Hydrogen Level Populations Research Articles

A possibly solar metallicity atmosphere escaping from HAT-P-32b revealed by Halpha and He absorption

This paper presents a hydrodynamic simulation that couples detailed non-local thermodynamic equilibrium (NLTE) calculations of the helium and hydrogen level populations to model the Halpha and He 10830 transmission spectra of the hot Jupiter HAT-P-32b. A Monte Carlo simulation was applied to calculate the number of Lyalpha resonance scatterings, which is the main process for populating H(2). In the examined parameter space, only models with H/He geq 99.5/0.5, $(0.5 3.0)$ times the fiducial value of XUV $, and spectral index $ 0.3)$, can explain the Halpha and He 10830 lines simultaneously. We found a mass-loss rate of $ $ g s$^ $, consistent with previous studies. Moreover, we found that the stellar Lyalpha flux should be as high as $4 $ erg cm$^ $ s$^ $, indicating high stellar activity during the observation epoch of the two absorption lines. Despite the fact that the metallicity in the lower atmosphere of HAT-P-32b may be super-solar, our simulations tentatively suggest it is close to solar in the upper atmosphere. Understanding the difference in metallicity between the lower and upper atmospheres is essential for future atmospheric characterisations.

Inverted level populations of hydrogen atoms in ionized gas

Context. Level population inversion of hydrogen atoms in ionized gas may lead to stimulated emission of hydrogen recombination lines, and the level populations can in turn be affected by powerful stimulated emissions. Aims. In this work the interaction of the radiation fields and the level population inversion of hydrogen atoms is studied. The effect of the stimulated emissions on the line profiles is also investigated. Methods. Our previous nl-model for calculating level populations of hydrogen atoms and hydrogen recombination lines is improved. The effects of line and continuum radiation fields on the level populations are considered in the improved model. By using this method the properties of simulated hydrogen recombination lines and level populations are used in analyses. Results. The simulations show that hydrogen radio recombination lines are often emitted from the energy level with an inverted population. The widths of Hnα lines can be significantly narrowed by strong stimulated emissions to be even less than 10 km s−1. The amplification of hydrogen recombination lines is more affected by the line optical depth than by the total optical depth. The influence of stimulated emission on the estimates of electron temperature and density of ionized gas is evaluated. We find that comparing multiple line-to-continuum ratios is a reliable method for estimating the electron temperature, while the effectiveness of the estimation of electron density is determined by the relative significance of stimulated emission.

Efficient Calculation of Non-local Thermodynamic Equilibrium Effects in Multithreaded Hydrodynamic Simulations of Solar Flares

Abstract Understanding the dynamics of the chromosphere is crucial to understanding energy transport across the solar atmosphere. The chromosphere is optically thick at many wavelengths and described by non-local thermodynamic equilibrium (NLTE), making it difficult to interpret observations. Furthermore, there is considerable evidence that the atmosphere is filamented, and that current instruments do not resolve small-scale features. In flares, it is likely that multithreaded models are required to describe the heating. The combination of NLTE effects and multithreaded modeling requires computationally demanding calculations, which has motivated the development of a model that can efficiently treat both. We describe the implementation of a solver in a hydrodynamic code for the hydrogen level populations that approximates the NLTE solutions. We derive an accurate electron density across the atmosphere that includes the effects of nonequilibrium ionization for helium and metals. We show the effects on hydrodynamic simulations, which are used to synthesize light curves using a postprocessing radiative transfer code. We demonstrate the utility of this model on IRIS observations of a small flare. We show that the Doppler shifts in Mg ii, C ii, and O i can be explained with a multithreaded model of loops subjected to electron beam heating, so long as NLTE effects are treated. The intensities, however, do not match the observed values very well, which is due to assumptions about the initial atmosphere. We briefly show how altering the initial atmosphere can drastically alter line profiles and derived quantities and suggest that it should be tuned to preflare observations.

Modeling the Hαline emission around classical T Tauri stars using magnetospheric accretion and disk wind models

Spectral observations of classical T Tauri stars show a wide range of line profiles, many of which reveal signs of matter inflow and outflow. Halpha is the most commonly observed line profile due to its intensity, and it is highly dependent on the characteristics of the surrounding environment of these stars. Our aim is to analyze how the Halpha line profile is affected by the various parameters of our model which contains both the magnetospheric and disk wind contributions to the Halpha flux. We used a dipolar axisymmetric stellar magnetic field to model the stellar magnetosphere and a modified Blandford & Payne model was used in our disk wind region. A three-level atom with continuum was used to calculate the required Hydrogen level populations. We use the Sobolev approximation and a ray-by-ray method to calculate the integrated line profile. Through an extensive study of the model parameter space, we have investigated the contribution of many of the model parameters on the calculated line profiles. Our results show that the Halpha line is strongly dependent on the densities and temperatures inside the magnetosphere and the disk wind region. The bulk of the flux comes, most of the time, from the magnetospheric component for standard classical T Tauri stars parameters, but the disk wind contribution becomes more important as the mass accretion rate, the temperatures and densities inside the disk wind increase. We have also found that most of the disk wind contribution to the Halpha line is emitted at the innermost region of the disk wind. Models that take into consideration both inflow and outflow of matter are a necessity to fully understand and describe classical T Tauri stars.

Radiative transfer in cylindrical threads with incident radiation

Context. Time-resolved observations of loops embedded in the solar corona show the existence of motions of matter inside these structures, as well as the global motions of these objects themselves. Aims. We have developed a modeling tool for cylindrical objects inside the solar corona, including 2-dimensional (azimuth-dependent) radiative transfer effects and 3-dimensional velocity fields. Methods. We used numerical methods to simultaneously solve the equations of NLTE radiative transfer, statistical equilibrium of hydrogen level populations, and electric neutrality. The radiative transfer equations were solved using cylindrical coordinates and prescribed solar incident radiation. In addition to the effects of anisotropic incident radiation, treated in previous papers, we took into account the Doppler shifts produced by a 3-dimension velocity field. Results. The effects of different types of velocity fields on hydrogen line profiles and intensities are described. Motions include loop oscillations, rotation, and longitudinal flows, which produce different deformations of profiles. Doppler brightening and dimming effects are also observed. Conclusions. This is a new step in the diagnostic of physical conditions in coronal loops, allowing the study of dynamical phenomena.

Non-equilibrium hydrogen ionization in 2D simulations of the solar atmosphere

The ionization of hydrogen in the solar chromosphere and transition region does not obey LTE or instantaneous statistical equilibrium because the timescale is long compared with important hydrodynamical timescales, especially of magneto-acoustic shocks. We implement an algorithm to compute non-equilibrium hydrogen ionization and its coupling into the MHD equations within an existing radiation MHD code, and perform a two-dimensional simulation of the solar atmosphere from the convection zone to the corona. Analysis of the simulation results and comparison to a companion simulation assuming LTE shows that: a) Non-equilibrium computation delivers much smaller variations of the chromospheric hydrogen ionization than for LTE. The ionization is smaller within shocks but subsequently remains high in the cool intershock phases. As a result, the chromospheric temperature variations are much larger than for LTE because in non-equilibrium, hydrogen ionization is a less effective internal energy buffer. The actual shock temperatures are therefore higher and the intershock temperatures lower. b) The chromospheric populations of the hydrogen n = 2 level, which governs the opacity of Halpha, are coupled to the ion populations. They are set by the high temperature in shocks and subsequently remain high in the cool intershock phases. c) The temperature structure and the hydrogen level populations differ much between the chromosphere above photospheric magnetic elements and above quiet internetwork. d) The hydrogen n = 2 population and column density are persistently high in dynamic fibrils, suggesting that these obtain their visibility from being optically thick in Halpha also at low temperature.

Radiative transfer in cylindrical threads with incident radiation

Context. Relatively cool and dense structures embedded in the solar corona (filaments, prominences, spicules, etc.) may be observed in hydrogen lines. Sometimes they last during several solar rotations. Aims. Our goal is to evaluate the lifetime of cool structures of the solar corona, determine their evolution from given physical conditions, and compute models in thermal equilibrium. Methods. We use numerical methods to simultaneously solve the equations of NLTE radiative transfer, statistical equilibrium of hydrogen level populations, and electric neutrality. Radiative transfer equations are solved using cylindrical coordinates and prescribed solar incident radiations. The computation of internal energy and radiative losses and gains yields the rates of temperature evolution. Results. For isothermal-isobaric cylinders with prescribed physical conditions, we determine the lifetimes and evolution rates for different positions along the radius. For models with prescribed diameter and pressure, we determine the run of temperature vs. radius corresponding to thermal equilibrium. This equilibrium is found to be stable for the whole range of parameters under investigation. Conclusions. The cores of large and high-pressure cylinders are found to evolve very slowly. This opens the possibility of observing these cool structures at temperatures somewhat different from that corresponding to theoretical radiative equilibrium.

Time-dependent hydrogen ionisation in 3D simulations of the solar chromosphere

Context. The hydrogen ionisation degree deviates substantially from statistical equilibrium under the conditions of the solar chromosphere. A realistic description of this atmospheric layer thus must account for time-dependent non-equilibrium effects. Aims. Advancing the realism of numerical simulations of the solar chromosphere by improved numerical treatment of the relevant physics will provide more realistic models that are essential for interpretation of existing and future observations. Methods. An approximate method for solving the rate equations for the hydrogen populations was extended and implemented in the three-dimensional radiation (magneto-)hydrodynamics code CO5BOLD. The method is based on a model atom with six energy levels and fixed radiative rates. It has been tested extensively in one-dimensional simulations. The extended method has been used to create a three-dimensional model that extends from the upper convection zone to the chromosphere. Results. The ionisation degree of hydrogen in our time-dependent simulation is comparable to the corresponding equilibrium value up to 500 km above optical depth unity. Above this height, the non-equilibrium ionisation degree is fairly constant over time and space, and tends to be at a value set by hot propagating shock waves. The hydrogen level populations and electron density are much more constant than the corresponding values for statistical equilibrium, too. In contrast, the equilibrium ionisation degree varies by more than 20 orders of magnitude between hot, shocked regions and cool, non-shocked regions. Conclusions. The simulation shows for the first time in 3D that the chromospheric hydrogen ionisation degree and electron density cannot be calculated in equilibrium. Our simulation can provide realistic values of those quantities for detailed radiative transfer computations.

Properties of the δ Scorpii Circumstellar Disk from Continuum Modeling

We present optical $WBVR$ and infrared $JHKL$ photometric observations of the Be binary system $\delta$ Sco, obtained in 2000--2005, mid-infrared (10 and $18 \mu$m) photometry and optical ($\lambda\lambda$ 3200--10500 \AA) spectropolarimetry obtained in 2001. Our optical photometry confirms the results of much more frequent visual monitoring of $\delta$ Sco. In 2005, we detected a significant decrease in the object's brightness, both in optical and near-infrared brightness, which is associated with a continuous rise in the hydrogen line strenghts. We discuss possible causes for this phenomenon, which is difficult to explain in view of current models of Be star disks. The 2001 spectral energy distribution and polarization are succesfully modeled with a three-dimensional non-LTE Monte Carlo code which produces a self-consistent determination of the hydrogen level populations, electron temperature, and gas density for hot star disks. Our disk model is hydrostatically supported in the vertical direction and radially controlled by viscosity. Such a disk model has, essentially, only two free parameters, viz., the equatorial mass loss rate and the disk outer radius. We find that the primary companion is surrounded by a small (7 $R_\star$), geometrically-thin disk, which is highly non-isothermal and fully ionized. Our model requires an average equatorial mass loss rate of $1.5\times 10^{-9} M_{\sun}$ yr$^{-1}$.

Molecular hydrogen emission from protoplanetary disks

We have modeled self-consistently the density and temperature profiles of gas and dust in protoplanetary disks, taking into account irradiation from a central star. Making use of this physical structure, we have calculated the level populations of molecular hydrogen and the line emission from the disks. As a result, we can reproduce the observed strong line spectra of molecular hydrogen from protoplanetary disks, both in the ultraviolet (UV) and the near-infrared, but only if the central star has a strong UV excess radiation.

A ready-made code for the computation of prominence NLTE models

A computer code is proposed for the computation of simple NLTE models of solar prominences. These models consist of plane-parallel slabs, with constant pressure and temperature, standing vertically above the solar surface. Each model is defined by five parameters: temperature, density, geometrical thickness, microturbulent velocity and height above the solar surface. The code computes the electron density, hydrogen level populations inside the slab, and determines the line profiles and continua emitted by the slab. An example of application of this code is given.

Non-LTE effects on the strength of the Lyman edge in quasar accretion disks

We have calculated UV/EUV (300 A which is less than or equal to lambda which is less than or equal to 1500 A) continuous energy distributions of accretion disks in the centers of active galactic nuclei (AGNs) for disk luminosities in the range 0.1 L(sub Edd) less than or equal to L(sub acc) less than 1.0 L(sub Edd) and central masses ranging from 10(exp 8) solar mass to 10(exp 9) solar mass. The vertical gas pressure structure of the disk and the disk height are obtained analytically; the temperature stratification and the resulting continuum radiation fields are calculated numerically. We have included non-Local Thermodynamic Equilibrium (LTE) effects of both the ionization equilibrium and the level populations of hydrogen and helium. We show that these non-LTE effects reduce the strength of the Lyman edge when comapred to the LTE case. In non-LTE we find that the edge can be weakly in emission or absorption for disks seen face-on, depending on the disk parameters.

Spectroscopic measurement of atomic hydrogen level populations and hydrogen dissociation degree in expanding cascaded arc plasmas

Optical absorption spectroscopy has been applied to measure the absolute population densities of the first excited levels of atomic hydrogen H*(n=2) and argon Ar*(4s) in an expanding cascaded arc plasma in hydrogen-argon mixture. It is demonstrated that the method allows us to determine both H*(n=2) and Ar*(4s) absolute density radial profiles for H2 admixtures in Ar ranging from 0.7% to 10% with good accuracy. The measured H*(n=2) densities are in the 1014–1016 m−3 range, and Ar*(4s) densities are in the range of 1015–1018 m−3. It has been shown, that the density of hydrogen excited atoms H*(n=2) serves as an indicator of the presence of argon ions and hydrogen molecules in the expanding plasma. A kinetic model is used to understand evolution of H*(n=2) density in the expansion, and to estimate the total atomic hydrogen population density and hydrogen dissociation degree in sub- and supersonic regions of the plasma.

Latitude dependent radiative wind model for Be stars: line profiles and intensity maps

We present theoretical line profiles and intensity maps from an axi-symmetric radiative wind model from a fast rotating Be star (Araújo & Freitas Pacheco, 1989; Araújo et al, 1993). The introduction of a viscosity parameter in the latitude dependent hydrodynamic code enables us to consider the effects of the viscous force in the azimuthal component of momentum equations. The line force is the same as Friend and Abbott (1986), but it is introduced a varying contribution of thin and thick lines from pole to equator by adopting latitude-dependent radiative parameters. The numerical calculation for parameters characteristic of early Be stars gives a density contrast between equator and pole of the order of 100. The total mass loss rates range from 10-8 to 10-7 solar mass per year. Furthermore, the “opening-angle” usually adopted in had-hoc models (Lamers and Waters, 1987; Waters et al., 1991) arises naturally as a result of our model. The velocity fields and density laws derived from the hydrodynamic equations have been used for solving the statistical equilibrium equations. By adopting the Sobolev approximation, we could easily obtain a good estimate of both electronic density and hydrogen level populations throughout the envelope (Stee and Araújo, 1994).

Spectral flux from low-density photospheres - Low-temperature results

Previous studies of low-density, hydrogen-dominated, quasi-static photospheres (such as those of Type II supernovae) are extended to effective temperatures Te = 4000-6000 K. These temperatures include those at which hydrogen recombines at the densities investigated, corresponding to scale heights Delta R = (1-100) x 10 to the 14th cm. In addition to presenting results for the emergent spectral flux (required to obtain supernova distances via the Baade method), the validity of some assumptions previously made is demonstrated. It is shown that the level populations of hydrogen are governed solely by a balance between photoionization directly to and recombination directly from the continuum.

Spectroscopic measurements of neutral hydrogen level populations in a multipole plasma H- source

Both active and passive spectroscopic techniques have been used to investigate the excited state populations in a multiple hydrogen plasma device typical of those under development for negative ion production. Laser fluorescence measurements have been used to obtain the hydrogen n=2 level population directly, and a simple model of the relevant atomic and molecular processes has been used to estimate from this the total atomic density. The density obtained is sufficiently high that lowering of the obtainable H- density by atomic processes cannot be ruled out. Measurements of emitted visible line radiation have demonstrated the existence of a strong atomic spectrum originating from close to the extraction aperture of the source. This may provide a spectroscopic method of measuring the H- density.

OBSERVATIONS OF OMI AND. III. A MODEL.

ABSTRACT A model is developed to account for the spectroscopic, photometric and polarimetric observations of the Be star Omicron Andromedae during the recent shell episode. A static solution for electron density and hydrogen level populations within a circumstellar envelope is first presented in which the shell is assumed static at the start of the shell episode, when shell density and polarization were high. Consideration of the time-dependent aspects of the problem then results in models for the waning of the shell in terms of a decrease in the mass loss rate, which is not able to account for the time difference in inner and outer shell density decreases, or a time-variable expansion velocity law, which was found to be more successful.

The role of particular collision and radiative processes in recombination

Rate equations for level populations of atomic hydrogen are solved to obtain collisional-radiative recombination coefficients. Several particular collision and radiative processes entering the rate equations have been accounted for or have been suppressed in order to obtain information on how strongly the solution depends on the different assumptions. Under certain conditions the results differ by orders of magnitude. The results also vary strongly with the number of quantum levelsp entering the rate equations. The critical quantum numbern * which defines the limit for partial L.T.E. cannot be considered as the critical number of rate equations above which the population coefficients become independent ofp.

Inversion of hydrogen level populations in stationary plasmas

Large inversions in the dense two-temperature hydrogen plasma are obtained by solving balance equations with inelastic collisions between heavy particles taken into account.

Measurements of the structure of an ionizing shock wave in a hydrogen-helium mixture

Shock structure during ionization of a hydrogen-helium mixture has been followed using hydrogen line and continuum emission measurements. A reaction scheme is proposed which includes hydrogen dissociation and a two-step excitation-ionization mechanism for hydrogen ionization by atom-atom and atom-electron collisions. Agreement has been achieved between numerical calculations and measurements of emission intensity as a function of time for shock velocities from 13 to 20 km/sec in a 0.208 H2-0.792 He mixture. The electron temperature was found to be significantly different from the heavy particle temperature during much of the ionization process. Similar time histories for Hβ and continuum emission indicate upper level populations of hydrogen in equilibrium with the electron concentration during the relaxation process. The expression for the rate constant for excitation of hydrogen by atom-atom collisions that best fit the data was kAA = 4.0×10−17 (8kT/πμ)1/2 exp (−10/kT) cm3 sec−1, where it has been assumed that the excitation cross section is the same for hydrogen and helium collision partners. The electron-atom excitation rate constant, k8 = 7.5 × 10−16 (8kTe/πμe)1/2 exp ( −10/kTe), determined from this investigation, was in agreement with recent electron beam cross-section measurements.