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SUMMARY:Thermohaline Mixing in Polluted White Dwarfs - Evan Bauer (UC Sant
 a Barbara)
DTSTART:20181004T100000Z
DTEND:20181004T110000Z
UID:TALK112303@talks.cam.ac.uk
CONTACT:Dr William Béthune
DESCRIPTION:Many isolated\, old white dwarfs (WDs) show surprising evidenc
 e of metals in their photospheres. Given that the timescale for gravitatio
 nal sedimentation is astronomically short\, this is taken as evidence for 
 ongoing accretion\, likely of tidally disrupted planetesimals. These pollu
 ted WDs require detailed modeling of surface mixing to understand accretio
 n rates and compositions of parent bodies that supply polluting material. 
 Much modeling of the pollution process relies on assuming an equilibrium b
 etween diffusive sedimentation and metal accretion supplied to the WD’s 
 surface convective envelope. However\, recent work is calling attention to
  the importance of additional mixing processes such as convective overshoo
 t and the thermohaline instability. We present results from a large grid o
 f MESA models indicating that this additional mixing can lead to inferred 
 accretion rates that are several orders of magnitude higher than previous 
 estimates. WDs with hydrogen atmospheres at effective temperatures above 1
 0\,000 K require rates as large as 10^13 g/s to explain observed Calcium a
 bundances. Mixing other than diffusion may also inhibit chemical separatio
 n\, resulting in systematic effects on inferences of compositions for the 
 rocky bodies that are the ultimate source of polluting metals.
LOCATION:MR2\,  Centre for Mathematical Sciences\, Wilberforce Road\, Camb
 ridge
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