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Serendipitous Discovery of a Double Helium-rich Subdwarf

Amir Ahmad and Simon Jeffery have been studying some rare stars known, loosely, as helium-rich subdwarfs. The challenge with all rare stars is to understand their evolutionary origin. In this case, our own observations had identified them as being possible products of white dwarf binary mergers. However, another group argued that the stars were less luminous and had an entirely different origin, possibly as single stars. New observations of the prototype (PG1544+488) obtained with the WHT showed that the strong helium absorption lines in its spectrum were unusual; the line shapes varied and appeared to split into two. Further study showed that PG1544+488 is in fact a double star with both stars having extremely helium-rich surfaces -- the first case known. This discovery explained why the star had appeared to be completely different when observed in the optical, where both stars have a similar brightness, and when observed in the far ultraviolet, where only the hotter star is seen. The discovery will be very important for understanding what happens to stars which, as they evolve, become so large that for a time they completely engulf their companions. As this phase of `common-envelope' evolution ends, the outer layers of the star are lost revealing the much-reduced cores of the original stars, stripped of their hydrogen-rich outer layers. PG1544+488 seems to be an extreme case of one of these naked double cores.