Cluster Assembly in Hierarchically Collapsing Clouds

We discuss the mechanism of cluster formation in hierarchically collapsing molecular clouds. Recent evidence, both observational and numerical, suggests that molecular clouds (MCs) may be undergoing global, hierarchical gravitational collapse. The "hierarchical" regime consists of small-scale collapses within larger-scale ones. The latter implies that the star formation rate increases systematically during the early stages of evolution, and occurs via filamentary flows onto "hubs" of higher density, mass, and velocity dispersion, and culminates a few Myr after than the small-scale collapses have started to form stars. In turn, the small-scale collapses occur in clumps embedded in the filaments, and are themselves falling into the larger potential well of the still-ongoing large-scale collapse. The stars formed in the early, small-scale collapses share the infall motion of their parent clumps towards the larger potential trough, so that the filaments feed both gaseous and stellar material to the hubs. This leads to the presence of older stars in a region where new protostars are still forming, to a scale-free or fractal structure of the clusters, in which each unit is composed of smaller-scale ones, and to the eventual merging of the subunits, explaining the observed structural features of open clusters.Comment: 6 pages, 7 figures, to appear in the proceedings of IAU Symposium 316: Formation, Evolution, and Survival of Massive Star Cluster

Cooling rate, heating rate and aging effects in glassy water

We report a molecular dynamics simulation study of the properties of the potential energy landscape sampled by a system of water molecules during the process of generating a glass by cooling, and during the process of regenerating the equilibrium liquid by heating the glass. We study the dependence of these processes on the cooling/heating rates as well as on the role of aging (the time elapsed in the glass state). We compare the properties of the potential energy landscape sampled during these processes with the corresponding properties sampled in the liquid equilibrium state to elucidate under which conditions glass configurations can be associated with equilibrium liquid configurations.Comment: to be published in Phys. Rev. E (rapid comunication

Aging near rough and smooth boundaries in colloidal glasses

We use confocal microscopy to study the aging of a bidisperse colloidal glass near rough and smooth boundaries. Near smooth boundaries, the particles form layers, and particle motion is dramatically slower near the boundary as compared to the bulk. Near rough boundaries, the layers nearly vanish, and particle motion is nearly identical to that of the bulk. The gradient in dynamics near the boundaries is demonstrated to be a function of the gradient in structure for both types of boundaries.Our observations show that wall-induced layer structures strongly influence aging.Comment: 8 pages, 7 figure

The Glass Transition Temperature of Water: A Simulation Study

We report a computer simulation study of the glass transition for water. To mimic the difference between standard and hyperquenched glass, we generate glassy configurations with different cooling rates and calculate the $T$ dependence of the specific heat on heating. The absence of crystallization phenomena allows us, for properly annealed samples, to detect in the specific heat the simultaneous presence of a weak pre-peak (``shadow transition''), and an intense glass transition peak at higher temperature. We discuss the implications for the currently debated value of the glass transition temperature of water. We also compare our simulation results with the Tool-Narayanaswamy-Moynihan phenomenological model.Comment: submitted to Phys. Re

The Accelerating Pace of Star Formation

We study the temporal and spatial distribution of star formation rates in four well-studied star-forming regions in local molecular clouds(MCs): Taurus, Perseus, $\rho$ Ophiuchi, and Orion A. Using published mass and age estimates for young stellar objects in each system, we show that the rate of star formation over the last 10 Myrs has been accelerating and is (roughly) consistent with a $t^2$ power law. This is in line with previous studies of the star formation history of molecular clouds and with recent theoretical studies. We further study the clustering of star formation in the Orion Nebula Cluster(ONC). We examine the distribution of young stellar objects as a function of their age by computing an effective half-light radius for these young stars subdivided into age bins. We show that the distribution of young stellar objects is broadly consistent with the star formation being entirely localized within the central region. We also find a slow radial expansion of the newly formed stars at a velocity of $v=0.17\,{\rm km\,s}^{-1}$, which is roughly the sound speed of the cold molecular gas. This strongly suggests the dense structures that form stars persist much longer than the local dynamical time. We argue that this structure is quasi-static in nature and is likely the result of the density profile approaching an attractor solution as suggested by recent analytic and numerical analysis.Comment: 7 pages, 4 figures, submitted to MNRA

Nonthermal radiation from relativistic electrons accelerated at spherically expanding shocks

We study the evolution of the energy spectrum of cosmic-ray electrons accelerated at spherically expanding shocks with low Mach numbers and the ensuing spectral signatures imprinted in radio synchrotron emission. Time-dependent simulations of diffusive shock acceleration (DSA) of electrons in the test-particle limit have been performed for spherical shocks with parameters relevant for typical shocks in the intracluster medium. The electron and radiation spectra at the shock location can be described properly by the test-particle DSA predictions with instantaneous shock parameters. However, the volume integrated spectra of both electrons and radiation deviate significantly from the test-particle power-laws, because the shock compression ratio and the flux of injected electrons at the shock gradually decrease as the shock slows down in time.So one needs to be cautious about interpreting observed radio spectra of evolving shocks based on simple DSA models in the test-particle regime.Comment: corrected typos and figures, 12 pages, 7 figures, Accepted for publication at Journal of Korean Astronomical Societ

Can Sgr A* flares reveal the molecular gas density PDF?

Illumination of dense gas in the Central Molecular Zone (CMZ) by powerful X-ray flares from Sgr A* leads to prominent structures in the reflected emission that can be observed long after the end of the flare. By studying this emission we learn about past activity of the supermassive black hole in our Galactic Center and, at the same time, we obtain unique information on the structure of molecular clouds that is essentially impossible to get by other means. Here we discuss how X-ray data can improve our knowledge of both sides of the problem. Existing data already provide: i) an estimate of the flare age, ii) a model-independent lower limit on the luminosity of Sgr A* during the flare and iii) an estimate of the total emitted energy during Sgr A* flare. On the molecular clouds side, the data clearly show a voids-and-walls structure of the clouds and can provide an almost unbiased probe of the mass/density distribution of the molecular gas with the hydrogen column densities lower than few $10^{23}\;{\rm cm^{-2}}$. For instance, the probability distribution function of the gas density $PDF(\rho)$ can be measured this way. Future high energy resolution X-ray missions will provide the information on the gas velocities, allowing, for example a reconstruction of the velocity field structure functions and cross-matching the X-ray and molecular data based on positions and velocities.Comment: 13 pages, 7 figures; Accepted for publication in MNRA

Time-dependent modeling of pulsar wind nebulae: Study on the impact of the diffusion-loss approximations

In this work, we present a leptonic, time-dependent model of pulsar wind nebulae (PWNe). The model seeks a solution for the lepton distribution function considering the full time-energy dependent diffusion-loss equation. The time-dependent lepton population is balanced by injection, energy losses, and escape. We include synchrotron, inverse Compton (IC, with the cosmic-microwave background as well as with IR/optical photon fields), self-synchrotron Compton (SSC), and bremsstrahlung processes, all devoid of any radiative approximations. With this model in place we focus on the Crab nebula as an example and present its time dependent evolution. Afterwards, we analyze the impact of different approximations made at the level of the diffusion-loss equation, as can be found in the literature. Whereas previous models ignored the escape term, e.g., with the diffusion-loss equation becoming advective, others approximated the losses as catastrophic, so that the equation has only time derivatives. Additional approximations are also described and computed. We show which is the impact of these approaches in the determination of the PWN evolution. In particular, we find the time-dependent deviation of the multi-wavelength spectrum and the best-fit parameters obtained with the complete and the approximate models.Comment: In press in MNRA

Constraining the ages of the fireballs in the wake of the dIrr galaxy VCC1217 / IC3418

A complex of Halpha emitting blobs with strong FUV excess is associated to the dIrr galaxy VCC1217 / IC3418 (Hester et al. 2010), and extends up to 17 Kpc in the South-East direction. These outstanding features can be morphologically divided into diffuse filaments and compact knots, where most of the star formation activity traced by Halpha takes place. We investigate the properties of the galaxy and the blobs using a multiwavelength approach in order to constrain their origin. We collect publicly available data in UV and Halpha and observe the scene in the optical U,g,r,i bands with LBT. The photometric data allows to evaluate the star formation rate and to perform a SED fitting separately of the galaxy and the blobs in order to constrain their stellar population age. Moreover we analyze the color and luminosity profile of the galaxy and its spectrum to investigate its recent interaction with the Virgo cluster. Our analysis confirms that the most plausible mechanism for the formation of the blobs is ram pressure stripping by the Virgo cluster IGM. The galaxy colors, luminosity profile and SED are consistent with a sudden gas depletion in the last few hundred Myr. The SED fitting of the blobs constrains their ages in < 400 Myr.Comment: 10 pages, 11 figures, accepted for publication in Astronomy and Astrophysic