By Hilmar Duerbeck at the Dutch 0.9-metre telescope at ESO
© ESO, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=6761844
Sakurai's Object (V4334 Sgr) is a star in the constellation of Sagittarius. It is thought to have previously been a white dwarf that, as a result of a very late thermal pulse, swelled and became a red giant. It is located at the center of a planetary nebula and is believed to currently be in thermal instability and within its final shell helium flash phase.
At the time of its discovery, astronomers believed Sakurai's Object to be a slow nova. Later spectroscopic analysis suggested that the star was not a nova, but had instead undergone a very late thermal pulse similar to that of V605 Aquilæ, causing it to vastly expand. V605 Aquilæ, which was discovered in 1919, is the only other star known to have been observed during the high luminosity phase of a very late thermal pulse, and models predict that Sakurai's Object, over the next few decades, will follow a similar life cycle.
Sakurai's Object and other similar stars are expected to end up as helium-rich white dwarfs after retracing their evolution track from the "born-again" giant phase back to the white dwarf cooling track. There are few other suspected "born-again" objects, one example being FG Sagittæ. Having erupted in 1995, it is expected that Sakurai's Object's final helium flash will be the first well-observed one.
An International Astronomical Union Circular sent on February 23, 1996 announced the discovery of a "possible 'slow' nova" of magnitude 11.4 by Yukio Sakurai, an amateur astronomer. Japanese astronomer Syuichi Nakano reported the discovery, drawing attention to the fact that the object had not been visible in images from 1993 nor in Harvard–Smithsonian Center for Astrophysics records for the years 1930-1951, despite it appearing to slowly brighten over the previous years. Nakano wrote that "While the outburst [suggests] a slow or symbiotic nova, the lack of obvious emission lines one year after brightening is very unusual."
Following the initial announcement, Hilmar Duerbeck published a study investigating the "possible final helium flash" seen by Sakurai. In it, they noted that the location of Sakurai's Object corresponded to a faint object detected in 1976 of magnitude 21, and discussed other observations in the years 1994-1996, by which time the magnitude had increased to around 11-15. By investigating the measured fluxes, angular diameter, and mass of the nebula, a distance of 5.5 kpc and luminosity of 38 Solar luminosity was determined. The researchers noted that this was in agreement with their appearance and model predictions and that the outburst luminosity was in the area of 3100 solar luminosities; lower than predicted by a factor of 3.
The first infrared observations were published in 1998, in which both near and far infrared spectroscopy data was presented. The collected data showed Sakurai's Object's steep brightening in 1996, followed by a sharp decline in 1999 as expected. It was later found that the Star's steep decline in light was due to the circumstellar dust located around the star, which was present at a temperature of ~680 K. Further infrared data recorded by the United Kingdom Infrared Telescope was published in 2000, in which findings of the changing absorption lines were discussed.
Observations from the United Kingdom Infrared Telescope (UKIRT) in 1999 revealed that the star is in a RCB-like phase with the release of dust and huge loss of mass.
Since 2005, it has been observed in the ejected particles of Sakurai's Object that photoionization of carbon is taking place.
Significance in astronomical research
A significant amount of new star formation and star destruction data is expected to be recorded from continued observation of Sakurai's Object, as well as be used as reference data in the future research of similar stars. The reason that stars such as Sakurai's Object and V605 Aquilæ exist, as well as experience a shorter lifespan compared to most stars, is largely unknown. Sakurai's Object and V605 Aquilæ have been observed experiencing born-again behavior for only 10 years, while FG Sagittæ has undergone such behavior for 120 years. It is hypothesized that this is due to Sakurai's Object and V605 Aquilæ evolving to the asymptotic giant branch of stars for the first time, while FG Sagittæ is undergoing the process a second time.