gd 358: the case for oblique pulsation and temperature change
DESCRIPTION
GD 358: The Case for Oblique Pulsation and Temperature Change. Mike Montgomery (UT-Austin, DARC), J. L. Provencal, A. Kanaan, A. S. Mukadam, S. E. Thompson, J. Dalessio, H. L. Shipman, D. E. Winget, S. O. Kepler, & D. Koester. (DARC = Delaware Asteroseismic Research Center). - PowerPoint PPT PresentationTRANSCRIPT
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GD 358: The Case for Oblique Pulsation and Temperature Change
Mike Montgomery (UT-Austin, DARC),
J. L. Provencal, A. Kanaan, A. S. Mukadam, S. E. Thompson, J. Dalessio, H. L. Shipman, D. E.
Winget, S. O. Kepler, & D. Koester
(DARC = Delaware Asteroseismic Research Center)
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A couple of recent developments…
Gabriel Montgomery, born Dec. 23rd, 2009
Mari Kleinman, born Feb. 25th, 2010
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GD358
• First single white dwarf to show evidence of a large change in Teff
(seen in WZ SGe systems)- accretion?
• First white dwarf to show evidence of oblique pulsation
(seen in roAp stars)- magnetic field?
Both questions can be addressed with non-linear light curve fits
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• Need a mechanism for producing non-linearities – convection zone is most likely candidate– can change thickness by » 10 during pulsations
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) Assumes all the nonlinearity is caused by the convection zone
Hybrid Approach Montgomery (2005) based on work of Brickhill (1992),
Wu & Goldreich (1998), and Ising & Koester (2001)
linear region(small amplitude)
nonlinear convection zone (larger amplitude)
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N » 90 for DAVs (Teff » 12000 K)
N » 23 for DBVs ( Teff » 25000 K)
Fph ´ photospheric flux, Fb ´ flux at base of convection zone
Depth of convection zone is very temperature dependent!
Fph =Fb +τ c
dFphdt
τ c ≡ thermal response timescale of CZ
~ τ 0 (Teff /T0 )−N
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Observations:Kleinman –1988
Dominant period: 615.15 s
Nonlinear light curvefitting of pulsations of
G29-38
For nearly mono-periodic pulsators, the fits are straightforward (from Montgomery 2005)
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l=1, m=1τ0= 150.1 secN=95.0θi= 65.5 degAmp= 0.259Res = 0.160
We derive convection zone parameters as well as constraints on l and m
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Normally, GD358 looks like this…(May 2006)
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However, it looked like this during the “whoopsie” or “sforzando” (Aug 1996)
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However, it looked like this during the “whoopsie” or “sforzando” (Aug 1996)
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So what is GD 358 normally like?
τ 0 à 50sec
θi ~52±5degrees
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GD358 during the May 2006 WET Run
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Simultaneous fit 29 high S/N runs:linear fit (12 periodicities – 36 parameters)
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Simultaneously fit 29 high S/N runs:nonlinear fit (only 3 additional parameters)
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Period (s) ell m422.561 1 1
423.898 1 -1
463.376 1 1
464.209 1 0
465.034 1 -1
571.735 1 1
574.162 1 0
575.933 1 -1
699.684 1 0
810.291 1 0
852.502 1 0
962.385 1 0
¿0 ~ 586 § 20 sec
µi ~ 47.5 § 2.5 degrees
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The difference in τ0 implies that GD 358 was ~ 3000 K hotter during the “sforzando”
Is there any other corroborating evidence?
τ 0 à 50sec
θi ~52±5degrees
τ 0 ~ 590sec
θ i ~ 47.5 ± 2.5degreesNormal state:
“sforzando”:
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Yes, there is…There were separate measurements of its relative brightness (which Judi dug out) before and after this event
McDonaldMt. Suhora
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Theoretical vs observed τ0 as a function of Teff
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Back to the 2006 WET run…oblique pulsation?
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Example of precession/oblique pulsations
m=1 m=0
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Could this be oblique pulsation?
• Need exactly evenly spaced triplets in the FT
• The phases of the members of the triplet have to “line up”:
• The amplitudes of the modes need to follow a given relation
ΔΦ =0 for m = 0
ΔΦ = π for | m |= 1
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Pre-whitening by 2 sets of equally spaced triplets
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For each triplet
Now lets fit the amplitudes…
ΔΦ / 2π ~ 0.5 ⇒ m = 1 modes
ΔΦ ≡2Φ0 − (Φ+ + Φ− )
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Amplitudes
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Amplitudes
The amplitudes fit very well: “98% significance level”
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Conclusions• The nonlinearities in GD358’s light curve can be
understood as originating in its convection zone• Compared to 2006, GD358 had a much thinner
convection zone during the “sforzando” (1996) about 3000 K hotter
• The oblique pulsator model provides an excellent match to the 6 peaks around k=12 (~575 sec):– frequencies– phases– amplitudes
• This provides important constraints on
the physics of convection in
white dwarf stars
Thanks!
⇒
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