A DYING STAR'S LAST GASP: SAKURAI OBJECT

In February 1995 a Japanese amateur astronomer discovered a nova in the constellation of Sagittarius (now known as V4334 Sagittarii). Its pre-discovery light curve indicated that it was unusual in that it had apparently been evolving only very slowly compared to a normal nova. Spectroscopic observations post discovered with the WHT showed the star to have little resemblance to any previously observed nova and in fact looked more like a solar type object shrouded in dust and with some level of hydrogen deficiency. Further observations revealed the presence of a nebula shell some 45 arcseconds in diameter. Thanks to a PATT award the ING has been monitoring this event since discovery and has witnessed gross spectral changes as the star has cooled.

The discovery of a Planetary Nebula at the WHT is important in that it indicates we are dealing with an evolved star. Planetary Nebula occur when a star evolves from red supergiant to a white dwarf expelling material. During this evolution the star rapidly heats up in 10,000–20,000 years reaching a surface temperature of 100,000 K or more, and this causes the expelled material to become visible. When the star becomes a white dwarf nuclear reactions no longer occur and the star simply fades and cools.

More recent work has shown that this may not be the end of the story, for some or even most stars. Just as the star reaches the white dwarf phase instabilities within its interior can cause an explosive event called a shell flash. In some objects this event can be so intense that material around the core of the star violently starts undergoing nuclear reactions. This can cause the star to go through a second supergiant phase and Planetary Nebula ejection before settling down to become a white dwarf. The time scale for this evolution is rapid taking anywhere from a few months to a few years to evolve from a white dwarf - red supergiant - hot Planetary Nebula central star. It is this evolution that Sakurai's object is currently undergoing.

During this century there is only one other object that is known to have undergone a shell flash of this magnitude: the central star of the old Planetary Nebula Abell 58 or V605 Aql. This object was first spotted as an unusually slow nova in 1918 and reaching about 10th magnitude in 1920.  During its slow fade the light curve underwent rapid and large fluctuations similar to those seen in R Corona Borealis stars. The star was finally lost to observers around 1923 and was essentially forgotten about. In 1989 the star was recovered again as a very hot Wolf-Rayet star shrouded in dust and gas and having a brightness of around the 22nd magnitude and its ejected nebula contains virtually no hydrogen. HST imaging shows this new nebula to be 0.5 arcsec in diameter and containing very non-uniformly distributed material.