The Abundances of Cobalt and Nickel in the Atmosphere of 78 Vir (A2p SrCrEu)

Cobalt and nickel abundances are rarely available for normal and Chemically Peculiar A stars because the strongest transitions of Co ii and Ni ii fall in the mid-UV. The abundances of cobalt and nickel are derived for 78 Vir using a mean mid-ultraviolet spectrum constructed by coadding 10 spectra collected with the Long Wave Prime and Long wavelength Redundant cameras over the 18 yr of the IUE mission. The strong transitions of Co ii at 2286.16 Å, 2307.86 Å, 2324.32 Å and 2580.33 Å and that of Ni ii et 2287.09 Å are present and more or less affected by blends. The least blended, λ 2286.16 Å, yields a mean overabundance of cobalt of 5 times the solar abundance, the Ni ii line at 2287.09 Å yields a 3 times solar overabundance. There is no convincing evidence that these lines varied in the spectra analyzed. The rotational period of 78 Vir estimated from its recent TESS lightcurve is 3.723 ± 0.055 days.

Atomic Iron and Nickel in the Coma of C/1996 B2 (Hyakutake): Production Rates, Emission Mechanisms, and Possible Parents

Two papers recently reported the detection of gaseous nickel and iron in the comae of over 20 comets from observations collected over two decades, including interstellar comet 2I/Borisov. To evaluate the state of the laboratory data in support of these identifications, we reanalyzed archived spectra of comet C/1996 B2 (Hyakutake), one of the nearest and brightest comets of the past century, using a combined experimental and computational approach. We developed a new, many-level fluorescence model that indicates that the fluorescence emissions of Fe I and Ni I vary greatly with heliocentric velocity. Combining this model with laboratory spectra of an Fe-Ni plasma, we identified 22 lines of Fe I and 14 lines of Ni I in the spectrum of Hyakutake. Using Haser models, we estimate the nickel and iron production rates as Q Ni = (2.6–4.1) × 1022 s−1 and Q Fe = (0.4–2.8) × 1023 s−1. From derived column densities, the Ni/Fe abundance ratio log10[Ni/Fe] = −0.15 ± 0.07 deviates significantly from solar abundance ratios, and it is consistent with the ratios observed in solar system comets. Possible production and emission mechanisms are analyzed in the context of existing laboratory measurements. Based on the observed spatial distributions, excellent fluorescence model agreement, and Ni/Fe ratio, our findings support an origin consisting of a short-lived unknown parent followed by fluorescence emission. Our models suggest that the strong heliocentric velocity dependence of the fluorescence efficiencies can provide a meaningful test of the physical process responsible for the Fe I and Ni I emission.

Elucidating the global distribution of reprocessing gas in NGC 1194

ABSTRACT A joint XMM–Newton and NuSTAR observation was conducted for the bright, local Seyfert 1.9 galaxy, NGC 1194. The hard spectral form of this active galactic nucleus (AGN) was modelled using the toroidal reprocessor mytorus. The decoupled model form provides a good description of the spectrum, with reflection arising from gas with a global average column density >4 × 1024 cm−2 and transmission of the continuum through an order-of-magnitude lower column. In this model, the reflection strength is a factor of ∼3 higher than expected from a simple torus. Such a result may indicate that much of the intrinsic X-ray continuum is hidden from view. An alternative model is that of a patchy torus, where 85 per cent of sightlines are obscured by Compton-thick gas and the remaining 15 per cent by Compton-thin gas. The patchy torus model is based on a solar abundance of Fe and is consistent with X-ray partial-covering results found in other AGN. That a patchy torus model would relieve the issue with the strength of the reflection signature is not an intuitive result: such an insight regarding the geometry of the global reprocessing gas could not have been obtained using ad hoc model components to describe the spectral form.

Non-local thermodynamical equilibrium abundance analysis of singly ionized praseodymium for the Sun

ABSTRACT Determinations of the solar abundance of praseodymium (Pr) depend critically on the local thermodynamical equilibrium (LTE) and non-local thermodynamical equilibrium (NLTE) techniques beyond the capabilities of a classical one-dimensional model atmosphere. Here, in this analysis, we adopt an atomic model atom of Pr consisting of 105 energy levels and 14 bound–bound transitions of singly ionized praseodymium (Pr ii) and the ground state of the Pr iii continuum limit. We briefly analyse the solar abundance of Pr taking the solar model atmospheres of Holweger & Müller (1974, Solar Physics, 39, 19) with the measured equivalent linewidths and invoking a microturbulent velocity treatment. We succeed in accurately selecting nearby clear sections of the spectrum for 14 spectral lines of Pr ii with the improved atomic data of high-quality oscillator strengths available from the laboratory measurements of several possible sources as well as accurate damping constants successfully determined from the literature. We find a Pr abundance revised to be downwards log ϵPr(NLTE) = 0.75 ± 0.09, which is in good agreement with the meteoritic value (log ϵPr = 0.76 ± 0.03). A comparison of the NLTE abundance corrections with the standard LTE analysis, log ϵPr(LTE) = 0.74 ± 0.08, reveals a positive correction of +0.01 dex, estimated from the selected solar Pr ii lines. The Pr abundance value is clearly superior following the classical one-dimensional model atmospheres of Holweger & Müller, the absolute scales of gf-values, the microturbulent velocity and the adopted equivalent linewidths.

Formation of aqua planets with water of nebular origin: effects of water enrichment on the structure and mass of captured atmospheres of terrestrial planets

ABSTRACT Recent detection of exoplanets with Earth-like insolation attracts growing interest in how common Earth-like aqua planets are beyond the Solar system. While terrestrial planets are often assumed to capture icy or water-rich planetesimals, a primordial atmosphere of nebular origin itself can produce water through oxidation of the atmospheric hydrogen with oxidizing minerals from incoming planetesimals or the magma ocean. Thermodynamically, normal oxygen buffers produce water comparable in mole number equal to or more than hydrogen. Thus, the primordial atmosphere would likely be highly enriched with water vapour; however, the primordial atmospheres have been always assumed to have the solar abundances. Here we integrate the 1D structure of such an enriched atmosphere of sub-Earths embedded in a protoplanetary disc around an M dwarf of 0.3$\, \mathrm{M}_\odot$ and investigate the effects of water enrichment on the atmospheric properties with focus on water amount. We find that the well-mixed highly enriched atmosphere is more massive by a few orders of magnitude than the solar-abundance atmosphere, and that even a Mars-mass planet can obtain water comparable to the present Earth’s oceans. Although close-in Mars-mass planets likely lose the captured water via disc dispersal and photoevaporation, these results suggest that there are more sub-Earths with Earth-like water contents than previously predicted. How much water terrestrial planets really obtain and retain against subsequent loss, however, depends on efficiencies of water production, mixing in the atmosphere and magma ocean, and photoevaporation, detailed investigation for which should be made in the future.

Galactic archaeology with asteroseismic ages

Context. With the wealth of information from large surveys and observational campaigns in the contemporary era, it is critical to properly exploit the data to constrain the parameters of Galactic chemical evolution models and quantify the associated uncertainties. Aims. We aim to constrain the two-infall chemical evolution models for the solar annulus using the measured chemical abundance ratios and seismically inferred age of stars in the APOKASC sample. Recently, in revised two-infall chemical evolution models, a significant delay of ∼4.3 Gyr has been invoked between the two episodes of gas accretion. In this work, we wish to test its robustness and statistically confirm and quantify the delay. Methods. We took a novel approach, using Bayesian framework based on Markov chain Monte Carlo methods to fit the two-infall chemical evolution models to the data. Results. In addition to fitting the data for stars in the APOKASC sample, our best fit models also reproduce other important observational constraints of the chemical evolution of the disk: i) present day stellar surface density; ii) present-day supernova and star formation rates; iii) the metallicity distribution function; and iv) solar abundance values. We find a significant delay between the two gas accretion episodes for various models explored with different values for the star formation efficiencies. The values for the delay lie in the range 4.5−5.5 Gyr. Conclusions. The results suggest that the APOKASC sample carries the signature of a delayed gas-rich merger, with dilution being the main process determining the shape of low-α stars in the abundance ratios space.

Remnants and ejecta of thermonuclear electron-capture supernovae

The explosion mechanism of electron-capture supernovae (ECSNe) remains equivocal: it is not completely clear whether these events are implosions in which neutron stars are formed, or incomplete thermonuclear explosions that leave behind bound ONeFe white dwarf remnants. Furthermore, the frequency of occurrence of ECSNe is not known, though it has been estimated to be of the order of a few per cent of all core-collapse supernovae. We attempt to constrain the explosion mechanism (neutron-star-forming implosion or thermonuclear explosion) and the frequency of occurrence of ECSNe using nucleosynthesis simulations of the latter scenario, population synthesis, the solar abundance distribution, pre-solar meteoritic oxide grain isotopic ratio measurements and the white dwarf mass–radius relation. Tracer particles from the 3d hydrodynamic simulations were post-processed with a large nuclear reaction network in order to determine the complete compositional state of the bound ONeFe remnant and the ejecta, and population synthesis simulations were performed in order to estimate the ECSN rate with respect to the CCSN rate. The 3d deflagration simulations drastically overproduce the neutron-rich isotopes 48Ca, 50Ti, 54Cr , 60Fe and several of the Zn isotopes relative to their solar abundances. Using the solar abundance distribution as our constraint, we place an upper limit on the frequency of thermonuclear ECSNe as 1−3% the frequency at which core-collapse supernovae (FeCCSNe) occur. This is on par with or 1 dex lower than the estimates for ECSNe from single stars. The upper limit from the yields is also in relatively good agreement with the predictions from our population synthesis simulations. The 54Cr/52Cr and 50Ti/48Ti isotopic ratios in the ejecta are a near-perfect match with recent measurements of extreme pre-solar meteoritc oxide grains, and 53Cr/52Cr can also be matched if the ejecta condenses before mixing with the interstellar medium. The composition of the ejecta of our simulations implies that ECSNe, including accretion-induced collapse of oxygen-neon white dwarfs, could actually be partial thermonuclear explosions and not implosions that form neutron stars. There is still much work to do to improve the hydrodynamic simulations of such phenomena, but it is encouraging that our results are consistent with the predictions from stellar evolution modelling and population synthesis simulations, and can explain several key isotopic ratios in a sub-set of pre-solar oxide meteoritic grains. Theoretical mass–radius relations for the bound ONeFe WD remnants of these explosions are apparently consistent with several observational WD candidates. The composition of the remnants in our simulations can reproduce several, but not all, of the spectroscopically-determined elemental abundances from one such candidate WD.