White dwarf merger origin for extreme helium star V652 Her
For thirty years an extraordinary star has baffled astronomers - until now. V652 Her has a surface made almost entirely of helium. Stars create helium in their centres but it is rarely exposed on the surface and then only with other products of nuclear burning such as carbon. But V652's surface doesn't have any other nuclear waste. Sometimes helium stars form by losing material to a companion -- but V652 has no companion. V652 Her shows another remarkable property. Every two and a half hours it expands and contracts by over 15,000 km (about 1% of its size). These pulsations enable us to measure V652's mass, which is essential if we want to know how it was formed. Moreoever, the pulsations are speeding up, which tells us that V652 Her is shrinking.
We have recently measured the composition of V652 Her in detail and proved that the surface helium is the product of hydrogen-burning alone. We have also determined the mass more precisely than before (Jeffery, Hill & Heber 1999, A&A 346, 491, abstract, full paper).
These observations led Saio & Jeffery (2000, MNRAS 313, 671, abstract, full paper) to an explanation of how V652 Her might have formed, V652's life-line is shown in the Figure. Many years ago, before the formation of the solar system, two Sun-like stars formed and began to evolve, first to become red giants and then helium white dwarfs. Billions of years later, they spiralled into one another until one was sucked onto the surface of its companion. With a new energy source, the merged star expanded to become a bright giant. Unusually, the new star started to burn from the outside in. The new models predict just the right luminosity, pulsation period and contraction rate for V652 Her. No other plausible models have yet succeeded in doing this
Stars like V652 Her are rare, but not unique, and pulsations in a similar helium giant were discovered recently (Kilkenny et al. 1999: MNRAS 310, 1119, abstract, full paper) . New observations with the 3.9m Anglo-Australian Telescope, the 4.2m William Herschel Telescope in La Palma and the Hubble Space Telescope are providing superb data on the rapidly changing surfaces of these stars. The Armagh group has predicted that several more helium stars should show similar pulsations (Jeffery & Saio 1999: MNRAS 308, 221, abstract, full paper) and will be searching for them in the coming year. This work demonstrates one of the intriguing ways in which stars in binary systems can be destroyed and reborn.
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