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Rogue Waves on Pulsating White Dwarf Stars

jjhermes
August 01, 2016

Rogue Waves on Pulsating White Dwarf Stars

Colloquium, 45 min. April 2016: University of Toronto, Toronto, Canada.

jjhermes

August 01, 2016
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  1. Rogue Waves on Pulsating
    White Dwarf Stars
    J.J. Hermes
    Hubble Fellow
    University of North Carolina at Chapel Hill
    jjhermes.web.unc.edu

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  2. Rogue Waves on Pulsating
    White Dwarf Stars
    U. Texas: Keaton J. Bell, Mike Montgomery, Don Winget
    U. Warwick: Boris Gaensicke, Paul Chote, Roberto Raddi, Nicola Gentile Fusillo, Dave Armstrong
    U. North Carolina: Chris Clemens, Bart Dunlap, Erik Dennihy, Josh Fuchs
    + Steve Kawaler, Alex Gianninas, Pier-Emmanuel Tremblay, Agnes Bischoff-Kim

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  3. “White dwarfs shed their complexity.”
    - Don Winget

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  4. Typical DA white dwarf photosphere
    White dwarfs are simple: Evolution is
    just cooling; chemically stratified due
    to strong surface gravity

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  5. DA white dwarf + metals
    (Deviations from
    simplicity yield
    insights into bulk
    exoplanet
    compositions!)

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  6. White dwarfs are compact: This
    0.25 M¤
    + 0.50 M¤
    WD+WD is the
    most compact detached binary
    system currently known
    Brown et al. 2011; Hermes et al. 2012
    Mean Earth-Moon
    separation
    (J0651+2844, 12.75-min)
    1 R¤

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  7. Some WD+WD binaries are so close that gravitational waves
    rob energy from the orbit, causing them to rapidly shrink
    J0651+2844 PSR B1913+16
    (aka Hulse-Taylor
    binary pulsar)
    dPorb
    /dt = -0.289 ms/yr dPorb
    /dt = -0.076 ms/yr
    Hermes et al. 2012 Weisberg et al. 2010

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  8. Kepler
    12 May 2009 –
    11 May 2013

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  9. Petigura et al. 2013

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  10. Kepler
    12 May 2009 –
    11 May 2013
    K2
    17 Jan 2014 –
    Ongoing

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  11. =
    Steve Howell
    The solar pressure on the Kepler spacecraft is ~50 µN m-2
    and acts as a third reaction wheel

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  12. Original Kepler Mission:
    20 WDs observed,
    6 pulsating WDs
    (just two >3 months)
    K2 through Campaign 6:
    >650 WDs observed
    146 @ minute-cadence
    22 pulsating WDs
    K2 through Campaign 13:
    >1150 WDs,
    >60 pulsating WDs (~200 known today)
    K1
    K2, today
    K2, by mid-2017

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  13. Vanderburg et al. 2015
    A disintegrating minor planet
    transiting a white dwarf in K2

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  14. Gänsickeet al. 2016; Rappaport et al. 2016
    Mark Garlick
    Ground-based follow-up: it really is disintegrating
    4RWD
    model

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  15. WD 1145+017 is also
    heavily metal-polluted

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  16. GD 1212, Hermes et al. 2014, ApJ, 789, 85
    K2 engineering test data
    of a pulsating white dwarf

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  17. DA (hydrogen atmosphere)
    WDs pulsate when H partially
    ionized (DAVs, aka ZZ Cetis)
    in Figure 3. The pulsating pre-white dwarf PG 1159 stars, the DOVs, around 75,
    170,000 K have the highest number of detected modes. The first class of pulsating st
    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 )
    Figure 3
    A 13-Gyr isochrone with z = 0.019 from Marigo et al. (2007), on which we have drawn the obser
    locations of the instability strips, following the nonadiabatic calculations of C´
    orsico, Althaus & Mi
    Bertolami (2006) for the DOVs, the pure He fits to the observations of Beauchamp et al. (1999) fo
    DBVs, and the observations of Gianninas, Bergeron & Fontaine (2006) and Castanheira et al. (200
    Annu. Rev. Astro. Astrophys. 2008.46:157-199. Downloa
    by University of Texas - Austin on 01/28/0
    Winget & Kepler 2008

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  18. Van Grootel et al. 2012
    The blue edge where
    pulsations turn on is well
    predicted
    (Brickhill 1991, Goldreich& Wu 1999)
    (1999). We have reproduced some of their results in Figure 9.
    These calculations were obtained through full nonadiabatic cal-
    culations of complete stellar models under the assumption that
    the outer convection zone (due to H recombination) reacts in-
    stantaneously to the perturbations caused by the oscillatory mo-
    tions in the star. This is appropriate because, as first pointed out
    by Brickhill (1983), the convective turnover timescale is about 2
    to 3 orders of magnitude smaller than the measured pulsation
    periods of ZZ Ceti stars near the blue edge. This is exactly
    the opposite of the frozen convection hypothesis used in all
    of the early nonadiabatic investigations of pulsating DA and
    DB white dwarfs. As indicated in Figure 9, the results do de-
    pend on the assumed version of the mixing-length theory used
    in the construction of the equilibrium stellar models. These
    should not be confused with the model atmospheres and the
    synthetic spectra computed by Bergeron et al. (1995) whereby
    a calibration of the mixing-length theory, the ML2=α ¼ 0:6
    parametrization, was achieved as mentioned above. That cali-
    bration, we recall, only applies in the atmospheric layers, in
    regions where the observable flux comes from.
    In comparison, the theoretical blue edge is most sensitive to
    the physical conditions found at the base of the convection zone
    (see the next section), well below the atmospheric layers. In fact,
    the location of the theoretical blue edge is a measure of the
    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
    the nonvariable stars are given by the open circles. The error cross in the lower
    1054 FONTAINE & BRASSARD 4000
    5000
    6000
    7000
    8000
    9000
    10000
    11000
    12000
    0
    500
    1000
    1500
    2000
    T
    eff
    (K)
    Period (s)
    TDC, β=(0,0), ML2/ α=1.0, 0.60 M
    s
    5000
    6000
    7000
    8000
    9000
    10000
    11000
    12000
    0
    500
    1000
    1500
    2000
    2500
    T
    eff
    (K)
    Period (s)
    TDC, β=(1,−1), ML2/ α=1.0, 0.60 M
    s
    Fig. 7. Periods (in seconds) of the excited l = 1 g-modes as functions
    of the effective temperature along the 0.6 M⊙
    evolutionary sequence
    computed with the detailed atmosphere modeling. The size of a dot is a
    measure of the excitation of that particular mode. Top panel: FC. Middle
    panel: TDC with β = (0, 0). Bottom panel: TDC with β = (1, −1).
    a
    2
    f
    T
    a
    b
    o
    i
    a
    p
    c
    o
    f
    o
    p
    d
    m
    t
    a
    d
    w
    m
    t
    o
    g
    T
    p
    m
    e
    s
    i
    e
    The red edge where pulsations turn
    off is observed several thousand K
    hotter than non-adiabatic calculations
    predict

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  19. 1000 s 200 s
    500 s
    Convection zone
    deepens as WD cools,
    driving longer-period
    pulsations

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  20. Mike Montgomery
    • Pulsations: periodic brightness
    changes, caused by surface
    temperature variations
    • White dwarfs only show
    nonradial pulsations
    (strong surface gravity)
    • g-modes with periods of
    100-1400 s (1.5-23 min)

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  21. E. L. Robinson
    Nather et al. 1990
    Gaps in data cause
    cycle-count confusion
    (aliasing)

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  22. Nather et al. 1990
    Coordinated
    global
    campaigns can
    minimize gaps

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  23. Used to Be, Getting Data Required
    Going to the Telescope

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  24. *PG 1159 star = hot pre-WD (aka DOV)

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  25. Vauclair et al. 2002
    Maidanak Observatory,
    Uzbekistan

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  26. THE MAIN LOG
    Observations at Maidanak observatory in Uzbekistan. Aug 1994
    Observers: E. Meistas, and local assistant Alexey V. Chernyshev

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  27. THE MAIN LOG
    Observations at Maidanak observatory in Uzbekistan. Aug 1994
    Observers: E. Meistas, and local assistant Alexey V. Chernyshev
    Jul 27th
    Uzbeks introduced new rules for the visas ... spent long 8 night hours
    in the old stinking Russian bus, which, using longest possible route
    and stopping more than ten times for the repairs, after which
    passengers were supposed to push the bus to start the engine, brought
    us to Shakhrisabz.
    Jul 28th
    Old military jeep, which exhaust went more inside than via its
    pipes, after 5 hours brought us to Maidanak [Observatory]. ...
    Some windows of our living house were broken, no clean sheets,
    they had nobody to prepare meals, and they were poor even with
    the food supply: no butter, meat, sugar. Running water system was
    not working anymore, not to mention hot water, which we had here
    always before.

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  28. THE MAIN LOG
    Observations at Maidanak observatory in Uzbekistan. Aug 1994
    Observers: E. Meistas, and local assistant Alexey V. Chernyshev
    Jul 29th
    I checked telescope; tracking and positioning were working, but
    telescope mirrors needed cleaning...
    Jul 30th
    Managed to repair distiller and to get 3 l of water late in the
    evening only. Decided to wash mirrors next day. Still lots of yellow
    Afghanistan dust in the sky.
    Jul 31st
    Washed mirrors, cleaned telescope inner surfaces from thick dust
    layer. Started the full scale system test.

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  29. THE MAIN LOG
    Observations at Maidanak observatory in Uzbekistan. Aug 1994
    Observers: E. Meistas, and local assistant Alexey V. Chernyshev
    Aug 1st
    All day no clouds, but wind increasing to the evening.
    Worked all night.
    Aug 3rd
    All day clear sky with some clouds. Quite strong wind in day time but
    diminished before the night.

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  30. THE MAIN LOG
    Observations at Maidanak observatory in Uzbekistan. Aug 1994
    Observers: E. Meistas, and local assistant Alexey V. Chernyshev
    Aug 5th
    It was first night there on the mountain without me. I was at that time in
    Kitab Hospital severely injured by the Tashkent Astrophysical Institute
    Director son Iskander Yuldashbaev, apparently mentally ill young man of
    about 21.

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  31. THE MAIN LOG
    Observations at Maidanak observatory in Uzbekistan. Aug 1994
    Observers: E. Meistas, and local assistant Alexey V. Chernyshev
    Aug 5th
    It was first night there on the mountain without me. I was at that time in
    Kitab Hospital severely injured by the Tashkent Astrophysical Institute
    Director son Iskander Yuldashbaev, apparently mentally ill young man of
    about 21.
    He did some cleaning, then came to my room ... suddenly saying no words
    grabbed my hair with his left hand and hit my throat with a broken knife
    from our kitchen. I jumped from my chair, ran in horror out of the house,
    but he managed to hit me twice into my back until I was out. I ran to the
    Russian house for the help all in the blood. It was no phone connection
    with outside world and two of them had to run all the way to Maidanak to
    soldiers, and in three hours at last I was delivered to Kitab hospital in
    rather weak condition.

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  32. THE MAIN LOG
    Observations at Maidanak observatory in Uzbekistan. Aug 1994
    Observers: E. Meistas, and local assistant Alexey V. Chernyshev
    Aug 5th
    It was first night there on the mountain without me. I was at that time in
    Kitab Hospital severely injured by the Tashkent Astrophysical Institute
    Director son Iskander Yuldashbaev, apparently mentally ill young man of
    about 21.
    He did some cleaning, then came to my room ... suddenly saying no words
    grabbed my hair with his left hand and hit my throat with a broken knife
    from our kitchen. I jumped from my chair, ran in horror out of the house,
    but he managed to hit me twice into my back until I was out. I ran to the
    Russian house for the help all in the blood. It was no phone connection
    with outside world and two of them had to run all the way to Maidanak to
    soldiers, and in three hours at last I was delivered to Kitab hospital in
    rather weak condition.
    ... He is in a custody now and cannot say the reason either, says he did
    not like the way I looked at him. But he was smart enough to steal before
    that event good sum of my money ... Until helicopter arrived I explained
    the basics of the work with the quilt program to Alexey - my assistant.
    Luckily I trained him in previous nights on almost everything...

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  33. THE MAIN LOG
    Observations at Maidanak observatory in Uzbekistan. Aug 1994
    Observers: E. Meistas, and local assistant Alexey V. Chernyshev
    Aug 8th
    All day clear sky, in the evening no clouds too, but bad seeing. I
    was in Kitab at russian astronomer place all throat bandaged,
    practically defected from Kitab hospital, where black bugs were
    running on the walls at night even in the patient's beds, over the
    face too. Throat is badly swollen and hurts.
    Aug 10th
    Alexey arrived from the Maidanak in the afternoon. Everything seems OK.
    Aug 11th Aug 12th
    I lived in the Russian hotel in Kitab ... working with data: writing
    logs, marking bad points, making .op files. Tomorrow night Uzbeks
    promised to bring me to the Samarkand airport.
    My throat is swollen, still hurts and ugly.
    END OF CAMPAIGN HERE IN THE UZBEKISTAN
    ------------------------------------------------------------------------

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  34. Today We Are Spoiled with
    Telescopes in Space

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  35. g=17.1 mag DAV in K2 Campaign 5
    Just 12 hr of a 78.8-day K2 light curve

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  36. g=17.1 mag DAV in K2 Campaign 5
    Just 12 hr of a 78.8-day K2 light curve

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  37. g=17.1 mag DAV in K2 Campaign 5
    Just 12 hr of a 78.8-day K2 light curve
    My throat is swollen, still hurts and ugly.
    END OF CAMPAIGN HERE IN THE UZBEKISTAN
    ------------------------------------------------------------------------

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  38. K2 is revolutionizing the way we
    look at pulsating white dwarfs

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  39. The First Kepler DAV Showed Something Funny
    2 Bell et al.
    Fig. 1.— Representative sections of the
    Kepler
    light curve of KIC 4552982 in units of days since the start of observations. The top pane
    shows the full Q11 light curve. The one-month shaded region in the top panel is expanded in the middle panel. The one-week shade
    region in the middle panel is expanded in the bottom panel. The solid line is the light curve smoothed with a 30-minute window. Th
    point-to-point scatter dominates the pulsation amplitudes in the light curve, so pulsations are not apparent to the eye. The dramati
    increases in brightness are discussed in detail in Section 3.
    to medium-resolution spectra for the white dwarf and fit
    the Balmer line profiles to models to determine its val-
    ues of Te↵ = 11, 129 ± 115 K, log g = 8.34 ± 0.06, and
    tion rate. We summarize our findings and conclude i
    Section 5.
    KIC 4552982: Bell et al. 2015
    3 months:
    1 month:
    1 week:
    Brightenings
    every ~2.7 d,
    4.0-25.0 hr
    durations

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  40. The First Kepler DAV Showed Something Funny
    2 Bell et al.
    Fig. 1.— Representative sections of the
    Kepler
    light curve of KIC 4552982 in units of days since the start of observations. The top pane
    shows the full Q11 light curve. The one-month shaded region in the top panel is expanded in the middle panel. The one-week shade
    region in the middle panel is expanded in the bottom panel. The solid line is the light curve smoothed with a 30-minute window. Th
    point-to-point scatter dominates the pulsation amplitudes in the light curve, so pulsations are not apparent to the eye. The dramati
    increases in brightness are discussed in detail in Section 3.
    to medium-resolution spectra for the white dwarf and fit
    the Balmer line profiles to models to determine its val-
    ues of Te↵ = 11, 129 ± 115 K, log g = 8.34 ± 0.06, and
    tion rate. We summarize our findings and conclude i
    Section 5.
    3 months:
    1 month:
    1 week:
    KIC 4552982: Bell et al. 2015
    1000 s 400 s
    Prot
    ≈ 0.73 d

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  41. A Second Case of Outbursts in a Cool DAV
    • In K2 Campaign 1 we saw another case
    • These outbursts are essentially rogue waves (or freak waves) on a
    pulsating star!
    PG 1149+057: Hermes et al. 2015, ApJ, 810, L5

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  42. A Second Case of Outbursts in a Cool DAV
    • No companion earlier than L3:
    This is happening on the white dwarf
    SDSS image
    K2 pixels
    11,060 K, log(g)=8.06 model
    (3σ uncertainties smaller than each point)
    PG 1149+057: Hermes et al. 2015, ApJ, 810, L5

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  43. A Second Case of Outbursts in a Cool DAV
    K2 Campaign 1 full light curve
    Prot
    ≈ 1.2 d
    PG 1149+057: Hermes et al. 2015, ApJ, 810, L5
    Outbursts every ~8 d,
    9.3-36.4 hr durations

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  44. Pulsations Persist in Outburst
    • White dwarf Teff
    = 11,060 K
    • é 14% mean flux = é 750 K
    • é >25% flux = é >1500 K
    Black line is
    30-min running mean
    Event 1
    Event 7
    Quiescence

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  45. A Second Case of Outbursts in a Cool DAV
    Pulsations are affected by outbursts
    PG 1149+057: Hermes et al. 2015, ApJ, 810, L5

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  46. A Second Case of Outbursts in a Cool DAV
    (3-day sliding window)
    PG 1149+057: Hermes et al. 2015, ApJ, 810, L5

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  47. Potential Outburst Mechanisms in Cool DAVs
    • Magnetic flares unlikely:
    τdynamical
    only a few s for WDs
    • Nuclear burning unlikely:
    T < 106 K at τthermal
    of recurrence
    timescale (~7.7 d)
    • Rocky accretion unlikely:
    No spectroscopic metal lines
    • Most likely related to pulsations
    Base of convection
    zone
    Surface
    Deeper

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  48. Potential Outburst Mechanisms in Cool DAVs
    • Possible mechanism:
    Nonlinear mode
    coupling, via
    parametric
    instability
    (Wu & Goldreich2001)
    • Mode density
    increases for cooler
    DAVs: more modes
    with which to couple
    The first two
    outbursting DAVs

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  49. Potential Outburst Mechanisms in Cool DAVs
    • Wu & Goldreich predicted nonlinear mode coupling could transfer
    energy into damped modes in the cool DAVs
    ωp
    = 2425.0 µHz
    (l=1, m=0, k=7)
    Wu & Goldreich2001, ApJ, 546, 469
    ωd1
    = 898.3 µHz
    (l=1, m=0, k=24)
    l=1
    l=2
    Model: 11,245 K, 0.632 M¤
    , 10-4.12 MH
    /MWD
    Observed: 11,060(170) K, 0.64(0.03) M¤
    (Romero et al. 2012)
    (Gianninas et al. 2011)
    ωd1
    + ωd2
    = ωp
    + δω
    Limit cycle if: δω < γd
    ωd2
    = 1521.4 µHz
    (l=1, m=0, k=13; damped)

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  50. Potential Outburst Mechanisms in Cool DAVs
    • Wu & Goldreich predicted nonlinear mode coupling could transfer
    energy into damped modes in the cool DAVs
    ωd1
    + ωd2
    = ωp
    + δω
    Limit cycle if: δω < γd
    ωp
    = 2425.0 µHz
    (l=1, m=0, k=7)
    Wu & Goldreich2001, ApJ, 546, 469
    ωd1
    = 898.3 µHz
    (l=1, m=0, k=24)
    l=1
    l=2
    l=3
    Model: 11,245 K, 0.632 M¤
    , 10-4.12 MH
    /MWD
    Observed: 11,060(170) K, 0.64(0.03) M¤
    (Romero et al. 2012)
    (Gianninas et al. 2011)
    HWHM of Lorentzian:
    γd
    ≃ 8.05 µHz (1.4 days)
    δω < γd
    if 1518.7 < ωd2
    < 1534.7 µHz
    ωd2
    = 1521.4 µHz
    (l=1, m=0, k=13; damped)

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  51. A Second Case of Outbursts in a Cool DAV
    (3-day sliding window)
    PG 1149+057: Hermes et al. 2015, ApJ, 810, L5
    • ~1034 erg per outburst
    • At least 1038 erg kinetic energy in ωp

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  52. 0 500 1000 1500 2000 2500
    Frequency (µHz)
    0.00
    0.02
    0.04
    0.06
    0.08
    0.10
    0.12
    0.14
    0.16
    Amplitude (%)
    1% FAP for a single peak
    1% FAP for three peaks
    500 s
    1000 s
    A Campaign 5 white dwarf
    showed at least 13 outbursts
    recurring every ~4.7 d,
    19.4-52.1 hr durations
    Bell, Hermes et al., in prep.
    A Third Case of Outbursts in a Cool DAV

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  53. Hot off the space telescope: A Campaign 6 white dwarf showed at least
    33 outbursts, recurring every ~2.4 d, 6.8-17.0 hr durations
    Bell, Hermes et al., in prep.
    1000 s 500 s
    A Fourth Case of Outbursts in a Cool DAV!

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  54. (3-day sliding window)
    Once again, outbursts affect pulsations!

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  55. 57 not outbursting from
    K2 30-min-cadence
    First 4 outbursting DAVs: Coolest DAVs, deepest convection zones

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  56. Why Have We Never Seen This Before?
    All ground-based photometry, including WET, traditionally divides out
    second-order polynomial to compensate for differential extinction
    GD66, taken 2007-11-07 from McDonald Observatory

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  57. Three Commonalities Among 4 Outbursting WDs
    1. Repeated, aperiodic outbursts; recurrence of days, duration of hours
    2. Temperatures at the cool, red edge of the DAV instability strip
    3. Long-period (800-1400 s) pulsations with a shorter-period (350-500 s)
    l=1 mode visible

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  58. 4.0
    9.3
    6.8 19.4
    Cooler (and less massive) WDs have longerthermal timescales
    at base of the convection zone; longer outburst durations?

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  59. • Confirmed that planets are disrupted onto white dwarfs
    • Outbursts occur in the coolest pulsating white dwarfs with the
    deepest convection zones. Is this how WD pulsations shut down?
    K2 is Changing the Way we Look at WDs

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  60. § Ensemble asteroseismology of white dwarfs
    § Diversity in envelope thicknesses?Affect on cooling ages.
    § Up to 100 white dwarf rotation rates
    § Incidence of magnetism in white dwarfs
    § More remnant planetary systems
    § More close, evolved binaries
    § Refining WDs as “Flux Standards”
    K2 begins observing Field 9 (towards Earth) in a week.
    It has fuel to make it beyond Field 18 (barring safe mode events).
    What More Can We Expect from K2?

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  61. • Spherical symmetry =
    spherical harmonics
    • Eases mode identification
    • 1 month Kepler data
    • Frequency splittingsyield
    Prot
    = 0.9 ± 0.2 day
    l = 1
    modes
    m = +1
    m = -1
    m = 0
    1000 s 200 s
    500 s 125 s

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  62. l = 0, m = 0 l = 2, m = 2
    l = 2, m = 0
    Radial Modes Nonradial Modes
    White Dwarf Pulsations in Observation
    • Pulsations: periodic brightness changes, caused by surface temperature variations
    • White dwarfs only show nonradial pulsations (strong surface gravity)
    Róbert Szabó

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