Degree Of Substructure Research Articles

Evolution of fractality and rotation in embedded star clusters

ABSTRACT More and more observations indicate that young star clusters could retain imprints of their formation process. In particular, the degree of substructuring and rotation are possibly the direct result of the collapse of the parent molecular cloud from which these systems form. Such properties can, in principle, be washed-out, but they are also expected to have an impact on the relaxation of these systems. We ran and analysed a set of 10 hydrodynamical simulations of the formation of embedded star clusters through the collapse of turbulent massive molecular clouds. We systematically studied the fractality of our star clusters, showing that they are all extremely substructured (fractal dimension D = 1.0–1.8). We also found that fractality is slowly reduced, with time, on small scales, while it persists on large scales on longer time-scales. Signatures of rotation are found in different simulations at every time of the evolution, even for slightly supervirial substructures, proving that the parent molecular gas transfers part of its angular momentum to the new stellar systems.

Microstructure and mechanical properties of a commercially pure Ti processed by warm equal channel angular pressing

Abstract A commercially pure (CP) Titanium alloy classified as Grade 1, was processed by Equal Channel Angular Pressing (ECAP) up to 4 passes in the temperature range of 450–150 °C. The resulting microstructures were observed by Electron Back-Scattered Diffraction, revealing a bimodal grain size distribution consisting of small recrystallized grains of submicrometer size, with an average value of 0.3 µm, and elongated bands of 1.4 µm with different degree of substructure. Additionally the fraction of restored and deformed grains were evaluated as a function of processing temperature following an internal grain misorientation criterion, leading to an overall fraction of recrystallized grains between 40% and 20% in samples ECAPed at 450 and 150 °C, respectively. The strengthening contributions of the grain size, equivalent oxygen content (O eq ) and Low Angle Grain Boundaries (LAGBs) to the yield stress were identified by the Hall Petch and Taylor equations. The strengthening coefficient k of the Hall–Petch relation was approximately 5 MPa mm −1/2 , with an increment of 0.44 MPa mm −1/2 per 0.1 O eq .-%, while the LAGB strengthening contribution was responsible approximately by half of the experimental yield stress values measured.

Statistics and implications of substructure detected in a representative sample of X-ray clusters

We present a morphological study of 35 X-ray luminous galaxy clusters at 0.15<z<0.3, selected in a similar manner to the Local Cluster Substructure Survey (LoCuSS), for which deep XMM-Newton observations are available. We characterise the structure of the X-ray surface brightness distribution of each cluster by measuring both their power ratios and centroid shift, and thus rank the clusters by the degree of substructure. These complementary probes give a consistent description of the cluster morphologies with some well understood exceptions. We find a remarkably tight correlation of regular morphology with the occurrence of cool cores in clusters. We also compare our measurements of X-ray morphology with measurements of the luminosity gap statistics and ellipticity of the brightest cluster galaxy (BCG). We check how our new X-ray morphological analysis maps onto cluster scaling relations, finding that (i) clusters with relatively undisturbed X-ray morphologies are on average more luminous at fixed X-ray temperature than those with disturbed morphologies, and (ii) disturbed clusters have larger X-ray masses than regular clusters for a given temperature in the M-T relation. We also show that the scatter in the ratio of X-ray and weak lensing based cluster mass measurements is larger for disturbed clusters than for those of more regular morphology. Overall, our results demonstrate the feasibility of assembling a self-consistent picture of the physical structure of clusters from X-ray and optical data, and the potential to apply this in the measurement of cosmological cluster scaling relations.

Importance of the initial conditions for star formation - III. Statistical properties of embedded protostellar clusters

We investigate the formation of protostellar clusters during the collapse of dense molecular cloud cores with a focus on the evolution of potential and kinetic energy, the degree of substructure, and the early phase of mass segregation. Our study is based on a series of hydrodynamic simulations of dense cores, where we vary the initial density profile and the initial turbulent velocity. In the three-dimensional adaptive mesh refinement simulations, we follow the dynamical formation of filaments and protostars until a star formation efficiency of 20%. Despite the different initial configurations, the global ensemble of all protostars in a setup shows a similar energy evolution and forms sub-virial clusters with an energy ratio $E_\mathrm{kin}/|E_\mathrm{pot}|\sim0.2$. Concentrating on the innermost central region, the clusters show a roughly virialised energy balance. However, the region of virial balance only covers the innermost $\sim10-30%$ of all the protostars. In all simulations with multiple protostars, the total kinetic energy of the protostars is higher than the kinetic energy of the gas cloud, although the protostars only contain 20% of the total mass. The clusters vary significantly in size, mass, and number of protostars, and show different degrees of substructure and mass segregation. Flat density profiles and compressive turbulent modes produce more subclusters then centrally concentrated profiles and solenoidal turbulence. We find that dynamical relaxation and hence dynamical mass segregation is very efficient in all cases from the very beginning of the nascent cluster, i.e., during a phase when protostars are constantly forming and accreting.

Y-STR loci diversity in native Alaskan populations

Y chromosome short tandem repeat (Y-STR) loci are important genetic markers for forensic biological evidence analyses. However, paternal inheritance, reduced effective population size, and lack of independence between loci can reduce Y-STR diversity and may yield greater population substructure effects on a locus-by-locus basis compared with the autosomal STR loci. Population studies are necessary to assess the genetic variation of forensically relevant markers so that proper inferences can be made about the rarity of DNA profiles. This study examined 16 Y-STRs in three sampled populations of Native Americans from Alaska: Inupiat, Yupik, and Athabaskan. Population genetic and statistical issues addressed were: (1) the degree of diversity at locus and haplotype levels, (2) determination of the loci that contribute more so to haplotype diversity, and (3) the effects of population substructure on forensic statistical calculations of the rarity of a Y-STR profile. All three population samples were highly polymorphic at the haplotype level for the 16 Y-STR markers; however, the Native Americans demonstrated reduced genetic diversity compared with major US populations. The degree of substructure indicated that the three populations were related and admixed in terms of paternal lineage. The examination of more polymorphic loci may be needed to increase the power of discrimination of Y-STR systems in these populations.

Biodemographic and Genetic Structure of Zamora Province (Spain): Insights from Surname Analysis

Located in the north-western extreme of the Iberian northern plateau with the Portugal border on the west, Zamora province is one of the current divisions of the autonomous region of Castile-Leon (Spain). According to natural boundaries and historical records, the province of Zamora can be subdivided into six different regions: Aliste, Benavente, Bajo Duero, Campos-Pan, Sanabria and Sayago. The geography of the area is configured by two major rivers: the Duero that crosses the province in a west-east direction and its affluent the Esla that crosses the province in a northwest-southeast course. In this work, we analyze both surnames of 166,349 individuals from Zamora province. The main goal of this study is to explore the differential weight of historical, demographic, geographic and sociocultural factors in shaping the bio-demographic and genetic structure of this specific population. The highest value of total consanguinity based on random isonymy is observed in Sanabria, which also has the lowest values of heteronymy and the highest intra-population a priori kinship. By contrast, Sayago exhibits the highest value of heteronymy and the lowest intra-population a priori kinship. The consanguinity values observed in Zamora are, in part, related to the population structure. Notwithstanding, the values of non-random consanguinity observed in some municipalities could be associated with particular mating behaviors that favor isonymic marriages. Finally, the results of analyses indicate that the levels of micro-differentiation in the province cannot be considered high; however, some degree of substructure could be deduced. It appears that the current population structure of Zamora has been determined not only by settlement history, geographic distance and geographic boundaries, but also, by land system distribution and population size.

The Accretion Origin of the Milky Way’s Stellar Halo

We have used data from the Sloan Digital Sky Survey (SDSS) Data Release 5 to explore the overall structure and substructure of the stellar halo of the Milky Way using ~4 million color-selected main-sequence turnoff stars with -->0.2 18.5 ? r 0.5 3.7 ? 1.2 ? 108 M?. The density profile of the stellar halo is approximately -->? r??, where ? -->2 > ? > ? 4. Yet, we found that all smooth and symmetric models were very poor fits to the distribution of stellar halo stars because the data exhibit a great deal of spatial substructure. We quantified deviations from a smooth oblate/triaxial model using the rms of the data around the model profile on scales 100 pc, after accounting for the (known) contribution of Poisson uncertainties. Within the DR5 area of the SDSS, the fractional rms deviation ?/total of the actual stellar distribution from any smooth, parameterized halo model is 40%: hence, the stellar halo is highly structured. We compared the observations with simulations of galactic stellar halos formed entirely from the accretion of satellites in a cosmological context by analyzing the simulations in the same way as the SDSS data. While the masses, overall profiles, and degree of substructure in the simulated stellar halos show considerable scatter, the properties and degree of substructure in the Milky Way's halo match well the properties of a typical stellar halo built exclusively out of the debris from disrupted satellite galaxies. Our results therefore point toward a picture in which an important fraction of the stellar halo of the Milky Way has been accreted from satellite galaxies.

The Effect of Substructure on Mass Estimates of Galaxies

Large galaxies are thought to form hierarchically, from the accretion and disruption of many smaller galaxies. Such a scenario should naturally lead to galactic phase-space distributions containing some degree of substructure. We examine the errors in mass estimates of galaxies and their dark halos made using the projected phase-space distribution of a tracer population (such as a globular cluster system or planetary nebulae) due to falsely assuming that the tracers are distributed randomly. The level of this uncertainty is assessed by applying a standard mass estimator to samples drawn from 11 random realizations of galaxy halos containing levels of substructure consistent with current models of structure formation. We find that substructure will distort our mass estimates by up to ~20% - a negligible error compared to statistical and measurement errors in current derivations of masses for our own and other galaxies. However, this represents a fundamental limit to the accuracy of any future mass estimates made under the assumption that the tracer population is distributed randomly, regardless of the size of the sample or the accuracy of the measurements.

High‐Resolution Simulations of Cluster Formation

The formation history of rich clusters is investigated using a hybrid N-body simulation in which high spatial and mass resolution can be achieved self-consistently within a small region of a very large volume. The evolution of three massive clusters is studied via mass accretion, spherically averaged density profiles, three-dimensional and projected shapes, and degree of substructure. Each cluster is resolved well and consists of ~4 × 105 particles at the present epoch. Although the clusters have similar masses, M(r = 1.5 h-1 Mpc) ~ 2 × 1015 h-1 M☉, and similar spherically averaged density profiles at the end of the simulations, markedly different formation histories are observed. No single, dominant pattern is apparent in the time variation of the mass accretion rate, the cluster shape, or the degree of substructure. Individually, the density profiles of the clusters are fitted well by Navarro, Frenk, & White (NFW) profiles over the course of the simulations. The values of the NFW concentration parameter that best reproduce the cluster profiles are, however, lower than the values predicted for halos with masses identical to those of the simulated clusters. The largest discrepancy between the observed and predicted concentration parameters is of order a factor of 2.5 and is most likely caused by substructure in the clusters on scales less than the virial radius, r200.