Similarities in Populations of Star Clusters

We compare the observed mass functions and age distributions of star clusters in six well-studied galaxies: the Milky Way, Magellanic Clouds, M83, M51, and Antennae. In combination, these distributions span wide ranges of mass and age: 10^2\lea M/M_{\odot}\lea10^6 and 10^6\lea\tau/yr \lea10^9. We confirm that the distributions are well represented by power laws: $dN/dM\propto M^{\beta}$ with $\beta \approx-1.9$ and $dN/d\tau\propto\tau^{\gamma}$ with $\gamma\approx -0.8$. The mass and age distributions are approximately independent of each other, ruling out simple models of mass-dependent disruption. As expected, there are minor differences among the exponents, at a level close to the true uncertainties, $\epsilon_{\beta}\sim\epsilon_{\gamma}\sim$~0.1--0.2. However, the overwhelming impression is the similarity of the mass functions and age distributions of clusters in these different galaxies, including giant and dwarf, quiescent and interacting galaxies. This is an important empirical result, justifying terms such as "universal" or "quasi-universal." We provide a partial theoretical explanation for these observations in terms of physical processes operating during the formation and disruption of the clusters, including star formation and feedback, subsequent stellar mass loss, and tidal interactions with passing molecular clouds. A full explanation will require additional information about the molecular clumps and star clusters in galaxies beyond the Milky Way.Comment: 20 pages, 4 figures, 2 tables; published in the Astrophysical Journal, 752:96 (2012 June 20

The Age Distribution of Massive Star Clusters in the Antennae Galaxies

We determine the age distribution of star clusters in the Antennae galaxies (NGC 4038/9) for two mass-limited samples (M > 3 x 10^4 M_{\odot} and M > 2 x 10^5 M_{\odot}). This is based on integrated broadband UBVI and narrowband H-alpha photometry from deep images taken with the Hubble Space Telescope. We find that the age distribution of the clusters declines steeply, approximately as dN/d\tau \propto \tau^{-1}. The median age of the clusters is ~10^7 yr, which we interpret as evidence for rapid disruption ("infant mortality"). It is very likely that most of the young clusters are not gravitationally bound and were disrupted near the times they formed by the energy and momentum input from young stars to the interstellar matter of the protoclusters. At least 20% and possibly all stars form in clusters and/or associations, including those that are unbound and short-lived.Comment: 11 pages, 2 figures. To appear in the ApJ Letters; Submitted 2004 July 29; accepted 2005 August

Density Dependence of the Mass Function of Globular Star Clusters in the Sombrero Galaxy and its Dynamical Implications

We have constructed the mass function of globular star clusters in the Sombrero galaxy in bins of different internal half-mass density rho_h and projected galactocentric distance R. This is based on the published measurements of the magnitudes and effective radii of the clusters by Spitler et al. (2006) in BVR images taken with the ACS on HST. We find that the peak of the mass function M_p increases with rho_h by a factor of about 4 but remains nearly constant with R. Our results are almost identical to those presented recently by McLaughlin & Fall (2007) for globular clusters in the Milky Way. The mass functions in both galaxies agree with a simple, approximate model in which the clusters form with a Schechter initial mass function and evolve subsequently by stellar escape driven by internal two-body relaxation. These findings therefore undermine recent claims that the present peak of the mass function of globular clusters must have been built into the initial conditions.Comment: Astrophysical Journal Letters, in press. 4 page

Crashworthiness of Highway Lamp Post against Vehicle Impact

Three dimensional numerical investigations has been carried out on steel lamp post against vehicle impact using ABAQUS/Explicit finite element code. The response of steel pole was studied against vehicle impact subjected to varying speed and mass of vehicle, and varying thickness and impact location of pole. The geometry of steel pole was considered on the basis of Indian standard recommendations and choice of pole is based on conventional highway lighting applications supporting luminaire mounting heights. The constitutive and fracture behavior of steel pole has been predicted using JC model available in ABAQUS/Explicit and the material parameters of JC model for the available in literature. The response of metallic pole was studied against varying speed, mass of vehicle and impact location considering the influential factors which affects the pole significantly. In addition to that, the response of pole was studied against the varying thickness, i.e. the effective thickness of cylindrical pole may be decreased at later stage due to corrosion and environmental effects. The responses of steel pole have been studied and presented in light of reaction forces, deflection and Von-Mises stresses

The Performance of Monolithic Reinforced Concrete Structure Includes Slab, Beam and Column against Blast Load

The numerical investigations has been carried out on reinforced concrete structures against blast loading to demonstrate the accuracy and effectiveness of the finite element based numerical models. The size of building was considered 3 ×3 × 3 m and whereas the size of beam and column was 0.3 m, arbitrary. The thickness of roof slab was 120 mm, whereas the reinforced concrete wall on all four sides was 0.2 m. The simulations were carried out through finite element code ABAQUS/CAE. The inelastic behavior of concrete has been incorporated through concrete damage plasticity model and the model includes compressive and tensile behavior. The elastic and plastic behavior of steel reinforcement bar has been incorporated using Johnson-cook model includes the effect of state of stress, temperature and strain rate. The simulations were carried out against varying standoff distance, mass of TNT, locations of blast origin and thickness of roof slab to examine the resistance of building. The response of structural elements were studied in light of deflection, impulsive velocity, von-Mises stresses, compression and tension damage of concrete. The results indicate that the standoff distance has great influence on the survivability of reinforced concrete slab and wall