Blue Straggler Stars

By Emilie Lafrance

Plan

• 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?• MBSS>MMSTO

• vsin(i) = 155km/sec (>100 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 aren’t in agreement with the standard star evolution model.

Color-magnitude diagram of M3

Origin of BSS (main theories)

• 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.

• 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.

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.

• 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.

Coalescence model

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

• 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 don’t we know about stellar evolution?

twin clusters

M13NBSS = 16

F = 0.07

M 3NBSS = 72

F = 0.28

References

• DRAKOS, Nikos, Blue Stragglers; A Study of Stellar Longevity, September 30, 1996, University of Leeds, http://casa.colorado.edu/~danforth/science/bss/

• SCHIAVON, Ricardo Piorno, Stellar Population in the Blue, 2005-07-27 ,http://www.astro.virginia.edu/~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.

The End

47 Tucanae