Transit Ly-α\alpha signatures of terrestrial planets in the habitable zones of M dwarfs

We modeled the transit signatures in the Lya line of a putative Earth-sized planet orbiting in the HZ of the M dwarf GJ436. We estimated the transit depth in the Lya line for an exo-Earth with three types of atmospheres: a hydrogen-dominated atmosphere, a nitrogen-dominated atmosphere, and a nitrogen-dominated atmosphere with an amount of hydrogen equal to that of the Earth. We calculated the in-transit absorption they would produce in the Lya line. We applied it to the out-of-transit Lya observations of GJ 436 obtained by the HST and compared the calculated in-transit absorption with observational uncertainties to determine if it would be detectable. To validate the model, we also used our method to simulate the deep absorption signature observed during the transit of GJ 436b and showed that our model is capable of reproducing the observations. We used a DSMC code to model the planetary exospheres. The code includes several species and traces neutral particles and ions. At the lower boundary of the DSMC model we assumed an atmosphere density, temperature, and velocity obtained with a hydrodynamic model for the lower atmosphere. We showed that for a small rocky Earth-like planet orbiting in the HZ of GJ436 only the hydrogen-dominated atmosphere is marginally detectable with the STIS/HST. Neither a pure nitrogen atmosphere nor a nitrogen-dominated atmosphere with an Earth-like hydrogen concentration in the upper atmosphere are detectable. We also showed that the Lya observations of GJ436b can be reproduced reasonably well assuming a hydrogen-dominated atmosphere, both in the blue and red wings of the Lya line, which indicates that warm Neptune-like planets are a suitable target for Lya observations. Terrestrial planets can be observed in the Lya line if they orbit very nearby stars, or if several observational visits are available.Comment: 17 pages, 12 figures, accepted for publication in Astronomy & Astrophysic

Intermediate regimes in granular Brownian motion: Superdiffusion and subdiffusion

Brownian motion in a granular gas in a homogeneous cooling state is studied theoretically and by means of molecular dynamics. We use the simplest first-principle model for the impact-velocity dependent restitution coefficient, as it follows for the model of viscoelastic spheres. We reveal that for a wide range of initial conditions the ratio of granular temperatures of Brownian and bath particles demonstrates complicated non-monotonous behavior, which results in transition between different regimes of Brownian dynamics: It starts from the ballistic motion, switches later to superballistic one and turns at still later times into subdiffusion; eventually normal diffusion is achieved. Our theory agrees very well with the MD results, although extreme computational costs prevented to detect the final diffusion regime. Qualitatively, the reported intermediate diffusion regimes are generic for granular gases with any realistic dependence of the restitution coefficient on the impact velocity

Inclusive and effective bulk viscosities in the hadron gas

We estimate the temperature dependence of the bulk viscosity in a relativistic hadron gas. Employing the Green-Kubo formalism in the SMASH (Simulating Many Accelerated Strongly-interacting Hadrons) transport approach, we study different hadronic systems in increasing order of complexity. We analyze the (in)validity of the single exponential relaxation ansatz for the bulk-channel correlation function and the strong influence of the resonances and their lifetimes. We discuss the difference between the inclusive bulk viscosity of an equilibrated, long-lived system, and the effective bulk viscosity of a short-lived mixture like the hadronic phase of relativistic heavy-ion collisions, where the processes whose inverse relaxation rate are larger than the fireball duration are excluded from the analysis. This clarifies the differences between previous approaches which computed the bulk viscosity including/excluding the very slow processes in the hadron gas. We compare our final results with previous hadron gas calculations and confirm a decreasing trend of the inclusive bulk viscosity over entropy density as temperature increases, whereas the effective bulk viscosity to entropy ratio, while being lower than the inclusive one, shows no strong dependence to temperature.Comment: 23 pages, 13 figure

Bubbly and Buoyant Particle-Laden Turbulent Flows

Fluid turbulence is commonly associated with stronger drag, greater heat transfer, and more efficient mixing than in laminar flows. In many natural and industrial settings, turbulent liquid flows contain suspensions of dispersed bubbles and light particles. Recently, much attention has been devoted to understanding the behavior and underlying physics of such flows by use of both experiments and high-resolution direct numerical simulations. This review summarizes our present understanding of various phenomenological aspects of bubbly and buoyant particle-laden turbulent flows. We begin by discussing different dynamical regimes, including those of crossing trajectories and wake-induced oscillations of rising particles, and regimes in which bubbles and particles preferentially accumulate near walls or within vortical structures. We then address how certain paradigmatic turbulent flows, such as homogeneous isotropic turbulence, channel flow, Taylor-Couette turbulence, and thermally driven turbulence, are modified by the presence of these dispersed bubbles and buoyant particles. We end with a list of summary points and future research questions.Comment: 29 pages, 14 figure

Characterizing the influence of neutron fields in causing single-event effects using portable detectors

The malfunction of semiconductor devices caused by cosmic rays is known as Single Event Effects(SEEs). In the atmosphere, secondary neutrons are the dominant particles causing this effect. The neutron flux density in atmosphere is very low. For a good statistical certainty, millions of device hours are required to measure the event rate of a device in the natural environment. Event rates obtained in such testings are accurate. To reduce the cost and time of getting the event rate, a device is normally taken to artificial accelerated neutron beams to measure its sensitivity to neutrons. Comparing the flux density of the beam and the flux density of a location in the atmosphere, the real time event rate can be predicted by the event rate obtained. This testing method was standardized as the neutron accelerated soft error rate (ASER) testing in JEDEC JESD89A standard. However, several life testings indicated that the neutron flux density predictions given by the accelerated testings can have large errors. Up to a factor of 2 discrepancy was reported in the literature. One of the major error sources is the equivalence of the absolute neutron flux density in the atmosphere and in accelerated beam. This thesis proposes an alternative accelerated method of predicting the real-time neutron error rate by using proxy devices. This method can avoid the error introduced by the uncertainty in the neutron flux density. The Imaging Single Event Effect Monitor (ISEEM) is one of the proxy devices. It is the instrument originally developed by Z. Török and his co-workers in the University of Central Lancashire. A CCD was used as the sensitive element to detect neutrons. A large amount of data sets acquired by Török were used in this work. A re-engineered ISEEM has been developed in this work to improve ISEEM performance in life testings. Theoretical models have been developed to analyze the response of ISEEM in a wide range of neutron facilities and natural environment. The agreement of the measured and calculated cross-sections are within the error quoted by facilities. Because of the alpha contamination and primary proton direct ionization effects, performance of ISEEM in life testings appeared to be weak

Droplet and cluster formation in freely falling granular streams

Particle beams are important tools for probing atomic and molecular interactions. Here we demonstrate that particle beams also offer a unique opportunity to investigate interactions in macroscopic systems, such as granular media. Motivated by recent experiments on streams of grains that exhibit liquid-like breakup into droplets, we use molecular dynamics simulations to investigate the evolution of a dense stream of macroscopic spheres accelerating out of an opening at the bottom of a reservoir. We show how nanoscale details associated with energy dissipation during collisions modify the stream's macroscopic behavior. We find that inelastic collisions collimate the stream, while the presence of short-range attractive interactions drives structure formation. Parameterizing the collision dynamics by the coefficient of restitution (i.e., the ratio of relative velocities before and after impact) and the strength of the cohesive interaction, we map out a spectrum of behaviors that ranges from gas-like jets in which all grains drift apart to liquid-like streams that break into large droplets containing hundreds of grains. We also find a new, intermediate regime in which small aggregates form by capture from the gas phase, similar to what can be observed in molecular beams. Our results show that nearly all aspects of stream behavior are closely related to the velocity gradient associated with vertical free fall. Led by this observation, we propose a simple energy balance model to explain the droplet formation process. The qualitative as well as many quantitative features of the simulations and the model compare well with available experimental data and provide a first quantitative measure of the role of attractions in freely cooling granular streams

Particle production and equilibrium properties within a new hadron transport approach for heavy-ion collisions

The microscopic description of heavy-ion reactions at low beam energies is achieved within hadronic transport approaches. In this article a new approach SMASH (Simulating Many Accelerated Strongly-interacting Hadrons) is introduced and applied to study the production of non-strange particles in heavy-ion reactions at $E_{\rm kin}=0.4-2A$ GeV. First, the model is described including details about the collision criterion, the initial conditions and the resonance formation and decays. To validate the approach, equilibrium properties such as detailed balance are presented and the results are compared to experimental data for elementary cross sections. Finally results for pion and proton production in C+C and Au+Au collisions is confronted with HADES and FOPI data. Predictions for particle production in $\pi+A$ collisions are made.Comment: 30 pages, 30 figures, replaced with published version; only minor change

Sedimentation of finite-size spheres in quiescent and turbulent environments

Sedimentation of a dispersed solid phase is widely encountered in applications and environmental flows, yet little is known about the behavior of finite-size particles in homogeneous isotropic turbulence. To fill this gap, we perform Direct Numerical Simulations of sedimentation in quiescent and turbulent environments using an Immersed Boundary Method to account for the dispersed rigid spherical particles. The solid volume fractions considered are 0.5-1%, while the solid to fluid density ratio 1.02. The particle radius is chosen to be approximately 6 Komlogorov lengthscales. The results show that the mean settling velocity is lower in an already turbulent flow than in a quiescent fluid. The reduction with respect to a single particle in quiescent fluid is about 12\% and 14\% for the two volume fractions investigated. The probability density function of the particle velocity is almost Gaussian in a turbulent flow, whereas it displays large positive tails in quiescent fluid. These tails are associated to the intermittent fast sedimentation of particle pairs in drafting-kissing-tumbling motions. The particle lateral dispersion is higher in a turbulent flow, whereas the vertical one is, surprisingly, of comparable magnitude as a consequence of the highly intermittent behavior observed in the quiescent fluid. Using the concept of mean relative velocity we estimate the mean drag coefficient from empirical formulas and show that non stationary effects, related to vortex shedding, explain the increased reduction in mean settling velocity in a turbulent environment.Comment: In press on Journal of Fluid Mechanic