When flux standards go wild: White dwarfs in the age of Kepler

Conference presentation, 7 min. January 2017: AAS Winter Meeting, Grapevine, TX, USA.

Transcript

1. http://jjherm.es
   J.J. Hermes
   Hubble Fellow
   University of North Carolina at Chapel Hill
   When flux standards go wild:
   White dwarfs in the age of Kepler
   

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2. white dwarfs are generally excellent flux standards
   caveats: binarity, magnetism, and pulsations
   beware weakly magnetic WDs >25,000 K
   summary slide (like eating breakfast for dinner)
   

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3. How many of our fellow
   AAS-goers see white
   dwarf stars: through
   the eyes of Oke
   

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4. G191-B2B
   DA0 (61,300 K)
   GD 71
   DA1 (32,300 K)
   GD 153
   DA1 (38,500 K)
   Bohlin, Colina & Finley 1995
   HZ 43
   DA1 (50,000 K)
   Nearby (3”) dM
   The HST White Dwarf Standards
   … JWST will likely use four primary white dwarfs as absolute flux standards
   … dark energy studies with Type Ia supernovae pushing towards <1% flux calibration
   “the primary reference
   standards that are
   recommended for all
   ultraviolet and optical
   absolute calibrations from
   1000 to 10,000 Å”
   

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5. Original Kepler Mission:
   20 white dwarfs observed,
   Six pulsating WDs
   (just two >3 months)
   K2 through Campaign 10:
   >1100 WDs observed
   43 pulsating WDs
   K2 through Campaign 16:
   >2000 WDs (if our GO proposal is !),
   >90 pulsating WDs (~280 known today)
   K1
   K2, today
   K2, by mid-2018
   K2 is providing a growing
   empirical test of white
   dwarf flux stability!
   

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6. K2 Shows: White Dwarfs Are Good Flux Standards
   >97% of all spectroscopically confirmed
   WDs in Kepler/K2 are flux constant to
   <1% on 30-min to 10-day timescales
   5/252 spectroscopic WDs >1% var.
   9/424 with WD colors/proper motions
   Kp
   < 19.0 mag
   Cut: Pulsating WDs (SC)
   & known SDSS WD+dM
   (sdss-wdms.org)
   Zhichao Xue & Brad Schaefer are exploring white dwarf stability at mmag levels
   

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7. Caveats: Binarity, Magnetism, Pulsations
   Reflection effect in close binary
   

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8. Caveats: Binarity, Magnetism, Pulsations
   

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9. (K2 Already Doubled WD Rotation Measurements)
   10 1 100 101 102
   White Dwarf Rotation Period (hr)
   0
   2
   4
   6
   8
   N
   K2 Asteroseismic
   Asteroseismic
   K2 Magnetic
   Magnetic
   0.5 d 1.0 d 5.0 d
   3 hr
   pre-K2: Kawaler et al. 2015
   

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10. EC 14012-1446, r= 15.7 mag 98.2% duty cycle for 78.9 days
    Caveats: Binarity, Magnetism, Pulsations
    

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11. We recently discovered
    pulsations and an apparent
    spot modulation in a hot
    (>31,000 K) DB:
    PG 0112+104
    No spectroscopic evidence
    for Zeeman splitting
    Hermes et al. 2017, arXiv: 1612.07807
    (Spots Appear on Weakly Magnetic WDs, As Well)
    10.17404 hr surface rotation period
    (peak-to-peak amplitude <0.3%)
    

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12. • Pulsation splittings consistent
    with 10.2-hr rotation
    • Pulsation symmetry requires
    <10 kG field
    Hermes et al. 2017, arXiv: 1612.07807
    (Spots Appear on Weakly Magnetic WDs, As Well)
    l=1 modes l=2 modes
    

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13. HST primary standards
    DAs surface convection <13,000 K
    DBs surface convection <30,000 K
    PG 0112+104
    

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14. white dwarfs are generally excellent flux standards
    caveats: binarity, magnetism, and pulsations
    beware weakly magnetic WDs >25,000 K
    summary slide (like eating dinner for dinner)
    >97% of isolated WDs are constant to <1% on 30-min to 10-d timescales
    reflection, eclipses, spots, pulsations, outbursts
    surface spots visible even with <10 kG magnetic fields
    

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15. Dufour et al. 2010
    PG 0112+104
    Teff
    = 31,300 ± 500 K
    log g = 7.8 ± 0.1
    (M = 0.52 ± 0.05 M
    )
    log N(H)/N(He) < −4.0
    several C II and C III lines
    

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