It has long been known that photoionization, whether by starlight or other sources, has difficulty in accounting for the observed spectra of the optical filaments that often surround central galaxies in large clusters. This paper builds on the first of this series in which we examined whether heating by energetic particles or dissipative magnetohydrodynamic (MHD) wave can account for the observations. The first paper focused on the molecular regions which produce strong H2 and CO lines. Here we extend the calculations to include atomic and low‐ionization regions. Two major improvements to the previous calculations have been made. The model of the hydrogen atom, along with all elements of the H‐like iso‐electronic sequence, is now fully nl‐resolved. This allows us to predict the hydrogen emission‐line spectrum including excitation by suprathermal secondary electrons and thermal electrons or nuclei. We show how the predicted H i spectrum differs from the pure‐recombination case. The second update is to the rates for H0–H2 inelastic collisions. We now use the values computed by Wrathmall et al. The rates are often much larger and allow the ro–vibrational H2 level populations to achieve a thermal distribution at substantially lower densities than previously thought. We calculate the chemistry, ionization, temperature, gas pressure and emission‐line spectrum for a wide range of gas densities and collisional heating rates. We assume that the filaments are magnetically confined. The gas is free to move along field lines so that the gas pressure is equal to that of the surrounding hot gas. A mix of clouds, some being dense and cold and others hot and tenuous, can exist. The observed spectrum will be the integrated emission from clouds with different densities and temperatures but the same pressure P/k=nT. We assume that the gas filling factor is given by a power law in density. The power‐law index, the only free parameter in this theory, is set by matching the observed intensities of infrared H2 lines relative to optical H i lines. We conclude that the filaments are heated by ionizing particles, either conducted in from surrounding regions or produced in situ by processes related to MHD waves.
We calculate the He i case B recombination cascade spectrum using improved radiative and collisional data. We present new emissivities over a range of electron temperatures and densities. The differences between our results and the current standard are large enough to have a significant effect not only on the interpretation of observed spectra of a wide variety of objects, but also on determinations of the primordial helium abundance. Subject headings: atomic data -atomic processes -ISM: atoms -ISM: clouds -plasmas 1. INTRODUCTION Helium is the second most abundant element in the universe, and its emission and opacity help us determine the structure of any interstellar cloud. Its abundance relative to hydrogen can be measured within a few percent since the emissivities of H i and He i lines have similar dependences on temperature and density. This makes it an indicator of both stellar and primordial nucleosynthesis (Pagel 1997).A good discussion of the history of calculations of the helium recombination spectra is given by Benjamin et al. (1999, hereafter B99), who present new calculations-the current standard in the field. Yet much progress has been made since the work by Smits (1991Smits ( , 1996 on which the B99 results depend. We implement these improvements, present a new set of predictions, and compare our results with those of B99. The differences are large enough to impact continuing attempts to estimate the primordial helium abundance (Peimbert et al. 2002). THE NEW MODEL HELIUM ATOMThe basic physical processes have been described by Brocklehurst (1972) and B99. Here we describe the differences between B99 and our new numerical representation of the helium atom, which is a part of the spectral simulation code CLOUDY (Ferland et al. 1998). This model resolves all terms, nlS, up to an adjustable maximum principal quantum number n max , followed by a pseudolevel, , in which all lS terms are assumed n ϩ 1 max to be populated according to statistical weight and "collapsed" into one. We set recombinations into the collapsed level equal to the convergent sum of recombinations from n p n ϩ 1 max to . In the low-density limit, the collapsed level increases the ϱ emissivities of our benchmark lines (the same 32 lines given in B99) by 0.4%, on average, with . The decays n p 100 max from states with are most sensitive to this correction l p n Ϫ 1 for system truncation. The strong optical line l5876 is corrected upward by 1.3%. At finite densities collisional processes force the populations of very highly excited states into local thermodynamic equilibrium (LTE). In this case the adequacy of the method used to compensate for truncation is unimportant. We find the corrections negligible for cm Ϫ3 and n p 100 e . Consequently, the uncertainties in the results pren p 100 max sented in § 3 are due to the uncertainties in atomic data, especially the often substantial uncertainties in collisional rates affecting terms not in LTE at given conditions.There are several differences in atomic data for radiative proc...
We update our prior work on the case B collisional-recombination spectrum of He I to incorporate ab initio photoionization cross-sections. This large set of accurate, self-consistent cross-sections represents a significant improvement in He I emissivity calculations because it largely obviates the piecemeal nature that has marked all modern works. A second, more recent set of ab initio cross-sections is also available, but we show that those are less consistent with bound-bound transition probabilities than our adopted set. We compare our new effective recombination coefficients with our prior work and our new emissivities with those by other researchers, and we conclude with brief remarks on the effects of the present work on the He I error budget. Our calculations cover temperatures 5000 ≤ T e ≤ 25 000 K and densities 10 1 ≤ n e ≤ 10 14 cm −3 . Full results are available online (see Supporting Information).
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