Keywords

We report Li–Be–B and Al–Mg isotopic compositions of Ca-Al-rich inclusions (CAIs) in Sayh al Uhaymir 290 (CH) and Isheyevo (CH/CB) metal-rich carbonaceous chondrites. All CAIs studied here do not show resolvable excesses in ²⁶Mg, a decay product of the short-lived radionuclide ²⁶Al, which suggests their formation occurred prior to the injection of ²⁶Al into the solar system from a nearby stellar source. The inferred initial ¹⁰Be/⁹Be ratios obtained for these CAIs range from 0.17 × 10⁻³ to 6.1 × 10⁻³, which tend to be much higher and more variable than those of CAIs in CV3 chondrites. The high ¹⁰Be/⁹Be ratios suggest that ¹⁰Be was most likely synthesized through solar cosmic-ray irradiation. The lithium isotopic compositions of these CAIs are nearly chondritic, independent of their initial ¹⁰Be/⁹Be ratios. This can be explained by the irradiation targets being of chondritic composition; in other words, targets were most likely not solid CAI themselves, but their precursors in solar composition. The larger variations in ¹⁰Be/⁹Be ratios observed in CH and CH/CB CAIs than in CV CAIs may reflect more variable cosmic-ray fluxes from the earlier, more active Sun at an earlier evolutionary stage (class 0-I) for the former, and a later, less active stage of the Sun (class II) for the latter. If this is the case, our new Be–B and Al–Mg data set implies that the earliest formed CAIs tend to be transported into the outer part of the solar protoplanetary disk, where the parent bodies of metal-rich chondrites likely accreted.

Suprathermal ion composition associated with corotating interaction regions (CIRs) exhibited a solar cycle variation during solar cycle 23 and the beginning of solar cycle 24. However, it is unclear if this variation would remain when considering all of solar cycle 24, or whether the variations in the CIR-associated suprathermal ion composition would change. Using 20 yr of Advanced Composition Explorer observations (1998–2018), we present a comparison of the suprathermal ion compositions for solar cycles 23 and 24. The energetic particle content for the two solar cycles is found to be remarkably similar. The observed solar cycle variations in 0.32–0.45 MeV/nuc Fe/O previously observed for solar cycle 23 was seen to be largely repeated in solar cycle 24, both in solar cycle phase and magnitude. A small enhancement in CIR-associated Fe/O during the declining phase was observed for both solar cycles. The CIR event-averaged intensities of Fe and O were also found to have a slight solar cycle dependence, with the Fe/O ratio being more closely bound to the intensity of Fe ions. Additionally, the elemental abundance versus O ratios compared to the Fe/C ratios were found to follow the same trends for both solar cycles, with high Fe/C ratio events occurring mostly during solar maximum.

We examine the contribution of electron-capture supernovae (ECSNe), low-mass SNe from collapsing Fe cores (FeCCSNe), and rotating massive stars to the chemical composition of the Galaxy. Our model includes contributions to chemical evolution from both thermonuclear ECSNe (tECSNe) and gravitational collapse ECSNe (cECSNe). We show that if ECSNe are predominantly gravitational collapse SNe but about 15% are partial thermonuclear explosions, the model is able to reproduce the solar abundances of several important and problematic isotopes including ${}^{48}\mathrm{Ca}$, ${}^{50}\mathrm{Ti}$, and ⁵⁴Cr together with ⁵⁸Fe, ⁶⁴Ni, ⁸²Se, and ⁸⁶Kr and several of the Zn–Zr isotopes. A model in which no cECSNe occur, only tECSNe with low-mass FeCCSNe or rotating massive stars, proves also very successful at reproducing the solar abundances for these isotopes. Despite the small mass range for the progenitors of ECSNe and low-mass FeCCSNe, the large production factors suffice for the solar inventory of the above isotopes. Our model is compelling because it introduces no new tensions with the solar abundance distribution for a Milky Way model—only tending to improve the model predictions for several isotopes. The proposed astrophysical production model thus provides a natural and elegant way to explain one of the last uncharted territories on the periodic table of astrophysical element production.

Plasma diagnostics and elemental abundance measurements are crucial to help us understand the formation and dynamics of the solar wind. Here we use a theoretical solar wind model to study the effect of nonequilibrium ionization (NEI) on plasma diagnostic techniques applied to line intensities emitted by the fast solar wind. We find that NEI almost always changes the spectral line intensities with up to 120% difference for the lighter elements and for higher charge states of Fe even below 1.5 solar radii (R_s). The measured plasma density, temperature, and differential emission measure are only slightly affected by NEI. However, NEI significantly affects the first-ionization potential (FIP) bias and abundance ratio measurements, producing an error of up to a factor 4 at 1.5 R_s for the Mg-to-Ne, Fe-to-S, and Ar-to-Fe ratios when EI is assumed. We conclude that it is very important to consider the NEI effect when spectral line intensities are synthesized and the FIP bias and elemental abundance are measured.

Helioseismic observations have revealed many properties of the Sun: the depth and helium abundance of the convection zone, the sound speed, and the density profiles in the solar interior. Those constraints have been used to judge the stellar evolution theory. With the old solar composition (e.g., GS98), the solar standard model is in reasonable agreement with the helioseismic constraints. However, a solar model with a revised composition (e.g., AGSS09) with a low abundance Z of heavy elements cannot be consistent with those constraints. This is the so-called "solar abundance problem," standing for more than 10 yr even with the recent upward revised Ne abundance. Many mechanisms have been proposed to mitigate the problem. However, there is still no low-Z solar model satisfying all helioseismic constraints. In this paper, we report a possible solution to the solar abundance problem. With some extra physical processes that are not included in the standard model, solar models can be significantly improved. Our new solar models with convective overshoot, the solar wind, and early mass accretion show consistency with helioseismic constraints, the solar Li abundance, and observations of solar neutrino fluxes.

We report new branching fractions (BFs) for 121 UV lines from the low-lying odd-parity levels of Fe ii belonging to the z⁶D^o, z⁶F^o, z⁶P^o, z⁴F^o, z⁴D^o, and z⁴P^o terms of the 3d⁶(⁵D)4p configuration. These lines range in wavelength from 2250 to 3280 Å and originate in levels ranging in energy from 38,459 to 47,626 cm⁻¹. In addition, we report BFs for 10 weak blue lines connecting to the z⁴D^o term that range in wavelength from 4173 to 4584 Å. The BFs are combined with radiative lifetimes from the literature to determine transition probabilities and log(gf) values. Comparison is made to selected experimental and theoretical data from the literature. Our new data are applied to iron abundance determinations in the Sun and in metal-poor star HD 84937. For the Sun, eight blue lines yield log ε(Fe) = 7.46 ± 0.03, in agreement with standard solar abundance estimates. For HD 84937 the observable wavelength range extends to the vacuum UV (λ ≥ 2327 Å), and from 75 lines we derive log ε(Fe) = 5.26 ± 0.01 (σ = 0.07), near to the metallicity estimates of past HD 84937 studies.

We examine the different element abundances exhibited by the closed loop solar corona and the slow speed solar wind. Both are subject to the first ionization potential (FIP) effect, the enhancement in coronal abundance of elements with FIP below 10 eV (e.g., Mg, Si, Fe) with respect to high-FIP elements (e.g., O, Ne, Ar), but with subtle differences. Intermediate elements, S, P, and C, with FIP just above 10 eV, behave as high-FIP elements in closed loops, but are fractionated more like low-FIP elements in the solar wind. On the basis of FIP fractionation by the ponderomotive force in the chromosphere, we discuss fractionation scenarios where this difference might originate. Fractionation low in the chromosphere where hydrogen is neutral enhances the S, P, and C abundances. This arises with nonresonant waves, which are ubiquitous in open field regions, and is also stronger with torsional Alfvén waves, as opposed to shear (i.e., planar) waves. We discuss the bearing these findings have on models of interchange reconnection as the source of the slow speed solar wind. The outflowing solar wind must ultimately be a mixture of the plasma in the originally open and closed fields, and the proportions and degree of mixing should depend on details of the reconnection process. We also describe novel diagnostics in ultraviolet and extreme ultraviolet spectroscopy now available with these new insights, with the prospect of investigating slow speed solar wind origins and the contribution of interchange reconnection by remote sensing.

We have analyzed spectral properties and abundances of ∼0.02–3.0 MeV nucleon⁻¹ suprathermal (ST) H–Fe ions in 41 stream interaction regions (SIRs) near 1 au observed by Wind and ACE spacecraft from 1995 January through 2008 December. We find that, (i) the event-averaged spectral index is γ ∼ 2.44, with a standard deviation (σ) of 0.67, (ii) γ's are poorly correlated with the magnetic compression ratios, and 17% of the events group around γ ∼ 1.5, (iii) γ's for both O and Fe at ∼0.02–0.09 MeV nucleon⁻¹ and 0.09–0.3 MeV nucleon⁻¹ are correlated, but do not exhibit any systematic steepening or flattening as a function of energy, (iv) the ST heavy ion abundance ratios remain constant with increasing energy, implying that the spectral rollovers, defined by the e-folding energy E₀, are independent of the ion's mass per charge (M/Q), and (v) SIR ST abundances are similar to the corresponding solar wind values, and do not exhibit any systematic behavior when plotted versus the ion's M/Q or first ionization potential. The above results pose challenges for (1) particle acceleration models that invoke either a corotating interaction region or SIR shocks between ∼3 and 5 au, (2) particle transport models that predict M/Q-dependent spectral rollovers due to interplanetary turbulence effects, and (3) the notion that SIR ST ions originate directly from the bulk solar wind. Instead, we suggest that the SIR ST ions are accelerated out of a pool of material that includes particles accelerated in solar energetic particle events and processed or heated solar wind ions.

Understanding elemental abundance variations in the solar corona provides an insight into how matter and energy flow from the chromosphere into the heliosphere. Observed variations depend on the first ionization potential (FIP) of the main elements of the Sun's atmosphere. High-FIP elements (>10 eV) maintain photospheric abundances in the corona, whereas low-FIP elements have enhanced abundances. Conversely, inverse FIP (IFIP) refers to the enhancement of high-FIP or depletion of low-FIP elements. We use spatially resolved spectroscopic observations, specifically the Ar xiv/Ca xiv intensity ratio, from Hinode's Extreme-ultraviolet Imaging Spectrometer to investigate the distribution and evolution of plasma composition within two confined flares in a newly emerging, highly sheared active region. During the decay phase of the first flare, patches above the flare ribbons evolve from the FIP to the IFIP effect, while the flaring loop tops show a stronger FIP effect. The patch and loop compositions then evolve toward the preflare basal state. We propose an explanation of how flaring in strands of highly sheared emerging magnetic fields can lead to flare-modulated IFIP plasma composition over coalescing umbrae which are crossed by flare ribbons. Subsurface reconnection between the coalescing umbrae leads to the depletion of low-FIP elements as a result of an increased wave flux from below. This material is evaporated when the flare ribbons cross the umbrae. Our results are consistent with the ponderomotive fractionation model for the creation of IFIP-biased plasma.

The influence of the new mass model Weizsäcker–Skyrme 4 (WS4) on the r-process abundance distribution is investigated using the site-independent classical r-process and the site-dependent dynamical r-process models. The dynamical r-process calculations are performed under the neutrino-driven wind scenario. In comparison with the finite-range droplet model (FRDM) often used in r-process calculations, better agreement between the calculated abundance and the observed solar r-process abundance is found in both the classical and dynamical calculations by using the mass model WS4. The abundance underestimations at the A ∼ 115, 140, and 200 mass regions encountered with the calculations using the FRDM is overcome to a large extent by using WS4.