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Rumblings in the stellar graveyard: White dwarf pulsations with K2 and TESS

jjhermes
July 18, 2017

Rumblings in the stellar graveyard: White dwarf pulsations with K2 and TESS

Conference presentation, 25 min. July 2017: TASC3 KASC 10 Workshop, Birmingham, UK.

jjhermes

July 18, 2017
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  1. http://jjherm.es
    J.J. Hermes
    Hubble Fellow
    University of North Carolina
    at Chapel Hill
    Rumblings in the Stellar Graveyard:
    White Dwarf Pulsations
    with K2 and TESS

    View full-size slide

  2. U. North Carolina: Chris Clemens, Bart Dunlap, Erik Dennihy, Josh Fuchs, Stephen Fanale
    U. Warwick: Boris Gaensicke, Roberto Raddi, N. P. Gentile Fusillo, P.-E. Tremblay, Paul Chote
    U. Texas: Keaton J. Bell, Mike Montgomery, Don Winget, Zach Vanderbosch
    + Steve Kawaler, Agnes Bischoff-Kim, Judi Provencal, S.O. Kepler, Alejandra Romero
    Rumblings in the Stellar Graveyard:
    White Dwarf Pulsations
    with K2 and TESS

    View full-size slide

  3. Kepler/K2 pulsating white dwarfs have provided opportunity to:
    • Establish the range of WD envelope masses
    • Empirically constrain the efficiency of convection in WDs
    • Witness nonlinear mode coupling
    • Measure the endpoints of angular momentum evolution
    • Test for radial differential rotation
    All without extremely detailed asteroseismic fits

    View full-size slide

  4. Sandra Greiss et al. 2016
    http://www2.warwick.ac.uk/fac/sci/physics/
    research/astro/research/catalogues/kis
    stars
    WDs
    Just two known WDs at Kepler launch
    In 2012 we started Kepler INT Survey
    (U,g,r,i,Hα
    ) down to 20th mag:
    Found 10 new pulsating WDs

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  5. Original Kepler Mission (4 years):
    Just 20 white dwarfs observed,
    6 pulsating WDs (just two >3 months)
    K2 through Campaign 10:
    >1000 white dwarf candidates observed
    35 more pulsating WDs
    K2 has given us hundreds of
    candidate pulsating white
    dwarfs to observe

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  6. GD 1212, Hermes et al. 2014
    Data from the 9-day K2 engineering test run
    V=13.3 mag

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  7. WET, and KASC4 Boulder (7/2011 so SIX YEARS AGO!)
    He
    P.I. zone
    (30-20 kK)
    H
    P.I. zone
    (13-10 kK)
    DBV
    aka V477 Her
    DAV
    aka ZZ Ceti

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  8. White Dwarfs are Finally Getting the Space Treatment
    PG1159-035, V=14.9 mag -- poster child for WET
    (March 1989, 9 sites, 90.8% duty cycle over 12.0 days)
    SDSSJ0106+0145, g=16.2 mag -- typical K2 light curve
    (K2 Campaign 8, 96.0% duty cycle over 78.7 days)
    Winget et al. 1991 Hermes et al. 2017, in prep.
    l = 1 l = 2
    l = 2
    l = 2
    l = 1
    l = 1

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  9. White Dwarfs Seismologist’s Dilemma: Often Few Modes
    For any one hot DAV:
    1. Small number of independent modes observed
    2. Best model hinges on mode identification
    3. Hidden free parameters (core profile, layer masses), with 8+ degrees of freedom
    e.g., WD0111+0018, 6 hr ground-based data e.g., WD0111+0018, 78.7-d K2 data
    Hermes et al. 2013 Hermes et al. 2017, in prep.

    View full-size slide

  10. White Dwarfs Seismologist’s Dilemma: Often Few Modes
    For any one hot DAV:
    1. Small number of independent modes observed
    2. Best model hinges on mode identification
    3. Hidden free parameters (core profile, layer masses), with 8+ degrees of freedom
    e.g., WD0111+0018, 6 hr ground-based data e.g., WD0111+0018, 78.7-d K2 data
    Hermes et al. 2013 Hermes et al. 2017, in prep.

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  11. Spectroscopy Yields Effective Temperatures and Masses
    SDSS
    SOAR spectroscopy
    yields
    WD mass
    We have
    obtained SOAR
    spectra of all
    DAVs observed
    by K2 so far:
    k2wd.org

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  12. The Kepler/K2 DAV Instability Strip
    Our DAVs span all edge to edge of DAV instability strip
    (We will address purity by the end of K2)
    Hermes et al. 2017, k2wd.org
    Josh Fuchs et al. 2017, in prep.
    Stay tuned: Recent UNC Ph.D. Josh Fuchs is
    exploring strip with minimal systematics
    (same instrument, methods, models)
    Empirical edges by Tremblay+ 2015

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  13. As Convection Zone Deepens, Longer Mode Periods Driven
    Known DAV
    from ground
    WMP > 500 s
    Outbursting
    DAV
    WMP > 500 s

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  14. Low-n Modes: Doing More with Less

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  15. 239 eigenperiods from 75 hot DAVs (mostly ground-based, no combination frequencies)
    Histogram of eigenperiods (again, no combination frequencies)
    0
    5
    10
    15
    20
    25
    30
    50 100 150 200 250 300 350 400 450 500
    0
    5
    10
    15
    20
    25
    30
    50 100 150 200 250 300 350 400 450
    Clemens et al. 2017, in prep.
    Insights from the Aggregated Periods of DAVs
    Mode Amplitude (ppt)
    N
    Mode Period (s)
    Mode Period (s)

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  16. 38 eigenperiods from 16 hot DAVs (l=1, m=0,
    no combination frequencies, mostly Kepler/K2)
    0
    1
    2
    3
    4
    5
    6
    7
    8
    50 100 150 200 250 300 350 400 450
    Clemens et al. 2017, in prep.
    Insights from the Aggregated Periods of DAVs
    n = 1
    l = 1
    n = 2
    l = 1
    n = 3
    l = 1
    Kepler makes mode identification relatively trivial
    Mode Period (s)
    N
    n=1 n=2 n=3 n=4

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  17. 0
    1
    2
    3
    4
    5
    6
    7
    8
    50 100 150 200 250 300 350 400 450
    l=1 hDAV periods, observed
    0
    1
    2
    3
    4
    5
    6
    7
    8
    50 100 150 200 250 300 350 400 450
    l=1 random MH
    simulation
    Clemens et al. 2017, in prep.
    Romero et al. 2012
    Comparing to a random distribution of
    models with thick (10-4 MH
    /M

    ) to thin
    (10-10 MH
    /M

    ) hydrogen layer masses,
    using spectroscopic Teff
    & masses
    Insights from the Aggregated Periods of DAVs

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  18. • The observed distribution supports a
    thick (canonical) hydrogen layers
    • Implications for improving white
    dwarf cooling ages (Gaia)
    0
    1
    2
    3
    4
    5
    6
    7
    8
    50 100 150 200 250 300 350 400 450
    l=1 hDAV periods, observed
    0
    1
    2
    3
    4
    5
    6
    7
    8
    50 100 150 200 250 300 350 400 450
    0
    1
    2
    3
    4
    5
    6
    7
    8
    50 100 150 200 250 300 350 400 450
    l=1 random MH
    simulation
    l=1 canonical MH
    simulation
    Clemens et al. 2017, in prep.
    Romero et al. 2012
    Insights from the Aggregated Periods of DAVs

    View full-size slide

  19. Long-Period Modes Lose Phase Coherence

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  20. Kepler Has Revealed a Dichotomy of Mode Linewidths
    Two modes within the same
    DAV show very different
    linewidths
    Many of the broadened
    modes appear relatively
    Lorentzian in shape
    (Absolutely no way to have
    made these measurements
    before Kepler)
    Hermes et al. 2017, in prep.

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  21. Kepler Has Revealed a Dichotomy of Mode Linewidths
    Results from fitting Lorentzians to
    the 27 DAVs through K2 Campaign 8:
    Clear dichotomy at ~800 s
    Hermes et al. 2017, in prep.
    Stephen Fanale

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  22. Kepler Has Revealed a Dichotomy of Mode Linewidths
    • Damping rather than driving
    important for broadening; phase incoherence
    Broadened modes: bounded by the
    base of the convection zone!
    Mike Montgomery et al. 2017, in prep.

    View full-size slide

  23. Kepler Has Revealed a Dichotomy of Mode Linewidths
    Mike Montgomery et al. 2017, in prep.
    0.4 0.6 0.8 1.0 1.2 1.4

    0.675
    0.700
    0.725
    0.750
    0.775
    0.800
    0.825
    0.850
    Fraction Correct
    0 200 400 600 800 1000 1200 1400
    Period (s)
    0
    1
    2
    3
    4
    5
    6
    HWHM (µHz)
    ML2/↵ = 0.6
    0 200 400 600 800 1000 1200 1400
    Period (s)
    0
    1
    2
    3
    4
    5
    6
    HWHM (µHz)
    ML2/↵ = 0.9 Given spectroscopic Teff
    /log(g), we
    can calculate the critical period for a
    mode reflecting off the base of the
    convection zone
    This empirically
    constrains ML2/α

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  24. Not the Whole Linewidth Story – Lots of Resonances
    First 5 days:
    885.243(0.057) µHz
    Last 5 days:
    888.285(0.067) µHz
    Some longer-period modes appear to
    change very quickly in frequency:
    Nonlinear mode coupling appears to
    be the only way the star can transfer that
    much energy so quickly!

    View full-size slide

  25. Not the Whole Linewidth Story – Lots of Resonances
    Weikai Zong et al. 2016
    Zong et al. have explored three-mode
    coupling to explain amplitude/phase
    changes in compact objects

    View full-size slide

  26. The Coolest DAVs Outburst

    View full-size slide

  27. A surprising discovery with Kepler: Aperiodic Outbursts
    Outbursting
    DAVs

    View full-size slide

  28. This outburst phenomenon never seen before in
    40+ years of pulsating white dwarf studies
    A surprising discovery with Kepler: Aperiodic Outbursts
    Quiescent pulsations
    (1151.9 s, 1160.8 s, …)
    In Outburst
    (999.9 s, 896.6 s, …)
    PG 1149+057: Hermes et al. 2015

    View full-size slide

  29. A surprising discovery with Kepler: Aperiodic Outbursts
    Keaton Bell et al. 2017
    We see outbursts in 6 of the 27 DAVs
    observed through Campaign 8
    These are aperiodic brightenings
    causing up to 15% mean flux
    increases (>750 K Teff
    increases)
    Pulsations persist in outburst,
    and are consistent with the star
    having a thinner convection zone

    View full-size slide

  30. A surprising discovery with Kepler: Aperiodic Outbursts
    Keaton Bell et al. 2017
    We see outbursts in 6 of the 27 DAVs
    observed through Campaign 8
    These are aperiodic brightenings
    causing up to 15% mean flux
    increases (>750 K Teff
    increases)
    Pulsations persist in outburst,
    and are consistent with the star
    having a thinner convection zone

    View full-size slide

  31. A surprising discovery with Kepler: Aperiodic Outbursts

    View full-size slide

  32. Bulk White Dwarf Rotation Rates

    View full-size slide

  33. Rotation Rates Usually Fall Readily from K2 Data

    View full-size slide

  34. 1 10 100
    White Dwarf Rotation Period (hr)
    0
    2
    4
    6
    8
    10
    N
    Kepler & K2
    Kawaler (2015)
    Kepler & K2 have doubled
    the number of white
    dwarfs with measured
    internal rotation
    periods using
    asteroseismology
    Hermes et al. 2017, in prep.
    None of the stars here are
    currently in binaries:
    Representative of
    single-star evolution
    Rotation Rates Usually Fall Readily from K2 Data
    0.5 d 1 d 2 d 4 d

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  35. 1 10 100
    WD Rotation Period (hr)
    0.4
    0.5
    0.6
    0.7
    0.8
    0.9
    WD Mass (M⊙
    )
    1.0
    1.5
    2.0
    2.5
    3.0
    3.5
    4.0
    ZAMS Progenitor Mass (M⊙
    )
    1 10 100
    White Dwarf Rotation Period (hr)
    0
    2
    4
    6
    8
    10
    N
    Kepler & K2
    Kawaler (2015)
    1 d 2 d 4 d
    Hermes et al. 2017, in prep.
    We Can Finally Probe WD Rotation as a Function of Mass
    The fastest-rotating pulsating white dwarf (1.13 hr) is also the most
    massive (0.87 M¤
    ) – descended from a single 4.0 M¤
    ZAMS progenitor
    Hermes et al. 2017c, ApJL, 841, L2; arXiv: 1704.08690

    View full-size slide

  36. >70% of Field WDs between
    0.51-0.73 M¤
    (evolved 1.7-3.0 M¤
    ZAMS)
    These WDs rotate at 0.5-2.2 d
    (WD Prot
    : 35 ± 28 hr)
    Link emerging between higher
    WD mass and faster rotation
    1 10 100
    0
    1
    2
    3
    4
    N
    1.7 2.0 M ZAMS
    WD Prot = 1.48 ± 0.94 d
    1 10 100
    0
    1
    2
    3
    4
    N
    2.0 2.5 M ZAMS
    WD Prot = 1.35 ± 0.74 d
    1 10 100
    0
    1
    2
    3
    4
    N
    2.5 3.0 M ZAMS
    WD Prot = 1.32 ± 1.04 d
    1 10 100
    White Dwarf Rotation Period (hr)
    0
    1
    2
    3
    4
    N
    3.5 4.0 M ZAMS
    WD Prot = 0.17 ± 0.15 d
    We Can Finally Probe WD Rotation as a Function of Mass

    View full-size slide

  37. Evolved Tests of Radial Differential Rotation

    View full-size slide

  38. PG 0112+104: Hermes et al. 2017a
    l=1 modes l=2 modes
    The Most Evolved Test of Radial Differential Rotation
    PG 0112+104 is a ~31,000 K
    pulsating He-atmosphere WD
    (DBV)

    View full-size slide

  39. The Most Evolved Test of Radial Differential Rotation
    PG 0112+104: Hermes et al. 2017a
    l=1 modes
    n
    (n)
    n=2
    n=3
    n=4
    n=5
    n=6
    Frequency splittings and
    overtone spacings behave in
    concert: Modes trapped to
    different depths
    Early hints: rigid rotation
    Period spacing
    difference (s)

    View full-size slide

  40. We also see a surface spot
    Surface: 10.17404 hr
    Towards core: 10.1±0.9hr
    PG 0112+104: Hermes et al. 2017a
    10.17404 hr surface spot rotation period
    The Most Evolved Test of Radial Differential Rotation
    Using l=1 and l=2 modes we
    measure a rotation period of
    10.1±0.9 hr in PG 0112+104
    (better asteroseismic modeling will
    improve this uncertainty)

    View full-size slide

  41. The Most Evolved Test of Radial Differential Rotation
    “PG0112+104
    rotates rigidlyover
    its outer 70% in radius
    with a period of
    Prot
    = 10.18 ± 0.27 hr”
    Based on full seismic
    model
    See Poster 2.7 by Noemi Giammichele et al.

    View full-size slide

  42. What Can We Expect from TESS?

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  43. WG8: TESS White Dwarf Candidate Follow-Up Ongoing!
    Zsófia Bognár et al., in prep.
    Time (d)
    Amplitude (mag)
    KonkolyObservatory:
    New I=14.3 mag DAV!
    S. Charpinet

    View full-size slide

  44. Conclusion: More White Dwarfs Yield Firmer Conclusions!
    Kepler/K2 pulsating white dwarfs provide opportunity to:
    • Establish the range of WD envelope masses
    § Most have canonically thick (10-4 MH
    /M

    ) hydrogen envelopes
    • Empirically constrain the efficiency of convection in WDs
    § ML2/α > 0.8 from mode linewidths bounded by base of convection zone
    • Witness nonlinear mode coupling
    § Outbursts (and frequency changes) on day-week timescales
    • Measure the endpoints of angular momentum evolution
    § Endpoints of 1.7-3.0 M¤
    stars rotate at 35 ± 28 hr, but >3.0 M¤
    faster
    • Test for radial differential rotation
    § White dwarfs appear to rotate rigidly, but more tests on the way!
    All of these constraints significantly improve by
    observing more pulsating WDs with K2 and TESS!

    View full-size slide

  45. >70% of Field WDs between
    0.51-0.73 M¤
    (evolved 1.7-3.0 M¤
    ZAMS)
    These WDs rotate at 0.5-2.2 d
    (WD Prot
    : 35 ± 28 hr)
    Link emerging between higher
    WD mass and faster rotation
    1 10 100
    0
    1
    2
    3
    4
    N
    1.7 2.0 M ZAMS
    WD Prot = 1.48 ± 0.94 d
    1 10 100
    0
    1
    2
    3
    4
    N
    2.0 2.5 M ZAMS
    WD Prot = 1.35 ± 0.74 d
    1 10 100
    0
    1
    2
    3
    4
    N
    2.5 3.0 M ZAMS
    WD Prot = 1.32 ± 1.04 d
    1 10 100
    White Dwarf Rotation Period (hr)
    0
    1
    2
    3
    4
    N
    3.5 4.0 M ZAMS
    WD Prot = 0.17 ± 0.15 d
    We Can Finally Probe WD Rotation as a Function of Mass

    View full-size slide