Evidence for two populations of Galactic globular clusters from the ratio of their half-mass to Jacobi radii

We investigate the ratio between the half-mass radii r_h of Galactic globular clusters and their Jacobi radii r_J given by the potential of the Milky Way and show that clusters with galactocentric distances R_{GC}>8 kpc fall into two distinct groups: one group of compact, tidally-underfilling clusters with r_h/r_J<0.05 and another group of tidally filling clusters which have 0.1 < r_h/r_J<0.3. We find no correlation between the membership of a particular cluster to one of these groups and its membership in the old or younger halo population. Based on the relaxation times and orbits of the clusters, we argue that compact clusters and most clusters in the inner Milky Way were born compact with half-mass radii r_h < 1 pc. Some of the tidally-filling clusters might have formed compact as well, but the majority likely formed with large half-mass radii. Galactic globular clusters therefore show a similar dichotomy as was recently found for globular clusters in dwarf galaxies and for young star clusters in the Milky Way. It seems likely that some of the tidally-filling clusters are evolving along the main sequence line of clusters recently discovered by Kuepper et al. (2008) and are in the process of dissolution.Comment: 8 pages, 4 figures, MNRAS in pres

Star Cluster Formation and Disruption Time-Scales -- I. An empirical determination of the disruption time of star clusters in four galaxies

We present a new method to derive the cluster disruption time in selected regions of galaxies from the mass or age distribution of magnitude-limited cluster samples. If the disruption time of clusters in a region of a galaxy depends on their initial mass as t_4 x (M_cluster/10^4 M_sun)^gamma and if the cluster formation rate is constant, then the mass and age distributions of the observed clusters will each show two powerlaw relations. The values of t_4 and gamma can be derived from these relations. We used this method to derive the cluster disruption time in specific regions in four galaxies: the inner region of M51, a region of M33, the SMC and the solar neighbourhood. The values of gamma are the same in the four galaxies within the uncertainty and the mean value is gamma= 0.62 +- 0.06. However the disruption time t_4 of a cluster of 10^4 M_sun is very different in the different galaxies. The clusters in the SMC have the longest disruption time, t_4 = 8 Gyr, and the clusters at 1 to 3 kpc from the nucleus of M51 have the shortest disruption time of t_4 = 0.04 Gyr. The disruption time of clusters 1 to 5 kpc from the nucleus of M33 is t_4 = 0.13 Gyr and for clusters within 1 kpc from the Sun we find t_4 = 1.0 Gyr.Comment: 18 pages, 18 figures. Accepted for publication by Monthly Notice

Spectral properties on a circle with a singularity

We investigate the spectral and symmetry properties of a quantum particle moving on a circle with a pointlike singularity (or point interaction). We find that, within the U(2) family of the quantum mechanically allowed distinct singularities, a U(1) equivalence (of duality-type) exists, and accordingly the space of distinct spectra is U(1) x [SU(2)/U(1)], topologically a filled torus. We explore the relationship of special subfamilies of the U(2) family to corresponding symmetries, and identify the singularities that admit an N = 2 supersymmetry. Subfamilies that are distinguished in the spectral properties or the WKB exactness are also pointed out. The spectral and symmetry properties are also studied in the context of the circle with two singularities, which provides a useful scheme to discuss the symmetry properties on a general basis.Comment: TeX, 26 pages. v2: one reference added and two update

Monte Carlo Simulations of Star Clusters - VII. The globular cluster 47 Tuc

We describe Monte Carlo models for the dynamical evolution of the massive globular cluster 47 Tuc (NGC 104). The code includes treatments of two-body relaxation, most kinds of three- and four-body interactions involving primordial binaries and those formed dynamically, the Galactic tide, and the internal evolution of both single and binary stars. We arrive at a set of initial parameters for the cluster which, after 12Gyr of evolution, gives a model with a fairly satisfactory match to surface brightness and density profiles, the velocity dispersion profile, the luminosity function in two fields, and the acceleration of pulsars. Our models appear to require a relatively steep initial mass function for stars above about turnoff, with an index of about 2.8 (where the Salpeter mass function has an index of 2.35), and a relatively flat initial mass function (index about 0.4) for the lower main sequence. According to the model, the current mass is estimated at 0.9 million solar masses, of which about 34% consists of remnants. We find that primordial binaries are gradually taking over from mass loss by stellar evolution as the main dynamical driver of the core. Despite the high concentration of the cluster, core collapse will take at least another 20Gyr.Comment: 16 pages, 16 figures, revised version submitted to MNRA

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$_{\odot}$ 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

Modulation of Aβ(42 )low-n oligomerization using a novel yeast reporter system

BACKGROUND: While traditional models of Alzheimer's disease focused on large fibrillar deposits of the Aβ(42 )amyloid peptide in the brain, recent work suggests that the major pathogenic effects may be attributed to SDS-stable oligomers of Aβ(42). These Aβ(42 )oligomers represent a rational target for therapeutic intervention, yet factors governing their assembly are poorly understood. RESULTS: We describe a new yeast model system focused on the initial stages of Aβ(42 )oligomerization. We show that the activity of a fusion of Aβ(42 )to a reporter protein is compromised in yeast by the formation of SDS-stable low-n oligomers. These oligomers are reminiscent of the low-n oligomers formed by the Aβ(42 )peptide in vitro, in mammalian cell culture, and in the human brain. Point mutations previously shown to inhibit Aβ(42 )aggregation in vitro, were made in the Aβ(42 )portion of the fusion protein. These mutations both inhibited oligomerization and restored activity to the fusion protein. Using this model system, we found that oligomerization of the fusion protein is stimulated by millimolar concentrations of the yeast prion curing agent guanidine. Surprisingly, deletion of the chaperone Hsp104 (a known target for guanidine) inhibited oligomerization of the fusion protein. Furthermore, we demonstrate that Hsp104 interacts with the Aβ(42)-fusion protein and appears to protect it from disaggregation and degradation. CONCLUSION: Previous models of Alzheimer's disease focused on unravelling compounds that inhibit fibrillization of Aβ(42), i.e. the last step of Aβ(42 )assembly. However, inhibition of fibrillization may lead to the accumulation of toxic oligomers of Aβ(42). The model described here can be used to search for and test proteinacious or chemical compounds for their ability to interfere with the initial steps of Aβ(42 )oligomerization. Our findings suggest that yeast contain guanidine-sensitive factor(s) that reduce the amount of low-n oligomers of Aβ(42). As many yeast proteins have human homologs, identification of these factors may help to uncover homologous proteins that affect Aβ(42 )oligomerization in mammals

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

Measuring Mass Loss Rates from Galactic Satellites

We present the results of a study that uses numerical simulations to interpret observations of tidally disturbed satellites around the Milky Way. When analysing the simulations from the viewpoint of an observer, we find a break in the slope of the star count and velocity dispersion profiles in our models at the location where unbound stars dominate. We conclude that `extra-tidal' stars and enhanced velocity dispersions observed in the outskirts of Galactic satellites are due to contamination by stellar debris from the tidal interaction with the Milky Way. However, a significant bound population can exist beyond the break radius and we argue that it should not be identified with the tidal radius of the satellite. We also develop and test a method for determining the mass loss rate from a Galactic satellite using its extra-tidal population. We apply this method to observations of globular clusters and dwarf spheroidal satellites of the Milky Way, and conclude that a significant fraction of both satellite systems are likely be destroyed within the next Hubble time. Finally, we demonstrate that this mass loss estimate allows us to place some limits on the initial mass function (IMF) of stars in a cluster from the radial dependence of its present day mass function (PDMF).Comment: 18 pages including 22 figures. Submitted to MNRA

Protein Folding Activity of the Ribosome is involved in Yeast Prion Propagation.

6AP and GA are potent inhibitors of yeast and mammalian prions and also specific inhibitors of PFAR, the protein-folding activity borne by domain V of the large rRNA of the large subunit of the ribosome. We therefore explored the link between PFAR and yeast prion [PSI(+)] using both PFAR-enriched mutants and site-directed methylation. We demonstrate that PFAR is involved in propagation and de novo formation of [PSI(+)]. PFAR and the yeast heat-shock protein Hsp104 partially compensate each other for [PSI(+)] propagation. Our data also provide insight into new functions for the ribosome in basal thermotolerance and heat-shocked protein refolding. PFAR is thus an evolutionarily conserved cell component implicated in the prion life cycle, and we propose that it could be a potential therapeutic target for human protein misfolding diseases

Structural parameters and blue stragglers in Sagittarius dwarf spheroidal galaxy globular clusters

We present BV photometry of four Sagittarius dwarf spheroidal galaxy globular clusters: Arp 2, NGC 5634, Palomar 12, and Terzan 8, obtained with the Danish telescope at ESO-La Silla. We measure the structural parameters of the clusters using a King profile fitting, obtaining the first reliable measurements of the tidal radius of Arp 2 and Terzan 8. These two clusters are remarkably extended and with low concentrations; with a concentration of only c = 0.41 +/- 0.02, Terzan 8 is less concentrated than any cluster in our Galaxy. Blue stragglers are identified in the four clusters, and their spatial distribution is compared to those of horizontal branch and red giant branch stars. The blue straggler properties do not provide evidence of mass segregation in Terzan 8, while Arp 2 probably shares the same status, although with less confidence. In the case of NGC 5634 and Palomar 12, blue stragglers are significantly less populous, and their analysis suggests that the two clusters have probably undergone mass segregation.Comment: 11 pages - 10 figures - MNRAS accepted || v2: minor changes to match published versio