New Insights on Interstellar Gas-Phase Iron (original) (raw)
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Ja n 20 07 Interstellar Iron and Silicon Depletions in Translucent Sight Lines 1
2007
We report interstellar Fe II and Si II column densities toward six translucent sight lines (AV & 1) observed with the Space Telescope Imaging Spectrograph (STIS). The abundances were determined from the absorption of Si II] at 2335 Å, and several weak Fe transitions including the first reported detections of the 2234 Å line. We derive an empirical f -value for the Fe II λ2234 Å transition of log(fλ) = −1.54 ± 0.05. The observed sight lines sample a variety of extinction characteristics as indicated by their RV values, which range from 2.6 5.8. The dust-phase abundances of both Si and Fe are positively correlated with the small-grain population (effective radii smaller than a few hundred μm) toward
Interstellar Iron and Silicon Depletions in Translucent Sight Lines
The Astrophysical Journal, 2007
We report interstellar Fe II and Si II column densities toward six translucent sight lines (A V 1) observed with the Space Telescope Imaging Spectrograph (STIS). The abundances were determined from the absorption of Si II] at 2335 A, and several weak Fe transitions including the first reported detections of the 2234Å line. We derive an empirical f -value for the Fe II λ2234Å transition of log(f λ) = −1.54 ± 0.05. The observed sight lines sample a variety of extinction characteristics as indicated by their R V values, which range from 2.6 -5.8. The dust-phase abundances of both Si and Fe are positively correlated with the small-grain population (effective radii smaller than a few hundred µm) toward
Astronomy & Astrophysics, 2011
Aims. We evaluate non-local thermodynamical equilibrium (non-LTE) line formation for the two ions of iron and check the ionization equilibrium between Fe i and Fe ii in model atmospheres of the cool reference stars based on the best available complete model atom for neutral and singly-ionized iron. Methods. We present a comprehensive model atom for Fe with more than 3000 measured and predicted energy levels. As a test and first application of the improved model atom, iron abundances are determined for the Sun and five stars with well determined stellar parameters and high-quality observed spectra. The efficiency of inelastic collisions with hydrogen atoms in the statistical equilibrium of iron is empirically estimated from inspection of their different influence on the Fe i and Fe ii lines in the selected stars. Results. Non-LTE leads to systematically depleted total absorption in the Fe i lines and to positive abundance corrections in agreement with the previous studies, however, the magnitude of such corrections is smaller compared to the earlier results. These non-LTE corrections do not exceed 0.1 dex for the solar metallicity and mildly metal-deficient stars, and they vary within 0.21 dex and 0.35 dex in the very metal-poor stars HD 84937 and HD 122563, respectively, depending on the assumed efficiency of collisions with hydrogen atoms. Based on the analysis of the Fe i/Fe ii ionization equilibrium in these two stars, we recommend to apply the Drawin formalism in non-LTE studies of Fe with a scaling factor of 0.1. For the Fe ii lines non-LTE corrections do not exceed 0.01 dex in absolute value over the whole range of stellar parameters that are considered. This study reveals two problems. The first one is that g f-values available for the Fe i and Fe ii lines are not accurate enough to pursue high-accuracy absolute stellar abundance determinations. For the Sun, the mean non-LTE abundance obtained from 54 Fe i lines is 7.56 ± 0.09 and the mean abundance from 18 Fe ii lines varies between 7.41 ± 0.11 and 7.56 ± 0.05 depending on the source of the g f-values. The second problem is that lines of Fe i give, on average, a 0.1 dex lower abundance compared with those of Fe ii lines for HD 61421 and HD 102870, even when applying a differential line-by-line analysis with regard to the Sun. A disparity between neutral atoms and first ions points to problems of stellar atmosphere modelling or/and effective temperature determination.
Iron emission lines in solar and laboratory plasmas
Journal of Physics: Conference Series, 2008
We present a spectrum of the Sun taken by the Cosmic Hot Interstellar Plasma Spectrometer (CHIPS) covering 150 to 275Å. This spectrum is dominated in the 200Å region by emission lines from intermediate charge states of iron (Fe viii -Fe xvi). We also made laboratory measurements of iron emission at temperatures relevent to solar emission on the EBIT-II electron beam ion trap and the Large Helical Device stellerator. We used our laboratory measurements to identify lines and check the CHIANTI database and then used CHIANTI to validate line identifications in the CHIPS spectrum.
The FERRUM Project: New f-value Data for Fe II and Astrophysical Applications
Physica Scripta, 2002
We present the FERRUM Project, an international collaboration aiming at a production and evaluation of oscillator strengths (transition probabilities) of selected spectral lines of singly ionized iron group elements, that are of astrophysical relevance.The results obtained include measurements and calculations of permitted and forbidden lines of Fe II. The data have been applied to both emission and absorption lines in astrophysical spectra. We make comparisons between experimental, theoretical and astrophysical f-values. We give a general review of the various measurements, and discuss the UV8 multiplet of Fe II around 1610 Ð in detail.
Non-LTE line formation for Fe I and Fe II in the atmospheres of A-F type stars
Non-local thermodynamical equilibrium (non-LTE) line formation for neutral and singly-ionized iron is considered through a range of spectral types when the effective temperature varies from 6500 K up to 8500 K, the gravity from log g = 4 down to log g = 3, and the metallicity is solar one. The non-LTE calculations were performed with a comprehensive model atom for iron which was treated in our earlier paper (Mashonkina et al. 2011, A&A, 528, A87). The departures from LTE lead to systematically depleted total absorption in the Fe I lines and positive abundance corrections, in qualitative agreement with the Rentzsch-Holm (1996, A&A, 312, 966) study. However, our predicted magnitude of the non-LTE effects is significantly smaller compared to the previous results due to the use of rather complete model atom of Fe I. The non-LTE abundance corrections do not exceed 0.1 dex for the dwarf models and 0.20 dex for the giant ones. Non-LTE leads to strengthening the Fe II lines, however, the ef...
Kinetic equilibrium of iron in the atmospheres of cool dwarf stars
Astronomy and Astrophysics, 2001
Line formation calculations of Fe i and Fe ii in the solar atmosphere are presented for atomic models of iron including all observed terms and line transitions with available f-values. Recent improved calculations of Fe i photoionization cross-sections are taken into account, and the influence of collision processes is investigated by comparing synthesized and observed solar line flux profiles. The background is represented by the opacity of all important non-iron elements with iron lines added. Using a representative sample of sufficiently unblended strong Fe i and Fe ii line profiles, it is evident that line formation is affected by (a) velocity fields and (b) deviations from local thermodynamic equilibrium (NLTE). The calculations are extended to a systematic analysis demonstrating that the ionization equilibrium of iron is recovered for solar parameters (T eff = 5780 K, log g = 4.44) either using the empirical atmospheric model of Holweger & Müller (1974) and assuming LTE for both Fe i and Fe ii or a line-blanketed theoretical atmospheric model with NLTE iron line formation. In the latter case the kinetic equilibrium of Fe i shows a substantial underpopulation of Fe i terms which depends sensitively on both the improved photoionization calculations and the choice of hydrogen collision rates while the Fe ii ion is well approximated by LTE. Although the source functions of most of the Fe i lines are nearly thermal, their formation is shifted deeper into the photosphere. NLTE wings of strong Fe i lines are therefore shallower than under the LTE assumption, whereas the cores of the strongest lines display the usual chromospheric contributions. Based on both calculated and laboratory f-values the abundances of 37 Fe ii lines range between log εFe ii, = 7.50 and 7.56, depending on atomic and atmospheric models, and those of 117 Fe i lines between log εFe ii, = 7.47 and 7.56, both with a relatively large scatter of 0.08. .. 0.12. The collisional coupling of Fe i levels is investigated. Electron collisions seem to play only a minor role. Hydrogen collisions are very important between terms of low excitation, and they efficiently thermalize the line source functions but not necessarily the populations of the lower levels that determine the line optical depth. Thermalization of those low-excitation terms that are responsible for most of the lines analyzed is achieved only if the collisional coupling among highly excited Fe i terms and their Fe ii parent terms is increased by large factors compared with standard collision rates. Solar flux profiles are reproduced under the assumption of both LTE or NLTE, with nearly all types of atomic and atmospheric models, because the Fe ionization equilibrium depends on the corresponding sets of f-values.
Monthly Notices of the Royal Astronomical Society, 2005
We have calculated the equivalent widths of the absorption lines produced by Fe XXV and Fe XXVI in a Compton-thin, low-velocity photoionized material illuminated by the nuclear continuum in active galactic nuclei. The results, plotted against the ionization parameter and the column density of the gas, are a complement to those presented by Bianchi & Matt for the emission lines from the same ionic species. As an extension to the work by Bianchi & Matt, we also present a qualitative discussion on the different contributions to the He-like iron emission line complex in the regimes where recombination or resonant scattering dominates, providing a useful diagnostic tool to measure the column density of the gas. Future high-resolution missions (e.g. Astro-E2) will allow us to fully take advantage of these plasma diagnostics. In the meantime, we compare our results with an up-to-date list of Comptonthick and unobscured (at least at the iron line energy) Seyfert galaxies with emission and/or absorption lines from Hand He-like iron observed with Chandra and XMM-Newton.
Highly Ionized Iron Absorption Lines from Outflowing Gas in the X-Ray Spectrum of NGC 1365
The Astrophysical Journal, 2005
We present the discovery of four absorption lines in the X-ray spectrum of the Seyfert Galaxy NGC 1365, at energies between 6.7 and 8.3 keV. The lines are detected with high statistical confidence (from > 20σ for the strongest to ∼ 4σ for the weakest) in two XMM-Newton observations 60 ksec long. We also detect the same lines, with lower signal-to-noise (but still > 2σ for each line) in two previous shorter (∼ 10 ksec) XMM observations. The spectral analysis identifies these features as FeXXV and FeXXVI Kα and Kβ lines, outflowing with velocities varying between ∼ 1000 to ∼ 5000 km s −1 among the observations. These are the highest quality detections of such lines so far. The high equivalent widths (EW(Kα) ∼ 100 eV) and the Kα/Kβ ratios imply that the lines are due to absorption of the AGN continuum by a highly ionized gas with column density N H ∼ 5 × 10 23 cm −2 at a distance of ∼ 50-100 R S from the continuum source.
Discovery of Iron in PG1159 Stars
AIP Conference Proceedings, 2010
The lack of Fe VII lines in PG1159 stars had led to the conclusion that in some objects iron must be strongly depleted. We have now detected Fe X lines in FUSE spectra of the very hottest PG1159 stars (T eff = 150 000-200 000 K; RX J2117.1+3412, K 1−16, NGC 246, Longmore 4). Surprisingly, we derive a solar iron abundance. It is conspicuous that they are among the most massive PG1159 stars (0.71-0.82 M ⊙), in contrast to those objects for which strongest Fe-deficiency was claimed (0.53-0.56 M ⊙). Based on new Fe VIII line identifications in SOHO/SUMER UV spectra of the Sun, we were able to detect these lines in FUSE spectra of several "cooler" (T eff < ∼ 150 000) objects, among them is the prototype PG1159−035. An abundance analysis is in progress.