Continuous Energy Distribution Research Articles

Link to the Physical Modelling with an Emphasis on Chemical Peculiarities

Frequently, the atmospheres of stars of spectral types B to F, on which the attention is particularly focussed in this Joint Discussion, are characterized by non-standard physical conditions: non-solar abundance patterns, surface in homogeneities, strong magnetic fields. The most extreme anomalies are found in the chemically peculiar (CP) stars but more moderate departures from standard atmospheres are observed in many stars in the considered temperature range. This can affect the derivation of fundamental stellar parameters in a number of ways, some of which will be presented in this contribution. The emphasis is set on those physical processes which are specific to CP stars. Due to lack of space, this review is necessarily incomplete: a selection of recent results of interest will be pointed out and some directions of investigation will be suggested.CP stars have an anomalous continuous energy distribution. Fitting this distribution by standard model atmospheres can only be achieved using models corresponding to different effective temperatures to represent different spectral ranges (e.g., Leone & Catalano 1991). This is obviously not physically meaningful: the effective temperature characterizes the total flux of a star, thus is unique. “Realistic” models must incorporate the anomalous elemental abundances. Such models are presently being developed by Kurucz (ATLAS12) and by the Vienna group (Weiss 1994, private communication). The latter is adapted from Muthsam’s (1979) code. One important result obtained by Muthsam is that the relation T(τ) appears steeper in magnetic Ap stars than in normal stars. This view recently received support from observations of rapid oscillations (Matthews et al. 1990). But even such realistic models may not represent satisfactorily CP star atmospheres, as some potentially important opacity sources may still be unknown. For instance, the broad continuum depressions observed in many magnetic CP stars have not been fully explained yet.

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.

The HST quasar absorption line key project. 4: HST faint-object spectrograph and ground-based observations of the unusual low-redshift broad absorption-line quasi-stellar object PG 0043+039

Hubble Space Telescope Faint Object Spectrograph (HST FOS) observations have shown that the spectrum of the low-redshift (z(sub em) approximately equal to 0.384) QSO PG 0043+039 exhibits weak broad absorption lines (BALs). The BALs were discovered during the course of UV spectrophotometry made for the HST Quasar Absorption Line Key Project. The HST data are analyzed along with ground-based optical and IUE spectrophotometry. The object is found to have a number of atypical properties relative to normal non-BAL QSOs. The observed continuum is atypical in the sense that it is much weaker than that of a normal optically selected QSO at rest wavelengths approximately less than 2200 A. Intrinsic reddening of E(B-V) approximately equal to 0.11 mag by dust similar to that found in the SMC at the redshift of PG 0043+039 conservatively accounts for the observed continuum shape moderately well. These observed characteristics are typical of low-ionization BAL QSOs, but convincing evidence for BALs due to low-ionization transitions of Mg II, Al III, Al II, or C II does not exist. Therefore, this object may be a misaligned BAL QSO having many of the characteristics of low-ionization BAL QSOs with the sight line passing through a putative dusty region, but evidently missing clouds of high enough column density to produce observable low-ionization BALs. If the intrinsic dust-extinction model is correct, the observations suggest that the dust is not confined to the presumably higher density, low-ionization BAL clouds, but that it has drifted to nearby high-ionization BAL regions. We also consider other possible mechanisms for producing the shape of the continuous energy distribution which cannot be ruled out. We compare the Fe II emission in PG 0043+039 with that in another Key Project QSO, NGC 2841-UB 3, which has optical Fe II emission comparable in strength to that in PG 0043+039, but has anomalously weak UV Fe II emission. In addition, from an analysis of UV and optical spectrophotometric data at 5 epochs over approximately 11 yr, there is tentative evidence that PG 0043+039 has varied in brightness by as much as 1.1 mag during this time interval. Two different interpretations of PG 0043+039 and the low-ionization BAL QSOs are considered. Various model scenarios for explaining the weak narrow-line (O III) emission are considered, but there is no definitive explanation.

Dynamics of the envelopes of Be stars and shell stars in the equatorial plane - A potential similarity and difference based upon the continuous energy distribution

Recently the continuous energy distribution from the infrared to the radio region has been obtained for six Be stars, three of which are Be stars and three of which are shell stars. Analysis of these continuous energy distributions by Waters et al. in terms of a disk model has yielded the density distribution and the radial component of velocity in the circumstellar material. In this paper we determine the additional force, F x (r), exclusive of those arising from gravity, rotation, and the gas pressure gradient, which is required in this disk model to produce the radial component of velocity for each of the stars studied. We find that F x (r) for the three Be stars and the three shell stars has the same behavior

Sorption and partial molar volume of small molecules in glassy polymers

The sorption of atoms (He, Ar) or molecules (N 2 , CO 2 , H 2 O, SO 2 , CH 4 , C 2 H 6 , C 3 H 8 ) in polymers is described by assuming a continuous distribution of dissolution energies. The energy distribution arises from a volume distribution of the intermolecular space yielding different elastic distortion energies when occupied by solute molecules. On the basis of statistical fluctuations of the free volume and Eshelby's elasticity theory, it will be shown that a gaussian distribution of sorption energies is appropriate for small molecules in glassy polymers. The width of the Gaussian distribution can be calculated and it agrees well with values obtained by fitting experimental sorption data

Passivation of shallow donors in GaAs by oxygen plasma exposure

Heavily doped ( N D ≈ 10 18 cm -3) GaAs substrates were exposed to an inductively coupled plasma in the temperature region 110–140°C. Formation of a thin GaAs plasma oxide layer is accompanied by a substantial reduction of the electrical activity of shallow Si donors, which finding could be supported by a series of capacitance versus bias measurements. As revealed by related deep-level transient spectroscopy (DLTS) data on Al-plasma oxide-GaAs diodes, the currently observed U-shaped continuous energy distribution of interface states is in fact composed of essentially two distinct branches. The occurence of either of the branches is controlled predominantly by the oxygen partial pressure. A true accumulation of electrons at forward biases is impeded by the highly resistive intermediate layer between the undisturbed GaAs substrate and the plasma oxide. A possible role of hydrogen present in the discharge is discussed with regard to the annealing kinetics of passivated donors. A stable GaAs isolation and an almost defect-free GaAs/oxide interface can be reached by selecting an optimum oxygen pressure.

The ultraviolet continuum and the fluorescent H2 lines in low-excitation Herbig-Haro objects

Long-exposure IUE observations are used here to determine the spatial distribution of the fluorescent H2 line emission in the low-excitation objects HH 43 and HH 47. In HH 43, the spatial distribution of the fluorescent H2 line emission has a width equal to or smaller than the width of the point-spread function of IUE, while both the short-wavelength UV continuum and the optical forbidden S II lines show distributions which are wider by more than a factor of two. In HH 47, the results are qualitatively analogous, but the differences between the fluorescent HH 23 emission distribution on the one hand, and the UV continuum and optical forbidden S II distribution on the other, are smaller. These distributions indicate that the appropriate conditions for the formation of fluorscent H2 lines are fulfilled only in rather narrow regions for these low-excitation objects. A continuous energy distribution in HH 43 and HH 47 is found which differs only slightly from that in the high-excitation objects HH 1 and HH 2.

Heavy-oil cracking: The case for nonhomogenous kinetics

The thermal cracking of heavy oils has been modelled on the basis of combinations of first-order reactions in order to describe the behaviour of pseudocomponents with time and temperature of reaction. An alternative formulation uses the concept of a time-varying rate constant of the form[Formula: see text]where β is the rate constant and γ is an exponent that has values of 0 < γ ≤ 1.0. This was used to derive a transform of temperature and time that we have called the reaction ordinate Rω, which enables the correlation of extensive kinetic data sets by means of three constants: β, γ, and the mean activation energy Ea. We demonstrate this correlation for two sets of published data. The values of γ were less than unity and are interpreted in terms of the Kohlrausch function, leading to the hypothesis of a continuous distribution of activation energies in the system. This form of correlation is similar to that of the continuous-time random-walk relaxation phenomena that has been explained in many ways including fractal time, and thus we propose that this is indirect evidence for the nonhomogenous nature of heavy-oil-cracking kinetics.

Electrical and acoustical relaxation in fast ionic conductors

The ultrasonic velocity, attenuation coefficient, electric conductivity and dielectric permittivity have been measured in wide temperature and frequency ranges in various crystalline, polycrystalline and glassy solid electrolytes. The high relaxational ultrasonic attenuation maxima and corresponding velocity dispersion have been observed. Such behaviour is supposed to be determined by acoustoionic interactions of two types: (i) the piezoelectric in the piezoelectric superionic; (ii) interaction caused by acoustic wave modulation of chemical potential felt by mobile ions in the centrosymmetric materials. In polycrystalline and glassy superionics the non-Debye behaviour for ultrasonic relaxation has been established. In such materials the characteristic frequency dependencies of ionic conductivity and dielectric permittivity were obtained. A simple theoretical model describing both frequency and temperature dependencies of electrical and acoustical properties of solid electrolytes is proposed. This model is based on continuous distribution of activation energies in a limited interval and describes the experimental data fairly well.

Effect of temperature on data retention of silicon-oxide-nitride-oxide-semiconductor nonvolatile memory transistors

The discharge behavior of silicon-oxide-nitride-oxide-semiconductor nonvolatile memory transistors is investigated for a range of programming and storage temperatures spanning −55 °C to 200 °C. A number of empirical observations strongly limit the nature of the mechanisms that govern charge injection and decay. Both electrons and holes contribute to the charge storage properties of the transistors, and the decay properties of both are thermally activated with a continuous distribution of activation energies (trap depths). Charge decay, for both charge states, is negligibly limited by mechanisms other than those which are strongly thermally activated. The programming temperature, relative to the storage temperature, significantly impacts the retention time of the excess electron state, while not affecting the long term decay of the excess hole state. The experimental results also have significant implications regarding proper retention screening techniques and nonvolatile ROM programming techniques.

Continuous distribution of αt relative kinetic energies in 7Li breakup reactions

A newly developed technique has been used to probe a previously inaccessible region of the continuum in the at channels of 7Li breakup reactions. This was used to investigate reactions of 63 MeV 7Li with 58Ni, 120Sn, 144Sm, and 208Pb targets. Breakup pairs of α-particles and tritons emitted within a finite opening angle were analyzed according to their magnetic rigidity using a K = 150 Enge split-pole spectrograph. The αt coincidences were taken with two single-wire proportional counters mounted at different positions along the focal plane of the spectrograph. The large energy acceptance (Emax/Emin ≈ 8) of the spectrograph is crucial to the present coincidence measurements involving two particles with different q/m values. Very small relative energy components were studied without any detection threshold. A pronounced bump was observed immediately above the αt particle threshold. No strong indication of the post Coulomb acceleration was observed. The excitation of the continuum states as well as the 7−2 state in 7Li is discussed in terms of the distorted-wave Born approximation. The astrophysical implications of the data are also discussed regarding the Coulomb excitation of the continuum. The present technique offers the promise of very high sensitivity at small relative energies if it were applied in conjunction with ray-tracing, a thin target, and a dispersion-matched beam.

Ionisation processes in collisions of open shell atoms. II. Oxygen and sulphur atom collisions with inert gases

For pt.I see ibid., vol.23, p.279 (1990). Results of an electron spectroscopy study of ionisation processes, in low keV collisions of oxygen and sulphur atoms with inert gases are presented. It is shown that ionisation close to threshold is dominantly due to population of singly excited autoionising states of the 2D(2P)nl series. At energies that are significantly above the threshold for excitation processes an additional ionisation channel which leads to a continuous electron energy distribution becomes important. A discussion in terms of the molecular orbital model is presented and indicates that quasimolecular Auger ionisation could be an efficient positive-ion production mechanism at high energies. A comparison with O- and S- collisions is conducted.

Ionisation processes in collisions of open-shell atoms. I. Fluorine atom collisions with inert gases and molecules

Results of an electron spectroscopy study of ionisation processes in collisions of F atoms with He, Ne, O2 and CO2 at low keV energies are presented. It is shown that in case of the He and Ne targets ionisation is partly due to population of singly-excited autoionising states of the 1D nl series and partly due to another mechanism which results in the production of a continuous electron energy distribution and becomes particularly important at high energies. A discussion in terms of the molecular orbital model is presented and indicates that quasimolecular Auger ionisation must be the other F+ production channel. In the case of molecular targets the authors observe a direct ionisation channel and also dissociative ionisation via the population of 2D nl autoionising states of fragment O atoms.

Thermoluminescence and phosphorescence with a continuous distribution of activation energies

Some theoretical features of phosphorescence decay curves and thermoluminescence (TL) glow curves have been investigated with some select depth distributions. In early studies a t -1 (inverse-time dependence) behaviour for isothermal decay was found assuming a uniform distribution of activation energies ranging from 0 to infinity. Additional studies explored the TL features resulting for Gaussian distributions of activation energies. For broad enough distributions the t -1 behavior for isothermal luminescent decay again followed. It thus appears that broad distributions will quite generally result in t -1 phosphorescence behavior. We have addressed the more mathematically tractable situation of a continuous uniform distribution within a finite range of energies, from E 1 to E 2. The circumstances under which the t -1 law can be accepted as a valid approximation, as well as the deviations from this law, are discussed using numerical calculations. Also, the expected features of TL glow curves under similar conditions for continuous trap distributions are discussed. In particular the effective values of the activation energies one can calculate using different known methods for its evaluation are studied. The relationship of these derived effective values to the distributions actually used in the calculations is discussed. The near- t -1 time dependence of phosphorescence resulting from a model of electrons and holes recombining through tunneling is contrasted to that for the standard first-order kinetics model with a broad excitation energy distribution also giving a near- t -1 dependence. The distinction appears in the curvature of yield curve with time during the onset of phosphorescence.

Computer simulations of photoionization in ionically bonded clusters

Photoionization spectra of clusters record a continuous distribution of ionization energies because each cluster has a different atomic arrangement at the moment of ionization. Using the technique of molecular dynamics we have generated photoionization spectra for (Cs2O)n clusters.

The ultraviolet spectrum of the binary system KS Persei (HD 30353)

Nine high- and low-resolution IEU spectra at different orbital phases during different cycles, of the hydrogen-poor single-lined spectroscopic binary KS per (HD 30353) have been studied. The near-UV spectrum is characterized by the presence of several lines of once ionized metals which show the same orbital RV shifts observed in the visual range. The MgII resonance lines present a sharp interstellar + circumstellar core and an emission peak at about 83 and 92 km s−1. The far UV spectrum is characterized by the presence of strong absorptions of the resonance lines ofNV,CIV, SiIV, SiIII. Direct determination of the IS extinction from the 2200 A feature givesE(B-V)=0.33. A comparison of corrected logarithmic continuous energy distribution with the theoretical models by Kurucz, indicates that the companion is a hot star,T e=15000 K; logg=4. A comparison with the recent UV work ofv Sgr, shows that, KS Per is very similar tov Sgr.

Shockwaves in Extended Near-Relativistic Jets

The origin of the sharp near infrared cutoff in the continuous energy distribution of many compact non-thermal sources (active nuclei or knots in jets) is considered under the assumption that particle acceleration takes place in shockwaves. Energy losses due to synchrotron emission and Compton interactions set upper limits to both electron and proton energies. In this case the upstream disturbance of the flow is dominated by the most energetic protons which are postulated, by analogy with the solar wind, to excite a turbulent wave spectrum of Kolmogorov type in this region. We predict for near relativistic flows a spectral cutoff near 3 1014 Hz independent of magnetic field. The observation of a sharp spectral cutoff near 3 1014 Hz is thus independent evidence for near–relativistic flows in jets.

Synchrotron emission from shock waves in active galactic nuclei

The origin of the sharp near infrared cutoff in the continuous energy distribution of many compact non-thermal sources (BL Lacs, OVVs, red quasars and certain jets) is considered under the assumption that particle acceleration takes place in shocks. This model predicts a highest frequency v* of electron synchrotron emission which depends principally on the shock velocity and the ratio a of photon to magnetic energy density in the acceleration region. For near relativistic flows and reasonable values of a a spectral cutoff is predicted in the range 3 1014 < v < 2 1015 Hz. The model thus leads to 1) near relativistic flows, 2) a gradual steepening of optical continuum spectra as one follows a jet outwards, 3) a correlation between spectral hardening and luminosity, 4) a correlation between optical spectrum and X-ray emission, 5) a possible synchrotron contribution to the X-ray emission in Quasars from secondary particles, and 6) the production of very high energy particles such as observed in cosmic rays, of up to ~1012 GeV.

Measurement of the grain-boundary states in semiconductors by deep-level transient spectroscopy.

The paper deals with standing issues regarding the application of deep-level transient spectroscopy (DLTS) to the measurement of the electron states at grain boundaries in semiconductors: (i) The relationship between the density of interface states and the associated DLTS spectra is worked out quantitatively for the case in which the levels form a continuous distribution in energy, leading to simple analytical expressions for the emission rate, the density of states, and the capture cross section at the quasi-Fermi level of the trapped carriers. This treatment will also apply, with minor modifications, to different physical systems where interface states are present, e.g., the oxide-semiconductor interfaces. The effect of field-enhanced emission on the DLTS spectra of the boundary states is also considered. (ii) As a practical illustration, the grain-boundary parameters are determined for the \ensuremath{\Sigma}=25 twin boundary in n- and p-type doped silicon bicrystals. In n-type silicon, a single boundary level is found at ${E}_{c}$-0.66 eV, with a density of 2.6\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. In p-type silicon, the boundary levels are continuously distributed in energy, with a density on the order of ${10}^{12}$ ${\mathrm{eV}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ in the energy range (${E}_{v}$+0.2 eV, ${E}_{v}$+0.6 eV). The density of states shows a sharp maximum at ${E}_{v}$+0.18 eV, associated with a single trap level, the density of which is on the order of 2\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. The capture cross sections of the traps are on the order of ${10}^{\mathrm{\ensuremath{-}}14}$ to a few ${10}^{\mathrm{\ensuremath{-}}13}$ ${\mathrm{cm}}^{2}$. These results are consistent with the data obtained from complementary capacitance-voltage and thermally-stimulated-capacitance experiments. The densities of states appear to be highly dependent on the thermal history of the specimens. Microanalytical investigations are currently in progress, aimed at clarifying the dependence of the electronic properties on impurity segregation at the grain boundaries in silicon.

Collision broadening of continuous electron spectra in energetic KrXe collisions

The continuous electron energy distribution from KrXe collision at incident energies between 75 and 1050 keV have been investigated. These are associated with transitions between the quasimolecular autoionizing state and the continuum. The present investigation is devoted to the dependence of collision broadening on the projectile velocity v. A velocity dependence proportional to v 2 3 is observed in agreement with theoretical predictions.