Blue Straggler Stars

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Blue Straggler
Stars

• By Emilie Lafrance

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Plan

• Discovery
• How BSS differ from regular stars
• Origin theories
• Observations
• Unanswered questions and Implications

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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.

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What distinguishes Blue Stragglers from other
stars?

• MBSSgtMMSTO
• vsin(i) 155km/sec
• (gt100 times that of the sun) in BSS19 in 47 Tuc
• 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 high-density
  regions of star clusters.
• Inside a globular cluster, star-forming material
  is scarce and the stars are usually formed at the
  same time. BSS arent in agreement with the
  standard star evolution model.

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Color-magnitude diagram of M3
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Origin of BSS (main theories)

• In1968, Sargent proposed they were stars in
  helium core burning state (HB). This theory
  didnt agree well with observations because BSS
  are usually much fainter than HB stars, amongst
  other things.
• 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.
• 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.

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Collision model

• 1. Stars 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.

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Coalescence model

• 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).
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Observed BSS
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Unanswered questions
twin clusters

• What are the conditions for higher BSS
  populations? (Different density of primordial
  binaries?)
• Just how many BSS are there
• Detection method?
• Red Straggler Stars??
• How much else dont we know about stellar
  evolution?

M 3 NBSS 72 F 0.28
M13 NBSS 16 F 0.07
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References

• DRAKOS, Nikos, Blue Stragglers A Study of
  Stellar Longevity, September 30, 1996, University
  of Leeds, http//casa.colorado.edu/danforth/scien
  ce/bss/
• SCHIAVON, Ricardo Piorno, Stellar Population in
  the Blue, 2005-07-27 ,http//www.astro.virginia.e
  du/rps7v/Models/ms/node30.html
• NEMIROFF(MTU), Robert and BONNELL, Jerry(USTA),
  Astronomy picture of the dayI, August 8th 2003,
  http//antwrp.gsfc.nasa.gov/apod/ap030808.html
• 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
• OSTLIE, D. and CARROLL, B., Modern Stellar
  Astrophysics, Addison-Wesley Publishing, 1996,
  P.531-534.

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The End

• 47 Tucanae