Atomic Diffusion in Post-Common Envelope Binary Stars

Last updated May 9, 2019 by Alessandro Ferraro

Thursday, May 9th 2019, 04:00 PM, MAPTC/0G/017

Speaker: C. Byrne (Armagh Observatory & Planetarium and TCD)

Atomic diffusion is a term which encompasses a group of chemical mixing processes
which are responsible for the alteration of the composition of the outer layers of a star, which
has a notable effect in many stars, particularly in hot subdwarf stars.

Hot subdwarfs are low-mass, core-helium-burning stars with low-mass hydrogen
envelopes. As well as showing interesting surface chemistries, they have an unusual history,
generally requiring close interaction in a binary star system to be produced. One such process
is common envelope ejection, where the companion star provides energy to remove most of
the hydrogen envelope from a red giant star. The most important diffusion process for hot
subdwarfs is radiative levitation, whereby the radiative flux of the star provides an upward
force to counteract that of gravity. This can explain the chemical peculiarities, as well as
enabling a mechanism to drive the brightness variations observed in some subdwarfs.

This work self-consistently investigates the evolution of post-common-envelope objects
as they transition from red giant to become hot subdwarfs. Evolutionary models were
produced testing the effects of atomic diffusion on their surface composition. The evolution
of these subdwarf progenitors was computed with the MESA (Modules for Experiments in
Stellar Astrophysics) stellar evolution code from the Main Sequence up to a point close to core
helium ignition on the red giant branch, at which point a common envelope event is initiated.
Then the simulation follows the subsequent evolution from immediately after envelope
ejection right up to the ignition of helium in the core, at which point the star becomes a hot
subdwarf.

It was found that atomic diffusion processes significantly alter the surface composition
of pre-subdwarf stars, with radiative levitation in particular causing significant enhancement
of heavy elements, while lighter elements get depleted. The abundances changes are more
pronounced than those seen in the observational data, implying that other mixing processes
must be present in order to counteract diffusion and reduce its effectiveness.

These methods were then applied to red giants with lower mass helium cores, in an
attempt to explain the origin of a newly-discovered class of variable star, knowns as blue
large-amplitude pulsators (BLAPs), where it was found that a 0.3 solar mass pre-white dwarf
can resemble a BLAP if the effects of atomic diffusion are considered. The period of
variability found in these models is also in agreement with the observed periods.


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