Joint Astronomy and EEPS Colloquium: Emilie Dunham

Date: Thu. October 24th, 2019, 3:00 pm-4:00 pm
Location: Sears 552

Meteoritic Implications for the Galactic Environment of Solar System Formation

Emilie Dunham (Arizona State)

Short-lived radionuclides (SLRs) once present in the solar nebula can be used to probe the Solar System’s galactic formation environment. Isotopic analyses reveal that the first solids formed in the Solar System, calcium- and aluminum-rich inclusions (CAIs) in chondritic meteorites, formed with the live SLRs ¹⁰Be (t_1/2 = 1.4 Myr) and ²⁶Al (t_1/2 = 0.7 Myr). Beryllium-10 is produced when high-energy ions, solar energetic particles or galactic cosmic rays (GCRs), spall nuclei in gas or dust. The most likely source of Solar System ¹⁰Be is inheritance of GCR-irradiated protosolar molecular cloud material, but only if all CAIs recorded the same initial ¹⁰Be abundance. In this talk, I assess the homogeneity of ¹⁰Be by measuring CAIs for ¹⁰Be–¹⁰B correlated to ²⁶Al–²⁶Mg isotope systematics.

I synthesized appropriate standards for secondary ion mass spectrometry (SIMS) measurements of ¹⁰Be–¹⁰B, necessary for accurate determination of the ¹⁰Be/⁹Be ratio. I then analyzed 32 CAIs for ¹⁰Be–¹⁰B as well as 6 CAIs for ²⁶Al–²⁶Mg and 5 CAIs for oxygen isotopes within this sample set, using SIMS techniques. Previous studies analyzed CAIs primarily from CV3 chondrites, which are known to have experienced thermal metamorphism and aqueous alteration. My work included a variety of CAIs (Type A, B, fine-grained, igneous) from CV3_oxidized, CV3_reduced, CO3, CR2, and CH/CB chondrites. Finally, after evaluating my data and literature data consistently, I statistically tested whether all CAIs belong to a single ¹⁰Be population. I find that the majority (~85%) of normal (i.e., without large isotope fractionations or anomalies), ²⁶Al-bearing CAIs recorded a single value, ¹⁰Be/⁹Be = (7.0 ± 0.2) × 10^-4. Although 6 CAIs recorded higher or lower values, these are plausibly explained by secondary processes. The galaxy-wide average value of ¹⁰Be/⁹Be from GCR interactions 4.56 billion years ago is predicted to be <2 × 10^-4; the value I measured is more than 3 times higher. Because GCRs trace supernovae and star formation, my results suggest a similarly enhanced star formation rate in the molecular cloud within ~1 kpc of the Sun, in the ~15 Myr prior to the Sun’s birth.