Download Sakurai`s Object - Department of Physics, HKU

Sakurai’s
Object
A Case Of
Superfast
Stellar
Evolution
Dr H F Chau
Department of
Physics
HKU
• 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
(17h52m32.69s,-17o41’07.7”).
• 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.
• 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.
• 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).
• 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.
• 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.
• 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.
• 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 dredgeup”, resulting in an increase of He, C and
N on the stellar surface.
• 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.
• 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.
2) The “on” phase: He shell burns very
strongly. Shell luminosity goes as high
as 108 Lsun. 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).
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.
• The rapid mass loss due to pulsating
instability or strong stellar wind blow this
carbon-rich
outer-layer
into
space,
producing a planetary nebula.
• 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.
• To summarize, Sakurai’s object was a dim
hot but rapidly cooling object about 20
years before its discovery in 1996. (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.)
• 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 1919-1921) and FG Sge (outburst
around 1900-1965) were possible born-again
AGB stars.
• Although Sakurai’s object is not visible in
the visible spectrum now, AAVSO advises
amateurs
to
monitor
this
object
occasionally.