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Kepler Asteroseismology of White Dwarf Stars

jjhermes
June 27, 2013

Kepler Asteroseismology of White Dwarf Stars

Conference presentation, 25 min. June 2013: KASC 6 Workshop, Sydney, Australia.

jjhermes

June 27, 2013
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  1. JJ Hermes!
    University of Texas at Austin!
    !
    Keaton J. Bell, Roy Østensen, Agnes Bischoff-Kim, Sean Moorehead,
    Sandra Greiss, Boris Gänsicke, Danny Steeghs, Mike Montgomery,

    D. E. Winget, James Dalessio, Judi Provencal, Barbara Castanheira!

    View full-size slide

  2. Pulsating White Dwarfs: The Future of the Sun
    •  > 97% of all stars in the Galaxy will evolve into white dwarfs, including our Sun!
    •  They exhibit g-modes excited to observable amplitudes by “convective driving”!
    •  The DAV instability strip is “pure” – all DA WDs should pulsate when they reach the
    appropriate temperature to foster a hydrogen partial ionization zone!
    Fontaine & Brassard 2008, PASP 120 1043
    (1
    Th
    cu
    th
    sta
    tio
    by
    to
    pe
    th
    of
    D
    pe
    in
    sh
    sy
    a
    pa
    br
    re
    th
    FIG. 9.—Instability domain in the log g À T diagram for the ZZ Ceti stars.
    1054 FONTAINE & BRASSARD
    82; Winget et al. 1982a). Theorists were finally able to find in their models the association of
    excitation by the zone of partial H ionization discovered by McGraw.
    The demonstration of driving from the H-partial-ionization zone led Winget (1981) and
    nget et al. (1982a) to investigate models of DB white dwarf stars for possible instabilities
    ing to the surface He partial ionization at a correspondingly higher temperature. They found
    tabilities in their models and predicted pulsations in DB white dwarf stars near the He I opacity
    ximum associated with the onset of significant partial ionization.
    Observations soon caught up. A systematic survey of the DB white dwarf stars demonstrated
    t the brightest DB with the broadest He I lines, GD 358, did indeed pulsate in nonradial
    modes—remarkably similar to the large-amplitude DAV pulsators (Winget et al. 1982b).
    The observed pulsating white dwarf stars lie in three strips in the H-R diagram, as indicated
    Figure 3. The pulsating pre-white dwarf PG 1159 stars, the DOVs, around 75,000 K to
    0,000 K have the highest number of detected modes. The first class of pulsating stars to be
    5.5 5.0 4.5
    Planetary Nebula
    Main
    sequence
    DOV
    DBV
    DAV
    4.0 3.5 3.0
    log [T
    eff
    (K)]
    4
    2
    0
    –2
    –4
    log (L/L )
    ure 3
    3-Gyr isochrone with z = 0.019 from Marigo et al. (2007), on which we have drawn the observed
    ations of the instability strips, following the nonadiabatic calculations of C´
    orsico, Althaus & Miller
    Winget & Kepler 2008, ARA&A 46 157

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  3. Pulsating Hydrogen-Atmosphere (DA) WDs
    •  Shown are a typical chemical
    profile (top) and propagation
    diagram (bottom) for a DAV
    model: 0.6 Msun
    , 11800 K,
    canonical He-layer mass,

    MHe
    /M★
    = 10-2 = 1%,

    MH
    /M★
    = 10-6!
    •  Typical g-mode pulsations
    observed in the range from
    100-1200 s!
    •  Note that even a 250 s pulsation
    propagates well into the CO-core!
    Fontaine & Brassard 2008, PASP 120 1043
    the g-modes observed in pulsating white dwarfs are relatively
    short ought to be considered a good thing. This is because a
    large number of pulsation cycles can be covered in a single night
    of observations and, as a consequence, it is easier to assess the
    multiperiodic character of a given light curve. On the downside,
    as indicated above, white dwarfs remain intrinsically faint and it
    is often difficult, observationally speaking, to reach comfortable
    S/Ns.
    Along with its compact nature, the chemical layering of a
    pulsating white dwarf is another mechanical property that bears
    a strong imprint on its period spectrum. Even though a typical
    white dwarf consists of a C/O core containing more than 99% of
    its mass, the thin He mantle that surrounds the core (containing
    at most <1% of the total mass), and the even thinner H layer that
    envelops a DA star (containing at most <0:01% of the total
    mass) play a major role in establishing the period distribution.
    This is because g-modes in white dwarfs have large amplitudes
    and propagate easily in these outermost layers. The modes are
    therefore quite sensitive to the details of the chemical stratifica-
    tion above the core. In fact, the layered structure produces a
    highly nonuniform period spectrum for a family of g-modes be-
    have very low amplitudes there. To a large extent, and this is
    particularly true for the most degenerate configuration repre-
    sented by ZZ Ceti stars, the degenerate interior of a white dwarf
    is refractory to asteroseismological probing. This poses a
    FIG. 6.—Chemical layering in a representative model of a ZZ Ceti pulsator.
    The location of the atmospheric layers is indicated through the values of the
    Rosseland optical depth.
    3 This is strictly valid only for purely radiative and chemically homogeneous
    models in the asymptotic regime of high radial order as demonstrated by Tassoul
    (1980).
    2008 PASP, 120:1043–1096
    N2
    L1
    2
    250 s

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  4. Ensemble Properties of DAVs (or ZZ Ceti stars)
    •  There is an observed trend of increasing mode period as DAVs cool!
    –  This is an expected consequence of the deepening convection zone with cooler temps.!
    •  There is also a slight trend of increasing amplitude as DAVs cool, until the
    amplitudes abruptly fall off around 11,000 K!
    •  Most WDs in these samples have log(g) ~ 8.0, so they are near the
    canonical WD mass of 0.6 M¤!
    Mukadam et al. 2006, ApJ 640 956
    2004) and Gianninas et al. (2005) have
    consistent temperatures and log g fits for
    r latest model atmospheres; we hereafter
    et of 39 DAVs as the BG04 ZZ Ceti sample.
    sponding set of 39 pulsation spectra from
    private communication with our colleagues.
    he BG04 ZZ Ceti sample were acquired by
    sing different instruments and telescopes.
    al amount of time-series data exists on most
    , and we utilized practically all published
    carefully derive well-averaged values of
    d and pulsation amplitudes, which we pre-
    2004a) show evidence of a relative shift
    een the SDSS and BG04 ZZ Ceti instabil-
    differ in shape and width. The spectra of
    samples were analyzed using different tech- Fig. 1.—Weighted mean period of 41 newly discovered ZZ Ceti stars from
    ENSEMBLE CHARACTERISTICS OF ZZ CETI STARS 957
    FIG. 24.—Correlation between excited period and effective temperature or
    Larger circle:
    Longer mode period
    Fontaine & Brassard 2008, PASP 120 1043

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  5. White Dwarfs at the Beginning of Kepler
    Østensen et al. 2011, MNRAS 414 2860
    •  White Dwarf stars were very hard to come by:!
    –  Just 14 DA white dwarfs were targeted by Kepler in its first year!
    •  Selected by GALEX UV-excess, the one SDSS stripe that crosses the Kepler field
    (no spectroscopy), and reduced proper motion!
    •  Very little U-band photometry on the field!
    •  Down to 17.5 mag limit, we expected 2-3 DAVs!
    •  Only 2 DAs come anywhere near

    the instability strip:!
    –  Spectroscopic Teff
    and log(g)

    fits by Østensen et al. 2011!
    (
    T
    c
    t
    s
    t
    b
    t
    p
    t
    o
    D
    p
    i
    s
    s
    a
    p
    b
    r
    t
    (
    t
    FIG. 9.—Instability domain in the log g À Teff
    diagram for the ZZ Ceti stars.
    The positions of the pulsators are indicated by the filled circles, while those of
    1054 FONTAINE & BRASSARD
    !DA ! ! ! Teff
    (K)! ! ! !log g (dex)!
    KIC 10420021 ! !16,200 ± 500 ! ! !7.8 ± 0.3!
    KIC 10198116 ! !14,200 ± 500 ! ! !7.9 ± 0.3!

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  6. Kepler’s Longest-Studied DA WD as of 2012
    •  KIC10420021!
    •  1 month in Q2, 

    6 months of SC
    in Q5 & Q6!
    •  NOV0.06!!
    (NOV @ 60 ppm)!
    KP
    = 16.2!
    •  This FT includes
    264,667 points,
    97.0% duty
    cycle!
    •  Teff
    = 16,200 ±
    ! !500 K!
    •  log g = 7.8 ± 0.3!

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  7. First DAV Discovered From High Proper Motion
    •  Rowell & Hambly 2011 catalog of ~10,000 possible WDs:!
    –  Selected from reduced proper motion!
    –  20 are on Kepler silicon, but 5 had already been observed by the spacecraft!
    •  WD1916+3938 (KIC4552982) had promising B-R, R-I colors!
    •  It was confirmed to vary at McDonald Observatory in May 2011!
    !
    Hermes et al. 2011, ApJ 741 L16

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  8. First DAV Discovered From High Proper Motion
    •  Excitement within KASC WG11 led to four different optical spectra!
    •  Spectral fits: Teff
    = 11,130 ± 120 K, log(g) = 8.34 ± 0.06, 0.82 ± 0.04 M¤!
    •  Ground-based light curve showed variability at 800—1450 s, consistent
    with cooler DAVs!
    •  Back to this KP
    =17.9 mag star later...!
    Hermes et al. 2011, ApJ 741 L16

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  9. The First (and only) DBV in the Kepler Field
    •  Discovered in Q7.2 (Oct. 2010)!
    •  Observed non-stop starting in Q10!
    •  KP
    = 18.1 mag!
    •  FT thru Q13, 8 independent modes!
    •  f1,0
    = 232.02 s!
    •  f2,0
    = 197.11 s!
    •  f3,0
    = 271.61 s!
    •  f4
    = 303.55 s!
    •  f5
    = 376.11 s!
    •  f6
    = 143.57 s!
    •  f7
    = 227.36 s!
    •  f8
    = 266.90 s!
    Østensen et al. 2011, ApJ 736 L39
    0
    1
    2
    3
    4
    5
    6
    2000 3000 4000 5000 6000 7000 8000
    Amplitude [mma]
    f1
    f2
    f3
    f4
    f5
    f6
    f7
    f8 8flc
    9flc
    0.0
    1.0
    2.0
    3.0
    4.0
    5.0
    4300 4310 4320
    Amplitude [mma]
    f1
    5070 5080
    f2
    3680 3690
    f3
    3290 3300
    f4
    0.0
    0.5
    1.0
    1.5
    2.0
    2650 2660
    Amplitude [mma]
    f5
    6960 6970
    Frequency [µHz]
    f6
    4390 4400
    f7
    3740 3750
    f8

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  10. First Seismological Results of the Kepler DBV
    •  Spectral fits originally found this to
    be a ~25,000 K DBV!
    •  Seismology shows this is instead a
    hot (~29,000 K) DBV, using the first
    five detected pulsation modes!
    Bischoff-Kim & Østensen 2011, ApJ 742 L16
    The Astrophysical Journal Letters, 742:L16 (5pp), 2011 November 20
    24000 25000 26000 27000 28000 29000 30000
    0.55
    0.56
    0.57
    0.58
    0.59
    0.6
    0.61
    0.62
    0.63
    0.64
    0.65
    Mass [Solar]
    0.3
    0.32
    0.34
    0.36
    0.38
    0.4
    0.42
    0.44
    0.46
    0.48
    0.5
    Edge of Homegeneous Core
    24000 25000 26000 27000 28000 29000 30000
    Effective Temperature [K]
    0.4
    0.5
    0.6
    0.7
    0.8
    0.9
    1
    Central Oxygen Abundance
    Figure 3. Best-fit models in three different planes of parameter space. The filled
    circles are models that fit the observed period spectrum with σrms 1.0 s.
    The best-fit models at 29,200 K is circled. In the top panel (mass vs. effective
    temperature), we also mark the spectroscopic value (diamond) and the formal
    1σ fitting error region around it.
    Letters, 736:L39 (5pp), 2011 August 1 Østensen et al.
    0
    200
    400
    600
    800
    1000
    3600 3800 4000 4200 4400 4600 4800 5000
    Response corrected counts [ADU/pix]
    Wavelength [Å]
    Teff
    = 24950 +/− 750 K
    log(g) = 7.91 +/− 0.07 dex
    Model fit:
    trum of GALEX J192904.6+444708. Overplotted is our best-fit DB model spectrum with Teff and log g as stated on the figure. Except
    , the model is an excellent fit to the observed spectrum.
    s available in the online journal.)

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  11. Currrent Seismological Results of the Kepler DBV
    Bischoff-Kim et al.,
    in preparation
    •  Additional observations have uncovered 8 significant, independent modes!
    •  Seismology still suggests this is a hot DBV, with a mass near 0.57 M¤!
    ((O-C)2)1/2 [0.1 s]
    Best Fit
    Worse Fit

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  12. Currrent Seismological Results of the Kepler DBV
    Bischoff-Kim et al.,
    in preparation
    •  The models show a good constraint on the helium layer mass!
    •  The best fits occur for a roughly canonical He-layer mass, MHe
    /M★
    ~ 10-2.8!
    ((O-C)2)1/2 [0.1 s]
    Log
    -
    -
    -
    -
    -
    -
    -
    -
    .
    .
    .
    .
    .
    .
    .
    .
    Best Fit
    Worse Fit

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  13. Currrent Seismological Results of the Kepler DBV
    Bischoff-Kim et al.,
    in preparation
    •  We continue to get similar results regarding the degeneracy boundary:

    The carbon-oxygen core extends to roughly 40% of mass!
    ((O-C)2)1/2 [0.1 s]
    Best Fit
    Worse Fit

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  14. The First (and only) DBV in the Kepler Field
    •  The 3 highest-amplitude modes are all well-defined triplets with an
    average frequency splitting of 3.3 μHz ≈ 0.3 cpd!
    •  This corresponds to a rotational period of Prot
    = 1.7 days in the limit that
    these are g-modes with Cnl
    ≈ 1/(ell(ell+1)) and assuming ell = 1!
    •  This is quite consistent with previous WD rotation rates derived from WD
    pulsational splittings!
    Østensen et al. 2011, ApJ 736 L39
    0
    1
    2
    3
    4
    5
    2000 3000 4000 5000 6000 7000
    Amplitude [mma]
    1
    f2
    f3
    f4
    f5
    f6
    f7
    f8 8flc
    9flc
    0.0
    1.0
    2.0
    3.0
    4.0
    5.0
    4300 4310 4320
    Amplitude [mma]
    f1
    5070 5080
    f2
    3680 3690
    f3
    2.0

    View full-size slide

  15. Running Fourier Transform of the Kepler DBV
    courtesy Roy Østensen

    View full-size slide

  16. O-C Diagrams of the f
    1
    triplet
    •  The two highest-amplitude modes are exceptionally stable in phase!

    View full-size slide

  17. O-C Diagrams of the f
    2
    triplet
    •  The two highest-amplitude modes are exceptionally stable in phase!

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  18. Hot DBVs Can Constrain Plasmon Neutrino Rates
    •  Neutrinos should be the
    dominant source of luminosity
    for this hot DBV!
    •  Measuring the rate of period
    change of the stable f1
    and f2
    components could differentiate
    between cooling with neutrino
    emission and cooling without
    neutrinos by 2016 or 2017!
    •  The ephemeris we have built
    with Kepler observations
    makes this possible within the
    decade; we have never had a
    DBV stable enough for this
    measurement!!
    Winget et al. 2004, ApJ 602 L109
    With Neurino Emission
    Without

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  19. Back to the DAVs in the Kepler Field
    •  The Kepler-INT Survey (KIS) is a U,g,r,i photometric survey of the Kepler
    field using the 2.5 m Isaac Newton Telescope on La Palma led by

    Sandra Greiss, Boris Gänsicke & Danny Steeghs at U. of Warwick!
    •  They have confirmed (spectra & time series) NINE new DAVs in the field!
    (unfortunately one is never on silicon)!
    Greiss et al. 2012, AJ 144, 1, 24
    Greiss et al. 2012, arXiv:1212.3613
    Google: “Kepler INT Survey”

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  20. Kepler Has Now Observed Five DAVs
    •  The first KIS-DAV, KIC 11911480 (KP
    =17.6 mag), was observed in Q12
    and also Q16-17!
    •  Spectroscopy shows Teff
    = 11,840 ± 200 K, log(g) = 8.07 ± 0.08!
    •  Four modes from

    135.8-324.3 s,

    three are nice

    triplets with an

    average freq.

    splitting of

    1.94 μHz,

    corresponding to

    Prot
    = 3.0 days

    if ell = 1 modes!
    •  Further analysis 

    is forthcoming

    on this DAV!
    Greiss et al. 2013, in prep.

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  21. Kepler Has Now Observed Five DAVs
    •  The second KIS-DAV, KIC 10132702 (KP
    =18.6), was observed in Q15!
    •  Spectroscopy shows Teff
    = 11,840 ± 300 K, log(g) = 8.25 ± 0.10!
    •  These data are very fresh, but we see at least 15 modes from 450-1000 s!!

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  22. Kepler Has Now Observed Five DAVs
    •  KIC 4357032 (KP
    =18.0) & KIC 7594781 (KP
    =18.2) were observed in Q16!
    •  White dwarf seismology suffered a blow with the loss of Kepler pointing,
    since we were

    unable to observe

    all nine DAVs

    from space!
    •  But as with all

    Kepler science,

    we still have

    plenty to explore,

    as one DAV has

    extensive coverage!!

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  23. KIC 4552982, Back to the First Kepler DAV
    Q11
    Q12 Q13
    Q14
    •  Kepler began observing this first DAV (KP
    =17.9 mag) in Q11!
    •  There is ~ 5% point-to-

    point scatter in the SC

    light curve of this DAV!
    •  The light curve also

    shows high-amplitude

    flux excursions every

    3 days or so!
    •  This has severely

    impeded our

    asteroseismic

    analysis, but they

    represent a unique

    new phenomena in

    pulsating WDs!!
    Bell et al. 2013, in prep.

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  24. KIC 4552982, Large-Scale Flux Excursions

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  25. KIC 4552982, The First Kepler DAV
    •  We did not see these events from the ground in our discovery data, about
    21 hr over 6 nights in May 2011 (~15% duty cycle)!
    •  These events wreak havoc on the Fourier transform, especially at low freq.!

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  26. KIC 4552982, Mean Event Profile
    •  We have
    averaged
    the profile of
    the 128
    events we
    see in the
    first year of
    data!
    •  This is most
    useful in
    showing the
    shape of the
    rise and fall
    of the
    events!

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  27. KIC 4552982, Probing Centroid Offsets
    •  For at least one
    month we see no
    appreciable offset
    in the centroid
    position in/out of
    quiescence!
    •  These events are
    most likely not a
    background
    source!
    •  For many months
    the data is not
    well behaved
    (centroid shifts
    with changes in
    mean flux levels)!
    •  Astrometry has
    been difficult with
    Kepler!

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  28. KIC 4552982, Energetics of the Events
    •  Event energy determined by comparing flux excess w.r.t. starʼs luminosity!

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  29. KIC 4552982, Are the Events Accretion?
    •  This could correspond to a small asteroidʼs mass, if accreted onto WD!

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  30. KIC 4552982, Are the Events Accretion?
    log g
    7.5
    8.0
    8.5
    9.0
    Koester & Wilken 2006, A&A 453 1051
    •  WDs have strong surface gravities, and metals relatively quickly settle out!
    •  However, convection zones slow the process down, and at 11,130 K we
    expect settling times

    of order 10 years!
    •  Roughly 25% of all

    WDs are polluted by

    metals, and have

    helped refine these

    settling timescales!
    •  We see no evidence

    of metal pollution

    onto this WD!

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  31. KIC 4552982, Are the Events Accretion?
    •  We obtained four separate spectra spread over 26 days in 2011 May/June,
    and we see no significant evidence for any metal lines!
    Hermes et al. 2011, ApJ 741 L16

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  32. KIC 4552982, Seismology Constrains the Events
    •  The Fourier transform of the region of pulsations (this is shown zoomed
    from roughly 1670 s to 770 s) is incredibly dense!
    •  This is ~90 days of ~97% duty cycle data (Q11), so the spectral window is
    incredibly sharp (basically the width of each peak)!

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  33. KIC 4552982, Seismology Constrains the Events
    courtesy Roy Østensen

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  34. KIC 4552982, Seismology Constrains the Events

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  35. KIC 4552982, Seismology Constrains the Events

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  36. KIC 4552982, Seismology Constrains the Events

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  37. KIC 4552982, Seismology Constrains the Events

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  38. KIC 4552982, Seismology Constrains the Events
    N2
    L1
    2
    362 s
    950 s
    •  The observed modes fit marginally best as k = 6, 17—22 (all ell = 1)!
    •  Best model: Teff
    = 11,200 K, M = 0.805 M¤
    , MHe
    = 10-2 M¤
    , MH
    = 10-5.5 M¤
    !
    •  Perhaps more usefully, seismology establishes that the events are
    occurring at the surface of this WD, and are more strongly affecting
    the pulsations nearer the surface!
    •  The isolated 362.6 s mode has lower k, even if has a different ell value,
    and propagates deeper in the star

    than the longer-period modes!
    •  This 362.6 s mode is thus more

    insulated from changes at the

    surface of the star, which can

    explain why the 362.6 s mode is

    changing much less dramatically

    over the timescale of our

    observations!
    = log (1-M(r)/M★
    )

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  39. KIC 4552982, Seismology Constrains the Events
    ΔP = 40.7 s$

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  40. Conclusions: Kepler Asteroseismology of WDs
    •  There are now 11 known pulsating white dwarfs known in the
    Kepler field, but only six were observed before May 2013!
    •  There is one DBV: It has more than 1.5 years of SC data!
    –  Seismology suggests this is a hot (29,000 K) DBV with a mass near 0.57 M¤
    –  Longer-term monitoring can constrain the neutrino luminosity from this WD!
    •  The first DAV in the Kepler field is a complicated mess of
    variability that will be difficult but exciting to interpret!
    –  There are unprecedented large-scale flux excursions every ~3 days!
    –  These events are very likely on the surface of the WD:!
    –  The low-k pulsation appears much less affected by these outbursts than
    the longer-period high-k modes!
    •  There may be more surprises as we investigate the other DAVs
    observed from space found by the Kepler-INT Survey!

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  41. KIC 4552982
    – 27 –

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  42. KIC 4552982
    – 40 –

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  43. GD 358: The “Whoopsie”
    Montgomery et al. 2010, ApJ 716 84
    No. 1, 2010 TEMPERATURE CHANGE AND OBLIQUE P
    Figure 7. Relative intensity of GD 358 as measured relative to its levels after the
    sforzando episode. The triangles are data from McDonald Observatory and the
    squares are data from Mt. Suhora Observatory. Both data sets indicate a jump
    in intensity near BJED ∼ 2450311.
    (A color version of this figure is available in the online journal.)
    One ef
    is to atte
    its base.
    amplitud
    where F0
    the instan
    at the pho
    of the mo
    If we a
    of the co
    Equation
    to an inc
    we would
    apparent
    a large fa
    increase

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