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Register a new user$^{26}$Al-$^{26}$Mg isotopic, mineralogy, petrography of a Hibonite-Pyroxene Spherule in Allan Hills 77307 (CO3.03): Implications for the origin and evolution of these objects
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Ritesh Kumar Mishra
Publication date
2020
fields
Physics
and research's language is
English
Authors
Ritesh Kumar Mishra
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10 Hibonite-pyroxene/glass spherules discovered hitherto are a rare suite of refractory inclusions that show the largest range of exotic isotopic properties (anomalies in neutron rich isotopes (e.g., $^{48}$Ca, $^{50}$Ti), abundance of $^{26}$Al) despite their defining simple spherical morphology and mineralogy consisting predominantly of few hibonites nestled within/with glassy or crystallised calcium, aluminium-rich pyroxene. $^{26}$Al-$^{26}$Mg chronological studies along with petrography and mineralogy of a relatively large (~120 micron diameter), found in Allan Hills 77307 (CO3.03) has been performed. Uniquely, both hibonite and pyroxene show discordant abundance of short-lived now-extinct radionuclide $^{26}$Al that suggest disparate and distinct regions of origin of hibonite and pyroxene. The pristine petrography and mineralogy of this inclusion allow discernment of their genesis and trend of alteration in hibonite-pyroxene/glass spherules.
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Hibonite-pyroxene spherules are an extremely rare kind of refractory inclusion that show a wide range of exotic isotopic properties despite their defining similarity and simplicity in morphology and mineralogy. One such, relatively large (about 120 micron diameter), inclusion has been found in one of the most pristine meteorites, Allan Hills 77307 (a carbonaceous chondrite of the Ornans group; Petrologic type 3.03). The inclusion consists of two central hibonite laths of about 30x15 micron surrounded by Al, Ca-rich pyroxene. The hibonite laths have uniform composition. The composition of pyroxene surrounding the hibonite is radially homogenously Al,-Ca rich up to about 50-60 microns which transitions to Mg, -Ti rich at the outer boundary. Hibonite-pyroxene spherule found in ALHA 77307 shares many similarities with the other previously found hibonite-pyroxene spherules. A distinguishing feature of the inclusion in ALHA77307 is the presence of two slivers/ wedges at the opposite outer edge of the hibonite- pyroxene spherule that consist of rapidly, poorly crystalized, sub-micron minerals with pristine textures. The pristine petrography and mineralogy of this inclusion allow discernment of the expected general trend of formation and alteration amongst hibonite-pyroxene spherules.
In contrast to the water-poor inner solar system planets, stochasticity during planetary formation and order of magnitude deviations in exoplanet volatile contents suggest that rocky worlds engulfed in thick volatile ice layers are the dominant family of terrestrial analogues among the extrasolar planet population. However, the distribution of compositionally Earth-like planets remains insufficiently constrained, and it is not clear whether the solar system is a statistical outlier or can be explained by more general planetary formation processes. Here we employ numerical models of planet formation, evolution, and interior structure, to show that a planets bulk water fraction and radius are anti-correlated with initial $^{26}$Al levels in the planetesimal-based accretion framework. The heat generated by this short-lived radionuclide rapidly dehydrates planetesimals prior to accretion onto larger protoplanets and yields a system-wide correlation of planet bulk abundances, which, for instance, can explain the lack of a clear orbital trend in the water budgets of the TRAPPIST-1 planets. Qualitatively, our models suggest two main scenarios of planetary systems formation: high-$^{26}$Al systems, like our solar system, form small, water-depleted planets, whereas those devoid of $^{26}$Al predominantly form ocean worlds, where the mean planet radii between both scenarios deviate by up to about 10%.
Recent work suggests that $^{26}$Al may determine the water budget in terrestrial exoplanets as its radioactive decay dehydrates planetesimals leading to rockier compositions. Here I consider the observed distribution of $^{26}$Al in the Galaxy and typical star-forming environments to estimate the likelihood of $^{26}$Al enrichment during planet formation. I do not assume Solar-System-specific constraints as I am interested in enrichment for exoplanets generally. Observations indicate that high-mass stars dominate the production of $^{26}$Al with nearly equal contributions from their winds and supernovae. $^{26}$Al abundances are comparable to those in the early Solar System in the high-mass star-forming regions where most stars (and thereby most planets) form. These high abundances appear to be maintained for a few Myr, much longer than the 0.7 Myr half-life. Observed bulk $^{26}$Al velocities are an order of magnitude slower than expected from winds and supernovae. These observations are at odds with typical model assumptions that $^{26}$Al is provided instantaneously by high velocity mass loss from supernovae and winds. Regular replenishment of $^{26}$Al especially when coupled with the small age differences that are common in high-mass star-forming complexes, may significantly increase the number of star/planet-forming systems exposed to $^{26}$Al. Exposure does not imply enrichment, but the order of magnitude slower velocity of $^{26}$Al may alter the fraction that is incorporated into planet-forming material. Together, this suggests that the conditions for rocky planet formation are not rare, nor are they ubiquitous, as small regions like Taurus that lack high-mass stars to produce $^{26}$Al may be less likely to form rocky planets. I conclude with suggested directions for future studies.
The rate of the $^{25}$Al($p$,$gamma$)$^{26}$Si reaction is one of the few key remaining nuclear uncertainties required for predicting the production of the cosmic $gamma$-ray emitter $^{26}$Al in explosive burning in novae. This reaction rate is dominated by three key resonances ($J^{pi}=0^{+}$, $1^{+}$ and $3^{+}$) in $^{26}$Si. Only the $3^{+}$ resonance strength has been directly constrained by experiment. A high resolution measurement of the $^{25}$Mg($d$,$p$) reaction was used to determine spectroscopic factors for analog states in the mirror nucleus, $^{26}$Mg. A first spectroscopic factor value is reported for the $0^{+}$ state at 6.256 MeV, and a strict upper limit is set on the value for the $1^{+}$ state at 5.691 MeV, that is incompatible with an earlier ($^{4}$He,$^{3}$He) study. These results are used to estimate proton partial widths, and resonance strengths of analog states in $^{26}$Si contributing to the $^{25}$Al($p$,$gamma$)$^{26}$Si reaction rate in nova burning conditions.
Proton captures on Mg isotopes play an important role in the Mg-Al cycle active in stellar H-burning regions. In particular, low-energy nuclear resonances in the $^{25}$Mg(p,$gamma$)$^{26}$Al reaction affect the production of radioactive $^{26}$Al$^{gs}$ as well as the resulting Mg/Al abundance ratio. Reliable estimations of these quantities require precise measurements of the strengths of low-energy resonances. Based on a new experimental study performed at LUNA, we provide revised rates of the $^{25}$Mg(p,$gamma$)$^{26}$Al$^{gs}$ and the $^{25}$Mg(p,$gamma$)$^{26}$Al$^{m}$ reactions with corresponding uncertainties. In the temperature range 50 to 150 MK, the new recommended rate of the $^{26}$Al$^{m}$ production is up to 5 times higher than previously assumed. In addition, at T$=100$ MK, the revised total reaction rate is a factor of 2 higher. Note that this is the range of temperature at which the Mg-Al cycle operates in an H-burning zone. The effects of this revision are discussed. Due to the significantly larger $^{25}$Mg(p,$gamma$)$^{26}$Al$^{m}$ rate, the estimated production of $^{26}$Al$^{gs}$ in H-burning regions is less efficient than previously obtained. As a result, the new rates should imply a smaller contribution from Wolf-Rayet stars to the galactic $^{26}$Al budget. Similarly, we show that the AGB extra-mixing scenario does not appear able to explain the most extreme values of $^{26}$Al/$^{27}$Al, i.e. $>10^{-2}$, found in some O-rich presolar grains. Finally, the substantial increase of the total reaction rate makes the hypothesis of a self-pollution by massive AGBs a more robust explanation for the Mg-Al anticorrelation observed in Globular-Cluster stars.
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