- ID:
- ivo://CDS.VizieR/J/ApJS/253/41
- Title:
- Nitrogen isotopic compositions in Isheyevo
- Short Name:
- J/ApJS/253/41
- Date:
- 17 Jan 2022 11:31:00
- Publisher:
- CDS
- Description:
- We report on the discovery of 33 oxygen-anomalous grains from the CH3/CB_b_3 chondrite Isheyevo and the CR2 chondrite Northwest Africa (NWA) 801. Oxygen isotopic compositions indicate the origin of the majority grains in stellar outflows of low-mass (~1.2 to ~2.2M_{sun}_), solar-metallicity red giant or asymptotic giant branch stars, while highly ^17^O-enriched grains probably have nova origins. Isotopic compositions of the eight ^18^O-rich grains, including an extremely ^18^O-rich grain (~16 times solar ^18^O/^16^O ratio), are reproduced by zone mixing of SNe II ejecta. Close-to-normal silicon, magnesium, and calcium isotopic compositions of grains are consistent with the isotope exchange in the interstellar medium or the meteorite parent body, while two grains with Si isotopic anomalies and one grain with Mg isotopic anomalies reflect the Galactic chemical evolution. An Isheyevo clast showed several hot spots with moderate to high ^15^N enrichments, including a hot spot with an extreme ^15^N excess of (7225+/-316)%0. However, no correlation between ^15^N enrichment and presolar oxygen-rich grain abundance is found. Grains with elliptical shapes probably indicate primary condensation feature. Two complex grains possibly display decoupling of the isotopic and elemental compositions in the grain formation environments. The low silicate-to-oxide abundance ratio for the fine-grained chondrule rims in NWA 801 likely reflects the preferential destruction of silicates due to terrestrial weathering. In NWA 801, the presolar O-rich grain abundance in fine-grained chondrule rims is higher than in the interchondrule matrix, similar to the trend observed for some aqueously altered chondrites of petrologic type 2.
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- ID:
- ivo://CDS.VizieR/J/A+A/654/A108
- Title:
- Triple-frequency meteor radar reflection coeff.
- Short Name:
- J/A+A/654/A108
- Date:
- 22 Feb 2022
- Publisher:
- CDS
- Description:
- Radar scattering from meteor trails depends on several poorly constrained quantities, such as electron line density, q, initial trail radius, r0, and ambipolar diffusion coefficient, D. The goal is to apply a numerical model of full wave backscatter to triple frequency echo measurements to validate theory and constrain estimates of electron radial distribution, initial trail radius, and the ambipolar diffusion coefficient. A selection of 50 transversely polarized and 50 parallel polarized echoes with complete trajectory information were identified from simultaneous tri-frequency echoes recorded by the Canadian Meteor Orbit Radar (CMOR). The amplitude-time profile of each echo was fit to our model using three different choices for the radial electron distribution assuming a Gaussian, parabolic exponential, and 1-by-r^2^ electron line density model. The observations were manually fit by varying, q, r0, and D per model until all three synthetic echo-amplitude profiles at each frequency matched observation. The Gaussian radial electron distribution was the most successful at fitting echo power profiles, followed by the 1/r^2^. We were unable to fit any echoes using a profile where electron density varied from the trail axis as an exponential-parabolic distribution. While fewer than 5% of all examined echoes had self-consistent fits, the estimates of r0 and D as a function of height obtained were broadly similar to earlier studies, though with considerable scatter. Most meteor echoes are found to not be described well by the idealized full wave scattering model.
