W.H. Goldstein

Laboratory measurement of opacity for stellar envelopes

Abstract We have measured the frequency dependent opacity of a low density iron plasma in Local Thermodynamic Equilibrium (LTE). The measured iron plasma conditions of 20 eV temperature and 10 −4 g/cc density, match those of stellar envelopes where iron dominates the radiative transport. Properties of the M-shell Δn = 0 transition arrays in iron are measured in this experiment, providing the first direct test of opacity models used in stellar pulsation and evolution calculations. We describe new methods to obtain LTE opacity data for plasmas at 100 times lower density than previous measurements. Experimental requirements include: high spectral resolution, large homogenous plasma sources, and Planckian radiation fields lasting tens of nanoseconds. These conditions were achieved using the 500 kJ SATURN facility at Sandia National Laboratory.

Configuration interaction in LTE spectra of heavy elements

Abstract We present a method for including the effects of configuration interaction (CI) between relativistic subconfigurations of an electron configuration in the calculation of emission and absorption spectra of plasmas in local thermodynamic equilibrium (LTE). Analytical expressions for the correction to the intensities, owing to CI, of an unresolved transition array (UTA) and of a supertransition array (STA) are obtained when the correction is small compared to the spin-orbit splitting, bypassing the need to diagonalize energy matrices. These expressions serve as working formulas in the STA model and, in addition, reveal a priori the conditions under which CI effects are significant. Examples of the effect are presented.

X-ray spectral signatures of photoionized plasmas. [Astrophysics]

Plasma emission codes have become a standard tool for the analysis of spectroscopic data from cosmic X-ray sources. However, the assumption of collisional equilibrium, typically invoked in these codes, renders them inapplicable to many important astrophysical situations, particularly those involving X-ray photoionized nebulae. This point is illustrated by comparing model spectra which have been calculated under conditions appropriate to both coronal plasmas and X-ray photoionized plasmas. It is shown that the (3s-2p)/(3d-2p) line ratios in the Fe L-shell spectrum can be used to effectively discriminate between these two cases. This diagnostic will be especially useful for data analysis associated with AXAF and XMM, which will carry spectroscopic instrumentation with sufficient sensitivity and resolution to identify X-ray photoionized nebulae in a wide range of astrophysical environments. 25 refs.

Calculation of the radiative cooling coefficient for molybdenum in a low density plasma

The calculated radiative cooling rate coefficient for krypton as a trace impurity in low to moderate density plasmas is calculated. Collisional-radiative line emission, dielectronic recombination, radiative recombination and bremsstrahlung are considered as the principal radiative loss channels. Collisional-radiative models and the calculated charge state distribution for krypton have been benchmarked against measured ion brightness profiles in the FTU plasma. The calculated radiative loss rate is compared with two measurements of the radiative cooling coefficient for krypton in the FTU plasma. The measurements differ in how the krypton density profile is experimentally constrained. The krypton density profile is found (to be flat) from spectroscopic observations and (to be increasing radially outwards) from measurements of visible bremsstrahlung emission. The calculations show excellent agreement with the former set of measurements. Polynomial fits to the total radiative cooling coefficient are provided for ease of use in plasma modelling codes. Tables of ion emissivities are provided for use in modelling radiative losses from non-equilibrium plasmas.

X-ray and VUV observations of brightness profiles from Alcator C-Mod plasmas

Brightness profiles of x-ray and VUV lines from eight molybdenum charge states between and have been measured in Alcator C-Mod plasmas. These spatial profiles agree very well with those predicted by a model which includes ionization, recombination, excitation and transport. Comparison with the profiles of many different charge states provides severe constraints upon the rates used in the model. The charge state density profiles are calculated using measured impurity transport coefficients, measured electron density and temperature profiles and newly calculated ionization and recombination rate coefficients. These new rate coefficients include direct collisional ionization, excitation - autoionization, dielectronic and radiative recombination. Excitation - autoionization is shown to be an important process, since the excellent agreement between the measurements and predictions is obtained only with its inclusion. Fits to newly calculated excitation rate coefficients for the transitions are also presented.

A burst model for line emission in the solar atmosphere. II - Coronal extreme ultraviolet lines

We review the spectroscopic evidence for explosive events, hereafter referred to as bursts, occurring in solar coronal plasmas. Problems in the interpretation of extreme ultraviolet spectra from Mg-like ions, particularly Fe XV in solar flares can be obviated to certain extent if these ions are produced in an ionizing plasma, with the 3s3p 3 P levels being excited directly by inner shell ionization of the Al-like ion. We believe that a similar mechanism should apply to Be-like ions

X-ray emission-line spectra of photoionized plasmas - Density sensitivity of the Fe L-shell series

The circumsource environments of accretion-powered X-ray sources are likely to support relatively dense (greater than 10 exp 11/cu cm) photoionized X-ray emission-line regions. The Fe L-shell ions provide a versatile class of plasma diagnostics in this regime, their multielectron structures resulting in diverse spectral phenomena. Attention is given to the spectral response of Fe L-shell ions to variations in electron density over the range 10 exp 11 to 10 exp 16/cu cm. It is found that density-sensitive line ratios exist in the wavelength interval 12-17 A for the ions Fe XVII-XXI. The prominent role of radiative recombination in the population kinetics distinguishes the density-sensitive Fe lines in photoionized plasmas from those which operate in coronal equilibrium plasmas. The results of detailed atomic modeling of these ions are presented and applications to spectroscopic observations of accretion-driven X-ray sources are discussed. 35 refs.

High-Resolution Spectroscopy of G191-B2B in the Extreme-Ultraviolet

We report a high-resolution (R = 3000-4000) spectroscopic observation of the DA white dwarf G191-B2B in the extreme-ultraviolet band 220-245 A. A low-density, ionized He component is clearly present along the line of sight, which if completely interstellar implies a He ionization fraction considerably higher than is typical of the local interstellar medium. However, some of this material may be associated with circumstellar gas, which has been detected by analysis of the C IV absorption-line doublet in a Hubble Space Telescope/Space Telescope Imaging Spectrograph spectrum. A stellar atmosphere model assuming a uniform element distribution yields a best fit to the data that includes a significant abundance of photospheric He. The 99% confidence contour for the fit parameters excludes solutions in which photospheric He is absent, but this result needs to be tested using models allowing abundance gradients.

Features of plasma produced by excimer laser at low intensities

A plasma, created at interaction of short-wavelength excimer laser radiation with flat targets was investigated (tlas = 12 ns, λlas = 0.308 μm, qlas = 4 – 8 × 1012 W/cm2) with the help of various x-ray spectroscopic methods. The comparison of shapes and intensities of some observable spectral lines of H-, He and Li-like ions of Na, Mg and Al with results of model calculations has allowed to determine space distributions of laser plasma parameters up to distances of 0.4 mm from the target surface. Comparison of obtained results with theoretical models of absorption of short-wavelength radiation in a plasma shows, that the absorption of short-wavelength laser radiation in a plasma (at considered values of laser flux density) is executed due to inverse bremsstrahlung process in the areas with Ne < Ne, crit..

Spectroscopic analysis of a recombining laser-produced aluminium plasma

The variation of Al XII resonance line intensities with the electron density has been measured in a recombining, laser-produced plasma. A detailed, quasi-steady-state model of the population kinetics for these ions is used to successfully describe the measured X-ray spectra. The collisional transfer of population between excited states must be included in detail to account for the observed line intensity dependence for late-time, recombining coronal plasmas.