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GD 358: The Case for Oblique Pulsation and Temperature Change

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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GD 358: The Case for Oblique Pulsation and Temperature Change

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  1. 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)

  2. A couple of recent developments… Mari Kleinman, born Feb. 25th, 2010 Gabriel Montgomery, born Dec. 23rd, 2009

  3. 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

  4. Need a mechanism for producing non-linearities • convection zone is most likely candidate • can change thickness by » 10 during pulsations

  5. Hybrid ApproachMontgomery (2005) based on work of Brickhill (1992), Wu & Goldreich (1998), and Ising & Koester (2001) ) Assumes all the nonlinearity is caused by the convection zone nonlinear convection zone (larger amplitude) linear region (small amplitude)

  6. Fph´ photospheric flux, Fb´ flux at base of convection zone N » 90 for DAVs (Teff» 12000 K) N » 23 for DBVs ( Teff» 25000 K) Depth of convection zone is very temperature dependent!

  7. For nearly mono-periodic pulsators, the fits are straightforward (from Montgomery 2005) Nonlinear light curve fitting of pulsations of G29-38 Observations: Kleinman –1988 Dominant period: 615.15 s

  8. We derive convection zone parameters as well as constraints on l and m l=1, m=1 τ0= 150.1 sec N=95.0 θi= 65.5 deg Amp= 0.259 Res = 0.160

  9. Normally, GD358 looks like this…(May 2006)

  10. However, it looked like this during the “whoopsie” or “sforzando” (Aug 1996)

  11. However, it looked like this during the “whoopsie” or “sforzando” (Aug 1996)

  12. So what is GD 358 normally like?

  13. GD358 during the May 2006 WET Run

  14. Simultaneous fit 29 high S/N runs: linear fit (12 periodicities – 36 parameters)

  15. Simultaneously fit 29 high S/N runs: nonlinear fit (only 3 additional parameters)

  16. ¿0 ~ 586 § 20 sec µi ~ 47.5 § 2.5 degrees

  17. Normal state: “sforzando”: The difference in τ0 implies that GD 358 was ~ 3000 K hotterduring the “sforzando” Is there any other corroborating evidence?

  18. Yes, there is… McDonald Mt. Suhora There were separate measurements of its relative brightness (which Judi dug out) before and after this event

  19. Theoretical vs observed τ0as a function of Teff

  20. Back to the 2006 WET run…oblique pulsation?

  21. Example of precession/oblique pulsations m=1 m=0

  22. 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

  23. Pre-whitening by 2 sets of equally spaced triplets

  24. For each triplet Now lets fit the amplitudes…

  25. Amplitudes

  26. Amplitudes The amplitudes fit very well: “98% significance level”

  27. 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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