Thermal Evolution Of Gas Research Articles

Protostellar-disc fragmentation across all metallicities

ABSTRACT Cosmic metallicity evolution possibly creates the diversity of star formation modes at different epochs. Gravitational fragmentation of circumstellar discs provides an important formation channel of multiple star systems, including close binaries. We here study the nature of disc fragmentation, systematically performing a suite of 2D radiation-hydrodynamic simulations, in a broad range of metallicities, from the primordial to the solar values. In particular, we follow relatively long-term disc evolution over 15 kyr after the disc formation, incorporating the effect of heating by the protostellar irradiation. Our results show that the disc fragmentation occurs at all metallicities 1–$0 \, \rm {Z}_{\odot }$, yielding self-gravitating clumps. Physical properties of the clumps, such as their number and mass distributions, change with the metallicity due to different gas thermal evolution. For instance, the number of clumps is the largest for the intermediate metallicity range of 10−2–$10^{-5} \, \rm {Z}_{\odot }$, where the dust cooling is effective exclusively in a dense part of the disc and causes the fragmentation of spiral arms, although the disc might fragment at a similar rate, also at lower metallicities 10−6–$0 \, \rm {Z}_{\odot }$ with higher spatial resolution. The disc fragmentation is more modest for 1–$0.1 \, \rm {Z}_{\odot }$, thanks to the disc stabilization by the stellar irradiation. Such metallicity dependence agrees with the observed trend that the close binary fraction increases with decreasing metallicity in the range of 1–$10^{-3} \, \rm {Z}_{\odot }$.

Disc fragmentation and intermittent accretion on to supermassive stars

ABSTRACT Supermassive stars (SMSs) with ∼104–105 M⊙ are candidate objects for the origin of supermassive black holes observed at redshift z > 6. They are supposed to form in primordial-gas clouds that provide the central stars with gas at a high accretion rate, but their growth may be terminated in the middle due to the stellar ionizing radiation if the accretion is intermittent and its quiescent periods are longer than the Kelvin–Helmholtz (KH) time-scales at the stellar surfaces. In this paper, we examine the role of the ionizing radiation feedback based on the accretion history in two possible SMS-forming clouds extracted from cosmological simulations, following their evolution with vertically integrated two-dimensional hydrodynamic simulations with detailed thermal and chemical models. The consistent treatment of the gas thermal evolution is crucial for obtaining the realistic accretion history, as we demonstrate by performing an additional run with a barotropic equation of state, in which the fluctuation of the accretion rate is artificially suppressed. We find that although the accretion becomes intermittent due to the formation of spiral arms and clumps in gravitationally unstable discs, the quiescent periods are always shorter than the KH time-scales, implying that SMSs can form without affected by the ionizing radiation.

Small-scale structure and the Lyman-α forest baryon acoustic oscillation feature

The baryon-acoustic oscillation (BAO) feature in the Lyman-$\alpha$ forest is one of the key probes of the cosmic expansion rate at redshifts z~2.5, well before dark energy is believed to have become dynamically significant. A key advantage of the BAO as a standard ruler is that it is a sharp feature and hence is more robust against broadband systematic effects than other cosmological probes. However, if the Lyman-$\alpha$ forest transmission is sensitive to the initial streaming velocity of the baryons relative to the dark matter, then the BAO peak position can be shifted. Here we investigate this sensitivity using a suite of hydrodynamic simulations of small regions of the intergalactic medium with a range of box sizes and physics assumptions; each simulation starts from initial conditions at the kinematic decoupling era (z~1059), undergoes a discrete change from neutral gas to ionized gas thermal evolution at reionization (z~8), and is finally processed into a Lyman-$\alpha$ forest transmitted flux cube. Streaming velocities suppress small-scale structure, leading to less violent relaxation after reionization. The changes in the gas distribution and temperature-density relation at low redshift are more subtle, due to the convergent temperature evolution in the ionized phase. The change in the BAO scale is estimated to be of the order of 0.12% at z=2.5; some of the major uncertainties and avenues for future improvement are discussed. The predicted streaming velocity shift would be a subdominant but not negligible effect (of order $0.26\sigma$) for the upcoming DESI Lyman-$\alpha$ forest survey, and exceeds the cosmic variance floor. It is hoped that this study will motivate additional theoretical work on the magnitude of the BAO shift, both in the Lyman-$\alpha$ forest and in other tracers of large-scale structure.

Star Formation in the Early Universe

AbstractIn low-metallicity environments, massive stars are more easily formed than in the solar neighborhood. In this article, we see the following examples of low-mass star formation. We first describe the first star formation in the universe and argue that they are typically ordinary-sized massive stars, rather than very massive (> 100M⊙) ones. Next, we see how the metal-enrichment changes the thermal evolution of gas, thereby causing the shift of characteristic stellar mass towards lower mass. Finally, we discuss the possibility of forming supermassive stars in some special conditions in young galaxies.

Star Formation at Very Low Metallicity. I. Chemistry and Cooling at Low Densities

We present a simplified chemical and thermal model designed to allow computationally efficient study of the thermal evolution of metal-poor gas within large numerical simulations. Our main simplification is the neglect of the molecular chemistry of the heavy elements. The only molecular chemistry retained within the model is the formation and destruction of molecular hydrogen. Despite this major simplification, the model allows for accurate treatment of the thermal evolution of the gas within a large volume of parameter space. It is valid for temperatures 50 < T < 10,000 K and metallicities 0 < Z < 0.1 Z?. In gas with a metallicity Z = 0.1 Z?, and in the absence of an incident ultraviolet radiation field, it is valid for hydrogen number densities nH 500/tchar cm-3, where tchar is the size in megayears of the characteristic physical timescale of interest in the problem. If Z 0.1 Z?, or if a strong ultraviolet radiation field is present, then the model remains accurate up to significantly higher densities. We also discuss some possible applications of this model.

Monte Carlo Modeling of Non-Gravitational Heating Processes in Galaxy Clusters

Abstract We consider non-gravitational heating effects on galaxy clusters based on the Monte Carlo modeling of merging trees of dark matter halos combined with the thermal evolution of gas inside each halo. Under the assumption of hydrostatic equilibrium and isothermal gas profiles, our model takes account of the metallicity evolution, metallicity-dependent cooling of gas, supernova energy feedback, and heating due to jets of radio galaxies in a consistent manner. The observed properties of galaxy clusters can be explained in models with higher non-gravitational heating efficiency than that in the conventional model. The possibilities include jet heating by the Fanaroff–Riley Type II radio galaxies, and the enhanced star formation efficiency and/or supernova energy feedback, especially at high redshifts.

Control of Photo-Induced Degradation in a-Si:H Prepared from Xe-Diluted Silane

ABSTRACTFilms were prepared by the Xe-dilution method in an attempt to mimic the microstructure found in low-photodegradation deuterated films which show a greater number of clustered deuterium and microvoids when compared to conventional hydrogenated films. Common features were found for the light soaking behaviors in the film properties characterized by the electron spin resonance, gas thermal evolution, Raman spectroscopy, and infrared spectroscopy among deuterated, conventional hydrogenated, and Xe-diluted films.

Thermal evolution of trapped gas from extended subsurface layers during a linear tempering schedule

Numerical computer calculations for the thermal evolution of gas trapped by defects in extended subsurface layers of solids or coatings under a linear tampering schedule have been carried out. The dependence of the shape and location of thermal evolution transients upon the extension (width) of the initially trapped gas distribution has been found in comparison with the case of an initial planar concentration of trapped gas some lattice units below the surface. A comparative study has been performed concerning the effect of the thermal release mechanism (for ‘tandem’ and pure diffuisve mechanism) upon the thermal evolution transients having a given initial extended distribution and specific ratios of the migration energy to detrapping energy.

Thermal evolution of implanted gas trapped by defects in extended subsurface layers

Abstract Numerical computer calculations are carried out for the evolution transient resulting from detrapping and subsequent diffusion of gas atoms wih extended initial distributions of gas/defect complexes. This arises, for instance, under polyenergetic ion implantation of the first wall of a thermonuclear device or of thin films, being deposited under simultaneous ion bombardment. The dependence was investigated of the main characteristics of the gas thermal evolution transients upon the ratio of activation energy for detrapping Q to the activation energy for diffusion E as well as upon the ramp rate according to a linear heating schedule. An essential influence of the initial distribution of complexes and activation energy of diffusion at E/Q ∼ 1 upon the thermal desorption spectra was revealed at the extension of the initial distribution of complexes by the order of hundreds of atomic layers. In this case the spectra resemble those conditioned by the pure diffusive mechanism for thermal evolution, and a special computer-based digital treatment of spectra is necessary to obtain the dissociation and diffusion parameters. At E/Q < 0.5 the thermal evolution transients are “degenerate”, i.e. do not depend on the activation energy for diffusion and the distribution of the complexes, so they are characterized only by the value of Q at a given ramp rate. In the case of light mobile gas atoms a possibility for revealing the complexes with close energetic characteristics by the thermal desorption spectra is obviously shown.

3770. Thermal evolution of gas from a solid following detrapping and diffusion during a linear tempering schedule. (GB): S E Donnelly and D C Ingram, Vacuum, 28 (2), 1978, 69–72

3770. Thermal evolution of gas from a solid following detrapping and diffusion during a linear tempering schedule. (GB): S E Donnelly and D C Ingram, Vacuum, 28 (2), 1978, 69–72

Thermal evolution of gas from a solid following detrapping and diffusion during a linear tempering schedule

A numerical analysis has been performed to determine the form of evolution rate transients resulting from detrapping and diffusion of gas implanted into a solid substrate (so-called substitutional pop-out) during a linear tempering schedule of the form T = T0 + at. The way in which the temperature for maximum release rate and the e−1 width of the transient vary as a function of the ratio of migration energy, E, to detrapping energy, Q, has been determined for a range of values of detrapping energy, Q, and diffusion starting depth, p. A general criterion has been derived which indicates that the diffusion process plays a significant role when E/Q ≳ 1−0.16 log p.

Thermal evolution of gas from a solid following detrapping and diffusion during a linear tempering schedule : S. E. Donnelly and D. C. Ingram. Vacuum28, (2) 69 (1978)

Thermal evolution of gas from a solid following detrapping and diffusion during a linear tempering schedule : S. E. Donnelly and D. C. Ingram. Vacuum28, (2) 69 (1978)

LVI. The disappearance of gas in the electric discharge (V.)

Summary The purpose of these experiments is to discover a relation between the rate of absorption of a gas in the electric discharge and the rate of ionization of its molecules. It is pointed out that the true electrical absorption, dependent simply on ionization or allied processes, is entangled with other causes of absorption and evolution of gas; the main difficulty is to disentangle it. The experiments show that, in nitrogen and carbon monoxide, the rate of absorption is proportional to the rate of ionization and, approximately at least, independent of the pressure, the speed of the ionizing electrons, and the arrangement of the electrodes. It depends very greatly on the condition, but not very greatly on the material, of the wails of the vessel. The number of molecules disappearing is often greater than the number ionized. In argon there is no evidence of an absorption simply related to ionization. In hydrogen the facts are obscure; but it is probable that there is au electrical absorption more rapid than in N2 or CO. An attempt is made to explain the facts. In spite of the simple relation between absorption and ionization, it does not seem possible to hold that the ions are absorbed. Alternative theories are discussed: (1) that monatomic molecules liberated by ionization, (2) that partially ionized or excited molecules are absorbed; but both are open to grave objection. The molecules almost certainly are absorbed by reaction with the walls of the vessel, but no suggestion can be made as to the nature of the reaction. Some observations, to be completed in a later paper, are described on the evolution of gas from glass vessels when heated, after they have been submitted to the usual processes of baking during exhaustion. It appears that there is no definite amount of gas that can be evolved by such heating, but rather that an equilibrium pressure is established. The paper consists of the following parts:—1. § 1–2. Statement of the Problem.2. § 3–4. Experimental Methods.3. § 5. General Nature of the Results.4. § 6–11. Detailed Results. Effects of anode voltage (6); Arrangement of electrodes (7); Nature of gas (8); Pressure of gas (9); Nature of walls (10); Thermal evolution of gas (11).5. § 12. Comparison with Previous Work.6. § 13–17. Discussion of Results. Absorption and ionization (13, 14); Absorption and monatomic molecules (15); Absorption and excited molecules (16); Nature of absorption (17).