Lying somewhere between young main-sequence stars and old white dwarfs, hot subdwarfs are blue stars with about half the mass, a tenth the diameter and ten times the brightness of the Sun. They generally have surfaces which are either extremely hydrogen-rich or helium-rich. The former are probably due to chemical diffusion in the stellar atmosphere, which encourages the heavier helium to sink. The latter are probably due to previous evolution, possibly the merger of two white dwarfs, which has led to the destruction of hydrogen.

In between, is a small group of stars with intermediate helium abundances — that is between 20 and 80% helium. In recent years, many of these have been discovered to show remarkable overabundances of certain iron-group and/or trans-iron elements, including zirconium and lead. The frequency with which these stars occur, the range of surface properties, and the reason for their extraordinary surface characteristics represent significant questions for subdwarf astronomy.

Over the past three years, Simon Jeffery (AOP) and Brent Miszalski (SALT) have been conducting a medium- and high-resolution spectroscopic survey of southern hot subdwarfs with the Southern African Large Telescope. They have discovered many unusual stars, of which EC 22536−5304 was the first SALT ‘heavy-metal’ subdwarf.

SALT spectra show strong absorption lines of triply ionized lead. The HRS spectrum and a follow-up SALT/RSS spectrum show EC 22536−5304 to have surface properties (temperature, gravity, helium/hydrogen ratio) similar to other heavy-metal subdwarfs. With lead in the star’s atmospheres being nearly 100,000 times more abundance that in the Sun, EC 22536−5304 is the most lead-rich intermediate helium subdwarf discovered so far. The atmosphere is moderately rich in carbon and oxygen, with the abundances of both exceeding that of nitrogen. Analyses of EC 22536−5304 and other heavy-metal subdwarfs with SALT and other telescope are ongoing. These aim to understand physical processes such as atomic diffusion currently at work in the stellar atmosphere, as well as the star’s history which led to the reduced hydrogen abundance.

Part of the SALT/RSS spectrum of EC 22536−5304 (black) showing the locations of two lines of triply ionised lead. The line at 4049A is clearly seen; Neither PbIV line is present in the model used in this figure (red). Other prominent lines are also labelled.

Article by: Prof. Simon Jeffery, Armagh Observatory and Planetarium