945,682 research outputs found
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
Changes in epigeic spider community in primary succession on a brown-coal dump
A descriptive model of primary succession of spiders on a brown-coal dump is presented. Multivariate methods (cluster analysis and detrended correspondence analysis), and community indexes were applied to evaluate changes in community composition of epigeic spiders. Two different rehabilitation age stages were investigated. The cluster analysis helped to determine a case of horizontal asynchronous succession. The DCA was able to distinguish divergent trends of succession from the initial stage. Successional trends in species replacements were observed. In all aspects of succession there was found to be directional towards a "ruderal steppe" subclimax
Metallicity in the Galactic Center: The Arches cluster
We present a quantitative spectral analysis of five very massive stars in the
Arches cluster, located near the Galactic center, to determine stellar
parameters, stellar wind properties and, most importantly, metallicity content.
The analysis uses a new technique, presented here for the first time, and uses
line-blanketed NLTE wind/atmosphere models fit to high-resolution near-infrared
spectra of late-type nitrogen-rich Wolf-Rayet stars and OfI+ stars in the
cluster. It relies on the fact that massive stars reach a maximum nitrogen
abundance that is related to initial metallicity when they are in the WNL
phase. We determine the present-day nitrogen abundance of the WNL stars in the
Arches cluster to be 1.6% (mass fraction) and constrain the stellar metallicity
in the cluster to be solar. This result is invariant to assumptions about the
mass-luminosity relationship, the mass-loss rates, and rotation speeds. In
addition, from this analysis, we find the age of the Arches cluster to be
2-2.5Myr, assuming coeval formation
How well do we know the age and mass distributions of the star cluster system in the Large Magellanic Cloud?
[ABRIDGED] The LMC star cluster system offers the unique opportunity to
independently check the accuracy of age and mass determinations based on a
number of complementary techniques, including isochrone analysis. Using our
sophisticated tool for star cluster analysis based on broad-band spectral
energy distributions (SEDs), we reanalyse the Hunter et al. (2003) LMC cluster
photometry. Our main aim is to set the tightest limits yet on the accuracy of
ABSOLUTE age determinations based on broad-band SEDs, and therefore on the
usefulness of such an approach. Our broad-band SED fits yield reliable ages,
with statistical absolute uncertainties within Delta[log(Age/yr)] = 0.4
overall. The systematic differences we find with respect to previous age
determinations are caused by conversions of the observational photometry to a
different filter system. The LMC's cluster formation rate (CFR) has been
roughly constant outside of the well-known age gap between ~3 and 13 Gyr, when
the CFR was a factor of ~5 lower. We derive the characteristic cluster
disruption time-scale, log(t_4^dis/yr) = 9.9 +- 0.1, where t_dis = t_4^dis
(M_cl/10^4 Msun)^0.62. This long characteristic disruption time-scale implies
that we are observing the INITIAL cluster mass function (CMF). We conclude that
the youngest mass and luminosity-limited LMC cluster subsets show shallower
slopes than the slope of alpha = -2 expected (at least below masses of a few x
10^3 Msun), which is contrary to dynamical expectations. This may imply that
the initial CMF slope of the LMC cluster system as a whole is NOT well
represented by a power-law, although we cannot disentangle the unbound from the
bound clusters at the youngest ages.Comment: 14 pages, 8 figures, resubmitted to MNRAS after responding to referee
repor
An empirical initial-final mass relation from hot, massive white dwarfs in NGC 2168 (M35)
The relation between the zero-age main sequence mass of a star and its
white-dwarf remnant (the initial-final mass relation) is a powerful tool for
exploration of mass loss processes during stellar evolution. We present an
empirical derivation of the initial-final mass relation based on spectroscopic
analysis of seven massive white dwarfs in NGC 2168 (M35). Using an internally
consistent data set, we show that the resultant white dwarf mass increases
monotonically with progenitor mass for masses greater than 4 solar masses, one
of the first open clusters to show this trend. We also find two massive white
dwarfs foreground to the cluster that are otherwise consistent with cluster
membership. These white dwarfs can be explained as former cluster members
moving steadily away from the cluster at speeds of <~0.5 km/s since their
formation and may provide the first direct evidence of the loss of white dwarfs
from open clusters. Based on these data alone, we constrain the upper mass
limit of WD progenitors to be >=5.8 solar masses at the 90% confidence level
for a cluster age of 150 Myr.Comment: 14 pages, 3 figures. Accepted for publication in the Astrophysical
Journal Letters. Contains some acknowledgements not in accepted version (for
space reasons), otherwise identical to accepted versio
On the Effects of Subvirial Initial Conditions and the Birth Temperature of R136
We investigate the effect of different initial virial temperatures, Q, on the
dynamics of star clusters. We find that the virial temperature has a strong
effect on many aspects of the resulting system, including among others: the
fraction of bodies escaping from the system, the depth of the collapse of the
system, and the strength of the mass segregation. These differences deem the
practice of using "cold" initial conditions no longer a simple choice of
convenience. The choice of initial virial temperature must be carefully
considered as its impact on the remainder of the simulation can be profound. We
discuss the pitfalls and aim to describe the general behavior of the collapse
and the resultant system as a function of the virial temperature so that a well
reasoned choice of initial virial temperature can be made. We make a correction
to the previous theoretical estimate for the minimum radius, , of the
cluster at the deepest moment of collapse to include a Q dependency,
, where is the number of particles.
We use our numerical results to infer more about the initial conditions of
the young cluster R136. Based on our analysis, we find that R136 was likely
formed with a rather cool, but not cold, initial virial temperature (). Using the same analysis method, we examined 15 other young clusters and
found the most common initial virial temperature to be between 0.18 and 0.25.Comment: Accepted for publication in MNRA
Spanish Tourist Behaviour: A Specific Objective-base Segmantation
This work uses data from the Spanish Tourism Demand Segments Survey (N=6900) conducted by the IESA-CSIC for Turismo Andaluz, SA. The objective of the paper is to develop a statistical segmentation or typology of Spanish tourists based on objective aspects of tourist behaviour measured in the survey including destinations visited, theme of the trip, lodging, transportation and travel group. Initial categorical data are reduced using multiple correspondence analysis and grouped through cluster analysis. Afterwards, identified segments are evaluated to analyse their tourist profiles with a view to examining sociological perspectives of tourist behaviour.Tourist Demand, Segmentation, Spain, Domestic Tourism, Multiple Correspondence Analysis, Cluster Analysis
Cluster growing process and a sequence of magic numbers
We present a new theoretical framework for modelling the cluster growing
process. Starting from the initial tetrahedral cluster configuration, adding
new atoms to the system and absorbing its energy at each step, we find cluster
growing paths up to the cluster sizes of more than 100 atoms. We demonstrate
that in this way all known global minimum structures of the Lennard-Jonnes (LJ)
clusters can be found. Our method provides an efficient tool for the
calculation and analysis of atomic cluster structure. With its use we justify
the magic numbers sequence for the clusters of noble gases atoms and compare it
with experimental observations. We report the striking correspondence of the
peaks in the dependence on cluster size of the second derivative of the binding
energy per atom calculated for the chain of LJ-clusters based on the
icosahedral symmetry with the peaks in the abundance mass spectra
experimentally measured for the clusters of noble gases atoms. Our method
serves an efficient alternative to the global optimization techniques based on
the Monte-Carlo simulations and it can be applied for the solution of a broad
variety of problems in which atomic cluster structure is important.Comment: 10 pages, 3 figure
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