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Complications to the Planetary Hypothesis for GD 66

jjhermes
January 10, 2013

Complications to the Planetary Hypothesis for GD 66

Conference presentation, 7 min. January 2013: AAS Winter Meeting, Long Beach, CA, USA.

jjhermes

January 10, 2013
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  1. Complications to the Planetary
    Hypothesis for GD 66
    JJ Hermes
    U. of Texas at Austin,
    McDonald Observatory
    221st AAS, Long Beach
    Fergal Mullally, D.E. Winget,
    Mike Montgomery,
    James Dalessio, et al.

    View Slide

  2. The UT/McDonald White Dwarf Planet Search
    •  Observations since at least 2003"
    –  Two stars since the 1970s"
    •  Monitor pulse arrival times for
    about a dozen DAVs (hydrogen-
    atmosphere WDs)"
    –  Similar to the pulsar timing
    method"
    •  > 95% of all stars in our Galaxy
    (including our Sun) will be DAVs"
    •  Pulsation periods 100-500 s"
    –  We expect the period change in
    these pulsations to be very slow

    (< 1 μs yr-1)"
    –  After 9 years we are sensitive to
    Jupiter-mass planets from

    2-10 AU"
    Mullally et al. 2008, ApJ 676 573
    JJ Hermes, UT-Austin, 221st AAS
    GD 244, typical DAV in our sample"

    View Slide

  3. G117-B15A: An Extremely Stable Optical Clock
    215.2 s mode"
    •  Some DAVs have been observed for 35+ years, including G117-B15A"
    •  The 215.2 s mode in that star produces an extremely stable rate of change
    of period with time"
    •  This dP/dt is in line

    with expectations of

    cooling for this

    ~12,000 K WD"
    •  The influence of

    a Jupiter-mass

    planet at 5 AU

    would cause an

    unmistakable

    10 s peak-to-peak

    modulation in the

    (O-C) diagram"
    JJ Hermes, UT-Austin, 221st AAS S.O. Kepler 2012, private communication

    View Slide

  4. G117-B15A: An Extremely Stable Optical Clock
    •  We can remove the secular
    trend from cooling and look
    for periodic modulation"
    •  We are nearly able to
    exclude a Uranus-mass
    planet at Uranusʼs distance"
    JJ Hermes, UT-Austin, 221st AAS Kepler et al. 2005, ApJ 634 1311
    Window"
    Residual Periodogram"
    (O-C) Residuals"

    View Slide

  5. WD 0111+0018: A Wrinkle in ΔPeriod/ΔTime
    JJ Hermes, UT-Austin, 221st AAS
    •  DA WD evolution
    should be simple,
    dictated by cooling"
    •  Expected rate of

    dP/dt < 10-14 s s-1

    ( < 0.3 μs yr-1 )

    for all radial order (k)
    and spherical degree (l) 

    (Bradley et al. 1992, ApJ 391 L33)"
    •  The high-amplitude
    mode in G117-B15A
    (and R548) behaves
    this way"
    •  However, the WDs
    have some surprises in
    store for us"

    View Slide

  6. WD 0111+0018: A Wrinkle in ΔPeriod/ΔTime
    JJ Hermes, UT-Austin, 221st AAS Hermes et al. 2013, ApJ submitted

    View Slide

  7. WD 0111+0018: A Wrinkle in ΔPeriod/ΔTime
    JJ Hermes, UT-Austin, 221st AAS Hermes et al. 2013, ApJ submitted
    •  The assumption that all modes in all DAVs are extremely stable

    (and that dP/dt < 10-15 s s-1) is not universal!
    •  There is a physical effect operating at a non-cooling timescale in this DAV"
    •  As with G117-B15A, we can remove this secular evolution to search for
    planetary companions by looking for periodicity in the four modes present"
    •  Again, we can exclude Jupiter-mass planets over a wide range of possible
    orbits (at least 3-10 AU)"
    Window"
    Residual Periodogram"

    View Slide

  8. GD 66: Complications to the Planetary Hypothesis
    JJ Hermes, UT-Austin, 221st AAS
    tant (Benvenuto et al. 2004), as well as provide useful
    on the mass of the hypothesized axion or other super-
    particles (Isern et al. 1992; Co
    ´rsicoet al. 2001; Bischoff-
    2007).
    et is in orbit around a star, the star’s distance from the
    ange periodically as it orbits the center of mass of the
    ystem. If the star is a stable pulsator like a hDAV, this
    a periodic change in the observed arrival time of the
    stable pulsations compared to that expected based on
    planet mass, MÃ is the mass of the WD, c is the speed of light,
    and i is the inclination of the orbit to the line of sight. In common
    with astrometric methods, the sensitivity increases with the orbital
    separation, making long-period planets easier to detect given data
    sets with sufficiently long baselines.
    In 2003 we commenced a pilot survey of a small number of
    DAVs in the hope of detecting the signal of a companion planet.
    We present here a progress report of the first 3Y4 yr of observa-
    tions on 12 objects, as well as presenting limits around three more
    objects based partly on archival data stretching as far back as
    1970. For one object we find a signal consistent with a planetary
    Sample FT of GD 66 from a single 6 hr run. The larger amplitude
    eled with their periods. The peaks at 271 and 198 s are composed of
    sely spaced modes separated by approximately 6.4 Hz that are not
    is FT.
    Fig. 2.—The OÀC diagram of the 302 s mode of GD 66. The solid line is a
    sinusoidal fit to the data.
    f2
    Mullally et al. 2008, ApJ 676 573
    •  The 302.77 s mode showed evidence for periodic behavior, and a 2MJ
    planet in a 4.5-year orbit was proposed by Mullally et. al 2008"
    •  Nearly five years later, how is “GD 66b” looking?"

    View Slide

  9. GD 66: Complications to the Planetary Hypothesis
    JJ Hermes, UT-Austin, 221st AAS Hermes et al. 2013, in prep.
    •  As “expected” the (O-C) diagram for f2
    has turned over, and there is clearly
    a periodic modulation to the arrival times of this pulsation mode"
    –  The period has since been refined slightly (defined by the best-fit linear trend)"
    •  This modulation is currently consistent with a 1.1 MJ
    sin i planet at 2.2 AU
    (4.1 yr); there is no amplitude modulation, especially on this timescale"

    View Slide

  10. GD 66: Complications to the Planetary Hypothesis
    JJ Hermes, UT-Austin, 221st AAS Hermes et al. 2013, in prep.
    •  We have been able to construct an (O-C) diagram for the highest-amplitude
    mode at 271.71 s, which is the m=0 component of a detected triplet

    (we simultaneously fit all 3 components, using several nights of data)"
    •  This mode also shows a 4.0-year modulation consistent with a

    1.2 MJ
    sin i planet!"

    View Slide

  11. GD 66: Complications to the Planetary Hypothesis
    JJ Hermes, UT-Austin, 221st AAS Hermes et al. 2013, in prep.
    •  Complication: The best-fit sine curves to f1
    and f2
    are nearly π out of phase"
    •  An external companion would modulate all modes identically!
    •  While discouraging for the planetary hypothesis, this is likely telling us
    something very interesting about the physics of pulsations in this star."

    View Slide

  12. EC 20058-5234: An Analogue to GD 66
    JJ Hermes, UT-Austin, 221st AAS Dalessio et al. 2013, ApJ in press
    – 14 –
    1995 2000 2005 2010 2015
    −250
    −200
    −150
    −100
    −50
    0
    50
    100
    150
    200
    250
    Time (Years)
    O−C (s)
    Fig. 2.— O − C of pulsation frequency D. The dashed lines are located at 0, P, and −P.
    The blue line is the best model fit for Π = 12.9yrs. The red lines indicate the boundaries
    of the one sigma likelihood prediction of the model not including the 1994 data. Note that
    a first order polynomial has been removed from the data.
    – 15 –
    1995 2000 2005 2010 2015
    −300
    −200
    −100
    0
    100
    200
    300
    Time (Years)
    O−C (s)
    Fig. 3.— O − C of pulsation frequency E. The dashed lines are located at 0, P, and −P.
    The blue line is the best model fit for Π = 12.9yrs. The red lines indicate the boundaries
    of the one sigma likelihood prediction of the model not including the 1994 data. Note that
    a first order polynomial has been removed from the data.
    •  GD66 is not the only pulsating
    white dwarf that shows such
    periodic behavior:"
    –  James Dalessio (U. Delaware) has
    observed a similar effect in a DBV
    (He-atmosphere), EC 20058-5234"
    •  Both the 204.6 s (top) and the
    256.9 s (bottom) modes have
    underlying 12.9-year periodic
    changes"
    –  Just as with GD 66, the changes
    are π out of phase with each other!"
    •  Again, this cannot be an external
    effect; it is rather evidence for
    some non-cooling timescale in

    pulsating WDs"
    204.6 s mode"
    256.9 s mode"

    View Slide

  13. Conclusion: The 4-yr Trend in GD 66 Is Not a Planet
    JJ Hermes, UT-Austin, 221st AAS
    Periodic modulation in the pulse arrival times of
    GD 66 had been interpreted as evidence of a
    possible planetary-mass companion."
    "
    Continued observations show that two modes in
    the star are in fact modulated at a 4-year period."
    "
    Unfortunately both modes are nearly exactly out
    of phase with each other. This cannot be space
    motion caused by a planetary companion."
    "
    Pulsating white dwarf stars can still be used to
    search for planets, but they are not as well
    behaved as we wanted them to be!"

    View Slide