Dreia Carrillo, Durham University
The detailed chemical abundance patterns of accreted halo stars from the optical to the infrared

Understanding the assembly of our Galaxy requires us to also characterize the systems that helped build it. In this work, we accomplish this by exploring the chemistry of accreted halo stars from Gaia-Enceladus/Gaia-Sausage (GES) selected in the infrared from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) Data Release 16. We use high resolution optical spectra for 62 GES stars to measure abundances in 20 elements spanning the α, Fe-peak, light, odd-Z, and notably, the neutron-capture groups of elements to understand their trends in the context of and in contrast to the Milky Way and other stellar populations. These stars have enhanced neutron-capture abundance trends compared to the Milky Way, and their [Eu/Mg] and neutron-capture abundance ratios (e.g. [Y/Eu], [Ba/Eu], [Zr/Ba], [La/Ba], and [Nd/Ba]) point to r-process enhancement and a delay in s-process enrichment. Their [α/Fe] trend is lower than the Milky Way trend for [Fe/H] > -1.5 dex, similar to previous studies of GES stars and consistent with the picture that these stars formed in a system with a lower rate of star formation.

Sabrina DeSoto, University of Denver
Asymmetry and Clumps and Jets! Oh My!: Revealing the Changing Structure of Type-Ib SN 2012au

SN 2012au is a key intermediate object between Type Ib supernovae (SNe), superluminous SNe and the highly-energetic, hypernovae. This unusual SN had a higher mass loss rate than other hypernovae but likely arose from a progenitor star of about 80 solar masses. These two findings contradict previous theories that more massive stars could not produce such an explosion, suggesting that a jet-powered mechanism caused SN 2012au. I present 6 epochs of spectropolarimetric data for SN 2012au observed by the Supernova Spectropolarimetry Project between 0 and 295 days post-maximum brightness. The unique combination of polarization information with multi-observation spectra gives us a detailed picture of the distribution of elements in the explosion and how these structures change over time. Spectropolarimetry is a powerful piece of the multi-messenger toolkit that enables us to identify individual components in the SN’s geometry, such as clumps and jets that otherwise cannot be resolved. Our spectropolarimetric data show that SN 2012au exhibited a dominant axis in early epochs, which is associated with a jet-like feature. We also see signatures in the helium line polarization that suggests this material is distributed in clumps with a different symmetry axis than the jets. By comparing our observations with radiative transfer models, we offer a picture of the structure of SN 2012au and discuss the implications our findings have for this unique breed of supernova.