${\rm He}
(1\,^1{\rm S},\, 2\,^3{\rm S},\, 2\,^1{\rm S},\, 2\,^3{\rm P} \to 
\, n\, ^{1,3} L)$: Thermally averaged electron collision strengths for 
$n \leq 5$

Bray, Igor; Burgess, A.; Fursa, Dmitry V.; Tully, J. A.

doi:10.1051/aas:2000277

Cited by 43 publications

(65 citation statements)

References 10 publications

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“…State-of-theart model atoms for hydrogen (Przybilla & Butler 2004, PB04) and helium (Przybilla 2005, P05) are utilised, which differ from older model atoms mostly by use of improved collision data from ab-initio computations for electron impact excitation processes. For He  these are the data from Bray et al (2000) and Sawey & Berrington (1993), and for H  the data from computations by Butler, as summarised in PB04 -see this work and P05 for further details on the modelling. For comparison, additional calculations are made using the helium model atom of Husfeld et al (1989, HBHD89).…”

Section: Model Calculationsmentioning

confidence: 99%

Extreme helium stars: non-LTE matters

Przybilla¹,

Butler

Heber³

et al. 2005

A&A

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Quantitative analyses of low-mass hydrogen-deficient (super-)giant stars -so-called extreme helium stars -to date face two major difficulties. First, theory fails to reproduce the observed helium lines in their entirety, wings and line cores. Second, a general mismatch exists for effective temperatures derived from ionization equilibria and from spectral energy distributions. Here, we demonstrate how the issue can be resolved using state-of-the-art non-LTE line-formation for these chemically peculiar objects. Two unique high-gravity B-type objects are discussed in detail, the pulsating variable V652 Her and the metal-poor star HD 144941. In the first case atmospheric parameters from published LTE analyses are largely recovered, in the other a systematic offset is found. Hydrogen abundances are systematically smaller than previously reported, by up to a factor ∼2. Extreme helium stars turn out to be important testbeds for non-LTE model atoms for helium. Improved non-LTE computations show that analyses assuming LTE or based on older non-LTE model atoms can predict equivalent widths, for the He  10 830 Å transition in particular, in error by up to a factor ∼3.

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Section: Model Calculationsmentioning

confidence: 99%

Extreme helium stars: non-LTE matters

Przybilla¹,

Butler

Heber³

et al. 2005

A&A

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“…For low-n transitions for which there are ab initio calculations, B99 uses the collision strengths of Sawey & Berrington (1993). We replace these, where available, with the results of the close-coupling calculation by Bray et al (2000), which include continuum states not considered in the R-matrix calculations by Sawey & Berrington.For l-changing collisions, B99 use two different treatments: Seaton (1962, hereafter S62) for low-l transitions and Pengelly & Seaton (1964, hereafter PS64) otherwise. Neither of these treatments allows for angular momentum transfers greater than one unit, and both apply when the projectile velocity is greater than the velocity of the bound electron.…”

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confidence: 99%

Theoretical He i Emissivities in the Case B Approximation

Porter¹,

Bauman²,

Ferland³

et al. 2005

ApJ

163

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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...

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“…both much larger than unity as has been observed ( Table 2). According to the model, with e − -collision coefficients from Bray et al (2000), the excitation of the He I 1.0830 µm line is dominated by collisions, so that in this case, the "10830 recombination line" is not really due to He recombination (compare the x He II and x He I regions in the figure). For purely thermal broadening, the line would be marginally optically thick, i.e.…”

Section: Discussionmentioning

confidence: 99%

X-ray and He I 1.0830μm emission from protostellar jets

Liseau¹

2006

A&A

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Context. The high energies of protostellar jets, implied by recent observations of X-rays from such flows, came very much as a surprise. Inferred shock velocities are considerably higher than what was previously known, hence putting even larger energy demands on the driving sources of the jets. The statistics of X-ray emitting jets are still poor, yet a few cases exist which seem to imply a correlation between the presence of He I 1.0830 µm emission and X-ray radiation in a given source. Aims. This tentative correlation needs confirmation and explanation. If the jet regions of He I 1.0830 µm emission are closely associated with those producing X-rays, high resolution infrared spectroscopy can be used to observationally study the velocity fields in the hot plasma regions of the jets. This would provide the necessary evidence to test and further develop theoretical models of intermediately fast (>500−1500 km s −1 ) interstellar shock waves. Methods. The HH 154 jet flow from the embedded protostellar binary L 1551 IRS 5 provides a case study, since adequate IR and X-ray spectroscopic data are in existence. The thermal X-ray spectrum is fed into a photoionization code to compute, in particular, the line emission of He I and H I and to account for the observed unusual line intensity ratios.Results. The advanced model is capable of accounting for most observables, but shows also major weaknesses. It seems not unlikely that these could, in principle, be overcome by a time dependent hydrodynamical calculation with self-consistent cooling. However, such sophisticated model development is decisively beyond the scope of the present work. Conclusions. Continued X-ray observations, coordinated with simultaneous high resolution infrared spectroscopy, are highly desirable.

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${\rm He} (1\,^1{\rm S},\, 2\,^3{\rm S},\, 2\,^1{\rm S},\, 2\,^3{\rm P} \to \, n\, ^{1,3} L)$: Thermally averaged electron collision strengths for $n \leq 5$

Cited by 43 publications

References 10 publications

Extreme helium stars: non-LTE matters

Extreme helium stars: non-LTE matters

Theoretical He i Emissivities in the Case B Approximation

X-ray and He I 1.0830μm emission from protostellar jets

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