Sakurai's Object Is Now a Wolf-Rayet Star. Astronomers Watched It Happen

A white dwarf that abruptly reignited in 1996 has now been confirmed as a Wolf-Rayet-type star, with new spectroscopy from the Very Large Telescope providing the first direct measurement of its surface temperature and stellar wind, according to a letter published Sept. 16 in Monthly Notices of the Royal Astronomical Society.

W. Marcolino of the Universidade Federal do Rio de Janeiro and colleagues used the VLT's FORS2 instrument along with non-local thermodynamic equilibrium atmosphere models to establish that Sakurai's Object has developed a [WC]-type stellar wind, a class of star defined by strong carbon emission lines driven by intense outflows. The strongest spectral features the paper identifies are due to carbon (C ii-iii) and helium (He i).

Stellar evolution normally unfolds over millions of years, on timescales no single observer follows. Sakurai's Object underwent a Very Late Thermal Pulse, a helium-shell ignition in a white dwarf that had already stopped fusing, and was first detected in that "born-again" state in 1996. Spectroscopic monitoring over the roughly 30 years since has now captured the full transition to a Wolf-Rayet object within a single scientific career.

The paper reports an effective temperature of about 30,500 K, which is substantially cooler than V605 Aquilae, the only other known born-again star with a comparably measured temperature (~95,000 K). V605 Aql erupted about 80 years before Sakurai's Object. The temperature is lower than models predict when convective mixing is suppressed during the thermal pulse, but is consistent with calculations for remnants of slightly lower mass (below roughly 0.6 solar masses).

Continued spectroscopic monitoring will be needed to trace how the star reheats over coming decades.