The effects of diffusion in hot subdwarf progenitors from the common envelope channel

TL;DR

This study models the impact of diffusion and radiative levitation on the surface composition of hot subdwarfs formed via the common envelope channel, revealing rapid helium depletion inconsistent with observations.

Contribution

It provides detailed stellar evolution models including diffusion and levitation effects, linking formation channels to surface chemistry in hot subdwarfs.

Findings

Diffusion depletes surface helium rapidly in models.

Models become more helium-depleted than observed.

Additional physics needed to match observed abundances.

Abstract

Diffusion of elements in the atmosphere and envelope of a star can drastically alter its surface composition, leading to extreme chemical peculiarities. We consider the case of hot subdwarfs, where surface helium abundances range from practically zero to almost 100 percent. Since hot subdwarfs can form via a number of different evolution channels, a key question concerns how the formation mechanism is connected to the present surface chemistry. A sequence of extreme horizontal branch star models was generated by producing post-common envelope stars from red giants. Evolution was computed with MESA from envelope ejection up to core-helium ignition. Surface abundances were calculated at the zero-age horizontal branch for models with and without diffusion. A number of simulations also included radiative levitation. The goal was to study surface chemistry during evolution from cool giant to hot subdwarf and determine when the characteristic subdwarf surface is established. Only stars leaving the giant branch close to core-helium ignition become hydrogen-rich subdwarfs at the zero-age horizontal branch. Diffusion, including radiative levitation, depletes the initial surface helium in all cases. All subdwarf models rapidly become more depleted than observations allow. Surface abundances of other elements follow observed trends in general, but not in detail. Additional physics is required.