Fe Ionization Research Articles

KELT-17: a chemically peculiar Am star and a hot-Jupiter planet

Context. There is very little information to be found in the literature regarding the detection of planets orbiting chemically peculiar stars. Aims. Our aim is to determine the detailed chemical composition of the remarkable planet host star KELT-17. This object hosts a hot-Jupiter planet with 1.31 MJup detected by transits, and it is one of the more massive and rapidly rotating planet hosts seen to date. We set out to derive a complete chemical pattern for this star, in order to compare it with those of chemically peculiar stars. Methods. We carried out a detailed abundance determination in the planet host star KELT-17 via spectral synthesis. Stellar parameters were estimated iteratively by fitting Balmer line profiles and imposing the Fe ionization balance using the SYNTHE program together with plane-parallel ATLAS12 model atmospheres. Specific opacities for an arbitrary composition and microturbulence velocity vmicro were calculated through the opacity sampling (OS) method. The abundances were determined iteratively by fitting synthetic spectra to metallic lines of 16 different chemical species using SYNTHE. The complete chemical pattern of KELT-17 was compared to the recently published average pattern of Am stars. We estimated the stellar radius using two methods: a) comparing the synthetic spectral energy distribution with the available photometric data and the Gaia parallax, and b) using a Bayesian estimation of stellar parameters using stellar isochrones. Results. We found over-abundances of Ti, Cr, Mn, Fe, Ni, Zn, Sr, Y, Zr, and Ba, together with subsolar values of Ca and Sc. Notably, the chemical pattern agrees with those recently published for Am stars, making KELT-17 the first exoplanet host whose complete chemical pattern is unambiguously identified with this class. The stellar radius derived by two different methods agrees to each other and with those previously obtained in the literature.

Radiative interaction between the relativistic jet and optically thick envelope in tidal disruption events

Reverberation observations have uncovered an Fe K{\alpha} fluorescence line in the tidal disruption event (TDE) Swift J1644+57 (Kara et al. 2016). The discovery paper used the lag spectrum to argue that the X-ray continuum source was located very close to the blackhole (~30 gravitational radii) and moved sub-relativistically. We reanalyze the lag spectrum, pointing out that dilution effects cause it to indicate a geometric scale an order of magnitude larger than previously inferred. If the X-ray continuum is produced by a relativistic jet, as suggested by rapid variability, high luminosity and hard spectrum, this larger scale predicts an Fe ionization state consistent with efficient K{\alpha} production. Moreover, the momentum of the jet radiation impinging on the surrounding accretion flow on this larger scale accelerates a layer of gas to speeds ~0.1-0.2c, consistent with the blueshifted line profile. Implications of our results on the global picture of jetted TDEs are discussed. A power-law {\gamma}/X-ray spectrum may be produced by external UV-optical photons being repetitively inverse-Compton scattered by cold electrons in the jet, although our model for the K{\alpha} reverberation does not depend on the jet radiation mechanism (magnetic reconnection in a Poynting jet is still a viable mechanism). The non-relativistic wind driven by jet radiation may explain the late-time radio rebrightening in Swift J1644+57. This energy injection may also cause the thermal UV-optical emission from jetted TDEs to be systematically brighter than in non-jetted ones.

The effect of laser contrast on generation of highly charged Fe ions by ultra-intense femtosecond laser pulses

Experimental studies on the formation of highly charged ions of medium-Z elements using femtosecond laser pulses with different contrast levels were carried out. Multiply charged Fe ions were generated by laser pulses with 35 fs duration and an intensity exceeding 1021 W/cm2. Using high-resolution X-ray spectroscopic methods, bulk electron temperature of the generated plasma has been identified. It is shown that the presence of a laser pre-pulse at a contrast level of 105–106 with respect to the main pulse drastically decreases the degree of Fe ionization. We conclude that an effective source of energetic, multiply charged moderate and high-Z ions based on femtosecond laser–plasma interactions can be created only using laser pulses of ultra-high contrast.

ON THE NATURE OF THE PROTOTYPE LUMINOUS BLUE VARIABLE AG CARINAE. II. WITNESSING A MASSIVE STAR EVOLVING CLOSE TO THE EDDINGTON AND BISTABILITY LIMITS

We show that the significantly different effective temperatures (Teff) achieved by the luminous blue variable AG Carinae during the consecutive visual minima of 1985–1990 (Teff ≃ 22,800 K) and 2000–2001 (Teff ≃ 17,000 K) place the star on different sides of the bistability limit, which occurs in line-driven stellar winds around Teff ∼ 21,000 K. Decisive evidence is provided by huge changes in the optical depth of the Lyman continuum in the inner wind as Teff changes during the S Dor cycle. These changes cause different Fe ionization structures in the inner wind. The bistability mechanism is also related to the different wind parameters during visual minima: the wind terminal velocity was 2–3 times higher and the mass-loss rate roughly two times smaller in 1985–1990 than in 2000–2003. We obtain a projected rotational velocity of 220 ± 50 km s−1 during 1985–1990 which, combined with the high luminosity (L⋆ = 1.5 × 106 L☉), puts AG Car extremely close to the Eddington limit modified by rotation (ΩΓ limit): for an inclination angle of 90°, ΓΩ ≳ 1.0 for M☉ ≲ 60. Based on evolutionary models and mass budget, we obtain an initial mass of ∼100 M☉ and a current mass of ∼60–70 M☉ for AG Car. Therefore, AG Car is close to, if not at, the ΩΓ limit during visual minimum. Assuming M = 70 M☉, we find that ΓΩ decreases from 0.93 to 0.72 as AG Car expands toward visual maximum, suggesting that the star is not above the Eddington limit during maximum phases.

Multispacecraft observations of a prominence eruption

Abstract. On 9 May 2007 a prominence eruption occurred at the West limb. Remarkably, the event was observed by the STEREO/EUVI telescopes and by the HINODE/EIS and SOHO/UVCS spectrometers. We present results from all these instruments. High-cadence (~37 s) data from STEREO/EUVI A and B in the He II λ304 line were used to study the 3-D shape and expansion of the prominence. The high spatial resolution EUVI images (~1.5"/pixel) have been used to infer via triangulation the 3-D shape and orientation of the prominence 12 min after the eruption onset. At this time the prominence has mainly the shape of a "hook" highly inclined southward, has an average thickness of 0.068 R⊙, a length of 0.43 R⊙ and lies, in first approximation, on a plane. Hence, the prominence is mainly a 2-D structure and there is no evidence for a twisted flux rope configuration. HINODE/EIS was scanning with the 2" slit the region where the filament erupted. The EIS spectra show during the eruption remarkable non-thermal broadening (up to ~100 km s−1) in the region crossed by the filament in spectral lines emitted at different temperatures, possibly with differences among lines from higher Fe ionization stages. The CME was also observed by the SOHO/UVCS instrument: the spectrograph slit was centered at 1.7 R⊙, at a latitude of 5° SW and recorded a sudden increase in the O VI λλ1032–1037 and Si XII λ520 spectral line intensities, representative of the CME front transit.

X-Ray Spectroscopy of SN 1006 with Suzaku

We report on observations of SN 1006 with Suzaku. We firmly detected K-shell emission from Fe, for the first time, and found that the Fe ionization state is quite low. The broad-band spectrum extracted from southeast of the remnant was well-fitted with a model consisting of three optically thin thermal non-equilibrium ionization plasmas and a power-law component. Two of the thermal models are highly overabundant in heavy elements, and hence are likely due to ejecta. These components have different ionization parameters, $n_{\rm e}t \sim 1.4\times 10^{10}$cm$^{-3}$s and $n_{\rm e}t \sim 7.7\times 10^8$cm$^{-3}$s; it is the later one that produces Fe-K emission. This suggests that Fe has been heated by reverse shock more recently than the other elements, consistent with a picture where the ejecta are stratified by composition with Fe in the interior. On the other hand, the third thermal component is assumed to be solar abundance, and we associate it with emission from the interstellar medium. The electron temperature ($kT_{\rm e} \sim$0.5keV) is lower than that expected from the shock velocity, which suggests a lack of collisionless electron heating at the forward shock. The extremely low ionization parameter and extreme non-equilibrium state are due to the low density of the ambient medium.

Acceleration and Transport Modeling of Solar Energetic Particle Charge States for the Event of 1998 September 9

The 1998 September 9 solar particle event was a 3He-rich solar particle event that showed a strong increase of Fe ionization states in the energy range below 1 MeV nucleon-1. We have investigated this event by fitting Wind and ACE observations using a model of acceleration and stripping near the Sun, followed by particle transport in the interplanetary medium taking into account particle focusing, pitch-angle scattering, adiabatic deceleration, and convection. The simulation provides a reconstruction of the injection function of the energetic particles released from the Sun and their time, energy, and charge dependence. We find that electrons and Fe ions are injected almost impulsively, whereas the injection of protons takes place on a much longer timescale or even consists of two distinct injection processes. We are able to obtain good overall fits to the observations. This suggests that our model can be used to obtain information about the conditions in the acceleration region such as density, temperature, and the timescales of the acceleration process, if sufficiently accurate modeling of the particle transport in the solar wind is possible.

Possible AGN Shock Heating in the Cool‐Core Galaxy Cluster Abell 478

We present a detailed X-ray study of the intracluster medium (ICM) of the nearby, cool-core galaxy cluster Abell 478, with Chandra and XMM observations. Using a wavelet smoothing hardness analysis, we derive detailed temperature maps of A478, revealing a surprising amount of temperature structure. The broad band Chandra spectral fits yield temperatures which are significantly hotter than those from XMM, but the Fe ionization temperature shows good agreement. We show that the temperature discrepancy is slightly reduced when comparing spectra from regions selected to enclose nearly isothermal gas. However, by simulating multi-temperature spectra and fitting them with a single temperature model, we find no significant difference between Chandra and XMM, indicating that non-isothermality cannot fully explain the discrepancy. We have discovered 4 hot spots located between 30--50 kpc from the cluster center, where the gas temperature is roughly a factor of 2 higher than in the surrounding material. We estimate the combined excess thermal energy present in these hot spots to be (3+/-1)x10^59 erg. The location of and amount of excess energy present in the hot spots are suggestive of a common origin within the cluster core, which hosts an active galactic nucleus. This cluster also possesses a pair of X-ray cavities coincident with weak radio lobes, as reported in a previous analysis, with an associated energy <10% of the thermal excess in the hot spots. The presence of these hot spots could indicate strong-shock heating of the ICM from the central radio source -- one of the first such detections in a cool core cluster. We also probe the mass distribution in the core and find it to be characterized by a logarithmic slope of -0.35+/-0.22, which is significantly flatter than an NFW cusp of -1. (abridged)

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

We examine the effects of low-temperature, or $\Delta$$n = 0$, dielectronic recombination (DR) on the ionization balance of the Fe M-Shell (Fe IX through Fe XVI). Since $\Delta$$n = 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 3rd row elements and Fe. For a range of ionization parameter and column density applicable to the intrinsic absorbers detected in {\it ASCA}, {\it Chandra}, and {\it 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 peakscan increase in some cases by factor $>$ 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 Seyferts, 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 3rd and 4th 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.

Feiv] Emission in Ionized Nebulae

This paper presents an analysis of [Fe IV] emission based on new identifications and previous measurements of [Fe IV] lines in 30 Doradus, IC 4846, M42, SMC N88A, and SBS 0335-052. The Fe abundances obtained by adding the abundances of the relevant Fe ions (mainly Fe^{++} and Fe^{3+}) are found to be lower, by factors in the range 2.6-5.9, than the Fe abundances implied by [Fe III] emission and an ionization correction factor derived from ionization models. The most likely explanation of this discrepancy is that either the collision strengths for [Fe IV] or the Fe ionization fractions predicted by models are unreliable. The available data neither allow one to distinguish between these two possibilities nor to exclude another possible explanation: that the discrepancy implies the presence of a gradient in the Fe abundance within the ionized gas. Further measurements of [Fe IV] lines and checks on the Fe^{3+} atomic data and ionization models are needed to reach a definitive conclusion. The discrepancy introduces an uncertainty in the determination of Fe abundances in ionized nebulae. This uncertainty has implications for our understanding of both the evolution of dust in ionized nebulae and the chemical history of low metallicity galaxies.

The Updated fe Ionization Equilibrium for the Electron K-Distributions

In the past few years new calculations of the ionization and recombination rates have been published. The new Fe ionization equilibrium for these new rates is available for a Maxwellian distribution. Therefore the updated Fe ionization equilibrium for the non-thermal κ-distribution with an enhanced number of particles in the high-energy tail is presented. Results for the various deviations from a Maxwellian distribution are given in tabular form and these are compared with previous ones. A method for the determination of an energy distribution different from the Maxwellian one is suggested.

Kinetic equilibrium of iron in the atmospheres of cool dwarf stars

NLTE line formation calculations of Fei in the solar atmosphere are extended to include weak lines in the visual spectrum of the Sun. Previously established atomic models are used to discriminate between different ways of treating collisional interaction processes. As indicated by the analysis of strong Fei lines, the influence of deviations from LTE in the solar atmosphere on the Fe abundance is small for all lines. To derive a common solar Fei abundance from both strong and weak lines fine-tuning of the microturbulence velocity parameter and the van der Waals damping constants is required. The solar Fei abundances based on all available f-values are dominated by the large scatter already found for the stronger lines. In particular the bulk of the data from the work of May et al. and O'Brian et al. is not adequate for accurate abundance work. Based on f-values measured by the Hannover and Oxford groups alone, the Fei LTE abundances are for the empirical and for the line-blanketed solar model. The solar Fe ionization equilibrium obtained for different atomic and atmospheric models rules out NLTE atomic models with a low efficiency of hydrogen collisions. At variance with Paper I, it is now in better agreement with laboratory Feii f-values for all types of line-blanketed models. Our final model assumptions consistent with a single unique solar Fe abundance calculated from NLTE line formation are (a) a line-blanketed solar model atmosphere, (b) an iron model atom with hydrogen collision rates times the standard value to compensate for the large photoionization cross-sections, (c) a microturbulence velocity km s-1, (d) van der Waals damping parameters decreased by as compared to Anstee & O'Mara's calculations, depending on SH, (e) Feii f-values as published by Schnabel et al., and (f) Fei f-values published by the Hannover and Oxford groups.

Kinetic equilibrium of iron in the atmospheres of cool dwarf stars

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 proles. The background is represented by the opacity of all important non-iron elements with iron lines added. Using a representative sample of suciently unblended strong Fe i and Fe ii line proles, it is evident that line formation is aected by (a) velocity elds 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 (Te = 5780 K, logg =4 :44) either using the empirical atmospheric model of Holweger & Muller (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 equi- librium 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 logFe ii; =7 :50 and 7:56, depending on atomic and atmospheric models, and those of 117 Fe i lines between logFe 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 eciently 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 proles 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.

Pulsation Hydrodynamics of Luminous Blue Variables and Pulsation-Driven Winds

AbstractMany physical factors, including radial and nonradial pulsation, rotation, radiation pressure, convection, magnetic fields, or dynamical instabilities may play important roles in the hydrodynamics of Luminous Blue Variables. We review the current status of hydrodynamic modeling of LBV envelopes, and describe results of our models using the one-dimensional nonlinear hydrodynamics code of Ostlie and Cox. We find that the models pulsate in several simultaneous radial modes, driven by the helium and Fe ionization к effect. The pulsations have quasi-periods between 5 and 80 days, with radial velocity amplitudes of 50-200 km/sec, and may be identified with the LBV microvariations. In some cases, depending on luminosity-to-mass ratio and helium abundance, deep layers in the model can periodically exceed the Eddington luminosity limit. The key to exceeding LE is the inclusion of the time dependence of convection: Near the regions of opacity peaks produced by Fe and helium ionization, convection is turning on and off during each pulsation cycle. If convection cannot turn on rapidly enough to transport the required luminosity through the region, the Eddington limit is exceeded. If this region of the star is sufficiently adiabatic, an “outburst” may occur. In the hydrodynamic models, an outburst is indicated by the photospheric radial velocity suddenly becoming very large, and the photospheric radius increasing monotonically over several pulsation cycles. Such pulsation-triggered outbursts may be responsible for the driving of variable, nonspherical winds. If large and infrequent enough, these outbursts may be identified with the classic LBV eruptions accompanied by episodic mass loss.

The extreme P-Cygni star HDE 316285

New line-blanketed models for the P-Cygni star HDE 316285 have been calculated. Representative model atoms for H, He I, He II, Ca I, Ca II, Mg I, Mg II, Al II, Al III, Fe II, and Fe III are included in the calculations. We confirm the basic parameters derived by Hillier et al. (1998) in an earlier unblanketed study, and also confirm that many of the optical metal lines are produced by continuum fluorescence. The charge exchange reaction Fe2+ + H ⇌ Fe+ + H + modifies the Fe ionization structure and significantly enhances the strength of the optical Fe II emission lines.

Further determinations of the gravities of cool giant stars using Mg I and MgH features

Surface gravities are deduced for Arcturus and five other cool giant stars. A temperature and an overall abundance are adopted for each object and the surface gravity is then found by requiring the magnesium abundance derived from MgH features to match that derived from MgH. The contamination of the MgH features by lines of the C 2 system is allowed for. The values obtained by this method are compared with those found by other spectroscopic methods that use Fe ionization equilibria and the profiles of strong lines. The errors inherent in the molecular equilibrium methods are discussed. The surface gravities found from radii derived from parallaxes and angular diameters, and an assumed mass of 1 M ○. , are compared with the spectroscopic results

Coronagraphic observations of an enhanced coronal region: I, Fexii and Nixv emission line data

Nine coronal emission lines representing five stages of Fe ionization and one stage of Ni were observed in an enhanced coronal region. The data from these observations are presented along with a density model of the enhanced region obtained from the FeXIII and NiXV emission line ratios as a function of position angle. The electron densities obtained from FeXIII lines range from N e = 108 to 109 cm−3, and are slightly lower for NiXV line data. Estimates of the variation of temperature over the enhanced region are inferred from the observed line intensities.

Coronagraph observations of the coronal condensation of 4 February 1962

Climax coronagraph observations of the accessible Fe lines, as well as the Caxv λ5694 line at the time of the 1962 total eclipse, are analyzed. The spectra show that the ionization equilibrium of iron is not substantially changed in an intense coronal condensation, at least for the stages x through xv. The only exception is Fexv λ7059, for which density effects are important. The stability of the ionization distribution is explained by the dynamic nature of the Fe ionization, with ions entering on the high side (Fexvi and up) due to rapid heating and then cooling through the visible stages. Comparison of the ionization distributions inferred from radiative and collisional excitation of the iron lines shows that the excitation must be by collisions everywhere at the heights examined (less than 50 000 km). The iron abundance in the corona is found to be 10−4 that of hydrogen, but this figure would be reduced by the amount of cyclic excitation. The peak electron density in the condensation is 8 × 109, and the peak value of the λ5694 line/ continuum ratio is 2.5, in good agreement with calculations by Chevalier and Lambert. The ratio of the infrared Fexiii lines is measured along the limb and found to vary with electron density as expected, the 10 747/10 798 ratio is 7 or less at densities much below 109 and saturates at a value of 2 for densities above that amount.

Calculations of Ionization Equilibria for Oxygen, Neon, Silicon, and Iron

O, Ne, Si and Fe ionization equilibria in low density plasma, including dielectronic recombination and autoionization processes in calculations