Congratulations, Dr. Emilie Dunham

The Center for Meteorite Studies congratulates Dr. Emilie Dunham, who successfully defended her doctoral thesis September 13th!

To Be or not to B: Meteoritic Implications for the Galactic Environment of Solar System Formation
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 (t1/2 = 1.4 Myr) and ²⁶Al (t1/2 = 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. The goal of this dissertation is to assess the homogeneity of ¹⁰Be by measuring CAIs for ¹⁰Be–¹⁰B isotope systematics, correlated to ²⁶Al–²⁶Mg and oxygen isotopes.

Dr. Dunham synthesized appropriate standards for secondary ion mass spectrometry (SIMS) measurements of ¹⁰Be–¹⁰B, necessary for accurate determination of the ¹⁰Be/⁹Be ratio. She 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. Dr. Dunham's 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 her data and literature data consistently, she statistically tested whether all CAIs belong to a single ¹⁰Be population. She found that the majority (~90%) of normal (i.e., without large isotopic 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 Dr. Dunham measured is more than 3 times higher. Because GCRs trace supernovae and star formation, her 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.