Helium abundances and the helium isotope anomaly of sdB stars

TL;DR

This study measures helium abundances and isotopic compositions in 46 hot subdwarf B stars, revealing helium-3 enrichment in certain stars and challenging existing diffusion models.

Contribution

It provides high-precision helium abundance measurements and identifies helium-3 enrichment in sdB stars, highlighting a phenomenon not fully explained by current diffusion theories.

Findings

Helium abundance correlates with temperature.

Two distinct helium abundance sequences are confirmed.

Helium-3 is strongly enriched in stars between 27,000 K and 31,000 K.

Abstract

Helium abundances and atmospheric parameters have been determined from high resolution spectra for a new sample of 46 bright hot subdwarf B (sdB) stars. The helium abundances have been measured with high accuracy. We confirm the correlation of helium abundance with temperature and the existence of two distinct sequences in helium abundance found previously. We focused on isotopic shifts of helium lines and found helium-3 to be strongly enriched in 8 of our programme stars. Most of these stars cluster in a small temperature range between 27000 K and 31000 K very similar to the known helium-3-rich main sequence B stars, which cluster at somewhat lower temperatures. This phenomenon is most probably related to diffusion processes in the atmosphere, but poses a challenge to diffusion models.