure
Star cluster formation and star formation: the role of environment and star-formation efficiencies
âThe original publication is available at www.springerlink.comâ. Copyright Springer. DOI: 10.1007/s10509-009-0088-5By analyzing global starburst properties in various kinds of starburst and post-starburst galaxies and relating them to the properties of the star cluster populations they form, I explore the conditions for the formation of massive, compact, long-lived star clusters. The aim is to determine whether the relative amount of star formation that goes into star cluster formation as opposed to field star formation, and into the formation of massive long-lived clusters in particular, is universal or scales with star-formation rate, burst strength, star-formation efficiency, galaxy or gas mass, and whether or not there are special conditions or some threshold for the formation of star clusters that merit to be called globular clusters a few billion years later.Peer reviewe
Tidal dwarf galaxies as a test of fundamental physics
Within the cold dark matter (CDM) framework tidal dwarf galaxies (TDGs)
cannot contain dark matter, so the recent results by Bournaud et al. (2007)
that 3 rotating TDGs do show significant evidence for being dark matter
dominated is inconsistent with the current concordance cosmological theory
unless yet another dark matter component is postulated. We confirm that the TDG
rotation curves are consistent with Newtonian dynamics only if either an
additional dark matter component is postulated, or if all 3 TDGs happen to be
viewed nearly edge-on, which is unlikely given the geometry of the tidal
debris. We also find that the observed rotation curves are very naturally
explained without any free parameters within the modified Newtonian dynamics
(MOND) framework if inclinations are adopted as derived by Bournaud et al. We
explore different inclination angles and two different assumptions about the
external field effect. The results do not change significantly, and we conclude
therefore that Newtonian dynamics has severe problems while MOND does
exceedingly well in explaining the observed rotation curves of the 3 TDGs
studied by Bournaud et al.Comment: Accepted for publication in A&A Letters, 5 pages, 3 figure
Systematic uncertainties in the analysis of star cluster parameters based on broad-band imaging observations
High-resolution Hubble Space Telescope (HST) imaging observations of star cluster systems provide a very interesting and useful alternative to spectroscopic studies for stellar population analyses with 8-m class telescopes. Here, we assess the systematic uncertainties in (young) cluster age, mass and (to a lesser extent) extinction and metallicity determinations, based on broad-band imaging observations with the HST. Our aim here is to intercompare the results obtained using a variety of commonly used modelling techniques, specifically with respect to our own extensively tested multidimensional approach. Any significant differences among the resulting parameters are due to the details of the various, independently developed, modelling techniques used, rather than to the stellar population models themselves. Despite the model uncertainties and the selection effects inherent to most methods used, we find that the peaks in the relative age and mass distributions of a given young (âČ109 yr) cluster system can be derived relatively robustly and consistently, to accuracies of ÏtâĄÎălog(age/yr)ă†0.35 and ÏMâĄÎălog(Mcl/Mâ)ă†0.14, respectively, assuming Gaussian distributions in cluster ages and masses for reasons of simplicity. The peaks in the relative mass distributions can be obtained with a higher degree of confidence than those in the relative age distributions, as exemplified by the smaller spread among the peak values of the mass distributions derived. This implies that mass determinations are mostly insensitive to the approach adopted. We reiterate that as extensive a wavelength coverage as possible is required to obtain robust and internally consistent age and mass estimates for the individual objects, with reasonable uncertainties. Finally, we conclude that the actual filter systems used for the observations should be used for constructing model colours, instead of using conversion equations, to achieve more accurate derivations of ages and masse
- âŠ