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K2's Revolutionary Eye on White Dwarfs

jjhermes
November 03, 2015

K2's Revolutionary Eye on White Dwarfs

Conference presentation, 25 min. November 2015: K2 Science Conference, Santa Barbara, CA, USA.

jjhermes

November 03, 2015
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  1. K2’s Revolutionary Eye
    on White Dwarf Stars
    JJ Hermes
    Hubble Fellow, University of North Carolina
    University of Warwick

    View Slide

  2. K1:
    20 WDs observed,

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  3. Kepler
    K2

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  4. Kepler
    K2

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  5. View Slide

  6. View Slide

  7. View Slide

  8. Planets!
    Supernovae!

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  9. White Dwarfs: Simple Stars, Simple Evolution
    •  >80%  of  white  dwarfs  have  hydrogen-­‐‑dominated  atmosphere  (DA)
    •  Vast  majority  of  DAs  ~0.6    M¤  
    •  Simple  evolution:  just  cooling
    Balmer line fits à masses
    Tremblay+ 2011
    MWD
     =  1.4  M¤
    MWD
     =  
    0.4  M¤

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  10. White Dwarfs: Not Every One is So Simple
    DZ  (atmospheric  metals)
    25-50% of white dwarfs
    are polluted with metals
    •  Metals  are  coming  from  remnant  planetary  systems!
    •  log(g)  =  8.0,  so  metals  
    should  quickly  sink  
    out  of  photosphere

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  11. •  Abundances:  debris  is  rocky  and  chemically  diverse,  like  solar  
    system  meteorites  (Gänsicke+12)
    •  Some  debris  is  rocky  &  water-­‐‑rich  (Farihi+13, Raddi+2015)
    •  Infrared  excesses  often  seen:  directly  detecting  debris  disks


    •  Exoplanet  
    compositions!
    The Scars of Tidally Disrupted Planetesimals

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  12. Vanderburg+ 2015
    K2: First Transiting Planetesimals Around WDs
    •  This  WD  has  
    metal  pollution  
    and  an  
    IR  excess  
    (dust  disk)
    •  See  talk  by  
    Andrew  
    Vanderburg  
    tomorrow:  A  
    disintegrating  
    minor  planet  
    transiting  a  
    white  dwarf

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  13. Either Way to Supernovae Ia: Need a White Dwarf

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  14. K2: Dozens of Short-Period WD Binaries
    •  By  the  end  of  K2  we  will  get  
    orbital  periods  for  dozens  of  
    pre-­‐‑  Cataclysmic  Variables  
    (WD+dM  binaries)
    Rebassa-Mansergas, Hermes+ in prep.
    99,860  K
    0.59  Msun  WD
    Reflection off dM
    every 19.9 hr

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  15. View Slide

  16. A ‘typical’ white dwarf
    electron degenerate

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  17. (Astero)Seismology
    White  Dwarf Earth

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  18. Convection Drives White Dwarf Pulsations
    •  DA  (hydrogen  
    atmosphere)  WDs  
    pulsate  when  H  partially  
    ionized  (DAVs,  aka  
    ZZ  Cetis)
    g-modes—remarkably similar to the large-amplitude DAV pulsators (Winget et al. 1
    The observed pulsating white dwarf stars lie in three strips in the H-R diagram,
    in Figure 3. The pulsating pre-white dwarf PG 1159 stars, the DOVs, around 7
    170,000 K have the highest number of detected modes. The first class of pulsating
    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 ob
    Annu. Rev. Astro. Astrophys. 2008.46:157-199. Downloaded f
    by University of Texas - Austin on 01/28/09. For
    Winget & Kepler 2008
    Partial ionization
    zone can store
    & release energy

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  19. Winget & Kepler 2008
    Fontaine & Brassard 2008
    Fractional  Mass  Depth:  log  q  –  log  (1-­‐‑M(r)/MWD
    )
    N2
    Ll
    2
    “Propagation  Diagram”
    Core Surface
    Seeing Inside a WD
    Takeaway:  Chemical  transitions  
    cause  a  “bump”  in  N2,  and  thus  a  
    detectable  asteroseismic  
    signature
    p-modes
    σ2  >  Ll
    2,  N2
    g-modes
    σ2  <  Ll
    2,  N2
    convection

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  20. 1000 s 200 s
    500 s

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

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  22. GD  1212,  Hermes  et  al.  2014,  ApJ,  789,  85
    The First K2 Pulsating White Dwarf
    Eng.  Run,  KP
     =  13.3  mag,  revisit  in  Field  12

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  23. 14+ hr

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  24. §  Core Angular Momentum Evolution
    §  How Pulsations Relate to WD Temperature
    §  Outbursts?!
    What Has K2 Taught Us So Far?

    View Slide

  25. White Dwarf Rotation Made Easy with Kepler
    •  Kp
     =  18.0  mag
    •  1  month  Kepler  data
    •  Frequency  spliiings  yield  
    Prot
     =  0.9  ±  0.3  d

    •  0.62  ±  0.05  M¤
     WD:  
    ~2.2  M¤
     (dA)  progenitor
    l  =  1  modes:
    m  =  +1
    m  =  -­‐‑1 m  =  0

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  26. A New K2 View on Common-Envelope Evolution
    •  WD+dM  in  K2  Campaign  1:  
    SDSS  J1136+0409
    SDSS
    SOAR VLT
    Hermes et al. 2015, MNRAS, 451, 1701

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  27. A New K2 View on Common-Envelope Evolution
    M-dwarf RV (VLT/FORS2)
    •  WD+dM  in  K2  Campaign  1:  
    SDSS  J1136+0409
    WD atmospheric parameters (SOAR)
    Teff
     =  12,330  ±  260  K
    log(g)  =  7.99  ±  0.06  (0.601  ±  0.036  M¤
    )
    SDSS
    SOAR VLT
    Hermes et al. 2015, MNRAS, 451, 1701
    Porb
     =  6.8976  hr

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  28. A New K2 View on Common-Envelope Evolution
    M-dwarf RV (VLT/FORS2)
    Porb
     =  6.8976  hr
    (Model:  Doppler  beaming,  reflection,  ellipsoidal  
    variations  using  spectroscopic  parameters)
    •  WD+dM  in  K2  Campaign  1:  
    SDSS  J1136+0409
    WD atmospheric parameters (SOAR)
    Teff
     =  12,330  ±  260  K
    log(g)  =  7.99  ±  0.06  (0.601  ±  0.036  M¤
    )
    SDSS
    SOAR VLT
    Hermes et al. 2015, MNRAS, 451, 1701
    Folded K2
    light curve

    View Slide

  29. A New K2 View on Common-Envelope Evolution
    10500
    11000
    11500
    12000
    12500
    White dwarf effective temperature (K)
    0.54
    0.56
    0.58
    0.60
    0.62
    0.64
    0.66
    0.68
    0.70
    White dwarf mass (Msun)
    10500
    11000
    11500
    12000
    12500
    White dwarf effective temperature (K)
    −5.6
    −5.4
    −5.2
    −5.0
    −4.8
    −4.6
    −4.4
    Hydrogen layer mass (log MH/Mstar)
    20
    22
    24
    26
    28
    30
    32
    34
    7 independent
    pulsation modes
    1-­‐‑σ  Spectroscopic  
    Teff  
    /  mass
    Best  
    asteroseismic  fit
    Hermes et al. 2015, MNRAS, 451, 1701

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  30. A New K2 View on Common-Envelope Evolution
    Hermes et al. 2015, MNRAS, 451, 1701
    J1136+0409  Prot
    :  
    2.49  ±  0.53  hr

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  31. A New K2 View on Common-Envelope Evolution
    10 1 100 101 102
    White Dwarf Rotation Period (hr)
    0
    1
    2
    3
    4
    5
    6
    N
    Non-magnetic CVs
    Pulsating white dwarfs
    J1136+0409
    J1136+0409  Prot
    :  
    2.49  ±  0.53  hr
    ~Days
    ~Minutes
    •  No  isolated  WD  rotates  this  fast
    •  No  accretion  history  in  J1136+0409
    •  RGB  core  evolution  influenced  by  
    common  envelope  ejection
    Hermes et al. 2015, MNRAS, 451, 1701

    View Slide

  32. §  Core Angular Momentum Evolution
    §  How Pulsations Relate to WD Temperature
    §  Outbursts?!
    What Has K2 Taught Us So Far?

    View Slide

  33. The Empirical DAV Instability Strip Today
    Gianninas+ 2011
    Tremblay+ 2011
    3D-­‐‑Corrected  Atmospheric  Parameters,  ML2/α  =  0.8
    Known pulsating
    white dwarfs
    Non-variable white dwarfs

    View Slide

  34. Convective Driving: WD Cools, Periods Increase

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  35. Convective Driving: WD Cools, Periods Increase
    Mukadam+ 2006
    V. Van Grootel et al.: The instab
    Fig. 2. Structure of the envelope of our representative evolving 0.6 M
    DA white dwarf. The ordinate gives the fractional mass depth in loga
    rithmic units. The small dots define “isocontours” of opacity, and som
    Surface
    Core
    Base  of  
    convection  zone  
    deepens  as  WD  cools
    Van Grootel+ 2012

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  36. 1000 s 200 s
    500 s

    View Slide

  37. §  Core Angular Momentum Evolution
    §  How Pulsations Relate to WD Temperature
    §  Outbursts?!
    What Has K2 Taught Us So Far?

    View Slide

  38. 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 p
    shows the full Q11 light curve. The one-month shaded region in the top panel is expanded in the middle panel. The one-week sh
    region in the middle panel is expanded in the bottom panel. The solid line is the light curve smoothed with a 30-minute window.
    point-to-point scatter dominates the pulsation amplitudes in the light curve, so pulsations are not apparent to the eye. The dram
    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-
    tion rate. We summarize our findings and conclud
    Section 5.
    KIC 4552982: Bell+ 2015
    3 months:
    1 month:
    1 week:

    View Slide

  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 p
    shows the full Q11 light curve. The one-month shaded region in the top panel is expanded in the middle panel. The one-week sh
    region in the middle panel is expanded in the bottom panel. The solid line is the light curve smoothed with a 30-minute window.
    point-to-point scatter dominates the pulsation amplitudes in the light curve, so pulsations are not apparent to the eye. The dram
    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-
    tion rate. We summarize our findings and conclud
    Section 5.
    KIC 4552982: Bell+ 2015
    3 months:
    1 month:
    1 week:
    e measured equivalent durations of the 186 outbursts
    at were recorded without interruption from gaps in the
    ta is displayed in Figure 4 and the continua used for
    e example outbursts are the dashed lines in Figure 3.
    e median outburst equivalent duration is 6.8 minutes
    he corresponding outburst is displayed in the second
    nel of Figure 3). Since the Kepler point-to-point scat-
    is 1.8% for this target, we are limited to detecting
    ly large outbursts by eye and so are undoubtedly in-

    View Slide

  40. A Second Case of Outbursts in a Cool DAV
    PG 1149+057: Hermes et al. 2015, ApJ, 810, L5
    1145.7 s, 998.1 s, 1052.8 s

    View Slide

  41. A Second Case of Outbursts in a Cool DAV
    PG 1149+057: Hermes et al. 2015, ApJ, 810, L5
    1145.7 s, 998.1 s, 1052.8 s

    View Slide

  42. A Second Case of Outbursts in a Cool DAV
    PG 1149+057: Hermes et al. 2015, ApJ, 810, L5
    •  No  companion  earlier  than  L3:  
    This  is  happening  on  the  white  dwarf
    SDSS image
    K2 pixels
    11,000  K,  log(g)=8.0  model  
    (3σ  uncertainties  smaller  than  each  point)

    View Slide

  43. (Aside: The Ecliptic is Full of Asteroids!)
    •  Always  check  the  target  
    pixels  if  you  see  a  
    blip  in  your  K2  light  curve
    Data  from  J1136+0409  
    (WD+dM  from  earlier)  
    h5p://barentsen.github.io/k2flix/
    K2flix

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  44. A Second Case of Outbursts in a Cool DAV
    PG 1149+057: Hermes et al. 2015, ApJ, 810, L5
    •  These  outbursts  are  essentially  rogue  waves  in  a  pulsating  star!
    •  The  pulsations  persist  in  outburst

    View Slide

  45. A Second Case of Outbursts in a Cool DAV
    PG 1149+057: Hermes et al. 2015, ApJ, 810, L5
    K2 Campaign 1 full light curve
    Prot
     ≈  1.2  d

    View Slide

  46. 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

    View Slide

  47. A Second Case of Outbursts in a Cool DAV
    PG 1149+057: Hermes et al. 2015, ApJ, 810, L5
    •  Pulsations  change  after  outburst

    View Slide

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

    View Slide

  49. Potential Outburst Mechanisms in Cool DAVs
    •  Magnetism  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  connected  to  
    pulsations
    Base of
    convection zone
    Surface
    Deeper

    View Slide

  50. Most Modes Bounded by Base of Convection Zone
    •  Growth  time  ~days:    
    γ-­‐‑1  ≈  nτω        
    •  For  ~1000  s  modes:
    –  Radial  order  n  ~  20
    –  τω  
    =  
    τthermal  at  top  
    of  mode  cavity  
    τω  
    ~  hrs
    Goldreich  &  Wu  I,  1999

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  51. Most Modes Bounded by Base of Convection Zone
    •  Growth  time  is  days  for  ~1000  s  (~1000  µμHz)  modes
    Full K2 light curve: PG1149+057

    View Slide

  52. Potential Outburst Mechanisms in Cool DAVs
    •  Nonlinear  mode  
    coupling,  via  
    parametric  
    instability?  
    (Wu & Goldreich 1999)
    •  Still  an  open  
    question!
    The  two  outbursting  
    DAVs

    View Slide

  53. Last Week We Discovered a Third Outbursting DAV

    View Slide

  54. K2’s Expanding View of Pulsating White Dwarfs
    •  So  far,  
    20  pulsating  white  dwarfs  with  
    >1  month  short-­‐‑cadence  Kepler  data
    Not variable
    with K2 SC
    Not outbursting
    from LC
    First 3 DAVs that
    outburst
    Ground-based
    DAVs

    View Slide

  55. §  Ensemble  Asteroseismology  of  White  Dwarfs
    §  White  Dwarf  Rotation  Rates
    §  Incidence  of  Magnetism  in  White  Dwarfs
    §  More  Remnant  Planetary  Systems
    §  Close,  Evolved  Binaries
    §  Refining  WDs  as  “Flux  Standards”

    What More Can We Expect from K2?

    View Slide

  56. K2’s Expanding View of Pulsating White Dwarfs
    •  Fresh  data  from  K2  Campaign  5!
    •  New  11720(340)  K,  log(g)  =  8.20(0.11)  pulsating  WD
    Kp
     =  17.1  mag

    View Slide

  57. Christmas in July (and October, and March)
    Campaign  3
    22  03  40.61  -­‐‑12  15  10.8
    Kp=17.6  mag

    No  spectra,  high  proper  motion

    View Slide

  58. 0.0 0.5 1.0 1.5 2.0
    Phase
    50
    0
    50
    100
    150
    200
    250
    300
    Radial Velocity (km/s)
    sssj2203
    1.137 hr
    Porb  =  68.22  min
    K1
     ~  69(8)  km/s  
    M2
     >  0.07  Msun.  
     
    For  i=60,  M2
     =  0.08  Msun

    Likely  a  WD+BD  binary
    very  close  to  Roche-­‐‑lobe  filling
    Christmas in July (and October, and March)
    2.5m  INT  spectroscopy:
    24,430(310)  K,  0.39(05)  Msun  WD

    View Slide

  59. Campaign  4
    Kp
     =  17.7  mag
    2.5m  INT  spectroscopy:
    8840  K,  0.50  Msun  WD
    Christmas in July (and October, and March)

    View Slide

  60. Φ  =  0.39 Φ  =  0.89

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  61. Mark Hollands, University of Warwick
    Rotation/observer  inclination:  45  ±  10  deg.
    Rotation/magnetism  colatitude:  38  ±  10  deg.
    Dipole  offset,  az
     =  -­‐‑0.31  ±  0.04

    View Slide

  62. White Dwarfs Aren’t All Great Flux Standards
    •  For  nonvariable  white  
    dwarfs,  stay  away  from  
    convective  surfaces:  
    DAs
     <  13,000  K  
    DBs
     <  25,000  K
    g-modes—remarkably similar to the large-amplitude DAV pulsators (Winget et al. 1
    The observed pulsating white dwarf stars lie in three strips in the H-R diagram,
    in Figure 3. The pulsating pre-white dwarf PG 1159 stars, the DOVs, around 7
    170,000 K have the highest number of detected modes. The first class of pulsating
    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 ob
    Annu. Rev. Astro. Astrophys. 2008.46:157-199. Downloaded f
    by University of Texas - Austin on 01/28/09. For
    Winget & Kepler 2008

    View Slide

  63. •  Confirmed  that  planets  are  disrupted  onto  white  dwarfs
    •  From  pulsations:  A  close  binary  WD  with  truncated  RGB  
    evolution  rotates  much  faster  than  isolated  WDs
    •  Outbursts  in  the  coolest  DAVs  with  very  deep  convection  zones
    What Have We Learned from K2 so Far?

    View Slide

  64. §  Ensemble  Asteroseismology  of  White  Dwarfs
    §  Can  test  for  diversity  in  envelope  thicknesses,  
    which  would  affect  cooling  ages
    §  White  Dwarf  Rotation  Rates
    §  Incidence  of  Magnetism  in  White  Dwarfs
    §  More  Remnant  Planetary  Systems
    §  Close,  Evolved  Binaries
    §  Refining  WDs  as  “Flux  Standards”

    What More Can We Expect from K2?

    View Slide

  65. K1:
    20 WDs observed,

    View Slide

  66. View Slide