Calculated rotational and vibrational excitation rates for electron–HeH+ collisions

Molecular R-matrix calculations are performed at a range of energies to give rotational and vibrational excitation and de-excitation cross-sections and, hence, rates for electron collisions with HeH⁺ up to electron temperatures of 20 000 K. Critical electron densities are also given. The rotational calculations include the Coulomb–Born completion of the cross-sections for high l values. Rates for the transition j  = 0 → 2, which have previously been assumed to be negligible, are found to be up to half those for j  = 0 → 1, raising the prospect of observing the HeH⁺ j  = 2 → 1 emission line at 74.8 μm.     

The broadening of strong lines of Ca+, Mg+ and Ba+ by collisions with neutral hydrogen atoms

The theory of Anstee &38; O'Mara is extended to the line broadening of transitions of singly ionized atoms by collisions with neutral hydrogen atoms. The theory is used to calculate broadening cross-sections for strong lines of singly ionized calcium, magnesium and barium. The broadening cross-sections calculated are compared with both theoretical and empirical results of other workers.     

State-to-state rotational transition rates of the HCO+ ion by collisions with helium

The rates of rotational transitions for HCO⁺, the most abundant ion in interstellar space, induced by collision with helium are obtained for temperatures ranging from 10 to 80 K. The calculations are based on a new potential energy surface for the He–HCO⁺ interaction and on a scattering matrix whose accuracy was checked by pressure broadening and shift measurements. The rates     decrease for increasing values of j and  Δ j   , with a temperature trend depending on the energy involved in the transitions: if it is small, the rates are almost constant, while an increase with T is found for other cases. Comparison with previous and less accurate results shows an agreement within 50 per cent. Comparison between state-to-state and pressure broadening cross-sections allows us to discuss importance and influence of elastic and inelastic collisions.     

On the identification of the C60+ interstellar features

The identity of the carriers of the diffuse interstellar bands (DIBs) is one of the most fascinating puzzles of modern spectroscopy. Over the last few years the number of known DIBs has grown substantially. In this paper we discuss the two recently discovered near-infrared weak interstellar features which have already been proposed as fingerprints of the buckminsterfullerene We present and discuss measurements of the two related DIBs within a larger sample of reddened targets, observed with different spectrometers, telescopes and site conditions. We provide additional arguments in favour of the interstellar origin of the two bands. We find evidence around the 9577-Å DIB of far-wing structures, which may affect broad-band measurements. We estimate corrections and errors for telluric and stellar blends, and show that the cores of the two DIBs are well correlated with a ratio near unity within 20 per cent. Finally, we discuss their relation to the laboratory spectra of and the search for two expected weaker transitions.     

H3+ in diffuse interstellar gas

A model is constructed of the material in front of the star Cygnus OB2 no. 12 in which dense cores are embedded in diffuse clumps of gas. The model reproduces the measured abundances of C₂ and CO, and predicts a column density of 910¹⁰ cm² for HCO⁺.     

C-type shocks in the interstellar medium: profiles of CH+ and CH absorption lines

We have computed optical absorption-line profiles of CH⁺ and CH, as predicted by a model of a C-type shock propagating in a diffuse interstellar cloud. Both these species are produced in the shock wave in the reaction sequence that is initiated by C⁺(H₂, H)CH⁺. Whilst CH⁺ flows at the ion speed, CH, which forms in the dissociative recombination reaction CH⁺₃(e⁻, H₂)CH, flows at a speed which is intermediate between those of the ions and the neutrals. The predicted velocity shift between the CH⁺ and CH line profiles is found to be no more than approximately 2 km s⁻¹, which is smaller than has previously been assumed. We also investigate OH and HCO⁺, finding that the correlation between their column densities, recently observed in the diffuse interstellar medium, can be reproduced by the model.     

Precursors and e± pair loading from erupting fireballs

Recent observations suggest that long-duration γ -ray bursts and their afterglows are produced by highly relativistic jets emitted in core-collapse explosions. As the jet makes its way out of the stellar mantle, a bow shock runs ahead and a strong thermal precursor is produced as the shock breaks out. Such erupting fireballs produce a very bright γ -ray precursor as they interact with the thermal break-out emission. The prompt γ -ray emission propagates ahead of the fireball before it becomes optically thin, leading to e^± pair loading and radiative acceleration of the external medium. The detection of such precursors would offer the possibility of diagnosing not only the radius of the stellar progenitor and the initial Lorentz factor of the collimated fireball, but also the density of the external environment.     

The H+ region contribution to [C ii] 158-μm emission

The [C  ii ]  158-μm  line is an important emission-line diagnostic in photodissociation regions (PDRs), but this emission line can also emerge from ionized gas. This work calculates the contribution of [C  ii ] emission from ionized gas over a wide range of parameter space by considering the simplified case of an H⁺ region and PDR in pressure equilibrium. Additionally, these calculations also predict the strong correlation observed between [N  ii ]  205-μm  emission and [C  ii ] discussed by previous authors. Overall, the results of these calculations have wide-ranging applications to the interpretation of [C  ii ] emission in astrophysical environments.     

Carbon abundances and 12C/13C from globular cluster giants

The behaviour of the  Δν= 2 CO  bands around 2.3 μm was examined by comparing observed and synthetic spectra in stars in globular clusters of different metallicity. Changes in the ¹²C/¹³C isotopic ratio and the carbon abundances were investigated in stars from 3500–4900 K in the galactic globular clusters M71, M5, M3 and M13, covering the metallicity range from −0.7 to −1.6. We found relatively low carbon abundances that are not affected by the value of oxygen abundance. For most giants, the ¹²C/¹³C ratios determined are consistent with the equilibrium value for the CN cycle. This suggests complete mixing on the ascent of the red giant branch, in contrast to the substantially higher values predicted across this range of parameters by the current generation of models. We found some evidence for a larger dispersion of ¹²C/¹³C in giants of M71 of metallicity  [μ]=[M/H]=−0.7  in comparison with the giants of M3, M5 and M13, which are more metal deficient. Finally, we show evidence for lower ¹²C/¹³C in giants of globular clusters with lower metallicities, as predicted by theory.     

Spectrum of hot water in the 4750–13 000 cm−1 wavenumber range (0.769–2.1 μm)

The high resolution laboratory spectrum of hot water vapour has been recorded in the 500–13 000 cm⁻¹ wavenumber range and we report on the analysis of the 4750–13 000 cm⁻¹ (0.769–2.1 μm) portion. The emission spectrum was recorded using an oxy-acetylene welding torch and a Fourier transform spectrometer. Line assignments in the laboratory spectrum as well as in an absorption spectrum of a sunspot umbra were made with the help of the BT2 line-list. Our torch spectrum is the first laboratory observation of the 9300 Å'steam bands' seen in M-stars and brown dwarfs.     

Effective collision strengths for fine-structure forbidden transitions among the 3s23p3 levels of K v

In wind-fed X-ray binaries the accreting matter is Compton-cooled and falls freely on to the compact object. The matter has a modest angular momentum l and accretion is quasi-spherical at large distances from the compact object. Initially small non-radial velocities grow in the converging supersonic flow and become substantial in the vicinity of the accretor. The streamlines with l >( GMR ∗)^1/2 (where M and R ∗ are the mass and radius of the compact object) intersect outside R ∗ and form a two-dimensional caustic which emits X-rays. The streamlines with low angular momentum, l <( GMR ∗)^1/2, run into the accretor. If the accretor is a neutron star, a large X-ray luminosity results. We show that the distribution of accretion rate/luminosity over the star surface is sensitive to the angular momentum distribution of the accreting matter. The apparent luminosity depends on the side from which the star is observed and can change periodically with the orbital phase of the binary. The accretor then appears as a 'Moon-like' X-ray source.