DIX Planetary Science Seminar

A white dwarf is the end stage of stellar evolution for stars with initial masses <8 solar masses. White dwarfs are degenerate stellar cores devoid of fusion, and their surface gravities are so high that anything heavier than H/He in their thin atmospheres should rapidly sink out of sight and become unobservable. Yet heavy elements are routinely detected in white dwarf spectra — the phenomenon known as "pollution" — implying recent or ongoing infall of exogenous material. The widely accepted scenario invokes accretion of tidally disrupted asteroids that survived post-main-sequence evolution and were later perturbed onto star-grazing or Roche-crossing orbits by massive planets. This interpretation has made polluted white dwarfs a probe of exoplanetary material and, by extension, exoplanet interiors. Such applications, however, require caution because key pieces of the pollution scenario remain unknown: Which minor bodies can survive to the white dwarf stage and remain dynamically active over white dwarfs' Gyr lifetimes? Which pathways can deliver material to the white dwarf efficiently over both Myr and Gyr timescales? Why are volatile-rich pollutants and tidally produced disks observed so rarely? In this talk, I will introduce our ongoing effort to explore an alternative route to pollution that begins earlier, during the Asymptotic Giant Branch phase immediately preceding white dwarf formation, when stellar mass loss via dusty winds, radiation forces, and gas drag reshape planetary systems while also raising the question of whether the polluting material is truly planetary or may be stellar.

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On January 7, 2025, the Eaton Fire erupted in the foothills of the San Gabriel Mountains in Southern California, destroying >9,000 structures across the Altadena neighborhood. Strong Santa Ana winds coupled with the release of burned material into the air also resulted in a large-scale dust plume stretching across LA County. Unlike traditional wildfires, urban firestorms can release toxic heavy metals (HM) from the burning of industrial/residential structures in fine particulates that can travel along wind patterns far beyond ignition sites and contaminate residential areas following settling. Here, we report on HM concentrations in dust samples collected ~1 month after the fire from 52 houses spatially distributed along the plume path and evaluate the effectiveness of basic surface cleaning at reducing HM levels. Samples were leached in a trace metal-free clean lab to release labile HM, and elemental concentrations measured via Q-ICP-MS. In our analyses, we focus on lead (Pb). Directly following the fire, highest Pb levels are traced along wind patterns, with homes miles away from the burn scar exhibiting Pb surface concentrations above EPA limits. Between windowsills and flat surfaces, most but not all cleaned surfaces contained reduced Pb below EPA limits. As the frequency of destructive urban fires increases historically, continuous and widespread evaluation of HM contamination is vital for developing effective emergency response and remediation strategies. In this respect, we also report on (preliminary) HM content from dust samples re-collected from 34 of the initial houses—displaying varying stages of remediation—~1 year after the fire.