Effect of Cluster Formation on Isospin Asymmetry in the Liquid-Gas Phase Transition Region

Nuclear matter within the liquid-gas phase transition region is investigated in a mean-field two-component Fermi-gas model. Following largely analytic considerations, it is shown that: (1) Due to density dependence of asymmetry energy, some of the neutron excess from the high-density phase could be expelled into the low-density region. (2) Formation of clusters in the gas phase tends to counteract this trend, making the gas phase more liquid-like and reducing the asymmetry in the gas phase. Flow of asymmetry between the spectator and midrapidity region in reactions is discussed and a possible inversion of the flow direction is indicated.Comment: 9 pages,3 figures, RevTe

Morphology of Shocked Lateral Outflows in Colliding Hydrodynamic Flows

Supersonic interacting flows occurring in phenomena such as protostellar jets give rise to strong shocks, and have been demonstrated in several laboratory experiments. To study such colliding flows, we use the AstroBEAR AMR code to conduct hydrodynamic simulations in three dimensions. We introduce variations in the flow parameters of density, velocity, and cross sectional radius of the colliding flows %radius in order to study the propagation and conical shape of the bow shock formed by collisions between two, not necessarily symmetric, hypersonic flows. We find that the motion of the interaction region is driven by imbalances in ram pressure between the two flows, while the conical structure of the bow shock is a result of shocked lateral outflows (SLOs) being deflected from the horizontal when the flows are of differing cross-section

Cooling and Instabilities in Colliding Radiative Flows with Toroidal Magnetic Fields

We report on the results of a simulation based study of colliding magnetized plasma flows. Our set-up mimics pulsed power laboratory astrophysical experiments but, with an appropriate frame change, are relevant to astrophysical jets with internal velocity variations. We track the evolution of the interaction region where the two flows collide. Cooling via radiative loses are included in the calculation. We systematically vary plasma beta ($\beta_m$) in the flows, the strength of the cooling ($\Lambda_0$) and the exponent ($\alpha$) of temperature-dependence of the cooling function. We find that for strong magnetic fields a counter-propagating jet called a "spine" is driven by pressure from shocked toroidal fields. The spines eventually become unstable and break apart. We demonstrate how formation and evolution of the spines depends on initial flow parameters and provide a simple analytic model that captures the basic features of the flow.Comment: 14 pages, 16 figures. Submitted to MNRA

W Hya : molecular inventory by ISO-SWS

Infrared spectroscopy is a powerful tool to probe the inventory of solid state and molecular species in circumstellar ejecta. Here we analyse the infrared spectrum of the Asymptotic Giant Branch star W Hya, obtained by the Short and Long Wavelength Spectrometers on board of the Infrared Satellite Observatory. These spectra show evidence for the presence of amorphous silicates, aluminum oxide, and magnesium-iron oxide grains. We have modelled the spectral energy distribution using laboratory measured optical properties of these compounds and derive a total dust mass loss rate of 3E-10 Msol/yr. We find no satisfactory fit to the 13 micron dust emission feature and the identification of its carrier is still an open issue. We have also modelled the molecular absorption bands due to H2O, OH, CO, CO2, SiO, and SO2 and estimated the excitation temperatures for different bands which range from 300 to 3000K. It is clear that different molecules giving rise to these absorption bands originate from different gas layers. We present and analyse high resolution Fabry-Perot spectra of the three CO2 bands in the 15 micron region. In these data, the bands are resolved into individual Q-lines in emission, which allows the direct determination of the excitation temperature and column density of the emitting gas. This reveals the presence of a warm (about 450K) extended layer of CO2, somewhere between the photosphere and the dust formation zone. The gas in this layer is cooler than the 1000K CO2 gas responsible for the low-resolution absorption bands at 4.25 and 15 micron. The rotational and vibrational excitation temperatures derived from the individual Q-branch lines of CO2 are different (450K and 150K, respectively) so that the CO2 level population is not in LTE.Comment: To appear in Astronomy and Astrophysics A reference is adde

The gas temperature in flaring disks around pre-main sequence stars

A model is presented which calculates the gas temperature and chemistry in the surface layers of flaring circumstellar disks using a code developed for photon-dominated regions. Special attention is given to the influence of dust settling. It is found that the gas temperature exceeds the dust temperature by up to several hundreds of Kelvins in the part of the disk that is optically thin to ultraviolet radiation, indicating that the common assumption that Tgas=Tdust is not valid throughout the disk. In the optically thick part, gas and dust are strongly coupled and the gas temperature equals the dust temperature. Dust settling has little effect on the chemistry in the disk, but increases the amount of hot gas deeper in the disk. The effects of the higher gas temperature on several emission lines arising in the surface layer are examined. The higher gas temperatures increase the intensities of molecular and fine-structure lines by up to an order of magnitude, and can also have an important effect on the line shapes.Comment: 14 pages, 10 figures, accepted for publication in A&

On the Use of Accelerated Molecular Dynamics to Enhance Configurational Sampling in Ab Initio Simulations

We have implemented the accelerated molecular dynamics approach (Hamelberg, D.; Mongan, J.; McCammon, J. A. J. Chem. Phys. 2004, 120 (24), 11919) in the framework of ab initio MD (AIMD). Using three simple examples, we demonstrate that accelerated AIMD (A-AIMD) can be used to accelerate solvent relaxation in AIMD simulations and facilitate the detection of reaction coordinates: (i) We show, for one cyclohexane molecule in the gas phase, that the method can be used to accelerate the rate of the chair-to-chair interconversion by a factor of ∼1 × 105, while allowing for the reconstruction of the correct canonical distribution of low-energy states; (ii) We then show, for a water box of 64 H2O molecules, that A-AIMD can also be used in the condensed phase to accelerate the sampling of water conformations, without affecting the structural properties of the solvent; and (iii) The method is then used to compute the potential of mean force (PMF) for the dissociation of Na−Cl in water, accelerating the convergence by a factor of ∼3−4 compared to conventional AIMD simulations.(2) These results suggest that A-AIMD is a useful addition to existing methods for enhanced conformational and phase-space sampling in solution. While the method does not make the use of collective variables superfluous, it also does not require the user to define a set of collective variables that can capture all the low-energy minima on the potential energy surface. This property may prove very useful when dealing with highly complex multidimensional systems that require a quantum mechanical treatment

A close look into the carbon disk at the core of the planetary nebula CPD-568032

We present high spatial resolution observations of the dusty core of the Planetary Nebula with Wolf-Rayet central star CPD-568032. These observations were taken with the mid-infrared interferometer VLTI/MIDI in imaging mode providing a typical 300 mas resolution and in interferometric mode using UT2-UT3 47m baseline providing a typical spatial resolution of 20 mas. The visible HST images exhibit a complex multilobal geometry dominated by faint lobes. The farthest structures are located at 7" from the star. The mid-IR environment of CPD-568032 is dominated by a compact source, barely resolved by a single UT telescope in a 8.7 micron filter. The infrared core is almost fully resolved with the three 40-45m projected baselines ranging from -5 to 51 degree but smooth oscillating fringes at low level have been detected in spectrally dispersed visibilities. This clear signal is interpreted in terms of a ring structure which would define the bright inner rim of the equatorial disk. Geometric models allowed us to derive the main geometrical parameters of the disk. For instance, a reasonably good fit is reached with an achromatic and elliptical truncated Gaussian with a radius of 97+/-11 AU, an inclination of 28+/-7 degree and a PA for the major axis at 345+/-7 degree. Furthermore, we performed some radiative transfer modeling aimed at further constraining the geometry and mass content of the disk, by taking into account the MIDI dispersed visibilities, spectra, and the large aperture SED of the source. These models show that the disk is mostly optically thin in the N band and highly flared.Comment: Paper accepted in A&

Global warming and ocean stratification : a potential result of large extraterrestrial impacts

We acknowledge the support of resources provided by UK National Centre for Atmospheric Science (NCAS), the High Performance Computing Cluster supported by the Research and Specialist Computing Support service at the University of East Anglia, UK Natural Environment Research Council (NERC), grants "CPE" (NE/K014757/1), and "Paleopolar" (NE/I005722/1). Data can be obtained from MJ on request. ACM acknowledges support from an AXA Postdoctoral Fellowship and the ERC ACCI grant Project No 267760, and NERC grant NE/M018199/1.The prevailing paradigm for the climatic effects of large asteroid or comet impacts is a reduction in sunlight and significant short-term cooling caused by atmospheric aerosol loading. Here we show, using global climate model experiments, that the large increases in stratospheric water vapor that can occur upon impact with the ocean cause radiative forcings of over +20 W m−2 in the case of 10 km sized bolides. The result of such a positive forcing is rapid climatic warming, increased upper ocean stratification, and potentially disruption of upper ocean ecosystems. Since two thirds of the world's surface is ocean, we suggest that some bolide impacts may actually warm climate overall. For impacts producing both stratospheric water vapor and aerosol loading, radiative forcing by water vapor can reduce or even cancel out aerosol-induced cooling, potentially causing 1–2 decades of increased temperatures in both the upper ocean and on the land surface. Such a response, which depends on the ratio of aerosol to water vapor radiative forcing, is distinct from many previous scenarios for the climatic effects of large bolide impacts, which mostly account for cooling from aerosol loading. Finally, we discuss how water vapor forcing from bolide impacts may have contributed to two well-known phenomena: extinction across the Cretaceous/Paleogene boundary and the deglaciation of the Neoproterozoic snowball Earth.Publisher PDFPeer reviewe

The variable mass loss of the AGB star WX Psc as traced by the CO J=1-0 through 7-6 lines and the dust emission

Low and intermediate mass stars lose a significant fraction of their mass through a dust-driven wind during the Asymptotic Giant Branch (AGB) phase. Recent studies show that winds from late-type stars are far from being smooth. Mass-loss variations occur on different time scales, from years to tens of thousands of years. The variations appear to be particularly prominent towards the end of the AGB evolution. The occurrence, amplitude and time scale of these variations are still not well understood. The goal of our study is to gain insight into the structure of the circumstellar envelope (CSE) of WX Psc and map the possible variability of the late-AGB mass-loss phenomenon. We have performed an in-depth analysis of the extreme infrared AGB star WX Psc by modeling (1) the CO J=1-0 through 7-6 rotational line profiles and the full spectral energy distribution (SED) ranging from 0.7 to 1300 micron. We hence are able to trace a geometrically extended region of the CSE. Both mass-loss diagnostics bear evidence of the occurrence of mass-loss modulations during the last ~2000 yr. In particular, WX Psc went through a high mass-loss phase (Mdot~5e-5 Msun/yr) some 800 yr ago. This phase lasted about 600 yr and was followed by a long period of low mass loss (Mdot~5e-8 Msun/yr). The present day mass-loss rate is estimated to be ~6e-6 Msun/yr. The AGB star WX Psc has undergone strong mass-loss rate variability on a time scale of several hundred years during the last few thousand years. These variations are traced in the strength and profile of the CO rotational lines and in the SED. We have consistently simulated the behaviour of both tracers using radiative transfer codes that allow for non-constant mass-loss rates.Comment: 12 pages, accepted for publication in A&

Modeling Molecular-Line Emission from Circumstellar Disks

Molecular lines hold valuable information on the physical and chemical composition of disks around young stars, the likely progenitors of planetary systems. This invited contribution discusses techniques to calculate the molecular emission (and absorption) line spectrum based on models for the physical and chemical structure of protoplanetary disks. Four examples of recent research illutrate these techniques in practice: matching resolved molecular-line emission from the disk around LkCa15 with theoertical models for the chemistry; evaluating the two-dimensional transfer of ultraviolet radiation into the disk, and the effect on the HCN/CN ratio; far-infrared CO line emission from a superheated disk surface layer; and inward motions in the disk around L1489 IRS.Comment: 6 pages, no figures. To appear in "The Dense Interstellar Medium in Galaxies", Procs. Fourth Cologne-Bonn-Zermatt-Symposiu