Empirical formulae to describe some physical properties of small groups of protogalaxies with multiplicity

By means of identical cubic elements, we generate a partition of a volume in which a particle-based cosmological simulation is carried out. In each cubic element, we determine the gas particles with a normalized density greater than an arbitrarily chosen density threshold. By using a proximity parameter, we calculate the neighboring cubic elements and generate a list of neighbors. By imposing dynamic conditions on the gas particles, we identify gas clumps and their neighbors, so that we calculate and fit some properties of the groups so identified, including the mass, size and velocity dispersion, in terms of their multiplicity(here defined simply as the number of member galaxies). Finally, we report the value of the ratio of kinetic energy to gravitational energy of such dense gas clumps, which will be useful as initial conditions in simulations of gravitational collapse of gas clouds and clusters of gas clouds.     

Roles of methane and carbon dioxide in geological processes on Mars

We discuss in this paper possible roles of methane and carbon dioxide in geological processes on Mars. These volatiles in the martian crust may migrate upward from their sources either directly or via various traps (structural, sedimentary, ground ice, gas hydrates). They are then likely emitted to the atmosphere by seepage or through diverse vent structures. Though gas hydrates have never been directly detected on Mars, theoretical studies favor their presence in the crust and polar caps; they could have played an important role as significant gas reservoirs in the subsurface. The martian gas hydrates would possibly be a binary system of methane and carbon dioxide occupying clathrate cavities. Landforms such as mud volcanoes with well-known linkage to gas venting are extensively distributed on Earth, and methane is the primary gas involved. Thus, identification of these landforms on Mars could suggest that methane and possibly carbon dioxide have contributed to geological processes of the planet. For example, we present a newly identified field in Chryse Planitia where features closely resembling terrestrial mud volcanoes occur widely, though with no observable activity. We also present results of a preliminary search for possible recent or present-day, methane-emission zones in the regions over which enrichments of atmospheric methane have been reported.     

Pyroclast loss or retention during explosive volcanism on asteroids: Influence of asteroid size and gas content of melt

Abstract– We review the conditions under which explosive volcanism took place on early‐forming differentiated asteroids. The pressure‐dependent solubility of typical asteroid volatiles in melts implies that the gas driving explosive volcanism on asteroids less than approximately 100 km in diameter was probably present mainly as a free phase capable of accumulating into large gas bodies and, thus, causing slug flow in melts approaching the surface. In contrast, in asteroids larger than approximately 100 km the gas was probably present largely as a dispersion of small bubbles. We show that these gas distributions have implications for the size distribution of the pyroclastic droplets produced in explosive eruptions at the surface. All pyroclastic melt droplets are accelerated by the expanding gases, but their speeds lag the gas speed by a finite amount that is a function of the droplet size and density and the asteroid size and, hence, acceleration due to gravity. We compute pyroclast speeds and, by comparing them with escape velocities, we identify the critical pyroclast diameter on a given‐size asteroid that distinguishes droplets lost to space from droplets that return to the surface. Identification of asteroidal pyroclasts and measurements of their sizes could throw light on the amounts of gas driving eruptions.     

Chemical evolution models of local dwarf spheroidal galaxies

We calculate chemical evolution models for four dwarf spheroidal (dSph) satellites of the Milky Way (Carina, Ursa Minor, Leo I and Leo II) for which reliable non-parametric star formation histories have been derived. In this way, the independently-obtained star formation histories are used to constrain the evolution of the systems we are treating. This allows us to obtain robust inferences on the history of such crucial parameters of galactic evolution as gas infall, gas outflows and global metallicities for these systems. We can then trace the metallicity and abundance ratios of the stars formed, the gas present at any time within the systems and the details of gas ejection, of relevance to enrichment of the galaxies environment. We find that galaxies showing one single burst of star formation (Ursa Minor and Leo II) require a dark halo slightly larger that the current estimates for their tidal radii, or the presence of a metal-rich selective wind that might carry away much of the energy output of their supernovae before this might have interacted and heated the gas content, for the gas to be retained until the observed stellar populations have formed. Systems showing extended star formation histories (Carina and Leo I), however, are consistent with the idea that their tidally-limited dark haloes provide the necessary gravitational potential wells to retain their gas. The complex time structure of the star formation in these systems remains difficult to understand. Observations of detailed abundance ratios for Ursa Minor strongly suggest that the star formation history of this galaxy might in fact resemble the complex picture presented by Carina or Leo I, but localized at a very early epoch.     

Accretion of gas on to nearby spiral galaxies

We present evidence for cosmological gas accretion on to spiral galaxies in the local universe. The accretion is seen through its effects on the dynamics of the extraplanar neutral gas. The accretion rates that we estimate for two nearby spiral galaxies are of the order of their star formation rates. Our model shows that most of the extraplanar gas is produced by supernova feedback (galactic fountain) and only 10–20 per cent comes from accretion. The accreting material must have low specific angular momentum about the disc's spin axis, although the magnitude of the specific angular momentum vector can be higher. We also explore the effects of a hot corona on the dynamics of the extraplanar gas and find that it is unlikely to be responsible for the observed kinematical pattern and the source of accreted gas. However, the interaction with the fountain flow should profoundly affect the hydrodynamics of the corona.     

Microdistribution of primordial Ne and Ar in fine‐grained rims,matrices, and dark inclusions of unequilibrated chondrites—Clues on nebular processes

Abstract— The low temperature fine‐grained material in unequilibrated chondrites, which occurs as matrix, rims, and dark inclusions, carries information about the solar nebula and the earliest stages of planetesimal accretion. The microdistribution of primordial noble gases among these components helps to reveal their accretionary and alteration histories. We measured the Ne and Ar isotopic ratios and concentrations of small samples of matrix, rims, and dark inclusions from the unequilibrated carbonaceous chondrites Allende (CV3), Leoville (CV3), and Renazzo (CR2) and from the ordinary chondrites Semarkona (LL3.0), Bishunpur (LL3.1), and Krymka (LL3.1) to decipher their genetic relationships. The primordial noble gas concentrations of Semarkona, and—with certain restrictions—also of Leoville, Bishunpur, and Allende decrease from rims to matrices. This indicates a progressive accretion of nebular dust from regions with decreasing noble gas contents and cannot be explained by a formation of the rims on parent bodies. The decrease is probably due to dilution of the noble‐gas‐carrying phases with noble‐gas‐poor material in the nebula. Krymka and Renazzo both show an increase of primordial noble gas concentrations from rims to matrices. In the case of Krymka, this indicates the admixture of noble gas‐rich dust to the nebular region from which first rims and then matrix accreted. This also explains the increase of the primordial elemental ratio 36Ar/ 20Ne from rims to matrix. Larger clasts of the noble‐gas‐rich dust form macroscopic dark inclusions in this meteorite, which seem to represent unusually pristine material. The interpretation of the Renazzo data is ambiguous. Rims could have formed by aqueous alteration of matrix or—as in the case of Krymka—by progressive admixture of noble gas‐rich dust to the reservoir from which the Renazzo constituents accreted. The Leoville and Krymka dark inclusions, as well as one dark inclusion of Allende, show noble gas signatures different from those of the respective host meteorites. The Allende dark inclusion probably accreted from the same region as Allende rims and matrix but suffered a higher degree of alteration. The Leoville and Krymka dark inclusions must have accreted from regions different from those of their respective rims and matrices and were later incorporated into their host meteorites. The noble gas data imply a heterogeneous reservoir with respect to its primordial noble gas content in the accretion region of the studied meteorites. Further studies will have to decide whether these differences are primary or evolved from an originally uniform reservoir.     

Impact of stochastic gas motions on galaxy cluster abundance profiles

The impact of stochastic gas motions on the metal distribution in cluster cores is evaluated. Peaked abundance profiles are a characteristic feature of clusters with cool cores, and abundance peaks are probably associated with the brightest cluster galaxies (BCGs), which dwell in cluster cores. However, the width of the abundance peaks is significantly broader than the BCG light distribution, suggesting that some gas motions are transporting metals originating from within the BCG. Assuming that this process can be treated as diffusive, and using the brightest X-ray cluster A426 (Perseus) as an example, we estimate that a diffusion coefficient of the order of  2 × 10²⁹ cm² s⁻¹  is needed to explain the width of the observed abundance profiles. Much lower (higher) diffusion coefficients would result in too peaked (too shallow) profiles. Such diffusion could be produced by stochastic gas motions, and our analysis provides constraints on the product of their characteristic velocity and their spatial coherence scale. We speculate that the activity of the supermassive black hole of the BCG is driving the stochastic gas motions in cluster cores. When combined with the assumption that the dissipation of the same motions is a key gas heating mechanism, one can estimate both the velocity and the spatial scale of such diffusive processes.     

Thermal state and gas production rate of rotating cometary nuclei

We formulate a completely three-dimensional nonstationary model of the thermal state and gas production rate of rotating spherical cometary nuclei moving in circular and elliptical orbits around the Sun. We perform a thermophysical analysis of the problem and formulate a system of similarity criteria. The possible thermal regimes of cometary nuclei are analyzed in the space of suggested similarity criteria. Only one criterion dependent on the nucleus spin period is shown to play a dominant role for rotating nuclei at a given heliocentric distance. This simplifies greatly a parametric study of the gas production rate of real cometary nuclei under conditions of uncertainty in their parameters. Based on the developed model, we numerically investigate the thermal state and gas production rate of rotating nuclei. The results of our calculations are in complete agreement with those of the similarity analysis for the problem. We perform a comparative analysis of the currently used simplified thermal models for cometary nuclei and determine the range of their applicability.     

The atmospheric entry of fine‐grained micrometeorites: The role of volatile gases in heating and fragmentation

The early stages of atmospheric entry are investigated in four large (250–950 μm) unmelted micrometeorites (three fine‐grained and one composite), derived from the Transantarctic Mountain micrometeorite collection. These particles have abundant, interconnected, secondary pore spaces which form branching channels and show evidence of enhanced heating along their channel walls. Additionally, a micrometeorite with a double‐walled igneous rim is described, suggesting that some particles undergo volume expansion during entry. This study provides new textural data which links together entry heating processes known to operate inside micrometeoroids, thereby generating a more comprehensive model of their petrographic evolution. Initially, flash heated micrometeorites develop a melt layer on their exterior; this igneous rim migrates inwards. Meanwhile, the particle core is heated by the decomposition of low‐temperature phases and by volatile gas release. Where the igneous rim acts as a seal, gas pressures rise, resulting in the formation of interconnected voids and higher particle porosities. Eventually, the igneous rim is breached and gas exchange with the atmosphere occurs. This mechanism replaces inefficient conductive rim‐to‐core thermal gradients with more efficient particle‐wide heating, driven by convective gas flow. Interconnected voids also increase the likelihood of particle fragmentation during entry and, may therefore explain the rarity of large fine‐grained micrometeorites among collections.     

The obscured growth of massive black holes

The mass density of massive black holes observed locally is consistent with the hard X-ray background provided that most of the radiation produced during their growth was absorbed by surrounding gas. A simple model is proposed here for the formation of galaxy bulges and central black holes in which young spheroidal galaxies have a significant distributed component of cold dusty clouds, which accounts for the absorption. The central accreting black hole is assumed to emit both a quasar-like spectrum, which is absorbed by the surrounding gas, and a slow wind. The power in both is less than the Eddington limit for the black hole. The wind, however, exerts the most force on the gas and, as earlier suggested by Silk & Rees, when the black hole reaches a critical mass it is powerful enough to eject the cold gas from the galaxy, so terminating the growth of both black hole and galaxy. In the present model this point occurs when the Thomson depth in the surrounding gas has dropped to about unity and results in the mass of the black hole being proportional to the mass of the spheroid, with the normalization agreeing with that found for local galaxies by Magorrian et al. for reasonable wind parameters. The model predicts a new population of hard X-ray and submm sources at redshifts above 1, which are powered by black holes in their main growth phase.     

Ram-pressure histories of cluster galaxies

Ram-pressure stripping can remove significant amounts of gas from galaxies that orbit in clusters and massive groups, and thus has a large impact on the evolution of cluster galaxies. In this paper, we reconstruct the present-day distribution of ram pressure and the ram-pressure histories of cluster galaxies. To this aim, we combine the Millennium Simulation and an associated semi-analytic model of galaxy evolution with analytic models for the gas distribution in clusters. We find that about one quarter of galaxies in massive clusters are subject to strong ram pressures that are likely to cause an expedient loss of all gas. Strong ram pressures occur predominantly in the inner core of the cluster, where both the gas density and the galaxy velocity are higher. Since their accretion on to a massive system, more than 64 per cent of galaxies that reside in a cluster today have experienced strong ram pressures of  >10⁻¹¹ dyn cm⁻²  which most likely led to a substantial loss of the gas.     

IRAS05391—0217和IRAS06572—0742源的热动性质

利用紫金山天文台青海观测站１３．７米射电望远镜对源ＩＲＡＳ０５３９１０２１７和ＩＲＡＳ０６５７２０７４２进行了１２ＣＯＪ＝１－０观测,获得了气体的相应参数;用ＩＲＡＳ及其他红外观测资料,获得了尘埃热结构;探讨了云中气体的热平衡．对于ＩＲＡＳ０５３９１０２１７中由远红外得出的Ｔｄ＜Ｔｋ的情形,考虑了几种可能的加热机制．光电加热对该云可能是比较重要的;激波可能是另一种加热途径．     

The Impact of Massive Stars on their Surrounding Interstellar Medium

In this paper, I present a brief overview of the observational evidence for the interaction between the winds of hot stars and their surrounding interstellar medium (ISM). As the star evolves, it will impinge on the gas in different ways depending on the nature of its wind and on the physical state of the interstellar gas. The three main evolutionary phases of massive stars, the main sequence O phase, the intermediate red supergiant or luminous blue variable phase and finally the Wolf-Rayet phase are discussed. The study of the ISM in the vicinity of hot stars is shown to be a powerful tool to study the history of the gas and gain more insight into the evolution of the star. This revised version was published online in July 2006 with corrections to the Cover Date.     

Weak lensing signal in unified dark matter models

This paper is an extension of the work done by Pierens & Nelson in which they investigated the behaviour of a two-planet system embedded in a protoplanetary disc. They put a Jupiter mass gas giant on the internal orbit and a lower mass planet on the external one. We consider here a similar problem taking into account a gas giant with mass in the range 0.5 to  1 M _J  and a Super-Earth (i.e. a planet with mass  ≤10 M _⊕  ) as the outermost planet. By changing disc parameters and planet masses, we have succeeded in getting the convergent migration of the planets which allows for the possibility of their resonant locking. However, in the case in which the gas giant has the mass of Jupiter, before any mean-motion first-order commensurability could be achieved, the Super-Earth is caught in a trap when it is very close to the edge of the gap opened by the giant planet. This confirms the result obtained by Pierens & Nelson in their simulations. Additionally, we have found that, in a very thin disc, an apsidal resonance is observed in the system if the Super-Earth is captured in the trap. Moreover, the eccentricity of the small planet remains low, while the eccentricity of the gas giant increases slightly due to the imbalance between Lindblad and corotational resonances. We have also extended the work of Pierens & Nelson by studying analogous systems in which the gas giant is allowed to take sub-Jupiter masses. In this case, after conducting an extensive survey over all possible parameters, we have succeeded in getting the 1:2 mean-motion resonant configuration only in a disc with low aspect ratio and low surface density. However, the resonance is maintained just for a few thousand orbits. Thus, we conclude that for typical protoplanetary discs the mean-motion commensurabilities are rare if the Super-Earth is located on the external orbit relative to the gas giant.     

Hale-Bopp: What Makes a Big Comet Different? Coma Dynamics: Observations and Theory

Comet Hale-Bopp was the largest comet by almost any definition, observed at least since the advent of modern observing techniques. In a more typical comet both the chemical and dynamical influences of collisional processes are limited by the short time a parcel of gas sublimated from the nucleus remains in the dense part of the coma. The resulting large size of the collisional coma in comet Hale-Bopp had important consequences on the dynamics of the coma, which in turn has important consequences on how observations are interpreted with standard models. Measured velocities of typical gas species (mostly the observed radicals) as well as dust were larger than normal comets. Conversely, velocities of super thermal atomic hydrogen were smaller than normal because of the samecollisional processes. Furthermore, as a consequence, dust particles, which are dragged by the outflowing gas, were also accelerated to larger velocities. Such larger velocities are not simply an interesting curiosity in their own right, because nearly all observations of dust and gas are interpreted with models of the coma that depend directly on some measurement or assumption with regard to velocity. In this presentation both observations and theory regarding the dynamical conditions in the coma of comet Hale-Bopp are summarized.     

The influence of interstellar medium on subsonic stellar motions

It is not a trivial problem to imagine how a spherical high-pressure balloon with supersonic gas jets leaving from pores densely distributed on its surface can be influenced by an ambient gas flow. The relative motion of such a balloon can be controlled by a corresponding rearrangement of the gas outflow into an aspherical configuration. A similar problem is connected with stars driving a supersonic stellar wind and moving relative to the interstellar medium. As we shall show, the adapted circumstellar flow leads to an upwind-downwind pressure asymmetry balancing the momentum loss that is braking such stars. The opposite process — i.e., acceleration — may occur if luminous stars are closely associated and their wind systems interfere with each other. This should lead to a mutual repulsion.     

Observations of Circumstellar Disks

This review presents recent results on protoplanetary disks obtained from angularly resolved observations. Observations with mm arrays show that disks are in Keplerian rotation, with radius as large as 1000 AU. Optical images show disks to be flared. Both type of observations imply the dust in disk has evolved and grown from interstellar dust. Measurement of the gas temperature from CO isotopes indicate temperature gradient, consistent with the disk flaring and heating by the central star. Disks which appear to have started to dissipate their initial gas content have also been discovered, but their very diverse aspects leaves the dissipation process unclear. Current data mostly concern the outer disk (>50 AU), although near-IR interferometry has started to unveil the innermost regions (<1 AU). The next generation of instruments (MIDI on VLTI, ALMA) will allow to probe the intermediate regime, where planet formation is expected to occur.     

On the fate of gas in ultraluminous infrared galaxies at low and high redshift

It is often suggested that the distant galaxies recently identified in 850-μm surveys with the SCUBA bolometer array on the James Clerk Maxwell Telescope are high-redshift analogues to local ultraluminous infrared galaxies, based on their similar spectral energy distributions and luminosities. We show that these two populations of objects must differ in at least one fundamental way from each other. This assertion is based on a consideration of the possible fates of gas in the high-redshift SCUBA galaxies, given the requirement that they most evolve into some subset of the low-redshift galaxy population with a comoving density of about 10⁻⁴ Mpc⁻³. One possibility is that the SCUBA galaxies have similar gas density profiles to local ultraluminous galaxies. If this is the case, then they must derive almost all their power from active galactic nuclei, which appears not to be the case for local ultraluminous galaxies, which are predominantly star-formation-powered. Another possibility is that the SCUBA galaxies have more extended gas density profiles than local ultraluminous galaxies. In this case they must be almost all star-formation-powered, and much of the star formation in the Universe can happen in these objects. Either way there is a significant difference between the low- and high-redshift populations.     

Structure and evolution of circumstellar shells formed by massive star winds

We follow the interaction of massive stars with their circumstellar gas over their entire life-times by combining hydrodynamic stellar evolution calculations for 35 and 60M stars and one- and two-dimensional gas dynamical calculations for the circumstellar medium.