Krister Emanuel Nielsen

X-Ray Spectral Variation of η Carinae through the 2003 X-Ray Minimum

We report the results of an observing campaign on η Car around the 2003 X-ray minimum, mainly using the XMM-Newton observatory. These are the first spatially resolved X-ray monitoring observations of the stellar X-ray spectrum during the minimum. The hard X-ray emission, associated with the wind-wind collision (WWC) in the binary system, varied strongly in flux on timescales of days, but not significantly on timescales of hours. The X-ray flux in the 2-10 keV band seen by XMM-Newton was only 0.7% of the flux maximum seen by RXTE. The slope of the X-ray continuum above 5 keV did not vary in any observation, which suggests that the electron temperature of the hottest plasma did not vary significantly at any phase. Through the minimum, the absorption to the stellar source increased by a factor of 5-10 to NH ~ (3-4) × 1023 cm-2. These variations were qualitatively consistent with emission from the WWC plasma entering into the dense wind of the massive primary star. During the minimum, X-ray spectra also showed significant excesses in the thermal Fe XXV emission line on the red side, while they showed only a factor of 2 increase in equivalent width of the Fe fluorescence line at 6.4 keV. These features are not fully consistent with the eclipse of the X-ray plasma and may suggest an intrinsic fading of the X-ray emissivity. The drop in the WWC emission revealed the presence of an additional X-ray component that exhibited no variation on timescales of weeks to years. This component may be produced by the collision of high-speed outflows at v ~ 1000-2000 km s-1 from η Car with ambient gas within a few thousand AU from the star.

The UV scattering halo of the central source associated with η carinae

We have made an extensive study of the UV spectrum ofCarinae and find that we do not directly observe the star and its wind in the UV. Because of dust along our line of sight, the UV light that we observe arises from bound-bound scattering at large impact parameters. We obtain a reasonable fit to the UV spectrum by using only the flux that originates outside 0B033. This explains why we can still observe the primary star in the UV despite the large optical extinction: it is due to the presence of an intrinsic coronagraph in theCar system and to the extension of the UV-emitting region. It is not due to peculiar dust properties alone. We have computed the spectrum of the purported companion star and show that it could only be directly detected in the UV spec- trum, preferentially in the FUSE spectral region (912-1175 8). However, we find no direct evidence for a com- panion star, with the properties indicated by X-ray studies and studies of the Weigelt blobs, in UV spectra. This might be due to reprocessing of the companions light by the dense stellar wind of the primary. Broad Fe ii and (Feii) emission lines, which form in the stellar wind, are detected in spectra taken in the southeastern lobe, 0B2 from the central star. The wind spectrum shows some similarities to the spectra of the B and D Weigelt blobs but also shows some marked differences in that lines pumped by Lyare not seen. The detection of the broad lines lends support to our interpretation of the UV spectrum and to our model forCar. Subject headingg stars: atmospheres — stars: early-type — stars: fundamental parameters — stars: individual (� Carinae) — stars: mass loss — ultraviolet: stars Online material: color figures

η Carinae across the 2003.5 minimum: Spectroscopic evidence for massive binary interactions

We have analyzed high spatial, moderate spectral resolution observations of η Carinae (η Car) obtained with the STIS from 1998.0 to 2004.3. The data were obtained at discrete times covering an entire 2024 day spectroscopic cycle, with focus on the X-ray/ionization low state that began in 2003 June. The spectra show prominent P Cygni lines in H I, Fe II, and He I, which are complicated by blends and contamination by nebular emission and absorption. All lines show phase- and species-dependent variations in emission and absorption. For most of the cycle the He I emission is blueshifted relative to the H I and Fe II P Cygni emission lines, which are centered at approximately system velocity. The blueshifted He I absorption components vary in intensity and velocity throughout the 2024 day period. We construct radial velocity curves for the absorption component of the He I and H I lines. The He I absorption shows significant radial velocity variations throughout the cycle, with a rapid change of over 200 km s-1 near the 2003.5 event. The H I velocity curve is similar to that of the He I absorption, although offset in phase and reduced in amplitude. We interpret the complex line profile variations in He I, H I, and Fe II to be a consequence of the dynamic interaction of the dense wind of η Car A with the less dense, faster wind plus the radiation field of a hot companion star, η Car B. We use the variations seen in He I and the other P Cygni lines to constrain the geometry of the orbit and the character of η Car B.

The FERRUM project: improved experimental oscillator strengths in CrII

We report absolute oscillator strengths for 119 Cr II transitions in the wavelength region 2050-4850 angstrom. The transition probabilities have been derived by combining radiative lifetimes, measured with time- resolved laser induced fluorescence, and branching fractions from intensity calibrated Fouirer transform spectrometer data. New radiative lifetimes for the 3d(4)(D-5)4p F-4, D-4 and P-6 terms are reported, adding up to a total of 25 energy levels with measured lifetimes used to derive this improved set of atomic data.

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.

Detection of high-velocity material from the wind-wind collision zone of Eta Carinae across the 2009.0 periastron passage

We report near-infrared spectroscopic observations of the Eta Carinae massive binary system during 2008-2009 using the CRIRES spectrograph mounted on the 8m UT 1 Very Large Telescope (VLT Antu). We detect a strong, broad absorption wing in He I lambda 10833 extending up to -1900 km s(-1) across the 2009.0 spectroscopic event. Analysis of archival Hubble Space Telescope/Space Telescope Imaging Spectrograph ultraviolet and optical data identifies a similar high-velocity absorption (up to -2100 km s(-1)) in the ultraviolet resonance lines of Si IV lambda lambda 1394, 1403 across the 2003.5 event. Ultraviolet resonance lines from low-ionization species, such as Si II lambda lambda 1527, 1533 and CII lambda lambda 1334, 1335, show absorption only up to -1200 km s(-1), indicating that the absorption with velocities -1200 to -2100 km s(-1) originates in a region markedly more rapidly moving and more ionized than the nominal wind of the primary star. Seeing-limited observations obtained at the 1.6m OPD/LNA telescope during the last four spectroscopic cycles of Eta Carinae (1989-2009) also show high-velocity absorption in He I lambda 10833 during periastron. Based on the large OPD/LNA dataset, we determine that material with velocities more negative than -900 km s(-1) is present in the phase range 0.976 = 1.049. Therefore, we constrain the duration of the high-velocity absorption to be 95 to 206 days (or 0.047 to 0.102 in phase). We propose that the high-velocity absorption component originates in shocked gas in the wind-wind collision zone, at distances of 15 to 45 AU in the line-of-sight to the primary star. With the aid of three-dimensional hydrodynamical simulations of the wind-wind collision zone, we find that the dense high-velocity gas is along the line-of-sight to the primary star only if the binary system is oriented in the sky such that the companion is behind the primary star during periastron, corresponding to a longitude of periastron of omega similar to 240 degrees-270 degrees. We study a possible tilt of the orbital plane relative to the Homunculus equatorial plane and conclude that our data are broadly consistent with orbital inclinations in the range i = 40 degrees-60 degrees. (Less)

Eta Carinae across the 2003.5 Minimum: The Character and Variability of the Ejecta Absorption in the Near-Ultraviolet

We present Hubble Space Telescope (HST ) Space Telescope Imaging Spectrograph (STIS) high-resolution near- ultraviolet (NUV) spectra ofCars central source recorded before, during and after the 2003.5 spectroscopic minimum. Our focus is on the narrow absorption lines formed in multiple circumstellar shells, superimposed on the broad P Cygni stellar wind features, and how they respond to the flux variations ofCar across the minimum. Over 30 separate narrow-line velocity components are identified: three around � 146 km s � 1 , many between � 323 and � 587kms � 1 ,andafewexceeding � 1000kms � 1 .Ingeneral,excitationdecreaseswithincreasingvelocityindicating that the primary excitation is by UV stellar photons and that the slower shells are located closer to the central source. Twowell-isolatedvelocitysystemsat � 146and � 513kms � 1 displayverydifferentspectralcharacteristicsregarding ionization/excitation and respond differently toCars spectroscopic minimum. The � 146 km s � 1 shell, associated with the (internal) Little Homunculus, is ionized across the broad spectroscopic maximum but relaxes during the few months long minimum. The � 146 km s � 1 component is joined by adjacent velocity components in lines of singly ionized iron-group elements. Ti ii and V ii absorptions appear during the minimum, most likely caused by a decrease in Lyman-ionizing flux. The � 513 km s � 1 component, part of the (outer) Homunculus, does not show significant changes in atomic absorptions, but intermediate-velocity components between � 369 and � 503 km s � 1 vary during the minimum. We present the NUV spectrum and describe its general characteristics, but we focus on identifications of thenebularabsorptionlinesandtheirvariationsacrossthe2003.5minimum.Thecompletespectrumbetween2380 and 3160 8 with line identifications is available in the electronic edition of this paper.

The FERRUM Project: laboratory-measured transition probabilities for Cr II

Aims: We measure transition probabilities for Cr II transitions from the z ^4H_J, z ^2D_J, y ^4F_J, and y ^4G_J levels in the energy range 63000 to 68000 cm^{-1}. Methods:Radiative lifetimes were m ...Aims. We measure transition probabilities for Cr ii transitions from the z HJ , z DJ , y FJ , and y GJ levels in the energy range 63 000 to 68 000 cm−1. Methods. Radiative lifetimes were measured using time-resolved laser-induced fluorescence from a laser-produced plasma. In addition, branching fractions were determined from intensity-calibrated spectra recorded with a UV Fourier transform spectrometer. The branching fractions and radiative lifetimes were combined to yield accurate transition probabilities and oscillator strengths. Results. We present laboratory measured transition probabilities for 145 Cr ii lines and radiative lifetimes for 14 Cr ii levels. The laboratory-measured transition probabilities are compared to the values from semi-empirical calculations and laboratory measurements in the literature.

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.

η Carinae's Dusty Homunculus Nebula from Near-infrared to Submillimeter Wavelengths: Mass, Composition, and Evidence for Fading Opacity

Infrared observations of the dusty, massive Homunculus Nebula around the luminous blue variable η Carinae are crucial to characterize the mass-loss history and help constrain the mechanisms leading to the Great Eruption. We present the 2.4 - 670 μm spectral energy distribution, constructed from legacy ISO observations and new spectroscopy obtained with the Herschel Space Observatory. Using radiative transfer modeling, we find that the two best-fit dust models yield compositions which are consistent with CNO-processed material, with iron, pyroxene and other metal-rich silicates, corundum, and magnesium-iron sulfide in common. Spherical corundum grains are supported by the good match to a narrow 20.2 μm feature. Our preferred model contains nitrides AlN and Si3N4 in low abundances. Dust masses range from 0.25 to 0.44 M ʘ but M tot ≥ 45 M ʘ in both cases due to an expected high Fe gas-to-dust ratio. The bulk of dust is within a 5″ × 7″ central region. An additional compact feature is detected at 390 μm. We obtain L IR = 2.96 × 106 L ʘ, a 25% decline from an average of mid-IR photometric levels observed in 1971-1977. This indicates a reduction in circumstellar extinction in conjunction with an increase in visual brightness, allowing 25-40% of optical and UV radiation to escape from the central source. We also present an analysis of 12CO and 13CO J = 5 - 4 through 9 - 8 lines, showing that the abundances are consistent with expectations for CNO-processed material. The [12C II] line is detected in absorption, which we suspect originates in foreground material at very low excitation temperatures.