Naslim N., C. S. Jeffery, A. Hibbert and N. T. Behara

Discovery of extremely lead-rich subdwarfs: does heavy metal signal the formation of subdwarf B stars?

ArtistòÀÙs impressions of the surface of HE2359-2844. The very high temperature (38000 degrees) makes the surface bluish in colour. An alien observer close to the star might be able to see the lead- and zirconium-rich cloud layers using specially chosen filters.
Images created using POV-Ray by C. S. Jeffery.

Abstract

Hot subdwarfs represent a group of low-mass helium-burning stars formed through binary- star interactions and include some of the most chemically peculiar stars in the Galaxy. Stellar evolution theory suggests that they should have helium-rich atmospheres but, because radiation causes hydrogen to diffuse upwards, a majority are extremely helium poor. Questions posed include: when does the atmosphere become chemically stratified and at what rate?

The existence of several helium-rich subdwarfs suggests further questions: are there distinct subgroups of hot subdwarf, or do hot subdwarfs change their surface composition in the course of evolution? Recent analyses have revealed remarkable surface chemistries amongst the helium-rich subgroup. In this paper, we analyse high-resolution spectra of nine intermediate helium-rich hot subdwarfs. We report the discovery that two stars, HE 2359òÈÒ2844 and HE 1256òÈÒ2738, show an atmospheric abundance of lead which is nearly 10 000 times that seen in the Sun. This is measured from optical Pb IV absorption lines never previously seen in any star. The lead abundance is 10 to 100 times that measured in normal hot-subdwarf atmospheres from ultraviolet spectroscopy. HE 2359òÈÒ2844 also shows zirconium and yttrium abundances similar to those in the zirconium star LS IVòÈÒ14ò×æ116. The new discoveries are interpreted in terms of heavily stratified atmospheres and the general picture of a surface chemistry in transition from a new-born helium-rich subdwarf to a normal helium-poor subdwarf.

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Last Revised: 2013 July 31st