gd 358: the case for oblique pulsation and temperature change
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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). - PowerPoint PPT PresentationTRANSCRIPT
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)
A couple of recent developments…
Gabriel Montgomery, born Dec. 23rd, 2009
Mari Kleinman, born Feb. 25th, 2010
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
• Need a mechanism for producing non-linearities – convection zone is most likely candidate– can change thickness by » 10 during pulsations
) 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)
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
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)
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
Normally, GD358 looks like this…(May 2006)
However, it looked like this during the “whoopsie” or “sforzando” (Aug 1996)
However, it looked like this during the “whoopsie” or “sforzando” (Aug 1996)
So what is GD 358 normally like?
τ 0 à 50sec
θi ~52±5degrees
GD358 during the May 2006 WET Run
Simultaneous fit 29 high S/N runs:linear fit (12 periodicities – 36 parameters)
Simultaneously fit 29 high S/N runs:nonlinear fit (only 3 additional parameters)
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
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”:
Yes, there is…There were separate measurements of its relative brightness (which Judi dug out) before and after this event
McDonaldMt. Suhora
Theoretical vs observed τ0 as a function of Teff
Back to the 2006 WET run…oblique pulsation?
Example of precession/oblique pulsations
m=1 m=0
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
Pre-whitening by 2 sets of equally spaced triplets
For each triplet
Now lets fit the amplitudes…
ΔΦ / 2π ~ 0.5 ⇒ m = 1 modes
ΔΦ ≡2Φ0 − (Φ+ + Φ− )
Amplitudes
Amplitudes
The amplitudes fit very well: “98% significance level”
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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