The Stream-Stream Collision after the Tidal Disruption of a Star Around a Massive Black Hole

A star can be tidally disrupted around a massive black hole. It has been known that the debris forms a precessing stream, which may collide with itself. The stream collision is a key process determining the subsequent evolution of the stellar debris: if the orbital energy is efficiently dissipated, the debris will eventually form a circular disk (or torus). In this paper, we have numerically studied such stream collision resulting from the encounter between a 10^6 Msun black hole and a 1 Msun normal star with a pericenter radius of 100 Rsun. A simple treatment for radiative cooling has been adopted for both optically thick and thin regions. We have found that approximately 10 to 15% of the initial kinetic energy of the streams is converted into thermal energy during the collision. The angular momentum of the incoming stream is increased by a factor of 2 to 3, and such increase, together with the decrease in kinetic energy, significantly helps the circularization process. Initial luminosity burst due to the collision may reach as high as 10^41 erg/sec in 10^4 sec, after which the luminosity increases again (but slowly this time) to a steady value of a few 10^40 erg/sec in a few times of 10^5 sec. The radiation from the system is expected to be close to Planckian with effective temperature of \~10^5K.Comment: 19 pages including 12 figures; Accepted for publication in Ap

N-Body Simulations of Compact Young Clusters near the Galactic Center

We investigate the dynamical evolution of compact young star clusters (CYCs) near the Galactic center (GC) using Aarseth's Nbody6 codes. The relatively small number of stars in the cluster (5,000-20,000) makes real-number N-body simulations for these clusters feasible on current workstations. Using Fokker-Planck (F-P) models, Kim, Morris, & Lee (1999) have made a survey of cluster lifetimes for various initial conditions, and have found that clusters with a mass <~ 2x10^4 Msun evaporate in ~10 Myr. These results were, however, to be confirmed by N-body simulations because some extreme cluster conditions, such as strong tidal forces and a large stellar mass range participating in the dynamical evolution, might violate assumptions made in F-P models. Here we find that, in most cases, the CYC lifetimes of previous F-P calculations are 5-30% shorter than those from the present N-body simulations. The comparison of projected number density profiles and stellar mass functions between N-body simulations and HST/NICMOS observations by Figer et al. (1999) suggests that the current tidal radius of the Arches cluster is ~1.0 pc, and the following parameters for the initial conditions of that cluster: total mass of 2x10^4 Msun and mass function slope for intermediate-to-massive stars of 1.75 (the Salpeter function has 2.35). We also find that the lower stellar mass limit, the presence of primordial binaries, the amount of initial mass segregation, and the choice of initial density profile (King or Plummer models) do not significantly affect the dynamical evolution of CYCs.Comment: 20 pages including 6 figures, To appear in ApJ, Dec 20 issu

The relationship between ethnic classroom composition and Turkish-origin and German students’ reading performance and sense of belonging

Past research on ethnic composition effects on migrant and ethnic majority students’ performance has reported inconclusive results: Some studies have found no relationship between the proportion of migrant students in school and students’ performance, some revealed positive effects, whereas others showed negative effects of the proportion of migrant students. Most of the studies did not consider whether an increase in the proportion of migrant students in the classroom has different effects on migrant and ethnic majority students’ performance. For this reason, the present study (N = 9215) extends previous research by investigating the cross-level interaction effect of the proportion of Turkish-origin students in classrooms on Turkish-origin and German students’ reading performance with data based on the German National Assessment Study 2008/2009 in the school subject German. In addition, we examined the cross-level interaction effect of Turkish-origin students’ proportion on sense of belonging to school of Turkish-origin and German students, as sense of belonging has been shown to be an important predictor of well-being and integration. No cross-level interaction effect on performance emerged. Only a small negative main effect of the Turkish-origin students’ proportion on all students’ performance was found. As predicted, we showed a cross-level interaction on sense of belonging. Only Turkish-origin students’ sense of belonging was positively related to the proportion of Turkish-origin students: The more Turkish-origin students there were in a classroom, the higher Turkish-origin students’ sense of belonging. German students’ sense of belonging was not related to the ethnic classroom composition. Implications of the results in the educational context are discussed

The Microvasculature of Human Oral Mucosa Using Vascular Corrosion Casts and India Ink Injection I. Tongue Papillae

The microvasculature of human tongue papillae originating from 9 males and 6 females, aged 0.5 to 2 years was studied by scanning electron microscopy (SEM) of vascular corrosion casts and by light microscopy (LM) of India ink injected specimens. All papillae showed a microvasculature characterized by primary, secondary and tertiary capillary loops. In the filiform papillae the loops were generally arranged in a corolla-like pattern with the tertiary loops demonstrating a hair-pin shape. The fungiform papillae showed basically a similar architectural pattern although the loops were somewhat more compact and complex in structure. A small, shallow depression of the tertiary loops at the top of these papillae was found to be occupied by a prominent rete ridge of the surface epithelium. There was a gradual transition from filiform to foliate papillae, the latter appearing as rows of coalesced filiform papillae. The circumvallate papillae easily identified by the surrounding furrow showed a rather complex and compact pattern of capillary loops of which typical hair-pin shaped tertiary loops dominated the periphery of the papilla. Small grooves or depressions in the vascular network were found to be occupied by rete ridges of the overlying mucosal epithelium

Development of Shear Modulus Reduction Curves Based on Lotung Downhole Ground Motion Data

In this study, equivalent shear moduli (or shear-wave velocities) and their variations with shearing strain at the Lotung seismic experiment site were back-calculated from recorded downhole array ground motions. Ground motion data for various levels of shaking (peak ground surface accelerations ranging from 0.03g to 0.21g) recorded during seven earthquakes were used in the analyses. Results show that downhole array ground motion data can be used to infer in-situ dynamic soil properties over a wide strain range