Sakurai's Object

Sakurai's Object
V4334 Sagittarii

Sakurai's Object, by Hilmar Duerbeck, ESO
Observation data Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation	Sagittarius
Right ascension	17h 52m 32.69s[1]
Declination	−17° 41′ 08.0″[1]
Characteristics
Spectral type	S...D[1]
Apparent magnitude (V)	10.90[1]
Apparent magnitude (J)	11.555 ± 0.022[1]
B−V color index	8.80[1]
Astrometry
Distance	1.8-5[2] pc
Details
Mass	0.6[3] M☉
Other designations
V4334 Sgr, V4334 Sagittarii, 2MASS J17523269-1741080, DPV 1, Sakurai's variable, PN G010.4+04.4, GSC2 S2202011182877, Sakurai's Object, AAVSO 1746-17
Database references
SIMBAD	'S OBJECT data

Coordinates: 17^h 52^m 32.69^s, −17° 41′ 08.0″

Sakurai's Object (V4334 Sgr) is a star in the constellation of Sagittarius named after Yukio Sakurai, a Japanese amateur astronomer, who discovered it in 1996. It is a star which has undergone a Very Late Thermal Pulse.

Sakurai's Object is thought to be a red giant star that was previously a white dwarf; a Very Late Thermal Pulse occurred during the white dwarf phase and caused it to swell back to a giant configuration. These types of objects, of which only a handful have been observed, become hydrogen deficient and enriched in helium and other metals.^[4] It is expected that these stars will end up as helium rich white dwarves after retracing their evolution track from the "born-again" giant phase back to the white dwarf cooling track.

Sakurai's Object is one of only two stars observed during the high luminosity phase of a Very Late Thermal Pulse, the other being V605 Aquilae which was discovered in 1919.^[2] There are few other suspected "born-again" objects; one example being FG Sagittae.^[4]

Observation history

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.^[5] 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 year. 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.”^[6]

Following the initial announcement, Hilmar Duerbeck et al. 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.^[7] By investigating the measured fluxes, angular diameter, and mass of the nebula, a distance of 5.5kpc and luminosity of 38L_☉ was determined. The researchers noted that this was in agreement with appearance and model predictions^[8] 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 was presented. The collected data showed Sakurai's Object brightening over the year of observations as expected and the researchers found that circumstellar dust was present at a temperature of ~680K.^[9] Further infrared data, taken using the United Kingdom Infrared Telescope, was published in 2000 in which findings of the changing absorption lines were discussed.^[10][11]

Properties

Sakurai's Object is classed as a post-asymptotic giant branch star which has, following a brief period on the white dwarf cooling track, undergone a helium shell flash (also known as a very late thermal pulse).^[12] The star is thought to have a mass of around 0.6M_☉.^[3]

Dust cloud

During the second half of 1998 an optically thick dust shell obscured Sakurai's Object, causing a rapid decrease in visibility of the star, until in 1999 it disappeared from optical wavelength observations altogether.^[13] Infrared observations showed that the dust cloud around the star is primarily carbon in an amorphous form.^[14] In 2009 it was discovered that the dust shell is strongly asymmetrical, as a disc with a major axis oriented at an angle of 134°, and inclination of around 75°. The disc is thought to be growing more opaque due to the fast spectral evolution of the source towards lower temperatures.^[15][16]

Planetary nebula

Sakurai's Object is surrounded by a planetary nebula created following the star's red giant phase around 8300 years ago.^[17] It has been determined that the nebula has a diameter of 44 arcseconds and expansion velocity of roughly 32 km/s.^[18]

See also

• FG Sagittae
• V605 Aquilae
• Very Late Thermal Pulse

References

External links

• AAVSO Variable Star of the Month. V4334 Sgr (Sakurai's Object): June 2002