Photoionization of fine structure levels of Ne III

Detailed study on the characteristic features in photoionization of Ne III, (hν + Ne III → Ne VI + e) is reported. The calculations were carried out in relativistic Briet–Pauli R-matrix (BPRM) method and close-coupling (CC) approximation. The CC expansion for the wavefunction of Ne III includes 58 fine structure levels of Ne IV from configurations 2s²2p³, 2s2p⁴, 2p⁵, 2s²2p²3s, 2s²2p²3p, 2s²2p²3d. The photoionization cross section (σ_PI) features exhibit i) presence of prominent resonances in the low energy region near the ionization thresholds of low lying levels, ii) lower energy region of extensive narrow Rydberg resonances and slow decaying background cross section in the high energy region for equivalent electron levels, and iii) presence of strong Seaton resonances due to photo-excitation-of-core (PEC) in the high energy region. The relativistic effects have almost no impact on the σ_PI in the high energy regions. However, for a number of low lying levels the coupling of fine structure channels, which is not allowed in LS coupling, introduces high-peak narrow resonances with almost zero background below the excitation threshold of 2s²2p³(²D^o) level of the residual ion. These will enhance quantities such as electron-ion recombination, opacity at very low temperature. The Seaton resonances due to n=3 PECs in the high energy region, which was not studied before, are more prominent than those for n=2 PECs. For complete modeling of astrophysical and laboratory plasmas, photoionization cross sections are presented for a large number of excited levels, in total 392, with n ≤ 10 and l ≤ 9.     

Photoionization of ground and excited states of Ti I

Detailed photoionization of ground and many excited states with autoionizing resonances of neutral Ti are presented. Ti I with 22 electrons forms a large number of bound states, the present work finds a total of 908 bound states with n $⩽$ 10 and $l⩽8$. Photoionization cross sections (${\sigma }_{\mathit{PI}}$) for all these bound states have been obtained. Calculations were carried out in the close-coupling R-matrix method using a wave function expansion that included 36 states of core ion Ti II. It is found that the resonances enhance the low energy region of photoionization of the ground and low lying excited states. The resonant features will increase the opacity, as expected of astrophysical observation, and hence play important role in determination of abundances in the elements in the astronomical objects. The excited states also show prominent structures of Seaton or photo-excitation-of-core resonances.     

Photoionization features of the ground and excited levels of Cl II and benchmarking with experiment

This report presents i) various characteristic features in photoionization cross sections (σ_PI) of Cl II + hν  →  Cl III + e for many fine structure levels of Cl II, 392 in total with n ≤ 10 and l ≤ 9, ii) comparison of features with those observed in an experiment carried out at the Advanced Light Source at Lawrence Berkeley National Lab, and iii) partial photoionization cross sections of the ground level for ionization leaving the core ion in to various excited levels for applications in plasma modeling. The features correspond to resonant structures, the shape of the background, and their interference effect in σ_PI of this near neutral ion Cl II with 16 electrons. σ_PI for the 5 levels of the ground configuration 3s²3p⁴(³P_0,1,2, ¹D₂, ¹S₀) of Cl II show regions of narrow Rydberg resonances at and near threshold energies, and resonant structures at higher energies in contrast to typical smooth decrease in the background. Various other features in σ_PI of levels of excited equivalent electron states and broad Seaton resonances in single valence electron excited levels are illustrated with examples. Comparison of calculated σ_PI of the 15 lowest levels with the combined features of the measured photoionization spectrum shows excellent agreement by reproducing and thus identifying them to the levels that they belong to. The calculations were carried out in relativistic Breit-Pauli R-matrix (BPRM) method using a close coupling wave function expansion of 45 levels up to 4s of the core ion Cl III. These levels were optimized using a set of 12 configurations going up to orbital 5s, 3s²3p³, 3s3p⁴, 3p⁵, 3s²3p²3d, 3s²3p²4s, 3s²3p²4p, 3s²3p²4d, 3s²3p²4f, 3s²3p²5s, 3s3p³3d, 3s²3p3d², 3p⁴3d producing 283 levels of Cl III. The autoionizing resonances are delineated with a fine energy mesh to observe the fine structure effects. The present results will provide high precision parameters for various applications involving this less studied ion.     

Continuous absorption and depression in the solar spectrum at wavelengths from 650 to 820 nm

Results of calculations of the cross-sections of the basic processes forming continuous absorption in the photospheres of solar-type stars in the visible and infrared spectral ranges are reported. (These processes are photoionization of H^– ions and excited hydrogen atoms, as well as absorption of photons by “free” electrons being in the partially ionized plasma of the photosphere.) The effective cross-section of hydrogen satisfying the observational data or the results of laboratory experiments was introduced, and its nonmonotonic behavior caused by photoionization of excited hydrogen atoms was ascertained in the spectral range of λ from 650 to 820 nm. For a plane-parallel model of the Sun, the continuous absorption coefficient κ _c (λ|z) was calculated as a function of the wavelength and coordinate. Its spectral features caused by the effective cross-section structure in the above-mentioned spectral range were for the first time analyzed. The spectral dependence of the radiation intensity in the solar disk center in the continuous spectral range of λ from 600 to 900 nm was studied. The calculation results were compared to the currently available data of observations. It has been shown that the deviation of the observed radiation intensity from the Planck distribution (i.e., the depression) is caused by the processes of photoionization of the excited hydrogen atoms in the states with a principal quantum number n = 3. In the range of λ from 650 to 820 nm, the mean relative deviation is approximately 4%. It has been established that the magnitude of the depression effect significantly depends on the effective temperature of the photosphere of a solar-type star.     

Photoionization Cross-Section of Chlorine-like Iron

Semi-relativistic calculations are performed for the photoionization of Fe X (an important coronal ion) from its ground state 3s²3p $^{5}(^{2}P^{0}_{3/2})$ and the first two excited states 3s²3p $^{5}(^{2}P^{0}_{1/2})$ and 3s3p $^{6}(^{2}S_{1/2})$ using the Breit?CPauli R-matrix method. A lowest 41 state eigenfunction expansion for Fe XI is employed to ensure an extensive treatment of auto ionizing resonances that affect the effective cross-sections. In the present calculations, we have considered all the important physical effects like channel coupling, exchange and short range correlation. The present calculations using the lowest 41 target levels of Fe XI in the LSJ coupling scheme are reported and we expect that the present results should enable more accurate modelling of the emission spectrum of highly excited plasma from the optical to the far UV region.     

Evidence for excited states of CO3−1 and NO3−1 from collisional dissociation processes

Excitation functions for collision-induced dissociation reactions of CO ₃^? and NO₃^? to give O^? and the corresponding neutral species have been studied using an in-line tandem mass spectrometer. When these ions were prepared from certain gaseous mixtures, larger cross-sections and lower thresholds were observed for the dissociation processes than those found for the same ions in their apparent ground states. These observations suggest the existence of long-lived excited states of CO₃^?1 and NO₃^?1. The heats of formation of these excited ionic states were determined to be ?4.8 ± 0.1 and ?0.3 ± 0.2 eV for CO₃^?1 and NO₃^?1, respectively. Possible implications of these findings with respect to the D -region negative ion reaction scheme are discussed.     

Energy transfer from excited N2 and O2 as a source of O(1S) in the Aurora

It is proposed that energy transfer from excited O₂ contributes to the production of O(¹S) in aurora. An analysis is presented of the OI5577 Å emission in an IBC II⁺ aurora between 90 and 130 km. The volume emission rate of the emission at these altitudes is consistent with the production rate of O(¹S) by energy transfer to O(³P) from N₂ in the A³Σ₂⁺ state and O₂ in the A³Σ_u⁺, C³Δc¹Σ_u^? states, the N₂A state being populated by direct electron impact excitation and B → A cascade and the excited O₂ states by direct excitation. Above the peak emission altitude (~105 km), energy transfer from N₂A is the predominant production mechanism for O(¹S). Below it, the contribution from quenching of the O₂ states becomes significant.     

Drift instabilities associated with the particles in ring current and inner edge of plasma sheet

Electromagnetic waves propagating transverse to the magnetic field, containing inhomogenous and loss cone plasma, may become unstable due to the excitation of resonant proton, resonant electron and drift cyclotron instabilities. Resonant proton instability gets excited in inhomogenous plasma, irrespective of the presence of temperature anisotropy, loss cone or temperature gradient. However, the growth rate of this instability is much smaller than the other two instabilities. The maximum growth rates of resonant electron instability are enhanced with the increase of loss cone index, gradients in transverse temperature and magnetic field, and with the decrease of temperature anisotropy and gradients in density and parallel temperature. The drift cyclotron instability exists in a bounded range of wave numbers and its growth rate increases with the increase of electron temperature, density and magnetic field gradient, and with the decrease of proton temperature and temperature anisotropy. In the region of ring current for beyond plasmapause the resonant proton and resonant electron instabilities have the characterstic frequencies around 0.1Ω_p and growth rates ~10^?6Ω_p and 10^?3Ω_p, respectively. In the ring current region the drift cyclotron instability is not excited whereas in the plasma sheet region the frequency and growth rate of this instability are around Ω_p and 10^?2Ω_p, respectively. These instabilities can accelerate the ring current particles along the magnetic field lines and dump them into the auroral region.     

Observations of solar wind stream with high abundance of heavy ions and relation with coronal conditions

Long intervals, during which heavy ions were detected in the high energy tail of the energy spectra of solar wind ions, were recorded by the plasma spectrometer SCS onboard the Prognoz-7 satellite. In particular, such a region with unusual features—low velocity, high density, low temperature of protons and, especially, low temperature of α-particles—was observed during 10–13 December 1978. The time dependence of these parameters makes it possible to recognize this event as “noncompressive density enhancement”. In this region heavy ions such as O⁺⁶, O⁺⁷, Si⁺⁷, Si⁺⁸, Si⁺⁹ and a group of iron from Fe⁺⁶ to Fe⁺¹³ were identified by the electrostatic analyzer.The abundance of these ions relative to protons was about ten times higher than had previously been observed. The coronal temperature, estimated from the ratios of the ion fluxes with different ionization states, is higher than that estimated earlier for the oxygen ions.     

Electric quadrupole transitions in the beryllium isoelectronic sequence

Electric quadrupole transition probabilities for transitions between the 2s ², 2s2p, and 2p ² states in the beryllium isoelectronic sequence are evaluated using configuration interaction wavefunctions for 6≤Z≤26. It is found that the contribution from the electric quadrupole transitions is negligible and will not be an important process in any laboratory or astrophysical process.     

The excited spin state of Comet 2P/Encke

Ways to rationalize the different periods (e.g., 15.08 h, Luu and Jewitt, 1990, Icarus 86, 69-81; 11.01 h, Fernández et al., 2004, Icarus, in this issue; Lowry et al., 2003, Lunar Planet. Sci. XXXIV, Abstract 2056) seen in near aphelion R-band light curves of Comet 2P/Encke are explored. We show that the comet is usually active at aphelion and it's observed light curves contain signal from both the nucleus and an unresolved coma. The coma contribution to the observed brightness is generally found to dominate with the nucleus providing from 28 to 87% of the total brightness. The amplitude of the observed variations cannot be explained by the nucleus alone and are due to coma activity. We show that some seven periodicities exist in the observed light curves at various times and that this is likely the result of an active nucleus spinning in an excited spin state. The changing periodicities are probably due to changes in the relative strengths of the active areas. We work out possible excited states based on experience with model light curves and by using an analogy to light curve observations of Comet 1P/Halley for which the spin state has been separately determined from spacecraft observations. There is a possibility of a fully relaxed principal axis spin state (0.538 d⁻¹; P=44.6 h) but, because it provides a poorer fit to the observed periodicities than the best fit excited state together with the absence of a peak near 1.08 d⁻¹ (2fφ) in the frequency spectrum of the Fernández et al. (2000, Icarus 147, 145-160) thermal IR lightcurve, we consider it unlikely. Both SAM and LAM excited states are allowed by the underlying periodicities and additional information is needed to choose between these. Our choice of a low excitation SAM state, i.e., one in which the instantaneous spin axis nutates around the total angular momentum vector in a motion that is characterized by limited angular oscillations around the long axis, is based on Sekanina's (1988, Astron J. 95, 911-924, 1988, Astron. J. 96, 1455-1475) interpretation of the fan coma that this comet often displays. We argue that possible LAM states are excluded either because they are too difficult to excite or because they would be inconsistent with the formation of the observed fan morphology. Two possible SAM states emerge that provide good fits to the observed periodicities, one with a precessional frequency for the long axis about the total angular momentum vector of 1.614 d⁻¹ (P?=14.9 h) and an oscillation frequency around the long axis of 0.539 d⁻¹ (Pψ=44.5 h) and a second with a precessional frequency of 2.162 d⁻¹ (P?=11.1 h) combined with an oscillation around the long axis of 0.502 d⁻¹ (Pψ=47.8 h). While either solution is possible, the latter is, in a least squares sense, more likely to be the actual spin state. In both cases the direction of the total angular momentum vector (αM,δM[J2000]=198.6, −0.3 deg) is assumed to be defined by the evolving geometry and morphology of the coma (Sekanina, 1988, Astron J. 95, 911-924, 1988, Astron. J. 96, 1455-1475; Festou and Barale, 2000, Astron J. 119, 3119-3132). We discuss the possible locations of the primary active areas found by Sekanina (1988, Astron J. 95, 911-924, 1988, Astron. J. 96, 1455-1475) and, while they are at high cometographic latitudes, they do not have to be physically located close the region were the axis of maximum moment of inertia pierces the surface (i.e., at high cometocentric latitude). We offer a new interpretation of the 10.7 μm data by Fernández et al. (2000, Icarus 147, 145-160) which yields an axial ratio a/b=2.04. This, with the two SAM states that we have found, requires that b/c>1.18 or >1.09 implying a significant asymmetry in the shape of the elongated nucleus. For the observed fan morphology to be maintained, the true axial ratio b/c cannot be much larger than these limiting values otherwise the amplitude of the oscillation about the long axis becomes too large and the fan morphology would be destroyed. The precise phasing of the spin modes, i.e., the value of the Euler angles at a particular time, is not determinable from the current data set, but a set of well sampled thermal infrared observations of the nucleus covering many periods and a wide range of observing geometries could provide this information in the future as well as clearly distinguishing between the two excited spin states.     

The nuclear structure and associated electron capture rates on odd-Z nucleus 51V in stellar matter

The Gamow-Teller strength distribution function, B(GT), for the odd Z parent ⁵¹V, N?Z=5, up to 30 MeV of excitation energy in the daughter ⁵¹Ti is calculated in the domain of proton-neutron Quasiparticle Random Phase Approximation (pn-QRPA) theory. The pn-QRPA results are compared against other theoretical calculations, (n, p) and high resolution (d, ²He) reaction experiments. For the case of (d, ²He) reaction the calibration was performed for 0≤E _j ≤5.0 MeV, where the authors stressed that within this excitation energy range the ΔL=0 transition strength can be extracted with high accuracy for ⁵¹V. Within this energy range the current pn-QRPA total B(GT) strength 0.79 is in good agreement with the (d, ²He) experiment’s total strength of 0.9±0.1. The pn-QRPA calculated Gamow-Teller centroid at 4.20 MeV in daughter ⁵¹Ti is also in good agreement with high resolution (d, ²He) experiment which placed the Gamow-Teller centroid at 4.1±0.4 MeV in daughter ⁵¹Ti. The low energy detailed Gamow-Teller structure and Gamow-Teller centroid play a sumptuous role in associated weak decay rates and consequently affect the stellar dynamics. The stellar weak rates are sensitive to the location and structure of these low-lying states in daughter ⁵¹Ti. The calculated electron capture rates on ⁵¹V in stellar matter are also in good agreement with the large scale shell model rates.     

d−α Radiative capture process by effective field theory

Two of the main components of nuclear fusion processes which occur in stars are the radiative capture of deuteron-alpha and its inverse reaction during which helium transforms into larger elements. One of the reactions occuring is the production of ⁶Li through the d(α, γ)⁶Li reaction that is supported by the Big Bang model. The process of radiation in the energy region 0.07 MeV ≤ E_cm ≤ 0.41 MeV has been studied in the framework of Effective Field Theory (EFT) at low energy. Further, within the EFT framework deuteron is assumed as a dibaryon and the coulomb effect between deuteron and alpha is ignored. The reaction amplitude for the P-wave initial states ³P₂, ³P₀ and ²P_3/2 and the astrophysical S-factor for sum of E₁ and E₂ transitions are found in the framework of EFT up to NLO. The results of this model for the astrophysical S-factor at low energies are comparable with the available experimental data and those of other theoretical models.     

Aeronomy of extra-solar giant planets at small orbital distances

One-dimensional aeronomical calculations of the atmospheric structure of extra-solar giant planets in orbits with semi-major axes from 0.01 to 0.1 AU show that the thermospheres are heated to over 10,000 K by the EUV flux from the central star. The high temperatures cause the atmosphere to escape rapidly, implying that the upper thermosphere is cooled primarily by adiabatic expansion. The lower thermosphere is cooled primarily by radiative emissions from H⁺₃, created by photoionization of H₂ and subsequent ion chemistry. Thermal decomposition of H₂ causes an abrupt change in the composition, from molecular to atomic, near the base of the thermosphere. The composition of the upper thermosphere is determined by the balance between photoionization, advection, and H⁺ recombination. Molecular diffusion and thermal conduction are of minor importance, in part because of large atmospheric scale heights. The energy-limited atmospheric escape rate is approximately proportional to the stellar EUV flux. Although escape rates are large, the atmospheres are stable over time scales of billions of years.     

Non-adiabatic interactions in excitedC 2 H molecules and their relationship toC 2 formation in comets

A unified picture of the photodissociation of theC ₂ H radical has been developed using the results from the latest experimental and theoretical work. This picture shows that a variety of electronic states ofC ₂ are formed during the photodissociation of theC ₂ H radical even if photoexcitation accesses only one excited state. This is because the excited states have many avoided corssings and near intersections where two electronic states come very close to one another. At these avoided crossings and near intersections, the excited radical can hop from one electronic state to another and access new final electronic states of theC ₂ radical. The complexity of the excited state surfaces also explains the bimodal rotational distributions that are observed in all of the electronic states studied. The excited states that dissociate through a direct path are limited by dynamics to produceC ₂ fragments with a modest amount of rotational energy, whereas those that dissociate by a more complex path have a greater chance to access all of phase space and produce fragments with higher rotational excitation. Finally, the theoretical transition moments and potential energy curves have been used to provide a better estimate of the photochemical lifetimes in comets of the different excited states of theC ₂ H radical. The photochemically active states are the 2²₊, 2²II, 3²II, and 3²₊, with photodissociation rate constants of 1.0×10^–6, 4.0×10^–6, 0.7×10^–6, and 1.3×10^–6s^–1, respectively. These rate constants lead to a total photochemical lifetime of 1.4×10⁵ s.     

Absorption features in the quasar HS 1603 + 3820 II. Distance to the absorber obtained from photoionisation modelling

We present the photoionisation modelling of the intrinsic absorber in the bright quasar HS 1603 + 3820. We constructed the broad-band spectral energy distribution using the optical/UV/X-ray observations from different instruments as inputs for the photoionisation calculations. The spectra from the Keck telescope show extremely high Civ to Hi ratios, for the first absorber in system A, named A1. This value, together with high column density of Civ ion, place strong constraints on the photoionisation model. We used two photoionisation codes to derive the hydrogen number density at the cloud illuminated surface. By estimating bolometric luminosity of HS 1603 + 3820 using the typical formula for quasars, we calculated the distance to A1. We could find one photoionization solution, by assuming either a constant density cloud (which was modelled using cloudy), or a stratified cloud (which was modelled using titan), as well as the solar abundances. This model explained both the ionic column density of Civ and the high Civ to Hi ratio. The location of A1 is 0.1 pc, and it is situated even closer to the nucleus than the possible location of the Broad Line Region in this object. The upper limit of the distance is sensitive to the adopted covering factor and the carbon abundance. Photoionisation modelling always prefers dense clouds with the number density n₀ = 10¹⁰ − 10¹² cm⁻³, which explains intrinsic absorption in HS 1603 + 3820. This number density is of the same order as that in the disk atmosphere at the implied distance of A1. Therefore, our results show that the disk wind that escapes from the outermost accretion disk atmosphere can build up dense absorber in quasars.     

The electron density in the direction of ζ Oph

It is shown that photoionization of vibrationally excited H₂ and photodissociation of the H ₂ ⁺ ions produced thereby constitute a significant electron production route in high UV flux situations. A significant fraction of the electron density in the direction of ζ Oph (?15 km s^?1 cloud) deduced from observations may be expected to arise in this way.     

Backscatter of solar resonance radiation—I

A simple model which essentially neglects temperature effects, but which includes gravity, radiation pressure, photoionization and charge exchange, is used to calculate the angular dependence of the intensity of solar Lyman α resonantly scattered from neutral interstellar hydrogen which has penetrated the solar system; the results are then compared with the observations. The resonant scattering of HeI λ584 is also treated.