Spectroscopic Challenges of Photoionized Plasmas

Ferland, Gary J.; Savin, D. W.

doi:10.1086/322922

Cited by 49 publications

(42 citation statements)

References 26 publications

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“…The reason for using two codes is to perform a consistency check of the results. Even though photoionization codes have been shown before to give similar results at subsolar metallicity (references in Ferland & Savin 2001), the results may diverge at supersolar metallicity because of numerical and physical instabilities under very extreme conditions. However, we find the same general conclusion from both photoionization codes, i.e., the inability to fit the observed emission line ratios with clusters formed 3-4 Myr ago containing stars more massive than M up !…”

Section: Input Parameters and Assumptionsmentioning

confidence: 94%

The Massive Stellar Content in the Starburst NGC 3049: A Test for Hot‐Star Models

Delgado

Leitherer

Stasińska

et al. 2002

ApJ

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The objective of this work is twofold. First, we seek evidence for or against the depletion of massive stars in metal-rich starbursts. A second, equally important goal is to perform a consistency test of the latest generation of starburst models in such a high-metallicity environment. We have obtained high spatial resolution ultraviolet and optical STIS spectroscopy and imaging of the metal-rich nuclear starburst in NGC 3049. The stellar continuum and the absorption-line spectrum in the ultraviolet are used to constrain the massive stellar population. The strong, blueshifted stellar lines of C iv and Si iv detected in the UV spectra indicate a metalrich, compact, massive ($10 6 M ) cluster of age 3-4 Myr emitting the UV-optical continuum. We find strong evidence against a depletion of massive stars in this metal-rich cluster. The derived age and the upper masslimit cutoff of the initial mass function are also consistent with the detection of Wolf-Rayet (W-R) features at optical wavelengths. As a second, independent constraint on the massive stellar content, the nebular emission-line spectrum is modeled with photoionization codes using stellar spectra from evolutionary synthesis models. The morphology of the nuclear starburst of NGC 3049 from the STIS images indicates a simple geometry for the nebular emission-line region. However, the nebular lines are badly reproduced by 3-4 Myr instantaneous bursts, as required by the UV line spectrum, when unblanketed W-R and/or Kurucz stellar atmospheres are used. The corresponding number of photons above 24 and 54 eV in the synthetic models is too high in comparison with values suggested by the observed line ratios. Since the ionizing spectrum in this regime is dominated by emission from W-R stars, this discrepancy between observations and models is most likely the result of incorrect assumptions about the W-R stars. Thus, we conclude that the nebular spectrum of high-metallicity starbursts is poorly reproduced by models for W-R-dominated populations. However, the new model set of Smith et al. with blanketed W-R and O atmospheres and adjusted W-R temperatures predicts a softer far-UV radiation field, providing a better match to the data.

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Section: Input Parameters and Assumptionsmentioning

confidence: 94%

The Massive Stellar Content in the Starburst NGC 3049: A Test for Hot‐Star Models

Delgado

Leitherer

Stasińska

et al. 2002

ApJ

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“…The physical state of this gas is interpreted by reference to large-scale numerical simulations; hence, it is critical that physical processes that occur in an ionized plasma be fully understood (the main topic of the conference ''Spectroscopic Challenges of Photoionized Plasmas'' held at the University of Kentucky in 2000 November ;Ferland & Savin 2001).…”

Section: Introductionmentioning

confidence: 99%

The Effects of Low‐Temperature Dielectronic Recombination on the Relative Populations of the Fe M‐Shell States

Kraemer

Ferland

Gabel

2004

ApJ

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We examine the effects of low-temperature, or Án ¼ 0, dielectronic recombination (DR) on the ionization balance of the Fe M shell (Fe ix-Fe xvi). Since Án ¼ 0 rates are not available for these ions, we have derived estimates based on the existing rates for the first four ionization states of the CNO sequence and newly calculated rates for L-shell ions of third-row elements and Fe. For a range of ionization parameter and column density applicable to the intrinsic absorbers detected in ASCA, Chandra, and XMM-Newton observations of Seyfert galaxies, we generated two grids of photoionization models, with and without DR. The results show that the ionization parameter at which the population of an Fe M-shell ion peaks can increase in some cases by a factor of more than 2 when these rates are included. More importantly, there are dramatic changes in the range in ionization parameter over which individual M-shell ions contain significant fractions of the total Fe (e.g., >10%) in the plasma. These results may explain the mismatch between the range of Fe ionization states detected in the X-ray spectra of Seyfert galaxies, identified by the energies of the M-shell unresolved transition array, and those predicted by photoionization models of the X-ray absorbers that reproduce lines of second-and third-row elements. The results suggest that care should be taken in using third-and fourth-row ions to constrain the physical conditions in photoionized X-ray plasmas until accurate DR rates are available. This underscores the importance of atomic physics in interpreting astronomical spectroscopy.

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“…Several dozens of independent photoionization codes suited for the study of PNe and H ii regions have been constructed over the years. Some of them have been intercompared at several workshops (Péquignot 1986and Ferland & Savin 2001. The codes mainly differ in the numerical treatment of the transfer of the ionizing photons produced in the nebula: on the spot reabsorption, outward-only approximation (most codes presently), full treatment (either with classical techniques as in Rubin 1968or Harrington 1968 or with Monte-Carlo techniques as in Och et al 1998).…”

Section: Photoionization Codesmentioning

confidence: 99%