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Sakurai’s Object Dr H F Chau Department of Physics HKU Dr H F Chau Department of Physics HKU A Case Of Superfast Stellar Evolution.

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Presentation on theme: "Sakurai’s Object Dr H F Chau Department of Physics HKU Dr H F Chau Department of Physics HKU A Case Of Superfast Stellar Evolution."— Presentation transcript:

1 Sakurai’s Object Dr H F Chau Department of Physics HKU Dr H F Chau Department of Physics HKU A Case Of Superfast Stellar Evolution

2 On 20th Feb 1996, amateur comet hunter Yukio Sakurai ( 櫻井幸雄 ) discovered a magnitude 12 variable star V4334 SGR. Position of the star is (17 h 52 m s,-17 o 41’07.7”). In the ESO/SERC sky survey image taken in 1976, a faint object of magnitude 21 was found in the same location in the J plate. The R plate of the same survey taken in 1984 shows no object at all, implying that V4334 SGR was at most about 20.5 in R magnitude. On 20th Feb 1996, amateur comet hunter Yukio Sakurai ( 櫻井幸雄 ) discovered a magnitude 12 variable star V4334 SGR. Position of the star is (17 h 52 m s,-17 o 41’07.7”). In the ESO/SERC sky survey image taken in 1976, a faint object of magnitude 21 was found in the same location in the J plate. The R plate of the same survey taken in 1984 shows no object at all, implying that V4334 SGR was at most about 20.5 in R magnitude.

3 Therefore, Sakurai believed that V4334 SGR was a nova. Spectrum taken soon after the discovery showed pure absorption lines of He , C , C , N , O , Si  and faint lines of H. In IAU Circular 6325 (26 Feb 1996), Benetti and others pointed out that the slow brightness evolution and the C -rich and H -poor spectrum suggest that this object may be undergoing its final helium flash stage. Therefore, Sakurai believed that V4334 SGR was a nova. Spectrum taken soon after the discovery showed pure absorption lines of He , C , C , N , O , Si  and faint lines of H. In IAU Circular 6325 (26 Feb 1996), Benetti and others pointed out that the slow brightness evolution and the C -rich and H -poor spectrum suggest that this object may be undergoing its final helium flash stage.

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5 In IAU Circular 6328 (29 Feb 1996), Durebeck and Pollacco found an almost circular planetary nebula around the central star V4334 SGR. At this point, it is clear that Sakurai has discovered an object undergoing a final helium flash. And for the first time, this object is referred to as the Sakurai’s Object ( 櫻井天體 ). In IAU Circular 6328 (29 Feb 1996), Durebeck and Pollacco found an almost circular planetary nebula around the central star V4334 SGR. At this point, it is clear that Sakurai has discovered an object undergoing a final helium flash. And for the first time, this object is referred to as the Sakurai’s Object ( 櫻井天體 ).

6 Recall that a main sequence star with about the mass of our Sun burns hydrogen steadily in the core. The core becomes helium-rich. Standard stellar evolution theory tells us that the star will then evolve to a red giant. Recall that a main sequence star with about the mass of our Sun burns hydrogen steadily in the core. The core becomes helium-rich. Standard stellar evolution theory tells us that the star will then evolve to a red giant.

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8 In the red giant phase, the core is made up of mainly non-burning helium which is surrounded by a hydrogen burning shell. The outer layer of this red giant star is convective. The convective layer extends very close to the hydrogen burning shell. In this “first dredge-up”, some He produced in the hydrogen burning shell will be transported to the stellar surface. In the red giant phase, the core is made up of mainly non-burning helium which is surrounded by a hydrogen burning shell. The outer layer of this red giant star is convective. The convective layer extends very close to the hydrogen burning shell. In this “first dredge-up”, some He produced in the hydrogen burning shell will be transported to the stellar surface.

9 At a later time, the contracting helium core ignites in the form of helium core flash and we have a star that burns helium in the core and hydrogen in a outer burning shell. But sooner or later, the core helium will be used up, leaving a carbon-oxygen-rich core. At a later time, the contracting helium core ignites in the form of helium core flash and we have a star that burns helium in the core and hydrogen in a outer burning shell. But sooner or later, the core helium will be used up, leaving a carbon-oxygen-rich core.

10 This marks the formation of an asymptotic giant branch (AGB) star. It consists of a non-burning carbon-oxygen core, a helium burning shell and a hydrogen burning shell. Shortly before the formation of an AGB, convective instability in some higher mass pre-AGBs may lead to the “second dredge- up”, resulting in an increase of He, C and N on the stellar surface. This marks the formation of an asymptotic giant branch (AGB) star. It consists of a non-burning carbon-oxygen core, a helium burning shell and a hydrogen burning shell. Shortly before the formation of an AGB, convective instability in some higher mass pre-AGBs may lead to the “second dredge- up”, resulting in an increase of He, C and N on the stellar surface.

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13 The hydrogen and helium shells burn at very different rates. Very quickly, the AGB star will set into the so-called thermally pulsing AGB phase (TP-AGB phase). The amplitude of the pulsation gradually increases. It is the unstable and oscillating TP-AGB phase that leads to a massive material ejection from the star. The hydrogen and helium shells burn at very different rates. Very quickly, the AGB star will set into the so-called thermally pulsing AGB phase (TP-AGB phase). The amplitude of the pulsation gradually increases. It is the unstable and oscillating TP-AGB phase that leads to a massive material ejection from the star.

14 A TP-AGB will undergo the so-called “third dredge-up”. According to the simulations by Iben together with Sugimoto and Nomoto, the process goes as follows: 1)The “off” phase: The re-ignition of the slow and steady H shell burning. This shell dominates the energy production.

15 2)The “on” phase: He shell burns very strongly. Shell luminosity goes as high as 10 8 L sun. An inter-shell convective zone is formed. 3)The “power down” phase: He burning declines. The inter-shell convective zone disappears. The inter-shell region expands. The H burning shell is extinguished (or very nearly so). 2)The “on” phase: He shell burns very strongly. Shell luminosity goes as high as 10 8 L sun. An inter-shell convective zone is formed. 3)The “power down” phase: He burning declines. The inter-shell convective zone disappears. The inter-shell region expands. The H burning shell is extinguished (or very nearly so).

16 The rapid mass loss due to pulsating instability or strong stellar wind blow this carbon-rich outer-layer into space, producing a planetary nebula. 4)The “dredge-up” phase: In response to the increasing luminosity coming out from the He burning shell, convective envelope extends down to the extinguished H burning shell. This brings a lot of C into the stellar surface.

17 Now back to the Sakurai’s object. Within the first 5 months after its discovery, its surface H abundance decreased rapidly. Also, its surface temperature decreased by several hundred K per month. The spectrum quickly reddened. Its optical emission eventually blocked by the dust and gas ejected from the outer layer although it remains bright in IR. Now back to the Sakurai’s object. Within the first 5 months after its discovery, its surface H abundance decreased rapidly. Also, its surface temperature decreased by several hundred K per month. The spectrum quickly reddened. Its optical emission eventually blocked by the dust and gas ejected from the outer layer although it remains bright in IR.

18 To summarize, Sakurai’s object was a dim hot but rapidly cooling object about 20 years before its discovery in (It was a new born white dwarf at that time.) It was red and increasingly carbon-rich. It also ejected a lot of dust since its discovery. (It was undergoing the helium shell flash and the third dredge-up around that time.) To summarize, Sakurai’s object was a dim hot but rapidly cooling object about 20 years before its discovery in (It was a new born white dwarf at that time.) It was red and increasingly carbon-rich. It also ejected a lot of dust since its discovery. (It was undergoing the helium shell flash and the third dredge-up around that time.)

19 The only reasonable explanation is that Sakurai’s object is undergoing its final helium shell flash before it dies and joins the rank of white dwarfs. This star is currently evolving at a timescale of a few 10s to at most 100 years --- a very rare situation in stellar evolution indeed. Besides Sakurai’s object, V605 Aql (outburst around ) and FG Sge (outburst around ) were possible born-again AGB stars. The only reasonable explanation is that Sakurai’s object is undergoing its final helium shell flash before it dies and joins the rank of white dwarfs. This star is currently evolving at a timescale of a few 10s to at most 100 years --- a very rare situation in stellar evolution indeed. Besides Sakurai’s object, V605 Aql (outburst around ) and FG Sge (outburst around ) were possible born-again AGB stars.

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24 Although Sakurai’s object is not visible in the visible spectrum now, AAVSO advises amateurs to monitor this object occasionally.


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