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This guest post was written by Arianna Dwomoh. Arianna Dwomoh is an undergraduate studying at Duke University. She completed this research at the Center for Astrophysics | Harvard & Smithsonian REU program under the supervision of Dr. Evan Bauer. She presented her work at AAS 241.
A white dwarf is a remnant of a low or intermediate-mass star. Polluted white dwarfs (WDs) provide a means of determining the composition and structure of the planetary bodies that previously orbited about the star. Once a planet gets sufficiently close to the WD, its tidally torn material becomes a circumstellar dust and gas disk that surrounds and accretes onto the WD. Theoretical models that account for various chemical mixing processes, such as diffusion and convection, within a WD allow us to learn more about the overall system (Koester 2009; Bauer & Bildsten 2018, 2019).
The particular WD considered here, SDSS J122859.93+104032.9, features a solid core fragment orbiting within the Roche radius of the WD debris disk (Manser et al. 2019). This means that the planetesimal is held together by its own internal strength and might be the core of a larger object that was stripped of its outer surface layers by the WD’s gravity (Bonsor et al. 2020; Buchan et al. 2022). We describe accreting WD models tuned to match the observed surface composition of SDSS J122859.93+104032.9 built with the open-source stellar evolution code, MESA, version r22.05.1 (Paxton et al. 2011, 2013, 2015, 2018, 2019). We calculate and model the accretion rates necessary to match the observed surface pollution levels, specifically comparing the difference in accretion rate with and without the presence of thermohaline mixing. Thermohaline mixing is induced when heavy elements accrete onto an atmosphere dominated by hydrogen, and leads to extra mixing of the accreted material (Deal et al. 2013). We consider thermohaline mixing here because SDSS J122859.93+104032.9 is expected to have no surface convection zone due to its high temperature. Thus, accreted heavy elements concentrate at the surface, which excites the thermohaline instability. We find that the accretion rate increases by approximately three orders of magnitude when accounting for thermohaline mixing relative to models just accounting for diffusion only. This implies that the solid planetary body supplying the polluting material is significantly larger than previously inferred.
The SAO REU program is funded in part by the National Science Foundation REU and Department of Defense ASSURE programs under NSF Grant no. AST-2050813, and by the Smithsonian Institution.
Figure: Time evolution of the thermohaline model with Ṁacc tuned to match observed mass fractions over a period of 1000 years. In order to match the observed mass fractions at the surface of the WD, the Ṁacc had to be increased by nearly three orders of magnitude. Credit: Dwomoh, et al., in prep.
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