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V471 Tauri:
Star Shows It Has the Right Stuff
The V471 Tauri system comprises a white dwarf star (the primary) in a
close orbit - one thirtieth of the distance between Mercury and the Sun
- with a normal Sun-like star (the secondary). The white dwarf star
was once a star several times as massive as the Sun. Chandra data on
this system provide the best evidence yet that a star can be engulfed by
its companion star and survive.
The illustration shows X-ray spectra made by Chandra's Low Energy
Transmission Grating Spectrometer of two individual stars and V471
Tauri: a red giant star (Beta Ceti, top panel), V471 Tauri, and a
Sun-like star (Epsilon Eridani). The peak in the spectrum due to carbon
ions is much smaller in the giant star than in the Sun-like star,
whereas the carbon peak in V471 is intermediate between the two. These
differences provide important clues to the different evolutionary
histories of the stars.
Nuclear fusion reactions in the core of such a star convert carbon into
nitrogen over a period of about a billion years. When the fuel in the
core of the star is exhausted, the core collapses, triggering more
energetic nuclear reactions that cause the star to expand and transform
into a red giant before eventually collapsing to become a white dwarf.
The carbon-poor material in the core of the red giant is mixed with
outer part of the star, so its atmosphere will have a deficit of carbon,
as compared with Sun-like stars, as shown in the figure. If a red giant
is part of binary system of closely orbiting stars, the evolution of the
secondary star can be dramatically affected.
Theoretical calculations indicate that the red giant can completely
envelop its companion star. During this common envelope phase, friction
causes the companion star to spiral inward rapidly where it will either
be destroyed by the red giant, or it will survive when much of the
envelope is spun away.
If the companion star manages to survive, it will bear the marks of its
ordeal in the form of contamination by carbon-poor material that it
accreted while it was inside the red giant envelope. The X-ray spectrum
of V471 Tauri in the middle panel shows just this effect - the carbon
peak is intermediate between that of a Sun-like star and an isolated red
giant star. The data indicate that about 10 percent of the star's mass
has been accreted from the red giant.
In the future the companion star can return the favor. It will expand
and dump material back onto the white dwarf. If enough material is
dumped on the white dwarf, it could cause the white dwarf to explode as
a supernova.
| Fast Facts for V471 Tauri:
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Credit
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NASA/CXC/SAO/J. Drake et al |
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Category
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Normal Stars & White Dwarf Stars |
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Coordinates
(J2000)
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RA 03h 50m 25.00s | Dec +17º
14' 47.40" |
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Constellation
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Taurus |
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Observation
Date
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January 24-25, 2002
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Observation
Time
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24 hrs |
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Obs.
ID
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2523 |
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Instrument
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HRC/LETG
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Reference
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J. Drake and M. Sarna, Astrophys. J. Letters, 594, L55 (2003) |
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Distance Estimate
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about 150 light years. |
| Release
Date |
January 30, 2004 |
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