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HOW MANY NEUTRON STARS ARE BORN RAPIDLY ROTATING?. NIKOLAOS STERGIOULAS. DEPARTMENT OF PHYSICS ARISTOTLE UNIVERSITY OF THESSALONIKI. ENTAPP, 23/1/2006. WHY DO WE NEED RAPID ROTATION?. Several GW emission mechanisms during NS formation rely on rapid rotation :.

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how many neutron stars are born rapidly rotating
HOW MANY NEUTRON STARS ARE BORN RAPIDLY ROTATING?

NIKOLAOS STERGIOULAS

DEPARTMENT OF PHYSICSARISTOTLE UNIVERSITY OF THESSALONIKI

ENTAPP, 23/1/2006

why do we need rapid rotation
WHY DO WE NEED RAPID ROTATION?

Several GW emission mechanisms during NS formation rely on rapid rotation:

  • Core-bounce signal in axisymmetric collapseFor slow rotation detectable only within Galaxy, but rapid rotation allows larger distances.Nonlinear couplings may enhance GW emission.
  • Dynamical bar-mode instabilityNeed T/|W|>0.24.If bar persists for many periods, signal detectable out to the Virgo cluster.
  • Low T/|W| m=2 instabilityNeed only T/|W|>0.01, but need a high degree of differential rotation. Has heff~10-22 at 100Mpc(!)
  • Low T/|W| m=1 instabilityNeed T/|W|>0.08 and a high degree of differential rotation. GWs through nonlinear m=2 mode excitation, only detectable in our Galaxy.
  • CFS f-mode instabilityNeeds T/|W|>0.08 to operate. If T/|W|>0.25 and α~1, detectable to 100Mpc!
  • r-mode instability in young strange starsNeeds millisecond initial periods. For α~10-3there may be several sourcesin our Galaxy at any time – detectable with a few weeks integration.

But, are NS born rapidly rotating?

typical progenitors
Typical Progenitors

A large fraction of progenitor stars are initially rapidly rotating:

The average rotation of OB type stars on the main sequence is 25% of break up speed.

About 0.3% of B stars have Ω > 67% of breakup, e.g.of Regulus in Leo: 86% of breakup.

But: Magnetic Torques can Spin Down the Core!

Spruit & Phinney 1998, Spruit 2002, Heger, Woosley & Spruit 2004

When the progenitor passes through the Red Supergiant (RSG) phase it has a huge envelope of several hundred times the initial radius.

The core’s differential rotation produces a magnetic field by dynamo action that couples the core to the outer layers, transferring away angular momentum. This leads to slowly rotating neutron stars at birth (~10-15ms).

Is there a way out of this?

by passing the rsg phase

Massive rapidly rotating cores=> millisecond NS => magnetars.

e.g. Wheeler et al.2000

By-Passing the RSG Phase

Massive Stars (M>25Msun) evolve very rapidly. Two advantages:

a) There is not sufficient time to slow down the core effectively!

b) A strong wind (WR phase) will expel the envelope, preventing slow down of core by magnetic torques.

A strong wind (high mass-loss rate) allows NS to be formed instead of a BH, but could also carry away a lot of angular momentum.

Mass-loss rate is lower if the star has low metallicity.

In addition, rapidly rotating WR stars may lose mass mainly at the poles (temperature is higher there) => angular momentum loss is lower.

Rapidly rotating coresproduced by right mixture ofhigh massandlow metallicity

Observational evidence: 1) magnetar produced by 30-40Msun progenitor 2) magnetar with > 40Msun progenitor in star cluster

Gaensler et al.2005

Muno et al.2005

additional paths to rapid rotation

4)Binary WD mergers

Suggested as ms pulsar formation mechanism in globular clusters.

(Middleditch 2003)

Also, suggested as alternative magnetar formation mechanism, with event rate0.3/year at ~ 40Mpc.

(Levan et al. 2006)

Additional Paths to Rapid Rotation

1)Rotational mixing in OB stars:

Woosley & Heger 2005

Rapid rotation in massive OB stars can induce deep rotational mixing, preventing the RSG phase (stars stay on main sequence).

Woosley & Heger (2005) estimate that 1% of all stars with mass >10Msun will produce rapidly rotating cores.

2)Loss of envelope in binary evolution:

If a binary companion strips the outer envelope of a massive star before core collapse, the RSG phase is avoided.

(see Fryer & Kalogera 2001, Pfahl et al. 2002, Podsiadlowski et al. 2003, Ivanova & Podsiadlowski 2003)

3)Fall-back accretion

(see e.g. Watts & Andersson, 2002)

conclusions
CONCLUSIONS
  • Typical core collapse will lead to slowly rotating NSs - most GW mechanisms not operating/not detectable at good event rates.
  • But, there are several ways to produce rapidly rotating NSs at birth, but only in ~1% of SN events.
  • Still, the strongest GW mechanisms (those detectable beyond theVirgo cluster) may have good event rates for advanced LIGO/VIRGO type of detectors.
  • Need to focus more on strongest GW mechanisms both theoretically and by narrow-banding/improving detectors in 1-3kHz range.