Asao Habe

Evolution of Dust in Primordial Supernova Remnants: Can Dust Grains Formed in the Ejecta Survive and Be Injected into the Early Interstellar Medium?

We investigate the evolution of dust that formed at Population III supernova (SN) explosions and its processing through the collisions with the reverse shocks resulting from the interaction of the SN ejecta with the ambient medium. In particular, we investigate the transport of the shocked dust within the SNR and its effect on the chemical composition, the size distribution, and the total mass of dust surviving in SNRs. We find that the evolution of the reverse shock, and hence its effect on the processing of the dust, depends on the thickness of the envelope retained by the progenitor star. Furthermore, the transport and survival of the dust grains depend on their initial radius, aini, and composition: for Type II SNRs expanding into the ISM with a density of nH,0 = 1 cm-3, small grains with aini 0.05 μm are completely destroyed by sputtering in the postshock flow, while grains with aini = 0.05-0.2 μm are trapped into the dense shell behind the forward shock. Very large grains of aini 0.2 μm are ejected into the ISM without decreasing their sizes significantly. We find that the total mass fraction of dust that is destroyed by the reverse shock ranges from 0.2 to 1.0, depending on the energy of the explosion and the density of the ambient ISM. The results of our calculations have significant impact on the abundance pattern of the second-generation stars that form in the dense shell of primordial SNRs.

Dust destruction in the high-velocity shocks driven by supernovae in the early universe

We investigate the destruction of dust grains by sputtering in the high-velocity interstellar shocks driven by supernovae(SNe)intheearlyuniversetorevealthedependenceofthetimescaleofdustdestructiononthegasdensity nH;0 in the interstellar medium (ISM), as well as on the progenitor mass Mpr and explosion energy E51 of SNe. The sputtering yields for the combinations of dust and ion species of interest to us are evaluated by applying the so-called universalrelationwithaslightmodification.Thedynamicsofdustgrainsandtheirdestructionbysputteringinshocks are calculated by taking into account the size distribution of each dust species, together with the time evolution of the temperature and density of the gas in spherically symmetric shocks. The results of the calculations show that the efficiency of dust destruction depends not only on the sputtering yield but also on the initial size distribution of each grain species. The efficiency of dust destruction increases with increasing E51 and/or increasing nH;0 but is almost independent of Mpr as long as E51 is the same. The mass of gas swept up by a shock is an increasing function of E51 andadecreasingfunctionof nH;0.Combiningtheseresults,wepresenttheapproximationformulaforthetimescaleof destruction for each grain species in the early universe as a function of E51 and nH;0. This formula is applicable for investigating the evolution of dust grains at the early epoch of the universe with the metallicity of Z P10 � 3 Z� . The effects of the cooling processes of gas on the destruction of dust are briefly discussed. Subject headingg dust, extinction — early universe — shock waves — supernova remnants — supernovae: general Online material: color figures

Formation and Evolution of Galactic Halos in Clusters of Galaxies

We investigate effects of time evolution of a rich cluster of galaxies on its member galactic halos in the standard cold dark matter (SCDM) universe using high resolution N-body simulations. We identify several hundred galactic halos within the virial radius of our simulated cluster. We also find that a large number of halos have been tidally disrupted at z=0. Therefore we improve a method of deriving merging history trees of galaxies taking account of tidally stripped galaxies. The main results are as follows: (1) At high redshift (z2), the mass function of the galactic halos that are in the cluster at z=0 is very similar to that obtained in the field region and agrees well with the Press-Schechter mass function. (2) The mass function of cluster galaxies that consist of both galactic halos and tidally stripped galaxies has hardly evolved since z2. This mass function at z=0 is well represented by the Press-Schechter mass function at z=2. (3) At high redshift (z>3), in the region that becomes the cluster, the fraction of galaxies that have undergone recent merger is larger than that in the field. After z~3, however, it rapidly decreases and becomes smaller than that in the field. (4) The fraction of strongly stripped galaxies among the cluster galaxies begins to increase from z0.5. At z=0, a clear correlation appears between this fraction and the distance from the center of the cluster. (5) Tidally truncated halos have steeper outer profiles than those of the model of Navarro, Frenk, & White.

Do Cloud-Cloud Collisions Trigger High-mass Star Formation? I. Small Cloud Collisions

We performed sub-parsec (~0.06pc) scale simulations of two idealised molecular clouds with different masses undergoing a collision. Gas clumps with density greater than 1e-20 g/cm3 (0.3e4 cm-3) were identified as pre-stellar cores and tracked through the simulation. The colliding system showed a partial gas arc morphology with core formation in the oblique shock-front at the collision interface. These characteristics support NANTEN observations of objects suspected to be colliding giant molecular clouds (GMCs). We investigated the effect of turbulence and collision speed on the resulting core population and compared the cumulative mass distribution to cores in observed GMCs. Our results suggest that a faster relative velocity increases the number of cores formed but that cores grow via accretion predominately while in the shock front, leading to a slower shock being more important for core growth. The core masses obey a power law relation with index gamma = -1.6, in good agreement with observations. This suggests that core production through collisions should follow a similar mass distribution as quiescent formation, albeit at a higher mass range. If cores can be supported against collapse during their growth, the estimated ram pressure from gas infall is of the right order to counter the radiation pressure and form a star of 100Msun.

Do giant molecular clouds care about the galactic structure

We investigate the impact of galactic environment on the properties of simulated giant molecular clouds formed in a M83-type barred spiral galaxy. Our simulation uses a rotating stellar potential to create the grand design features and resolves down to 1.5 pc. From the comparison of clouds found in the bar, spiral and disc regions, we find that the typical GMC is environment independent, with a mass of 5e+5 Msun and radius 11 pc. However, the fraction of clouds in the property distribution tails varies between regions, with larger, more massive clouds with a higher velocity dispersion being found in greatest proportions in the bar, spiral and then disc. The bar clouds also show a bimodality that is not reflected in the spiral and disc clouds except in the surface density, where all three regions show two distinct peaks. We identify these features as being due to the relative proportion of three cloud types, classified via the mass-radius scaling relation, which we label A, B and C. Type A clouds have the typical values listed above and form the largest fraction in each region. Type B clouds are massive giant molecular associations while Type C clouds are unbound, transient clouds that form in dense filaments and tidal tails. The fraction of each clouds type depends on the cloud-cloud interactions, which cause mergers to build up the GMA Type Bs and tidal features in which the Type C clouds are formed. The number of cloud interactions is greatest in the bar, followed by the spiral, causing a higher fraction of both cloud types compared to the disc. While the cloud types also exist in lower resolution simulations, their identification becomes more challenging as they are not well separated populations on the mass-radius relation or distribution plots. Finally, we compare the results for three star formation models to estimate the star formation rate and efficiency in each region.

THE ROLE OF DUST IN THE EARLY UNIVERSE. I. PROTOGALAXY EVOLUTION

We develop one-zone galaxy formation models in the early universe, taking into account dust formation and evolution by supernova (SN) explosions. We focus on the time evolution of dust size distribution, because H2 formation on the dust surface plays a critical role in the star formation process in the early universe. In the model, we assume that star formation rate (SFR) is proportional to the total amount of H2. We consistently treat (1) the formation and size evolution of dust, (2) the chemical reaction networks including H2 formation both on the surface of dust and in gas phase, and (3) the SFR in the model. First, we find that, because of dust destruction due to both reverse and forward shocks driven by SNe, H2 formation is more suppressed than in situations without such dust d ]

Isolating signatures of major cloud-cloud collisions using position-velocity diagrams

Collisions between giant molecular clouds are a potential mechanism for triggering the formation of massive stars, or even super star clusters. The trouble is identifying this process observationally and distinguishing it from other mechanisms. We produce synthetic position-velocity diagrams from models of: cloud-cloud collisions, non-interacting clouds along the line of sight, clouds with internal radiative feedback and a more complex cloud evolving in a galactic disc, to try and identify unique signatures of collision. We find that a broad bridge feature connecting two intensity peaks, spatially correlated but separated in velocity, is a signature of a high velocity cloud-cloud collision. We show that the broad bridge feature is resilient to the effects of radiative feedback, at least to around 2.5Myr after the formation of the first massive (ionising) star. However for a head on 10km/s collision we find that this will only be observable from 20-30 per cent of viewing angles. Such broad-bridge features have been identified towards M20, a very young region of massive star formation that was concluded to be a site of cloud-cloud collision by Torii et al (2011), and also towards star formation in the outer Milky Way by Izumi et al (2014).

The Effect of the Self-Gravity of Gas on Gas Fueling in a Barred Galaxy with a Supermassive Black Hole

In our previous paper, we showed that a gas disk in the nuclear region of a barred galaxy that contains a central supermassive black hole (SMBH) rapidly evolves into a nuclear gas ring because of an additional inner Lindblad resonance caused by the SMBH. In this paper, we investigate the fate of the gas ring, which involves the self-gravity of gas, using two-dimensional hydrodynamical simulations. We find that the gas ring becomes gravitationally unstable for the surface density of the gas above a critical value and fragments into several gas clumps. Some denser clumps increase their mass via the accretion of the surrounding gas and collisions with other clumps, and finally a very massive gas clump (M ~ 107 M☉) is formed. Because of the torque from the massive clump, a part of the gas in the ring loses its angular momentum and falls into the galactic center. As a result, a nuclear gas disk (R ~ 50 pc) is formed around the SMBH. The accretion rate for R < 50 pc reaches about 0.1 M☉ yr-1 for 3.5 × 107 yr. At the final phase of the bar-driven fueling, self-gravity is crucial for the angular momentum transfer of the gas. This is a new mechanism for gas fueling to the vicinity of the SMBH.

Sequential explosions of supernovae in an ob association and formation of a superbubble

From the standpoint of view that the early type stars are formed sequentially at an OB association, it is expected that the supernova explosions will also occur sequentially. We study the expansion law of a supernova remnant, which is formed by sequential explosions of supernovae. The superbubbles and supershells with the radii 200∼1000 pc are naturally explained by this model. Assuming that the sequential explosion of supernovae occurs at every OB association, we deduce the star formation rate in our Galaxy.

FORMATION HISTORY OF METAL-POOR HALO STARS WITH THE HIERARCHICAL MODEL AND THE EFFECT OF INTERSTELLAR MATTER ACCRETION ON THE MOST METAL-POOR STARS

We investigate star formation and chemical evolution in the early universe by considering the merging history of the Galaxy in the Λ cold dark matter scenario according to the extended Press-Schechter theory. We give some possible constraints from comparisons with observation of extremely metal-poor (EMP) stars, made available by the recent large-scale surveys and by the follow-up high-resolution spectroscopy. We demonstrate that (1) the hierarchical structure formation can explain the characteristics of the observed metallicity distribution function including a break around [Fe/H] = –4; (2) a high-mass initial mass function (IMF) of peak mass ~10 M ☉ with the contribution of binaries, derived from the statistics of carbon-enhanced EMP stars, predicts the frequency of low-mass survivors consistent with the number of EMP stars observed for –4 [Fe/H] –2.5; (3) the stars formed from primordial gas before the first supernova (SN) explosions in their host mini-halos are assigned to the hyper metal-poor (HMP) stars with [Fe/H] ~ –5; and (4) there is no indication of significant changes in the IMF and the binary contribution at metallicities –4 [Fe/H] –2.5, or even larger, as far as the field stars of the Galactic halo are concerned. We further study the effects of surface pollution through the accretion of interstellar matter (ISM) along the chemical and dynamical evolution of the Galaxy for low-mass Population III and EMP survivors. Because of the shallower potential of smaller halos, the accretion of ISM in the mini-halos in which these stars were born dominates the surface metal pollution. This can account for the surface iron abundances as observed for the HMP stars if the cooling and concentration of gas in their birth mini-halos are taken into account. We also study the feedback effect from the very massive Population III stars. The metal pre-pollution by pair-instability SNe is shown to be compatible with the observed lack of their nucleosynthetic signatures when some positive feedback on gas cooling works and changes the IMF from being very massive to being high mass.