ALBERT

Description: Hydrogen and helium emission lines in nebulae form by radiative recombination. This is a simple process which, in principle, can be described to very high precision. Ratios of He i and H i emission lines can be used to measure the He + /H + abundance ratio to the same precision as the recombination rate coefficients. This paper investigates the controversy over the correct theory to describe dipole l -changing collisions ( nl -〉 nl ' =  l  ± 1) between energy-degenerate states within an n -shell. The work of Pengelly & Seaton has, for half-a-century, been considered the definitive study which ‘solved’ the problem. Recent work by Vrinceanu et al. recommended the use of rate coefficients from a semiclassical approximation which are nearly an order of magnitude smaller than those of Pengelly & Seaton, with the result that significantly higher densities are needed for the nl populations to come into local thermodynamic equilibrium. Here, we compare predicted H i emissivities from the two works and find widespread differences, of up to 10 per cent. This far exceeds the 1 per cent precision required to obtain the primordial He/H abundance ratio from observations so as to constrain big bang cosmologies. We recommend using the rate coefficients of Pengelly & Seaton for l -changing collisions, to describe the H recombination spectrum, based-on their quantum mechanical representation of the long-range dipole interaction.

Description: Recent atomic physics calculations for Si ii are employed within the cloudy modelling code to analyse Hubble Space Telescope ( HST ) STIS ultraviolet spectra of three cool stars, β Geminorum, α Centauri A and B, as well as previously published HST /GHRS observations of α Tau, plus solar quiet Sun data from the High Resolution Telescope and Spectrograph. Discrepancies found previously between theory and observation for line intensity ratios involving the 3s 2 3p 2 P J –3s3p 2 4 P $_{J^{\prime }}$ intercombination multiplet of Si ii at ~ 2335 Å are significantly reduced, as are those for ratios containing the 3s 2 3p 2 P J –3s3p 2 2 D $_{J^{\prime }}$ transitions at ~1816 Å. This is primarily due to the effect of the new Si ii transition probabilities. However, these atomic data are not only very different from previous calculations, but also show large disagreements with measurements, specifically those of Calamai et al. for the intercombination lines. New measurements of transition probabilities for Si ii are hence urgently required to confirm (or otherwise) the accuracy of the recently calculated values. If the new calculations are confirmed, then a long-standing discrepancy between theory and observation will have finally been resolved. However, if the older measurements are found to be correct, then the agreement between theory and observation is simply a coincidence and the existing discrepancies remain.

Description: Many massive galaxies at the centres of relaxed galaxy clusters and groups have vast reservoirs of warm (~10 000 K) and cold (100 K) gas. In many such low-redshift systems this gas is lifted into the hot interstellar medium in filamentary structures, which are long lived and are typically not forming stars. Two important questions are how far do these reservoirs cool and if cold gas is abundant what is the cause of the low star formation efficiency? Heating and excitation of the filaments from collisions and mixing of hot particles in the surrounding X-ray gas describes well the optical and near infrared line ratios observed in the filaments. In this paper we examine the theoretical properties of dense, cold clouds emitting in the far infrared and sub-millimetre through the bright lines of [C  ii ] 157 μm , [O  i ] 63 μm and CO, exposed to such energetic ionizing particles. We find that optical depth effects and thermal pressure support alone cannot account for the line ratios; however, a very modest additional pressure support can fit the observed [O  i ] 63 μm/[C  ii ] 157 μm line ratios by decreasing the density of the gas. This may also help stabilize the filaments against collapse leading to the low rates of star formation. We make predictions for the line ratios expected from cold gas under these conditions and present diagnostic diagrams for comparison with further observations. We provide our code as an Appendix.

Description: We present an analysis of the far-infrared and submillimetre molecular emission-line spectrum of the luminous M-supergiant VY CMa, observed with the Spectral and Photometric Imaging Receiver (SPIRE) and Photodetector Array Camera and Spectrometer for Herschel spectrometers aboard the Herschel Space Observatory . Over 260 emission lines were detected in the 190–650 μm SPIRE Fourier Transform Spectrometer spectra, with one-third of the observed lines being attributable to H 2 O. Other detected species include CO, 13 CO, H $_2^{18}$ O, SiO, HCN, SO, SO 2 , CS, H 2 S and NH 3 . Our model fits to the observed 12 CO and 13 CO line intensities yield a 12 C/ 13 C ratio of 5.6 ± 1.8, consistent with measurements of this ratio for other M-supergiants, but significantly lower than previously estimated for VY CMa from observations of lower- J lines. The spectral line energy distribution for 20 SiO rotational lines shows two temperature components: a hot component at ~1000 K, which we attribute to the stellar atmosphere and inner wind, plus a cooler ~200 K component, which we attribute to an origin in the outer circumstellar envelope. We fit the line fluxes of 12 CO, 13 CO, H 2 O and SiO, using the smmol non-local thermodynamic equilibrium (LTE) line transfer code, with a mass-loss rate of 1.85 10 –4 M yr –1 between 9 R * and 350 R * . We also fit the observed line fluxes of 12 CO, 13 CO, H 2 O and SiO with smmol non-LTE line radiative transfer code, along with a mass-loss rate of 1.85 10 –4 M yr –1 . To fit the high rotational lines of CO and H 2 O, the model required a rather flat temperature distribution inside the dust condensation radius, attributed to the high H 2 O opacity. Beyond the dust condensation radius the gas temperature is fitted best by an r –0.5 radial dependence, consistent with the coolant lines becoming optically thin. Our H 2 O emission-line fits are consistent with an ortho:para ratio of 3 in the outflow.

Description: When modelling an ionized plasma, all spectral synthesis codes need the thermally averaged free–free Gaunt factor defined over a very wide range of parameter space in order to produce an accurate prediction for the spectrum. Until now no data set exists that would meet these needs completely. We have therefore produced a table of relativistic Gaunt factors over a much wider range of parameter space than has ever been produced before. We present tables of the thermally averaged Gaunt factor covering the range 10 log 2  = –6 to 10 and 10 log u  = –16 to 13 for all atomic numbers Z  = 1 through 36. The data were calculated using the relativistic Bethe–Heitler–Elwert (BHE) approximation and were subsequently merged with accurate non-relativistic results in those parts of the parameter space where the BHE approximation is not valid. These data will be incorporated in the next major release of the spectral synthesis code cloudy . We also produced tables of the frequency integrated Gaunt factor covering the parameter space 10 log 2  = –6 to +10 for all values of Z between 1 and 36. All the data presented in this paper are available online.

Description: This work follows Lykins et al. discussion of classic plasma cooling function at low density and solar metallicity. Here, we focus on how the cooling function changes over a wide range of density ( n H 〈10 12 cm –3 ) and metallicity ( Z  〈 30 Z ). We find that high densities enhance the ionization of elements such as hydrogen and helium until they reach local thermodynamic equilibrium. By charge transfer, the metallicity changes the ionization of hydrogen when it is partially ionized. We describe the total cooling function as a sum of four parts: those due to H&He, the heavy elements, electron–electron bremsstrahlung and grains. For the first three parts, we provide a low-density limit cooling function, a density dependence function, and a metallicity-dependent function. These functions are given with numerical tables and analytical fit functions. We discuss grain cooling only in the interstellar medium case. We then obtain a total cooling function that depends on density, metallicity and temperature. As expected, collisional de-excitation suppresses the heavy elements cooling. Finally, we provide a function giving the electron fraction, which can be used to convert the cooling function into a cooling rate.

Description: We announce a new facility in the spectral code cloudy that enables tracking the evolution of a cooling parcel of gas with time. For gas cooling from temperatures relevant to galaxy clusters, earlier calculations estimated the [Fe  xiv ] 5303/[Fe  x ] 6375 luminosity ratio, a critical diagnostic of a cooling plasma, to slightly less than unity. By contrast, our calculations predict a ratio of ~3. We revisit recent optical coronal line observations along the X-ray cool arc around NGC 4696 by Canning et al., which detected [Fe  x ] 6375, but not [Fe  xiv ] 5303. We show that these observations are not consistent with predictions of cooling flow models. Differential extinction could in principle account for the observations, but it requires extinction levels ( A V  〉 3.625) incompatible with previous observations. The non-detection of [Fe  xiv ] implies a temperature ceiling of 2.1 million K. Assuming cylindrical geometry and transonic turbulent pressure support, we estimate the gas mass at ~1 million M . The coronal gas is cooling isochorically. We propose that the coronal gas has not condensed out of the intracluster medium, but instead is the conductive or mixing interface between the X-ray plume and the optical filaments. We present a number of emission lines that may be pursued to test this hypothesis and constrain the amount of intermediate-temperature gas in the system.

Description: Modern spectral synthesis codes need the thermally averaged free–free Gaunt factor defined over a very wide range of parameter space in order to produce an accurate prediction for the spectrum emitted by an ionized plasma. Until now no set of data exists that would meet this need in a fully satisfactory way. We have therefore undertaken to produce a table of very accurate non-relativistic Gaunt factors over a much wider range of parameters than has ever been produced before. We first produced a table of non-averaged Gaunt factors, covering the parameter space 10 log i  = –20 to +10 and 10 log w  = –30 to +25. We then continued to produce a table of thermally averaged Gaunt factors covering the parameter space 10 log 2  = –6 to +10 and 10 log u  = –16 to +13. Finally, we produced a table of the frequency integrated Gaunt factor covering the parameter space 10 log 2  = –6 to +10. All the data presented in this paper are available online.

Description: We analyse the remnants of CK Vul (Nova Vul 1670) using optical imaging and spectroscopy. The imaging, obtained between 1991 and 2010, spans 5.6 per cent of the lifetime of the nebula. The flux of the nebula decreased during the last two decades. The central source still maintains the ionization of the innermost part of the nebula, but recombination proceeds in more distant parts of the nebula. Surprisingly, we discovered two stars located within 10 arcsec of the expansion centre of the radio emission that are characterized by pronounced long-term variations and one star with high proper motion. The high proper motion star is a foreground object, and the two variable stars are background objects. The photometric variations of two variables are induced by a dusty cloud ejected by CK Vul and passing through the line of sight to those stars. The cloud leaves strong lithium absorption in the spectra of the stars. We discuss the nature of the object in terms of recent observations.