Chemical Stratification of the Neutron-Star Merger Ejecta in GW170817

The binary neutron star merger GW170817 remains the only confirmed joint gravitational wave-electromagnetic discovery thus far. The kilonova associated with GW170817 exhibited two distinct components to its ejecta — the disk wind ejecta and the dynamical ejecta, the latter of which is stratified into lanthanide-rich (LR) and lanthanide-poor (LP) components. Recently, proper motion measurements made with the Hubble Space Telescope and Very Long Baseline Interferometry constrained GW170817’s viewing angle very precisely to 19-25 deg (Mooley et al. 2018, Mooley et al. 2022). These measurements enable us to make better inferences about the properties of the kilonova ejecta masses, velocities and compositions. We present a new kilonova model grid from the radiative transfer code POSSIS (Bulla et al. 2019) with heating rates consistent with numerical simulations, and parameterized by dynamical ejecta mass and velocity, disk wind ejecta mass and velocity, and the electron fraction at which the dynamical ejecta is stratified into LP and LR regions. Equipped with the precise viewing angle constraint and our new model grid from POSSIS, we derive new constraints on the ejecta properties of GW170817. I will discuss both our updated inference results, and broader implications on r-process nucleosynthesis from kilonovae.