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We briefly discuss the history of pumping schemes for the common 22-GHz maser transition in o-H2O, and note that radiative mechanisms have been considered in the past, as well as the better known collisional pumping mechanisms associated with shock waves. Substantial IR irradiation is indeed destructive to inversion at 22-GHz under a wide range of physical conditions. We identify a small number of transitions, including 22 GHz, that can be pumped by both collisional and radiative schemes, and, through radiative transfer modelling over a grid of values in kinetic temperature, number density, and dust temperature, identify loci in density-kinetic temperature space where these schemes lead to strong inversions. The radiative pumping scheme generally operates at higher dust temperature, and lower kinetic temperature, than the collisional scheme. We identify a small network of transitions that form a radiative pumping scheme for the 22-GHz maser, involving radiative transitions of wavelength approximately 6  |$\mu$| m. This network is capable of supplying more than 50 per cent of the 22-GHz inversion under typical radiative pumping conditions, and it is consistent with the need for high dust and low kinetic temperatures determined from the modelling. We identify a probable case of radiative pumping in a massive star-forming region from the observation of a positive correlation between the flux densities of 22-GHz H2O masers and 6.7-GHz methanol masers. We discuss possibilities for finding radiatively pumped H2O maser lines in other source types. [ABSTRACT FROM AUTHOR]

The population of the 2 s -level of hydrogen formed in the early Universe is studied within a standard and a non-standard quasi-molecular mechanism of recombination. We demonstrate that, if hydrogen molecular ion |$H_2^+$| temporarily formed during a collision directly dissociates into a hydrogen atom and a proton, both mechanisms of recombination lead to the same probabilities of population. These probabilities differ significantly, however, when attractive quasi-molecular channels are uninvolved and only repulsive channels remain in the treatment. The probabilities of the direct and indirect radiative transitions into the 2 s -level of hydrogen are evaluated. The dominant process in the population of 2 s -level of hydrogen is the direct free–bound radiative transition. [ABSTRACT FROM AUTHOR]

In our recent papers, a non-standard quasi-molecular mechanism was suggested and applied to treat the cosmological recombination. It was assumed that, in the pre-recombination stage of evolution of the Universe, an electron combined with two neighbouring protons and created a hydrogen molecular ion, |$\mathrm{ H}_2^+$| in a highly excited state, which then descended into the lower lying state or dissociated. In this work, we implement a quantitative analysis of this quasi-molecular mechanism of recombination; namely, we elaborate the scheme of calculation for a free–bound radiative transition. We show that the quasi-molecular mechanism played a significant role in the pre-recombination and recombination stages of evolution of the early Universe, and hence must be included into the consideration of the description of a thermal history of the Universe. Together with the earlier developed treatment of bound–bound radiative transitions in |$\mathrm{ H}_2^+$|⁠ , the elaborated scheme of calculation can be used for the design of a rapid and complete cosmological recombination code. [ABSTRACT FROM AUTHOR]

We present a new method aimed at improving the efficiency of component by component ionization modelling of intervening quasar absorption-line systems. We carry out cloud-by-cloud, multiphase modelling making use of cloudy and Bayesian methods to extract physical properties from an ensemble of absorption profiles. Here, as a demonstration of method, we focus on four weak, low-ionization absorbers at low redshift, because they are multiphase but relatively simple to constrain. We place errors on the inferred metallicities and ionization parameters for individual clouds, and show that the values differ from component to component across the absorption profile. Our method requires user input on the number of phases and relies on an optimized transition for each phase, one observed with high resolution and signal-to-noise ratio. The measured Doppler parameter of the optimized transition provides a constraint on the Doppler parameter of H  i , thus providing leverage in metallicity measurements even when hydrogen lines are saturated. We present several tests of our methodology, demonstrating that we can recover the input parameters from simulated profiles. We also consider how our model results are affected by which radiative transitions are covered by observations (for example, how many H  i transitions) and by uncertainties in the b parameters of optimized transitions. We discuss the successes and limitations of the method, and consider its potential for large statistical studies. This improved methodology will help to establish direct connections between the diverse properties derived from characterizing the absorbers and the multiple physical processes at play in the circumgalactic medium. [ABSTRACT FROM AUTHOR]

In the framework of a quasi-molecular approach, the formation of hydrogen atom in the pre-recombination period of evolution of the Universe is analysed quantitatively. Calculations in an adiabatic multilevel representation enable estimates of probabilities of radiative transitions. The quasi-molecular mechanism of recombination allows the formation of hydrogen molecular ion, |${\mathrm{ H}_2}^+$|⁠ , in its ground state. The probability of this process is comparable with the probability of the creation of atomic hydrogen. The participation of a second proton in the recombination increases the binding energy of an electron and decreases the rate of recombination of hydrogen. [ABSTRACT FROM AUTHOR]

In this work, we report a list of 91 strong Th  ii spectral lines in the visible wavelength region that could be used as cosmochronometers in future investigations to determine stellar ages from thorium radioactive decay. This list was established on the basis of a pseudo-relativistic Hartree–Fock model including core-polarization corrections (HFR+CPOL) allowing us to compute the corresponding radiative decay parameters, the latter being found to be in good overall agreement with the most accurate experimental data when available. Besides this semi-empirical approach, the fully relativistic ab initio multiconfiguration Dirac–Hartree–Fock (MCDHF) method was attempted, showing large discrepancies with both the present HFR+CPOL and the most recent experimental oscillator strengths, although the calculated wavelengths from theoretical energy levels agreed with observed spectral line wavelengths within 2.4  per cent. [ABSTRACT FROM AUTHOR]

The inner parsec of our Galaxy contains tens of Wolf–Rayet stars whose powerful outflows are constantly interacting while filling the region with hot, diffuse plasma. Theoretical models have shown that, in some cases, the collision of stellar winds can generate cold, dense material in the form of clumps. However, their formation process and properties are not well understood yet. In this work, we present, for the first time, a statistical study of the clump formation process in unstable wind collisions. We study systems with dense outflows (⁠|${\sim }10^{-5}\rm \ M_{\odot }\ yr^{-1}$|⁠), wind speeds of 500– |$1500\rm \ km\ s^{-1}$|⁠ , and stellar separations of ∼20– |$200\rm \ au$|⁠. We develop three-dimensional high-resolution hydrodynamical simulations of stellar wind collisions with the adaptive-mesh refinement grid-based code ramses. We aim at characterizing the initial properties of clumps that form through hydrodynamic instabilities, mostly via the non-linear thin-shell instability (NTSI). Our results confirm that more massive clumps are formed in systems whose winds are close to the transition between the radiative and adiabatic regimes. Increasing either the wind speed or the degree of asymmetry increases the dispersion of the clump mass and ejection speed distributions. Nevertheless, the most massive clumps are very light (∼10−3– |$10^{-2}\rm \ M_{\oplus }$|⁠), about three orders of magnitude less massive than theoretical upper limits. Applying these results to the Galactic Centre, we find that clumps formed through the NTSI should not be heavy enough either to affect the thermodynamic state of the region or to survive for long enough to fall on to the central supermassive black hole. [ABSTRACT FROM AUTHOR]

We consider the structure of mildly relativistic shocks in dense media, taking into account the radiation and pair loading, and diffusive radiation energy transfer within the flow. For increasing shock velocity (increasing post-shock temperature), the first important effect is the efficient energy redistribution by radiation within the shock that leads to the appearance of an isothermal jump, whereby the flow reaches the final state through a discontinuous isothermal transition. The isothermal jump, on scales much smaller than the photon diffusion length, consists of a weak shock and a quick relaxation to the isothermal conditions. Highly radiation-dominated shocks do not form isothermal jump. Pair production can mildly increase the overall shock compression ratio to ≈10 (4 for matter-dominated shocks and 7 of the radiation-dominated shocks). [ABSTRACT FROM AUTHOR]

We present simultaneous optical and near-infrared (IR) photometry of the millisecond pulsar PSR J1023+0038 during its low-mass X-ray binary phase. The r'- and Ks-band light curves show rectangular, flat-bottomed dips, similar to the X-ray mode-switching (active-passive state transitions) behaviour observed previously. The cross-correlation function (CCF) of the optical and near-IR data reveals a strong, broad negative anticorrelation at negative lags, a broad positive correlation at positive lags, with a strong, positive narrow correlation superimposed. The shape of the CCF resembles the CCF of black hole X-ray binaries but the time-scales are different. The features can be explained by reprocessing and a hot accretion flow close to the neutron star's magnetospheric radius. The optical emission is dominated by the reprocessed component, whereas the near-IR emission contains the emission from plasmoids in the hot accretion flow and a reprocessed component. The rapid active-passive state transition occurs when the hot accretion flowmaterial is channelled on to the neutron star and is expelled from its magnetosphere. During the transition the optical reprocessing component decreases resulting in the removal of a blue spectral component. The accretion of clumpy material through the magnetic barrier of the neutron star produces the observed near-IR/optical CCF and variability. The dip at negative lags corresponds to the suppression of the near-IR synchrotron component in the hot flow, whereas the broad positive correlation at positive lags is driven by the increased synchrotron emission of the outflowing plasmoids. The narrow peak in the CCF is due to the delayed reprocessed component, enhanced by the increased X-ray emission. [ABSTRACT FROM AUTHOR]

The rate coefficient for the formation of aluminium monochloride, AlCl, from the radiative association of aluminium and chlorine atoms is estimated as a function of temperature. The coupling of the Al and Cl atoms through the A1Π molecular electronic state, which undergoes radiative transition to the X1Σ+ ground state, is the most efficient transition to form AlCl. The rate constant was found to vary with temperature according to the expressions k(T) = 1.22 × 10-16(T/300)0.40exp (-748/T) cm3 s-1 for temperatures between 300 and 1000 K, and k(T) = 2.20 × 10-16(T/300)0.175exp (-1067/T) cm3 s-1 for temperatures between 1000 and 14 000 K. [ABSTRACT FROM AUTHOR]

The radiative lifetime measurements by the time-resolved laser-induced fluorescence technique are reported for 24 levels of Co I with the energy range of 283 45.86-55 922.3 cm-1, amongst which the lifetimes of 20 levels are reported for the first time. The branching fraction measurements by the emission spectrum of a hollow cathode lamp were performed for 11 levels of them together with other two levels reported in the literature, and branching fractions of 39 transitions were obtained. By combining them with lifetime values, the transition probabilities and absolute oscillator strengths of these lines were determined. [ABSTRACT FROM AUTHOR]

The determination of radiative transition probabilities or oscillator strengths is of common interest in astrophysics. The analysis of the high-resolution stellar spectra is now available in order to estimate the stellar abundances. In this paper, 93 experimentally obtained transition probability values (Aki) for singly ionized krypton spectral lines belonging to the ultraviolet (UV) wavelength region (208-360) nm are presented. These data, expressed in absolute units, were derived from the measurements of relative spectral line intensities and the values of transition probability data taken from the literature. The results obtained extend considerably the transition probability data base. As a light source, a plasma from a low-pressure pulsed arc was used. Its electron density was in the range of (1.5-3.4) × 1022 m-3, while the temperature was between 28 000 and 35 000 K. A detailed analysis of the results is also given. Only a few relative and a few absolute transition probabilities from other authors, for the mentioned spectral region, are available in the literature. [ABSTRACT FROM AUTHOR]

Iron emission lines at 6.4-6.97 keV, identified with Ka radiative transitions, are among the strongest discrete features in the X-ray band. These are one of the most powerful probes to infer the properties of the plasma in the innermost part of the accretion disc around a compact object. In this paper, we present a recent Suzaku observation, 100-ks effective exposure, of the atoll source and X-ray burster 4U 1705-44, where we clearly detect signatures of a reflection component which is distorted by the high-velocity motion in the accretion disc. The reflection component consists of a broad iron line at about 6.4 keV and a Compton bump at high X-ray energies, around 20 keV. All these features are consistently fitted with a reflection model, and we find that in the hard state the smearing parameters are remarkably similar to those found in a previous XMM-Newton observation performed in the soft state. In particular, we find that the inner disc radius is Rin = 17 ± 5Rg (where Rg is the gravitational radius, GM/c²), the emissivity dependence from the disc radius is r-2.5 ± 0.5, the inclination angle with respect to the line of sight is i = 43° ± 5°, and the outer radius of the emitting region in the disc is Rout > 200Rg. We note that the accretion disc does not appear to be truncated at large radii, although the source is in a hard state at ~3 percent of the Eddington luminosity for a neutron star. We also find evidence of a broad emission line at low energies, at 3.03 ± 0.03 keV, compatible with emission from mildly ionized argon (Ar XVI-XVII). Argon transitions are not included in the self-consistent reflection models that we used and we therefore added an extra component to our model to fit this feature. The low-energy line appears compatible with being smeared by the same inner disc parameters found for the reflection component. [ABSTRACT FROM AUTHOR]

We present the results of a large-scale survey of the very dense (n > 106 cm−3) gas in the Perseus molecular cloud using HCO+ and HCN (J = 4 → 3) transitions. We have used this emission to trace the structure and kinematics of gas found in pre- and protostellar cores, as well as in outflows. We compare the HCO+/HCN data, highlighting regions where there is a marked discrepancy in the spectra of the two emission lines. We use the HCO+ to identify positively protostellar outflows and their driving sources, and present a statistical analysis of the outflow properties that we derive from this tracer. We find that the relations we calculate between the HCO+ outflow driving force and the Menv and Lbol of the driving source are comparable to those obtained from similar outflow analyses using 12CO, indicating that the two molecules give reliable estimates of outflow properties. We also compare the HCO+ and the HCN in the outflows, and find that the HCN traces only the most energetic outflows, the majority of which are driven by young Class 0 sources. We analyse the abundances of HCN and HCO+ in the particular case of the IRAS 2A outflows, and find that the HCN is much more enhanced than the HCO+ in the outflow lobes. We suggest that this is indicative of shock enhancement of HCN along the length of the outflow; this process is not so evident for HCO+, which is largely confined to the outflow base. [ABSTRACT FROM AUTHOR]

ABSTRACT We combine the galaxy formation model galform with the photon-dominated region code ucl−pdr to study the emission from the rotational transitions of 12CO (CO) in galaxies from z = 0 to z = 6 in the Λcold dark matter framework. galform is used to predict the molecular (H2) and atomic hydrogen (H i) gas contents of galaxies using the pressure-based empirical star formation relation of Blitz & Rosolowsky. From the predicted H2 mass and the conditions in the interstellar medium, we estimate the CO emission in the rotational transitions 1-0 to 10-9 by applying the ucl−pdr model to each galaxy. We find that deviations from the Milky Way CO-H2 conversion factor come mainly from variations in metallicity, and in the average gas and star formation rate surface densities. In the local universe, the model predicts a CO(1-0) luminosity function (LF), CO-to-total infrared (IR) luminosity ratios for multiple CO lines and a CO spectral line energy distribution (SLED) which are in good agreement with observations of luminous and ultra-luminous IR galaxies. At high redshifts, the predicted CO SLED of the brightest IR galaxies reproduces the shape and normalization of the observed CO SLED. The model predicts little evolution in the CO-to-IR luminosity ratio for different CO transitions, in good agreement with observations up to z ≈ 5. We use this new hybrid model to explore the potential of using colour-selected samples of high-redshift star-forming galaxies to characterize the evolution of the cold gas mass in galaxies through observations with the Atacama Large Millimetre Array. [ABSTRACT FROM AUTHOR]

ABSTRACT We present optical photometry and spectroscopy of the peculiar Type IIn/Ibn supernova (SN) 2011hw. Its optical light curve exhibits a slower decline rate than that of normal SNe Ibc, with a peak absolute magnitude of −19.5 (unfiltered) and a secondary rise 20-30 d later of −18.3 mag ( R). Spectra of SN 2011hw are highly unusual compared to those of normal SN types, most closely resembling the spectra of SNe Ibn. We centre our analysis on comparing SN 2011hw to the well-studied Type Ibn SN 2006jc. While the two SNe have many important similarities, the differences are quite telling: compared to SN 2006jc, SN 2011hw has weaker He i and Ca ii lines and relatively stronger H lines, its light curve exhibits a higher visual-wavelength luminosity and slower decline rate, and emission lines associated with the progenitor's circumstellar material (CSM) are narrower. One can reproduce the unusual continuum shape of SN 2011hw with roughly equal contributions from a 6000-K blackbody and a spectrum of SN 2006jc. We attribute this blackbody-like emission component and many other differences between the two SNe to a small amount of additional H in SN 2011hw, analogous to the small H mass that makes SNe IIb differ from SNe Ib. Slower speeds in the CSM and somewhat elevated H content suggest a connection between SN 2011hw's progenitor and Ofpe/WN9 stars, which have been associated with luminous blue variables (LBVs) in their hot quiescent phases, and are H poor - but not H free like classical Wolf-Rayet (WR) stars. Comparisons between SN 2011hw and SN 2006jc can be largely understood if their progenitors exploded at different points in the transitional evolution from an LBV to a WR star. [ABSTRACT FROM AUTHOR]

ABSTRACT The formation of stars is a key process in the early universe with far-reaching consequences for further cosmic evolution. While stars forming from truly primordial gas are thought to be considerably more massive than our Sun, stars in the universe today have typical masses below one solar mass. The physical origin of this transition and the conditions under which it occurs are highly debated. There are two competing models: one based on metal-line cooling as the primary agent and one based on dust cooling. The recent discovery of the extremely metal poor star SDSS J1029151+172927 provides a unique opportunity to distinguish between these two models. Based on simple thermodynamic considerations we argue that SDSS J1029151+172927 was more likely formed as a result of dust continuum cooling rather than cooling by metal lines. We conclude that the masses of extremely metal poor stars are determined by dust-induced fragmentation. [ABSTRACT FROM AUTHOR]

ABSTRACT Parity forbidden radiative transitions from metastable levels are observed in spectra of low-density astrophysical plasmas. These lines are used as probes of the physical conditions, made possible due to the long lifetime of their upper level. In a joint effort, the FERRUM project aims to obtain new and accurate atomic data for the iron-group elements, and part of this project concerns forbidden lines. The radiative lifetimes of the metastable energy levels 3 d4(a 3 D)4 s c4 D 5/2 and 3 d4(a 3 D)4 s c4 D 7/2 of singly ionized chromium have been measured. The experiment has been performed at the ion storage ring CRYRING. We employed a laser-probing technique developed for measuring long lifetimes. In this article, we present the lifetimes of these levels to be τ5/2= 1.28(16) s and τ7/2= 1.37(7) s, respectively. A comparison with previous theoretical work shows good agreement and the result is discussed in a theoretical context. [ABSTRACT FROM AUTHOR]

ABSTRACT We investigate the effect of including diffuse field radiation when modelling the radiatively driven implosion of a Bonnor-Ebert sphere (BES). Radiation-hydrodynamical calculations are performed by using operator splitting to combine Monte Carlo photoionization with grid-based Eulerian hydrodynamics that includes self-gravity. It is found that the diffuse field has a significant effect on the nature of radiatively driven collapse which is strongly coupled to the strength of the driving shock that is established before impacting the BES. This can result in either slower or more rapid star formation than expected using the on-the-spot approximation depending on the distance of the BES from the source object. As well as directly compressing the BES, stronger shocks increase the thickness and density in the shell of accumulated material, which leads to short, strong, photoevaporative ejections that reinforce the compression whenever it slows. This happens particularly effectively when the diffuse field is included as rocket motion is induced over a larger area of the shell surface. The formation and evolution of 'elephant trunks' via instability is also found to vary significantly when the diffuse field is included. Since the perturbations that seed instabilities are smeared out elephant trunks form less readily and, once formed, are exposed to enhanced thermal compression. [ABSTRACT FROM AUTHOR]

The annihilation of huge quantities of captured dark matter (DM) particles inside low-mass stars has been shown to change some of the stellar properties, such as the star's effective temperature or the way the energy is transported throughout the star. While in the classical picture, without DM, a star of is expected to have a radiative interior during the main sequence, the same star evolving in a halo of DM with a density GeV cm will develop a convective core in order to evacuate the energy from DM annihilation in a more efficient way. This convective core leaves a discontinuity in the density and sound-speed profiles that can be detected by the analysis of the stellar oscillations. In this paper we present an approach towards the use of asteroseismology to detect the signature produced by the presence of DM inside a star, and we propose a new methodology to infer the properties of a DM halo from the stellar oscillations (such as the product of the DM density and the DM particle-nucleon scattering cross-section). [ABSTRACT FROM AUTHOR]

Stars can produce steady-state winds through radiative driving as long as the mechanical luminosity of the wind does not exceed the radiative luminosity at its base. This upper bound on the mass-loss rate is known as the photon-tiring limit. Once above this limit, the radiation field is unable to lift all the material out of the gravitational potential of the star, such that only part of it can escape and reach infinity. The rest stalls and falls back towards the stellar surface, making a steady-state wind impossible. Photon-tiring is not an issue for line-driven winds since they cannot achieve sufficiently high mass-loss rates. It can, however, become important if the star exceeds the Eddington limit and continuum interaction becomes the dominant driving mechanism. This paper investigates the time-dependent behaviour of stellar winds that exceed the photon-tiring limit, using one-dimensional numerical simulations of a porosity-moderated, continuum-driven stellar wind. We find that the regions close to the star show a hierarchical pattern of high-density shells moving back and forth, unable to escape the gravitational potential of the star. At larger distances, the flow eventually becomes uniformly outward, though still quite variable. Typically, these winds have a very high density but a terminal flow speed well below the escape speed at the stellar surface. Since most of the radiative luminosity of the star is used to drive the stellar wind, such stars would appear much dimmer than expected from the super-Eddington energy generation at their core. The visible luminosity typically constitutes less than half of the total energy flow and can become as low as 10 per cent or less for those stars that exceed the photon-tiring limit by a large margin. [ABSTRACT FROM AUTHOR]

We continue a systematic study of chemical abundances of the strontium filament found in the ejecta of η Carinae. To this end we interpret the emission spectrum of Sc ii and Cr ii using multilevel non-local thermodynamic equilibrium models. Since the atomic data for these ions were previously unavailable, we carry out ab initio calculations of radiative transition rates and electron impact excitation rate coefficients. The observed spectrum is consistent with an electron density of the order of 107 cm−3 and a temperature between 6000 and 7000 K, conditions previously determined from [Ni ii], [Ti ii] and [Sr ii] diagnostics. The observed spectrum indicates an abundance of Sc relative to Ni more than 40 times the solar value, while the Cr/Ni abundance ratio is roughly solar. Various scenarios of depletion and dust destruction are suggested to explain such abnormal abundances. [ABSTRACT FROM AUTHOR]

We present solutions for the structure and evolution of bubbles created by slow stellar winds, during their fully and partially radiative phases. Exact limiting forms are derived for the partially radiative case, and the transition between the two phases is analysed under the assumption of steady spherical flow. We compute the factor by which the bubble's radial momentum is amplified relative to that of the wind, and provide accurate formulae for its expansion across the fully-to-partially radiative transition. Our results will be useful in the study of protostellar outflow cocoons and bubbles driven by mass-loaded winds from star clusters. [ABSTRACT FROM AUTHOR]

Recent observations of thermally emitting isolated neutron stars revealed spectral features that could be interpreted as radiative transitions of He in a magnetized neutron star atmosphere. We present Hartree–Fock calculations of the polarization-dependent photoionization cross-sections of the He atom in strong magnetic fields ranging from 1012 to 1014 G. Convenient fitting formulae for the cross-sections are given along with the related oscillator strengths for various bound–bound transitions. The effects of finite nucleus mass on the radiative absorption cross-sections are examined using perturbation theory. [ABSTRACT FROM AUTHOR]

In this paper, we report the detection of a series of radio recombination lines (RRLs) in absorption near 26 MHz arising from the largest bound carbon atoms detected in space. These atoms, which are more than a million times larger than the ground-state atoms, are undergoing delta transitions in the cool tenuous medium located in the Perseus arm in front of the supernova remnant, Cassiopeia A (Cas A). Theoretical estimates had shown that atoms which recombined in tenuous media are stable up to quantum levels . Our data indicate that we have detected radiation from atoms in states very close to this theoretical limit. We also report high signal-to-noise ratio detections of and γ transitions in carbon atoms arising in the same clouds. In these data, we find that the increase in linewidths with quantum number (approximately proportional to n5) due to pressure and radiation broadening of lines is much gentler than that expected from existing models which assume a power-law background radiation field. This discrepancy had also been noted earlier. The model linewidths had been overestimated since the turnover in radiation field of Cas A at low frequencies had been ignored. In this paper, we show that, once the spectral turnover is included in the modelling, the slower increase in linewidth with quantum number is naturally explained. [ABSTRACT FROM AUTHOR]

The rate coefficients for the formation of carbon monophosphide (CP) and silicon monophosphide (SiP) by radiative association are estimated for temperatures ranging from 300 to 14 100 K. In this temperature range, the radiative association rate coefficients are found to vary from to and from to for CP and SiP, respectively. In both cases, rate coefficients increase slowly with the increase in temperature. [ABSTRACT FROM AUTHOR]

The measured values of the glitch healing parameter, Q, of the Vela pulsar are found to be inconsistent with the starquake mechanism for glitch generation in various neutron star (NS) models, based upon the parametrized equations of state (EOSs) of dense nuclear matter. Such models correspond to an unrealistic mass range ≤0.5 M⊙ for the pulsar, if the observational constraints of the fractional moment of inertia of the core component which is called the glitch healing parameter, Q, according to the starquake model, are imposed on these models. However, we show that these observational constraints yield a realistic mass range for NS models, corresponding to a core given by the stiffest equation of state, d P/d E= 1 (in geometrized units), and the envelope is characterized by the well-known EOS of an adiabatic polytrope , if the continuity of the adiabatic speed of sound , together with the pressure ( P), the energy density ( E) and the two metric parameters (ν and λ), is assured at the core–envelope boundary of the models and this boundary is worked out on the basis of the ‘compatibility criterion’ for hydrostatic equilibrium. The models yield a stable sequence of NS masses in the range 1.758 ≤ M≤ 2.2 M⊙, corresponding to the glitch healing parameter range 0 ≤ Q≤ 0.197, for a choice of the ‘transition density’ at the core–envelope boundary. The maximum stable value of 2.2 M⊙ in this sequence, in fact, corresponds to the lowest possible upper bound on NS masses calculated in the literature, on the basis of modern EOSs for NS matter. The models yield the surface redshift and mass M≃ 2.153 M⊙ for the ‘central’ weighted mean value, Q= 0.12 ± 0.07, of the glitch healing parameter of the Vela pulsar. This value of mass can increase slightly up to M≃ 2.196 M⊙, whereas the surface redshift can increase up to the value [which represents an ultracompact object ], if the observational constraint of the upper weighted mean value of Q≃ 0.19 is imposed on these models. However, for the lower weighted mean value of Q≃ 0.05, the mass and surface redshift can decrease to the values of zR≃ 0.6066 and M≃ 2.052 M⊙ respectively. These results set the lower bound on the energy of gravitationally redshifted radiation in the rather narrow range of 0.291–0.302 MeV. The observation of the lower bound on the energy of a γ-ray pulse at about 0.30 MeV from the Vela pulsar in 1984 is in excellent agreement with this result, provided that this energy can be interpreted as the energy of gravitationally redshifted electron–positron annihilation radiation from the surface of the star. [ABSTRACT FROM AUTHOR]

Radiative parameters for transitions depopulating the levels belonging to the 4d85s2, 4d96s and 4d95d configurations of Ag ii have been obtained from a combination of theoretical lifetimes and experimental branching fractions. On the experimental side, a laser-produced plasma was used as a source of Ag+ ions. The light emitted by the plasma was analysed by a grating monochromator coupled with a time-resolved optical multichannel analyser system. Spectral response calibration of the experimental system was performed using a deuterium lamp in the wavelength range from 200 to 400 nm, and a standard tungsten lamp in the range from 350 to 600 nm. The transition probabilities were obtained from measured branching ratios and theoretical radiative lifetimes of the corresponding states calculated with a relativistic Hartree–Fock approach including core-polarization effects and configuration interaction in an extensive way. Theoretical and experimental data have been compared and the new data have also been compared with the few previous results available in the literature. [ABSTRACT FROM AUTHOR]

Examines the estimation of the radiative association rate coefficient for the formation of LiH during Li[sup +] + H[sup -] collisions. Description of the mutual neutralization collision process; Calculation of cross-sections at fixed continuum energy; Behavior of the radiative association cross-section.

Examines the oscillator strength of Germanium 1 transitions. Measurement of radiative lifetimes using laser-produced plasma levels; Use of diffraction grating based emission spectrometer to measure the transitions; Comparison of values within pseudo-relativistic Hartree-Fock approach.

A correction to the article "Radiative Association in Li+H Revisited: The Role of Quasi-Bound States," published in a 2003 issue is presented.