E. Verner

The Binarity of η Carinae Revealed from Photoionization Modeling of the Spectral Variability of the Weigelt Blobs B and D

We focus on two Hubble Space Telescope Space Telescope Imaging Spectrograph (HST STIS) spectra of the Weigelt blobs B and D, extending from 1640 to 10400 A, one recorded during the 1998 minimum (1998 March) and the other recorded in 1999 February, early in the following broad maximum. The spatially resolved spectra suggest two distinct ionization regions. One structure is the permanently low-ionization cores of the Weigelt blobs B and D, located several hundred AU from the ionizing source. Their spectra are dominated by emission from H I, [N II], Fe II, [Fe II], Ni II, [Ni II], Cr II, and Ti II. The second region, relatively diffuse in character and located between the ionizing source and the Weigelt blobs, is more highly ionized with emission from [Fe III], [Fe IV], N III], [Ne III], [Ar III], [Si III], [S III], and He I. Through photoionization modeling, we find that the radiation field from the more massive B-star companion supports the low-ionization structure throughout the 5.54 yr period. The radiation field of an evolved O star is required to produce the higher ionization emission seen across the broad maximum. This emission region is identified with slow-moving condensations photoionized by the O star and located in the extended mass flow emanating from the B-star primary. Comparison between the models and observations reveals that the high-ionization region is physically distinct (nH ≈ 107 cm-3 and Te ~ 104 K) from the B and D blobs (nH ≈ 106 cm-3 and Te ~ 7000 K).

Waiting in the wings: Reflected X-ray emission from the Homunculus nebula

We report the first detection of X-ray emission associated with the Homunculus nebula that surrounds the supermassive star η Car. The emission is characterized by a temperature in excess of 100 MK and is consistent with scattering of the time-delayed X-ray flux associated with the star. The nebular emission is bright in the northwestern lobe and near the central regions of the Homunculus, and fainter in the southeastern lobe. We also report the detection of an unusually broad Fe K fluorescent line, which may indicate fluorescent scattering off the wind of a companion star or some other high-velocity outflow. The X-ray Homunculus is the nearest member of the small class of Galactic X-ray reflection nebulae, and the only one in which both the emitting and reflecting sources are distinguishable.

The Absorption Spectrum of High-Density Stellar Ejecta in the Line of Sight to η Carinae

Using the high-dispersion near-UV (NUV) mode of the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope (HST) to observe η Carinae, we have resolved and identified over 500 sharp circumstellar absorption lines of iron-group singly ionized and neutral elements with ≈20 velocity components ranging from -146 to -585 km s-1. These lines are from transitions originating from ground and metastable levels as high as 40,000 cm-1 above ground. The absorbing material is located either in dense inhomogeneities in the stellar wind, in the warm circumstellar gas immediately in the vicinity of η Car, or within the cooler foreground lobe of the Homunculus nebula. We have used classical curve-of-growth analysis to derive atomic level populations for Fe II at -146 km s-1 and for Ti II at -513 km s-1. These populations, plus photoionization and statistical equilibrium modeling, provide electron temperatures, Te, densities, nH, and constraints on distances from the stellar source, d. For the -146 km s-1 component, we derive Te = 6400 K, nH ≥ 107-108 cm-3, and d ≈ 1300 AU. For the -513 km s-1 component, we find a much cooler temperature, Te = 760 K, with nH ≥ 107 cm-3; we estimate d ≈ 10,000 AU. The large distances for these two components place the absorptions in the vicinity of identifiable ejecta from historical events, not near or in the dense wind of η Car. Further analysis, in parallel with obtaining improved experimental and theoretical atomic data, is underway to determine what physical mechanisms and elemental abundances could explain the large number of strong circumstellar absorption features in the spectrum of η Car.

Revisited Abundance Diagnostics in Quasars: Fe II/Mg II Ratios

Both the Fe II UV emission in the 2000-3000 A region [Fe II(UV)] and resonance emission-line complex of Mg II at 2800 A are prominent features in quasar spectra. The observed Fe II(UV)/Mg II emission ratios have been proposed as means to measure the buildup of the Fe abundance relative to that of the α-elements C, N, O, Ne, and Mg as a function of redshift. The current observed ratios show large scatter and no obvious dependence on redshift. Thus, it remains unresolved whether a dependence on redshift exists and whether the observed Fe II(UV)/Mg II ratios represent a real nucleosynthesis diagnostic. We have used our new 830 level model atom for Fe+ in photoionization calculations, reproducing the physical conditions in the broad-line regions of quasars. This modeling reveals that interpretations of high values of Fe II(UV)/Mg II are sensitive not only to Fe and Mg abundance, but also to other factors such as microturbulence, density, and properties of the radiation field. We find that the Fe II(UV)/Mg II ratio combined with Fe II(UV)/Fe II(optical) emission ratio, where Fe II(optical) denotes Fe II emission in 4000-6000 A band, can be used as a reliable nucleosynthesis diagnostic for the Fe/Mg abundance ratios for the physical conditions relevant to the broad-line regions of quasars. This has extreme importance for quasar observations with the Hubble Space Telescope and also with the future James Webb Space Telescope.

Fe II Diagnostic Tools for Quasars

The enrichment of Fe relative to α-elements such as O and Mgrepresents a potential means to determine the age of quasars and probethe galaxy formation epoch. To explore how Fe II emission in quasars islinked to physical conditions and abundance, we have constructed an 830level Fe II model atom and investigated through photoionizationcalculations how Fe II emission strengths depend on nonabundancefactors. We have split Fe II emission into three major wavelength bands,Fe II (UV), Fe II (Opt1), and Fe II (Opt2), and explore how the Fe II(UV)/Mg II, Fe II (UV)/Fe II (Opt1), and Fe II (UV)/Fe II (Opt2)emission ratios depend on hydrogen density and ionizing flux in thebroad-line regions (BLRs) of quasars. Our calculations show that (1)similar Fe II (UV)/Mg II ratios can exist over a wide range of physicalconditions, (2) the Fe II (UV)/Fe II (Opt1) and Fe II (UV)/Fe II (Opt2)ratios serve to constrain ionizing luminosity and hydrogen density, and(3) flux measurements of Fe II bands and knowledge of the ionizing fluxprovide tools to derive distances to BLRs in quasars. To derive all BLRphysical parameters with uncertainties, comparisons of our model withobservations of a large quasar sample at low redshift (z<1) aredesirable. The STIS and NICMOS instruments aboard the Hubble SpaceTelescope offer the best means to provide such observations. (Less)

Discovery of CH and OH in the –513 km s–1 Ejecta of η Carinae

The very massive star η Carinae (η Car) is enshrouded in an unusual complex of stellar ejecta, which is highly depleted in C and O and enriched in He and N. This circumstellar gas gives rise to distinct absorption components corresponding to at least 20 different velocities along the line of sight. The velocity component at -513 km s-1 exhibits very low ionization with predominantly neutral species of iron-peak elements. Our statistical equilibrium/photoionization modeling indicates that the low temperature (T = 760 K) and high density (nH ~ 107 cm-3) of the -513 km s-1 component is conducive to molecule formation including those with the elements C and O. Examination of echelle spectra obtained with the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST) confirms the models predictions. The molecules H2, CH, and most likely OH have been identified in the -513 km s-1 absorption spectrum. This paper presents the analysis of the HST STIS spectra with the deduced column densities for CH, OH, and C I and an upper limit for CO. It is quite extraordinary to see molecular species in a cool environment at such a high velocity. The sharp molecular and ionic absorptions in this extensively CNO-processed material offer us a unique environment for studying the chemistry, dust formation processes, and nucleosynthesis in the ejected layers of a highly evolved massive star.

Fe II emission spectra in AGN: observations and theoretical interpretation

The enrichment of Fe, relative to alpha elements such as O and Mg, represents a potential means to measure the ages of quasi-stellar object (QSO) host galaxies and probe nucleosynthesis in the early universe. QSOs exhibit prominent Fe II features and Mg II 2800 angstrom resonance doublet emission in the ultraviolet. Although chemical evolutionary models predict that the Fe/Mg abundance ratio decreases with increasing redshift, measurements of QSO Fe II (UV)/Mg II emission line ratios show large scatter from 1 to 20, with no redshift dependency up to z similar to 6.4. Before using Fe II emission as an abundance indicator, one must ascertain how Fe II emission varies with physical conditions. We have constructed an 830-level model atom for Fe II and used it in a photoionization code to calculate Fe II emission. This model is more sophisticated than previous efforts, and uses the most recent laboratory atomic data and includes the numerous transitions that are sensitive to the strong radiation field in QSOs. Predicted Fe II(UV)/Mg II ratios and fluxes strongly depend on non-abundance factors such as microturbulence, ionizing flux, and hydrogen density; all must be taken into account before any accurate abundance can be derived. Our calculations show that Fe II is the dominant coolant at densities found in active galactic nucleus (AGN) broad emission line regions (BLRs), and must be included in photoionization modelling. Our close collaboration with spectroscopists at Lund University has been highly beneficial for further development of our Fe ii model, most importantly through atomic data studies that link high-energy levels in Fe ii. Additional studies of the atomic structure of Fe ii are necessary to improve our understanding of the AGN continua by accounting for the effects of the Fe ii pseudo-continuum, which blanket QSO spectra from 1000 to 10 000 angstrom. Predicted Fe ii emission spectra, suitable for BLRs in AGN, are available at http://iacs.cua.edu/people/verner/FeII. (Less)

Elucidating the Correlation of the Quasar Fe II/Mg II Ratio with Redshift

Interpretation of the Fe II(UV)/Mg II emission ratios from quasars has a major cosmological motivation. Both Fe and Mg are produced by short-lived massive stars. In addition, Fe is produced by accreting white dwarf supernovae somewhat after star formation begins. Therefore, we expect that the Fe/Mg ratio will gradually decrease with redshift. We have used data from the Sloan Digital Sky Survey to explore the dependence of the Fe II(UV)/Mg II ratio on redshift and on luminosity in the redshift range of 0.75 < z < 2.20, and we have used predictions from our 830-level model for the Fe II atom in photoionization calculations to interpret our findings. We have split the quasars into several groups based on the value of their Fe II(UV)/Mg II emission ratios and then checked to see how the fraction of quasars in each group varies with the increase of redshift. We next examined the luminosity dependence of the Fe II(UV)/Mg II ratio, and we found that beyond a threshold of Fe II(UV)/Mg II = 5, and M2500 < -25 mag, the Fe II(UV)/Mg II ratio increases with luminosity, as predicted by our model. We interpret our observed variation of the Fe II(UV)/Mg II ratio with redshift as a result of the correlation of redshift with luminosity in a magnitude-limited quasar sample.

The Fe/Mg Abundance Ratio: A Diagnostic of Nucleosynthesis in the Early Universe?

Mt. Stromlo Observatory, Canberra, ACT 2611, Australiae-mail: peterson@mso.anu.edu.auAbstract. The Fe/Mg abundance ratio may be one of the fundamental indicators for nucle-osynthesis in the Early Universe. Even at the highest redshift, QSO broad-lined regions (BLRs)exhibit prominent 2000-3000˚A Fe II(UV) band and Mg II 2800 ˚A resonance doublet emission inthe restframe UV. The Mg is formed in Type-II SNe, while Fe has been traditionally thoughtto be produced in Type Ia SNe. These different origins imply a sharp falloff in Fe abundanceat very high-z. However, these predictions are clouded by uncertainties about the nature of thefirst stars and in the nuclear yields from supernovae models. Our theoretical studies of Fe IIin QSO BLRs show that Fe and Mg abundance cannot be directly deduced from the observedFe II(UV)/Mg II, because it is sensitive to luminosity and microturbulence, as well as abun-dance. Observationally, support for a luminosity dependence comes from SDSS data for QSOsthat show a Fe II(UV)/Mg II correlation with luminosity at z