The Solar Argon Abundance

The solar argon abundance cannot be directly derived by spectroscopic observations of the solar photosphere. The solar Ar abundance is evaluated from solar wind measurements, nucleosynthetic arguments, observations of B stars, HII regions, planetary nebulae, and noble gas abundances measured in Jupiter's atmosphere. These data lead to a recommended argon abundance of N(Ar) = 91,200(+/-)23,700 (on a scale where Si = 10^6 atoms). The recommended abundance for the solar photosphere (on a scale where log N(H) = 12) is A(Ar)photo = 6.50(+/-)0.10, and taking element settling into account, the solar system (protosolar) abundance is A(Ar)solsys = 6.57(+/-)0.10.Comment: 14 pages, 1 figure, 1 table; submitted to Astrophysical Journa

Oldhamite in enstatite achondrites (Aubrites)

Properties of oldhamite (ideally CaS) in aubrites are summarized and compared to oldhamite in enstatite chondrites. The origin of the high REE abundances in aubritic oldhamite and the diversity of REE abundance patterns is addressed. Low CaS/silicate partition coefficients indicate that oldhamite in enstatite achondrites cannot have gained its high REE concentrations during igneous differentiation processes. However, the observed REE abundance patterns in oldhamite can be explained by REE condensation into CaS grains in the solar nebula. The very high melting point of oldhamite plausibly led to its preservation and prevented major exchange reactions of the oldhamite with other aubritic minerals during the short differentiation and metamorphism period on the aubrite parent body. Thus, oldhamite in aubrites is probably a slightly metamorphosed relict condensate, as suggested earlier

Exoplanet Chemistry

The terrestrial and gas-giant planets in our solar system may represent some prototypes for planets around other stars; the exoplanets because most stars have similar overall elemental abundances as our sun. The solar system planets represent at least four chemical planet types, depending on the phases that make them: Terrestrial-like planets made of rock (metal plus silicates), Plutonian planets made of rock and ice, Neptunian giant planets of rocky, icy with low H and He contents, and Jovian gas-giant planets of rocky, icy planets with near-solar H and He contents. The planetary compositions are linked to the chemical fractionation in the planetary accretion disks. Chemical tracers of these fractionations are described. Many known exoplanets are gas-giant planets with up to several Jupiter-masses and their atmospheric chemistry is compared to that of brown dwarfs. Exoplanets in close orbits around their host stars may resemble hot brown dwarfs (L-dwarfs). Planets receiving less radiation form their host may compare more to the methane-rich T dwarfs. The cloud layers resulting from condensation of oxides, metal, sulfides, and salts in these hot and cool gas giant planets and their chemical tracers are described.Comment: 33 pages, 8 figures, 2 tables, in: Formation and Evolution of Exoplanets, R. Barnes (ed.), Wiley, Berlin, in pres

Atmospheric Chemistry in Giant Planets, Brown Dwarfs, and Low-Mass Dwarf Stars III. Iron, Magnesium, and Silicon

We use thermochemical equilibrium calculations to model iron, magnesium, and silicon chemistry in the atmospheres of giant planets, brown dwarfs, extrasolar giant planets (EGPs), and low-mass stars. The behavior of individual Fe-, Mg-, and Si-bearing gases and condensates is determined as a function of temperature, pressure, and metallicity. Our results are thus independent of any particular model atmosphere. The condensation of Fe metal strongly affects iron chemistry by efficiently removing Fe-bearing species from the gas phase. Monatomic Fe is the most abundant Fe-bearing gas throughout the atmospheres of EGPs and L dwarfs and in the deep atmospheres of giant planets and T dwarfs. Mg- and Si-bearing gases are effectively removed from the atmosphere by forsterite (Mg2SiO4) and enstatite (MgSiO3) cloud formation. Monatomic Mg is the dominant magnesium gas throughout the atmospheres of EGPs and L dwarfs and in the deep atmospheres of giant planets and T dwarfs. Silicon monoxide (SiO) is the most abundant Si-bearing gas in the deep atmospheres of brown dwarfs and EGPs, whereas SiH4 is dominant in the deep atmosphere of Jupiter and other gas giant planets. Several other Fe-, Mg-, and Si-bearing gases become increasingly important with decreasing effective temperature. In principle, a number of Fe, Mg, and Si gases are potential tracers of weather or diagnostic of temperature in substellar atmospheres.Comment: 42 pages, 15 figures, submitted to the Astrophysical Journa

Solar System Abundances and Condensation Temperatures of the Halogens Fluorine, Chlorine, Bromine, and Iodine

We review a large body of literature for concentrations of halogens in chondrites and stellar halogen data used for solar system abundances (i.e., representative abundances of the solar system at the time of its formation) and associated analytical problems. Claims of lower solar system chlorine, bromine and iodine abundances from recent analyses of CI-chondrites are untenable because of incompatibility of such low values with nuclear abundance systematics and measurements of halogens in the sun and other stars. We suspect analytical problems associated with these peculiar rock types caused lower analytical results in several studies. Mass concentrations in CI-chondrites are F=92+-20 ppm, Cl=717+-110 ppm, Br=3.77+-0.90 ppm, and I=0.77+-0.31 ppm, and abundances normalized to N(Si) =10^6 atoms are N(F)=1270+-270, N(Cl)=5290+-810, N(Br)=12.3+-2.9, and N(I)=1.59+-0.64. Meteoritic values scaled to present-day photospheric abundances with log N(H)=12 are A(F)=4.61+-0.09, A(Cl)=5.23+-0.06, A(Br)=2.60+-0.09, and A(I)=1.71+-0.15. These recommended present-day solar system abundances compare to the sunspot values of N(F)=776+-260, A(F)=4.40+-0.25, and N(Cl)=5500+-810, A(Cl)=5.25+-0.12 and are consistent with F and Cl abundance ratios in other stars and other astronomical environments. The chlorine abundance of 776+-21 ppm by Yokoyama et al. (2022) for the CI-chondrite-like asteroid Ryugu is consistent with the chlorine abundance evaluated for CI-chondrites here. Updated equilibrium 50% condensation temperatures from our previous work (Lodders 2003, Fegley & Schaefer 2010, Fegley & Lodders 2018) considering solid-solution and kinetic inhibition effects are 713K (F), 427K (Cl), 392K (Br) and 312K (I) at 10^-4 bar total pressure. Condensation temperatures computed with lower halogen abundances do not represent the correct condensation temperatures from a solar composition gas. (abridged)Comment: 59 pages, 5 figures, 18 table

Water Clouds in Y Dwarfs and Exoplanets

The formation of clouds affects brown dwarf and planetary atmospheres of nearly all effective temperatures. Iron and silicate condense in L dwarf atmospheres and dissipate at the L/T transition. Minor species such as sulfides and salts condense in mid-late T dwarfs. For brown dwarfs below Teff=450 K, water condenses in the upper atmosphere to form ice clouds. Currently over a dozen objects in this temperature range have been discovered, and few previous theoretical studies have addressed the effect of water clouds on brown dwarf or exoplanetary spectra. Here we present a new grid of models that include the effect of water cloud opacity. We find that they become optically thick in objects below Teff=350-375 K. Unlike refractory cloud materials, water ice particles are significantly non-gray absorbers; they predominantly scatter at optical wavelengths through J band and absorb in the infrared with prominent features, the strongest of which is at 2.8 microns. H2O, NH3, CH4, and H2 CIA are dominant opacity sources; less abundant species such as may also be detectable, including the alkalis, H2S, and PH3. PH3, which has been detected in Jupiter, is expected to have a strong signature in the mid-infrared at 4.3 microns in Y dwarfs around Teff=450 K; if disequilibrium chemistry increases the abundance of PH3, it may be detectable over a wider effective temperature range than models predict. We show results incorporating disequilibrium nitrogen and carbon chemistry and predict signatures of low gravity in planetary- mass objects. Lastly, we make predictions for the observability of Y dwarfs and planets with existing and future instruments including the James Webb Space Telescope and Gemini Planet Imager.Comment: 23 pages, 20 figures, Revised for Ap