Transcript Blue Straggler Stars - University of Ottawa

Blue
Straggler
Stars
By Emilie Lafrance
Plan
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Discovery
How BSS differ from regular stars
Origin theories
Observations
Unanswered questions and
Implications
Discovery
• A few Blue Straggler
Stars were first
observed in 1953 by
Allan Sandage in the
globular cluster M3.
• Their properties were
those of young spectral
type A stars with
stronger Balmer lines.
What distinguishes Blue
Stragglers from other stars?
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MBSS>MMSTO
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vsin(i) = 155km/sec
(>100 times that of the sun) in BSS19 in 47 Tuc
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BSS are bluer and more massive than most other stars in a
cluster. Their position on a H-R diagram is usually off from the
main-sequence stars. We find them more often in highdensity regions of star clusters.
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Inside a globular cluster, star-forming material is scarce and
the stars are usually formed at the same time. BSS aren’t in
agreement with the standard star evolution model.
Color-magnitude diagram of M3
Origin of BSS (main theories)
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In1968, Sargent proposed they were stars in helium core
burning state (HB). This theory didn’t agree well with
observations because BSS are usually much fainter than HB
stars, amongst other things.
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It was then proposed that BSS were spawn from a different
generation than the main-sequence (MS) stars. This is
unlikely since globular clusters have little available material
for star formation.
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In 1996, Leonard detected that the ratio of binary BSS in M67
was much higher than for regular stars. This hints at a binary
origin for BSS. This theory explains the fact that BSS are
more numerous at the core of globular clusters rather than on
the outskirts. Since star density is higher, binary systems are
expected to be more common.
Collision model
• 1. Star’s gravitational forces
interact.
• 2. Low-mass stars approach.
• 3.They start to merge
• 4. Debris are ejected
• 5. We now have a hot, massive
reborn star with high angular
velocity.
• 6. Becomes a red giant and loses
speed through magnetic activity.
• 7. It shrinks, heats up and becomes
a slow rotating Blue Straggler.
Coalescence model
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1.Two stars with fast rotation
(most likely primordial) are in
close contact and slowly merge
into one star.
2. The most massive cannibalizes
the other, forming a even more
massive star.
3.This new star rotates at
velocities much higher than
average.
So far the slow coalescence model is favored. It seems to agree
better with observations. This model gives a BSS with fast rotation.
In 1995, astronomers found a BSS in the center of 47 Tucanae that
rotates about 75 times faster than the Sun. However, the model of
collision explains well why BSS population is greater in cluster
centers (~3x).
Observed BSS
Unanswered questions
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What are the conditions for
higher BSS populations?
(Different density of
primordial binaries?)
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Just how many BSS are
there
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Detection method?
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Red Straggler Stars??
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How much else don’t we
know about stellar
evolution?
twin clusters
M3
NBSS = 72
F = 0.28
M13
NBSS = 16
F = 0.07
References
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DRAKOS, Nikos, Blue Stragglers; A Study of Stellar Longevity, September 30,
1996, University of Leeds, http://casa.colorado.edu/~danforth/science/bss/
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SCHIAVON, Ricardo Piorno, Stellar Population in the Blue, 2005-07-27
,http://www.astro.virginia.edu/~rps7v/Models/ms/node30.html
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NEMIROFF(MTU), Robert and BONNELL, Jerry(USTA), Astronomy picture of
the dayI, August 8th 2003, http://antwrp.gsfc.nasa.gov/apod/ap030808.html
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P. Benvenuti, F. D. Macchetto, and E. J. Schreier ,M3: An Ideal Laboratory for
Testing Stellar Evolution and Dynamics,
http://www.stsci.edu/stsci/meetings/shst2/fusipeccif1.html
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OSTLIE, D. and CARROLL, B., Modern Stellar Astrophysics, Addison-Wesley
Publishing, 1996, P.531-534.
47 Tucanae
The End