A comparative study between the mineralogy of rocky objects around white dwarfs and FGM stars
Abstract
We compare the mineralogy of rocky objects orbiting so-called polluted White Dwarfs (WD), to theoretical rocky exoplanet compositions obtained for FGM stars in the Hypatia Catalog. Earlier work on FGM stars (Putirka and Rarick 2019) used "major oxides" (oxides of Na, Mg, Al, Si, P, K, Ca, Ti, Cr, Fe, Mn, Ni) to estimate the mineralogy and rock types of exoplanet interiors. WD catalogs only report Mg, Si, Ca, and Fe, but our earlier work shows that for FGM stars, the ratio (FeO+MgO)/SiO2 can characterize a planet's mantle rock type (these three oxides comprise >80% of the non-volatile fractions of expected FMG exoplanets. To compare WDs with Hypatia, we must also re-normalize Hypatia compositions to MgO + SiO2 + FeO + CaO = 1. And since mantle mineralogy also depends upon how much Fe is segregated to its core, we take the following ratio: α = (Fe-in-mantle)/(Fe-in-whole planet) as 0.3, which approximates the case for Earth, but in our solar system varies from 0 (Mercury) to 0.5 (Mars). We find that major oxides, and hence the rock types that would comprise the mantle's of WD rocky objects, are quite similar to those expected to orbit Hypatia stars. The few stars that we have in the WD database (n=37) fills the range of what is observed for their Hypatia counterparts, with the only difference being that WDs exhibit a wider range of FeO than Hypatia, although such contrasts are less than certain. Given that the Hypatia catalog is so much larger (n>4,000), and because some WD compositions are reported as upper limits, it could be that the WDs are equally similar in their range in FeO. The similar (FeO+MgO)/SiO2 ratios between the two populations implies a similar range in development of crust type and plate tectonics. A possibly interesting, but perhaps spurious, finding is that both catalogs contain some very high SiO2 cases. If the very high SiO2 cases are real, such planets would have small metallic cores, rocky mantles that consist largely of quartz and, with the addition of water, crusts composed of opal or other hydrated SiO2 phases. But more careful observations may tighten rather than broaden the compositional range of both the WD and Hypatia catalogs. But in any case, our comparisons show that our own solar system is quite normal" falling in the middle of the WD and FGM compositional distributions.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.P51G3431P
- Keywords:
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- 5205 Formation of stars and planets;
- PLANETARY SCIENCES: ASTROBIOLOGY;
- 6295 Venus;
- PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS;
- 6296 Extra-solar planets;
- PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS;
- 5430 Interiors;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS