Formation Of Emission Lines Research Articles

The physics of radiation transport in dense plasmas

Radiation transport redistributes energy within a medium through the emission and reabsorption of photons. These processes also have a pronounced effect on the spectrum of radiation that escapes the medium. As the deliverable energies of plasma drivers such as lasers and pulsed-power generators steadily increase, denser and/or more massive plasmas can be created. Such plasmas are more absorptive to their own emitted radiation, with portions of the line spectrum frequently being highly opaque. Thus, radiation transport becomes more important, along with the need to consider its impact on the design of experiments and their diagnosis. This tutorial paper covers the basic theory and equations describing radiation transport, its physical effects, experimental examples of transport phenomena, and current challenges and issues. Among the specific topics discussed are requirements for local thermodynamic equilibrium (LTE), conditions for diffusion and the use of the diffusion approximation, the formation of emission and absorption lines, the approach of an emitted spectrum to the Planck limit, and diagnostic applications of transport effects.

The region of Fe II emission line formation of the symbiotic nova RR Telescopii

In our investigation of the region of formation of the Fe II emission lines in RR Tel, we have applied the SAC method to the optical Fe II emission line fluxes measured by Crawford et al. (1999). It is possible to determine physical parameters of the line emitting region, because of the presence both of lines with large and much smaller optical thicknesses. The values we obtained, given in Tables 3 and 4, are limits in most cases. The upper limit to the excitation temperature of the metastable levels in the permitted line region of 6600 K, is close to the value of this temperature in the forbidden line region. Other excitation temperature limits corresponding to level population ratios were also found. The permitted lines are formed in a region with a radius between 1:8 10 12 cm and 1:6 10 14 cm, while the forbidden lines are formed in a region with a minimum radius of 5:3 10 12 cm. The populations of the lower levels of the permitted lines indicate a minimum column density of Fe+ of 3:0 10 18 atoms cm 2 and, by assuming cosmic abundances, to a minimum column density of 7:5 10 22 atoms cm 2 for H. If we assume a minimum radius of 1:0 10 14 cm, which is supposed to be that of dust condensation of the cool Mira component of this symbiotic binary, a minimum mass loss rate of 3:510 6 Mo yr 1 is found. Our results support a model according to which both the permitted and the forbidden lines are formed in the wind of the cool Mira component above the regions where absorption by dust is important. The forbidden and the permitted lines are not formed in quite the same region, the former being most probably formed further out in the part of an accelerated wind which is ionized by the hot component.

Radiation Hydrodynamics of 2-D Accretion Disks

To examine the structure and dynamics of thick accretion disks, we use a two-dimensional viscous hydrodynamic code coupled with radiation transport. The -model and the full viscous stress-tensor description for the kinematic viscosity are used. The radiation transport is treated in the gray, flux-limited diffusion approximation. The finite difference methods used are based on an explicit-implicit method. We apply the numerical code to the Super-Eddington black-hole model for SS 433.@The result for a very small viscosity parameter a reproduces well the characteristic features of SS 433, such as the relativistic jets with 0.26c, the small collimation degree of the jets, the mass-outflow rate of , and the formation of the X-ray iron emission lines.

Ion cyclotron diffusion of velocity distributions in the extended solar corona

The Ultraviolet Coronagraph Spectrometer aboard the Solar and Heliospheric Observatory has revealed strong kinetic anisotropies and extremely large perpendicular temperatures of heavy ions in the extended solar corona. These observations have revived interest in the idea that the high‐speed solar wind is heated and accelerated by the dissipation of ion cyclotron resonant Alfvén waves. This process naturally produces departures from Maxwellian and bi‐Maxwellian velocity distributions. Here it is argued that these departures must be taken into account in order to understand the resonant velocity space diffusion, the wave damping, and the formation of ultraviolet emission lines. Time‐dependent ion velocity distributions are computed for a fixed spectrum of waves in a homogeneous plasma, and the moments of the distributions are compared with simple bi‐Maxwellian models. The existence of a boundary, in parallel velocity space, between resonance and nonresonance produces an effective saturation of the velocity space diffusion that bi‐Maxwellian models could not predict. The damping of an input wave spectrum is computed for a coronal population of 1000 ion species with the above saturation effect included. For realistic levels of fluctuation power, it is concluded that waves propagating solely from the coronal base would not be able to heat and accelerate the ions that have been observed to exhibit strong energization and that local wave generation is required. Ultraviolet emission line profiles are computed for the derived non‐Maxwellian distributions, and possible unique identifiers of the ion cyclotron resonance mechanism are noted.

Evidence for an Accelerating Wind as the Broad‐Line Region in NGC 3516

Spectroscopic data in wavelengths 900-3000 A have been obtained in a low flux state of the nucleus of the Seyfert 1 galaxy NGC 3516. The line profiles show P Cygni characteristics, particularly in O VI λ1032, and are compared with data from an earlier and higher state. The profiles are suggestive of and consistent with an accelerating wind driven by a disk continuum source in which the ionization radii have changed. This scenario may apply to the formation of other broad emission lines in active galactic nuclei.

Broadening of electron cyclotron maser emission lines in a nonuniform magnetic field

The formation of cyclotron maser emission lines in a non-uniform (regular or random) magnetic field is studied. In the presence of sufficiently small inhomogeneity, the line shape can be described by a broadened Gaussian profile. In the case of stronger inhomogeneity, the initial Gaussian profile splits into two Gaussian components, which could be observationally perceived as “harmonics.” A relation between the distribution of local magnetic trap sizes and the distribution of the spectral widths of solar radio spikes is derived. Possible applications of the results to the interpretation of solar radio spikes and related problems are discussed.

On the Formation of Lithium Emission Lines in Nova Shells

The role of nova outbursts in the galactic lithium production is still controversial. Photoionization model calculations of nova shells are presented in this contribution, aiming to identify the optimal post-outburst phase for detecting lithium emission lines from nova ejecta which may confirm its lithium enhancement. Predictions for the Li I λ6708 Â flux are made as a function of the envelope geometry, its physical parameters and properties of the central ionizing source.

On Inferring the Properties of Dynamic Plasmas from Their Emitted Spectra: The Case of the Solar Transition Region

We reexamine the issue of inferring physical properties of solar plasmas using EUV and UV observations. We focus on the question of whether one can determine if typical structures seen as bright in typical "transition-region" lines are formed in the thermal interface between the coronal and chromospheric plasmas. Since 1983, Feldman and colleagues have proposed, based upon Skylab and other data, that much of the transition-region emission is formed in so-called unresolved fine structures (UFS) that are magnetically and thermally disconnected from the corona. This has led others to consider theoretical models of the transition region that differ from classical models. We examine the evidence cited in support of the UFS picture, specifically by relaxing the implicit assumption of a static atmospheric structure. Noting that observational data alone do not contain the information necessary to infer essential properties of the emitting plasmas, we argue that additional information must be added through forward calculations using physical models. MHD models of coronal flux tubes are then examined with explicit assumptions and boundary conditions, not as an attempt to "fit" observed data, but in order to study the formation of emission lines in dynamically evolving plasmas that are unresolved in space and time. We show that incorrect conclusions can be drawn by applying reasonable and traditional diagnostic methods to spectral data when unresolved dynamic evolution of the emitting plasma is important but not accounted for. In the particular case of the transition region, we show that the UFS interpretation is not unique, and is likely to be incorrect in the presence of unresolved dynamics. Most or all of the evidence for UFS is amenable to a different, equally reasonable interpretation, in which the transition-region emission is at all times formed in the time-varying thermal interface between the corona and the chromosphere. This work is likely to be important for a wider range of astrophysical plasmas than simply in the solar transition region. At stake is our basic ability to correctly diagnose physical conditions of plasmas for which heating mechanisms are not yet understood, but which are likely to be time dependent.

The role of an extended corona in the formation of emission lines and continuum in active galactic nuclei

We study a model of an accretion disc surrounded by an extended corona of the temperature 10^7 - 10^8 K. This corona modifies the disk spectrum since it redirects a significant fraction of the emission from the central parts towards the more distant parts of the disk. The same corona is indirectly the source of the broad emission lines because we expect the formation of cool clouds at the basis of the corona due to thermal instabilities. We model the number of the clouds and their motion through the corona adopting a few different physically sound assumptions. Comparing the predicted optical/UV continua and emission line ratios and profiles with the observed values we favor a particular model of a typical quasar. It radiates at ~0.5 of the Eddington luminosity, the corona surrounding the disk is additionally heated in excess of the Inverse Compton heating, and the broad line clouds are most probably destroyed when accelerated vertically above the sound speed within the corona.

On Schuster's mechanism of emission line formation

We investigate the influence of scattering processes in the continuum on the formation of lines in a static, one-dimensional atmosphere. It is shown that the Schuster mechanism of emission line formation can work under certain physical conditions. We derive formulae for the calculation of the line profiles in atmospheres with an ad hoc temperature distribution.

On the Formation of Hydrogen Emission Lines in Miras

AbstractWe investigate non-LTE effects in the hydrogen recombination zone of Long-Period Variables of Mira type. We propose that the Balmer hydrogen emission lines could be formed in this deep layer. We show how the spectral cyclic variations of Miras could be mimicked with this interpretation. As a result the radius of a Mira in the Balmer transitions could be much smaller than its radius in the molecular absorptions, which are formed in higher layers.

On the problem of the formation of quasar emission lines by the Cherenkov mechanism

In a series of papers of You, Cheng, et al. (cf., for example, [2–4]) the Cherenkov mechanism of radiation was proposed as the explanation of the wide emission lines in the spectra of quasars. This mechanism acts because the refractive index of the plasma in a certain frequency range near a spectral line can exceed one. It has been found that the role of Cherenkov radiation becomes significant for an optically thick medium when the density of the packet of relativistic electrons ne is 104–106 cm−3. However, the question of the formation of a spectral line as the result of ordinary (recombination and collision) processes under the same physical conditions has not been considered, and hence no comparison has been made between the intensities of lines formed by different mechanisms. Moreover, a large number of effects that may have significant influence on the profile of a line (multiple scattering in the medium, redistribution of radiation over frequencies within a line, redistribution of energy between a line and the continuous spectrum) have remained unexamined by these authors.

On the formation of Balmer emission lines in the model envelopes of Be stars

We consider the formation of the Balmer emission lines and the decrement Hα/Hβ/Hγ, by solving the non-LTE problems in elementary regions of the envelope divided by equal line-of-sight velocities. These envelope-elements are characterized by different optical depths in the Balmer lines and by different dilution factors for the incident stellar radiation. It is shown that the decrements sensitively depend on these parameters of the envelope-elements. We show that the observed spectral-type dependence and large scatter of the decrements among Be stars can be explained in terms of the variation of these physical parameters.

Energy approach to consistent QED theory for calculation of electron-collision strengths: Ne-like ions.

An energy approach based on consistent QED theory is outlined which presents the energy of any decaying atomic state in complex form with the imaginary part responsible for different types of decays. The aim is to study, in a uniform manner, elementary processes responsible for emission-line formation in plasmas. The energy approach is generalized here for the collisional problem. Numerical results on electron collisional strengths and rate coefficients for Ne-like ions are presented. It is shown that collisional rate coefficients are close to their maxima at an electron temperature \ensuremath{\sim}0.30${\mathit{E}}_{\mathrm{threshold}}$ for iron and \ensuremath{\sim}0.15${\mathit{E}}_{\mathrm{threshold}}$ for argon.

Winds from accretion disks - Ultraviolet line formation in cataclysmic variables

Winds from accretion disks in cataclysmic variable stars are ubiquitous. Observations by IUE reveal P Cygni-shaped profiles of high-ionization lines which are attributed to these winds. We have studied the formation of UV emission lines in cataclysmic variables by constructing kinematical models of biconical rotating outflows from disks around white dwarfs. The photoionization in the wind is calculated taking into account the radiation fields of the disk, the boundary layer, and the white dwarf. The 3D radiative transfer is solved in the Sobolev approximation. Effects on the line shapes of varying basic physical parameters of the wind are shown explicitly. We identify and map the resonant scattering regions in the wind which have strongly biconical character regardless of the assumed velocity and radiation fields. Rotation at the base of the wind introduces a radial shear which decreases the line optical depth and reduces the line core intensity. We find that it is possible to reproduce the observed P Cygni line shapes and make some predictions to be verified in high-resolution observations.

Observational and theoretical spectra of supernovae

Progress in nuclear astrophysics by means of quantitative supernova spectroscopy is discussed with special concentration on type Ia, Ib and Ic and on SN 1987A. Spectral calculations continue to support an exploding C/O white dwarf as the best model of a SN Ia. Deflagration model W7 produces good maximum light spectra of SN Ia and seems to have a better composition distribution compared to delayed detonation models, but proper treatment of opacity remains a problem and the physical basis of SN Ia explosions is still not completely understood. All models for SN Ia predict large quantities of 56Co in the ejecta, but it is not clear that observations confirm this. Although the evolutionary origin of SN Ia remains uncertain, there is recent evidence that transfer of hydrogen in a binary system may be involved, as long suspected. There has been progress in comparing dynamical models with the optical/IR spectra of SN 1987A. The evolution of the [OI] λλ6300, 6364 feature and the presence of strong persistent HeI λ10 830 indicate that both the envelope and core material contribute substantially to the formation of emission lines in the nebular phase and that neither the core nor the envelope can be neglected. Blending with nearby hydrogen lines may affect both of these spectral features, thereby complicating the analysis of the lines. The effects of continuum transfer and photoionization have been included and are under study. The discrepancies between theoretical and observed spectra are due primarily to the one-dimensional hydrodynamic models. The spectral data are not consistent with the high density “spike” (in radial coordinate) of the core material that is predicted by all such models. Analysis of the light curves of SN Ib and SN Ic supernovae implies that there are significant differences in their physical properties. Some SN Ib have considerably more ejecta mass than SN Ic events. SN Ib require He-rich atmospheres to produce the observed strong optical lines of HeI somewhat after maximum. SN Ic events require a considerable depletion, if not absence, of helium. Calculations of the nebular phase after about 200 days show that the optical spectra of SN Ib/c will not reveal HeI even if helium is present. The spectra at that phase are rather insensitive to variations in the mass and composition. The similarity of the optical spectra of SN Ib and Ic events at late times thus does not mean that they are physically very similar. Observations of the HeI λ10 830 line could provide a good diagnostic of the atmospheric composition of Sn Ib and SN Ic.

Formation of emission lines of hydrogen, calcium, and magnesium in expanding envelopes of T Tau stars

Formation of emission lines of hydrogen, calcium, and magnesium in expanding envelopes of T Tau stars

On the interpretation of chromospheric emission lines

This paper reexamines the formation of ultraviolet emission lines in stellar chromospheres, using detailed radiative transfer calculations and relatively simple methods based on approximate line cooling rates, including escape probabilities. Approximations for the cooling integrals are demonstrated to work well for 'effectively thin' chromospheric lines. Two cases are identified, whose behavior can be understood using Ayres's (1979) chromospheric scaling laws relating the chromospheric structure to stellar properties: those of inactive stars, like cool giants, and more active stars like the sun.

HeI emission line formation in symbiotic stars and novae

The HeI triplet to singlet line ratio Λ = F(λ5876)/F(λ6678) in symbiotic stars and novae is often much lower than the planetary nebulae value of ≈ 3.5, which is also the low density value Λ0 predicted by recombination theory.Fig.1 shows that the dereddened HeI line ratios ^ are different for S type and D type symbiotic stars. The spectrophotometric data are mainly from Allen (1984, personal communication) and Blair et al.(1983). The S and D type classification divides the symbiotic stars according to the IR continuum into objects with a stellar continuum or dust emission. Practically all D types have low nebular densities Ne ≲ 107cm−3 and a rich spectrum of forbidden lines, while in S type objects these lines are suppressed by collisional deexcitation due to the high electron densities Ne ≳ 107cm−3. The observed HeI line ratio ^ in S type symbiotics, which is systematically too low when compared with the theoretical low density value Λ0 = 3.5, can therefore be attributed to the high nebular densities in these objects.

The emission and forbidden lines in quasars

We use the Cerenkov line radiation and the moving model of quasar to discuss the formation of some broad emission lines of quasars. Our calculated profiles and relative intensities of the lines are in good agreement with the observations. The Cerenkov line radiation may therefore be important in the formation of the emission lines.