Arshad ALI^1,2, Iffat JABEEN², Neil R. BANERJEE², Gordon R. OSINSKI^2,3, Ian NICKLIN⁴, David GREGORY⁵, and Patrick HERRMANN⁶
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13072]
¹Earth Sciences Research Centre (ESRC), Sultan Qaboos University, Al-Khoudh, Muscat 123, Sultanate of Oman
²Department of Earth Sciences/Centre for Planetary Science and Exploration (CPSX), University of Western Ontario, London,Ontario, Canada
³Department of Physics and Astronomy, University of Western Ontario, London, Ontario, Canada
⁴Department of Natural History, Royal Ontario Museum, Toronto, Ontario N5R 4P5, Canada
⁵230 First Ave., Suite 108, St. Thomas, Ontario, Canada
⁶Pallasite.ca, Toronto, Ontario, Canada
Published by arrangement with John Wiley & Sons

Oxygen isotope measurements of olivine in main group (MG) pallasites by traditional laser fluorination method are associated with some uncertainties including terrestrial weathering, incomplete olivine reaction, and sample state. We improved our laser fluorination approach by pretreating olivine grains with acid to remove terrestrial weathering products and by modifying the sample holder for an efficient and complete laser reaction. Our experiments on Brahin olivine demonstrate that acid‐washing successfully removes the terrestrial weathering with <0.1‰ variation in δ¹⁸O value and, at the same time, improving the ∆¹⁷O value significantly. We also achieved a complete olivine fluorination by employing a custom‐designed sample holder with “V”‐shaped profile having rounded bottom because incomplete/partial reaction of olivine gives comparatively lighter δ¹⁸O values. Using these new techniques, we present precise triple oxygen isotope data (N = 72) of 25 olivine samples separated from main group pallasites. The data are, on average, ~0.5‰ heavier in δ¹⁸O relative to the values published in the literature for the same samples. Critically, the ∆¹⁷O values of MG pallasites and to some extent their Fo‐contents suggest that there are at least two populations of olivine. Based on our improved data set, we propose that MG pallasites potentially have high‐∆¹⁷O‐ and low‐∆¹⁷O‐bearing subgroups that are statistically distinct. The subgroups present average ∆¹⁷O values of −0.166 ± 0.003 (2SE; N = 16) and −0.220 ± 0.003 (2SE; N = 9), respectively. Furthermore, the high‐∆¹⁷O‐bearing subgroup samples trend toward lower Fo‐contents compared to the other subgroup. Taken together, our data provide evidence that argues against a single parent body origin for MG pallasites.

Christian Iliadis^1,2, Lori N. Downen^1,2, Jordi José^3,4, Larry R. Nittler⁵, and Sumner Starrfield⁶
Astrophysical Journal 855, 76 Link to Article [DOI: 10.3847/1538-4357/aaabb6]
¹Department of Physics & Astronomy, University of North Carolina, Chapel Hill, NC 27599-3255, USA
²Triangle Universities Nuclear Laboratory, Durham, NC 27708-0308, USA
³Departament de Física, EEBE, Universitat Politècnica de Catalunya, c/Eduard Maristany 10, E-08930 Barcelona, Spain
⁴Institut d’Estudis Espacials de Catalunya, c/Gran Capità 2-4, Ed. Nexus-201, E-08034 Barcelona, Spain
⁵Department of Terrestrial Magnetism, Carnegie Institution for Science, Washington, DC 20015, USA
⁶Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA

About 30%–40% of classical novae produce dust 20–100 days after the outburst, but no presolar stardust grains from classical novae have been unambiguously identified yet. Although several studies claimed a nova paternity for certain grains, the measured and simulated isotopic ratios could only be reconciled, assuming that the grains condensed after the nova ejecta mixed with a much larger amount of close-to-solar matter. However, the source and mechanism of this potential post-explosion dilution of the ejecta remains a mystery. A major problem with previous studies is the small number of simulations performed and the implied poor exploration of the large nova parameter space. We report the results of a different strategy, based on a Monte Carlo technique, that involves the random sampling over the most important nova model parameters: the white dwarf composition; the mixing of the outer white dwarf layers with the accreted material before the explosion; the peak temperature and density; the explosion timescales; and the possible dilution of the ejecta after the outburst. We discuss and take into account the systematic uncertainties for both the presolar grain measurements and the simulation results. Only those simulations that are consistent with all measured isotopic ratios of a given grain are accepted for further analysis. We also present the numerical results of the model parameters. We identify 18 presolar grains with measured isotopic signatures consistent with a CO nova origin, without assuming any dilution of the ejecta. Among these, the grains G270_2, M11-334-2, G278, M11-347-4, M11-151-4, and Ag2_6 have the highest probability of a CO nova paternity.

Yuri Aikawa¹, Kenji Furuya², Ugo Hincelin³, and Eric Herbst³
Astrophysical Journal 855, 119 Link to Article [DOI: 10.3847/1538-4357/aaad6c]
¹Department of Astronomy, The University of Tokyo, Japan
²Center for Computational Sciences, University of Tsukuba, Japan
³Department of Chemistry, The University of Virginia, Charlottesville, VA, USA

We investigate deuterium chemistry coupled with the nuclear spin-state chemistry of H₂ and in protoplanetary disks. Multiple paths of deuterium fractionation are found; exchange reactions with D atoms, such as HCO⁺ + D, are effective in addition to those with HD. In a disk model with grain sizes appropriate for dark clouds, the freeze-out of molecules is severe in the outer midplane, while the disk surface is shielded from UV radiation. Gaseous molecules, including DCO⁺, thus become abundant at the disk surface, which tends to make their column density distribution relatively flat. If the dust grains have grown to millimeter size, the freeze-out rate of neutral species is reduced and the abundances of gaseous molecules, including DCO⁺ and N₂D⁺, are enhanced in the cold midplane. Turbulent diffusion transports D atoms and radicals at the disk surface to the midplane, and stable ice species in the midplane to the disk surface. The effects of turbulence on chemistry are thus multifold; while DCO⁺ and N₂D⁺ abundances increase or decrease depending on the regions, HCN and DCN in the gas and ice are greatly reduced at the innermost radii, compared to the model without turbulence. When cosmic rays penetrate the disk, the ortho-to-para ratio (OPR) of H₂ is found to be thermal in the disk, except in the cold (10 K) midplane. We also analyze the OPR of and H₂D⁺, as well as the main reactions of H₂D⁺, DCO⁺, and N₂D⁺, in order to analytically derive their abundances in the cold midplane.