Recent Work on the Lowest-Mass White Dwarfs

70d4f7eb14525537a3fd6c15a33a8ac1?s=47 jjhermes
November 19, 2014

Recent Work on the Lowest-Mass White Dwarfs

Colloquium, 45 min. November 2014: Keele University, Keele, UK.

70d4f7eb14525537a3fd6c15a33a8ac1?s=128

jjhermes

November 19, 2014
Tweet

Transcript

  1. JJ Hermes University of Warwick

  2. Mean Earth--Moon

  3. Motivation and Outline •  Extremely Low-Mass (ELM, <0.3 M¤ )

    White Dwarfs (WDs) –  Set Galactic gravitational wave foreground –  eLISA verification binaries –  Progenitors of Galactic exotica: merged WDs and subdwarfs, AM CVn systems,
  4. •  WDs: Burnt-out cores of all low-mass stars initially <8-10

    M¤ •  WDs are the endpoints of stellar evolution –  Their progenitors lost considerable mass White Dwarfs, the Quantum Dots
  5. Kleinman  et  al.  2013,  ApJS,  204,  5 •  All WDs

    discussed today have pure hydrogen atmospheres (DA) –  4/5 of WDs are DA; strong gravitational settling •  Estimate WD masses from observed Balmer line profiles: Teff /log(g) He-Core WDs CO-Core WDs ONe-Core WDs Mass Distribution of Known White Dwarfs
  6. •  The Galaxy is not old enough for a single

    star to evolve into a
  7. Latest  ELM  Survey  release: Brown  et  al.  2013,  ApJ,  769,

     66 •  ELM Survey: u-g, g-r color selection from Sloan Digital Sky Survey (SDSS) •  Discovery spectroscopy from
  8. •  ELM Survey: SDSS u-g, g-r color selection •  Discovery

    spectroscopy from
  9. Discovery spectroscopy determines the binary and atmospheric parameters blue-shifted red-shifted

    Brown  et  al.  2012,  ApJ,  744,  142 T eff  =  10,540  ±  170  K log(g)  =  6.01  ±  0.06 P orb  =  87.996  ±  0.006  min K 1  =  508  ±  4  km  s-­‐‑1 M 2  >  1.10  M¤      if  M 1  =  0.17  M¤ t merge  <  170  Myr J1741+6526: An 88-min WD+WD binary
  10. Asin(φ) = 0.50 ± 0.08 % Doppler beaming Acos(2φ) =

    1.30 ± 0.08 % Ellipsoidal variations J1741+6526: An 88-min WD+WD binary •  Follow-up photometry yields further physical constraints
  11. •  Doppler beaming: Radiation is beamed toward our line of

    sight, proportional to how fast the source is moving (V): •  (Also a small factor for the
  12. •  Ellipsoidal variations: Changing projected area of a tidally distorted

    star •  Tidal bulge rotates once per orbit –  We see its oblique (fat) side
  13. J0751-0141: A New Eclipsing WD+WD Binary Kilic  et  al.  2014,

      MNRAS,  438,  L26 P orb  =  115.22  min 3.2% rel. amplitude
  14. Yellow: Our eight new tidally distorted ELM WDs Black: Eclipsing

    WD+WD binaries Pink: Eclipsing
  15. Some Open Questions Regarding ELM WDs – CNO flashing episodes and

    HR-diagram loops – Hydrogen-layer masses in He-core WDs – The ubiquity of metals in the lowest-gravity WDs – Tidal torques on binary inspiral
  16. •  Lowest-mass WDs (≤0.18 M¤ ) have

  17. ELM WDs and Predicted CNO Flashes Althaus  et  al.  2013,

     A&A,  557,  A19 •  Two low-mass WDs of different masses often cross the same points in a T eff —log(g)  diagram •  There is a non-uniqueness to using T eff ,log(g) for ELM WD mass
  18. ELM WDs and Predicted CNO Flashes Althaus  et  al.  2013,

     A&A,  557,  A19 •  For example, take a 10,000 K, log(g) = 6.60 WD:
  19. Some Open Questions Regarding ELM WDs – CNO flashing episodes and

    HR-diagram loops – Hydrogen-layer masses in He-core WDs – The ubiquity of metals in the lowest-gravity WDs – Tidal torques on binary inspiral
  20. Discovery of Pulsations in Low-Mass WDs Hermes  et  al.  2012,

     ApJ,  750,  L28 Hermes  et  al.  2013,  ApJ,  765,  102 Hermes  et  al.  2013,  MNRAS,  436,  3573 Kilic  et  al.  2015,  MNRAS,  446,  26 •  CNO flashes erode the hydrogen layer mass of ELM WDs •  An observational test would come from pulsating WDs: asteroseismology •  Since October 2011 we discovered the first five pulsating low-mass,
  21. Pulsating Hydrogen-Atmosphere (DA) WDs •  Global g-mode pulsations driven by

    a hydrogen partial ionization zone
  22. G117-B15A: A 0.59 M¤ Pulsating CO-Core WD •  Stable pulsating

    WD •  dP/dt ~ 4 x 10-15 s s-1 •  Main pulsation modes: –  P1 = 215.2 s –  P2 = 304.1 s –  P3 = 270.5 s •  Can multiply the star’s frequencies by 300,000 to convert to audible range: Kepler  et  al.  2005,  ApJ,  634,  1311 target comparison
  23. J1614+1912: A 0.20 M¤ Pulsating ELM WD •  The pulsating

    ELM WD with the shortest-period variability •  Main pulsation modes: –  P1 = 1262.7 s –  P2 = 1184.1 s •  Scaling frequencies by 300,000 to an audible range: Hermes  et  al.  2013,  MNRAS,  436,  3573 target comparison
  24. J2228+3623: A 0.16 M¤ Pulsating ELM WD •  The pulsating

    ELM WD with the longest-period variability •  Main pulsation modes: –  P1 = 4181 s –  P2 = 3252 s –  P3 = 6229 s •  Scaling frequencies by 300,000 to an audible range: target comparison Hermes  et  al.  2013,  MNRAS,  436,  3573
  25. J1112+1117: A 0.17 M¤ Pulsating ELM WD Hermes  et  al.

     2013,  ApJ,  765,  102 •  Main pulsation modes: –  P1 = 2258.5 s –  P2 = 2539.7 s –  P3 = 1884.6 s –  P4 = 2855.7 s –  P5 = 1792.9 s •  Scaling frequencies by 300,000 to an audible range: target comparison
  26. Full Seismology Will Reveal ELM WD Structure •  We are

    currently only able to qualitatively match the periods to WD models •  Near to having a large grid of He-core WD models with different hydrogen layer masses to perform asteroseismology Van  Grootel  et  al.  2013,  ApJ,  762,  57 MH /M* = 10-4 MH /M* = 10-2 J1840 J1518 J1518 J1112 Theoretical periods for ell=1 g-modes modes vs. Observed Periods
  27. Some Open Questions Regarding ELM WDs – CNO flashing episodes and

    HR-diagram loops – Hydrogen-layer masses in He-core WDs – The ubiquity of metals in the lowest-gravity WDs – Tidal torques on binary inspiral
  28. Lowest-Gravity WDs All Show Metals Hermes  et  al.  2014,  MNRAS,

     444,  1674
  29. •  Roughly 1 in every 2-3 WDs we find has

    some metal pollution •  Metals should settle out of the high-surface-gravity atmosphere very quickly (of order days) •  Consensus: Metals are from accreted, tidally disrupted debris •  Abundances match bulk
  30. •  Ca II lines phase with the ~288 km/s RV

    of the Balmer lines –  Metals are photospheric, not interstellar •  We obtained an HST/COS
  31. •  This ELM WD is carbon deficient, just like planetary

    debris •  BUT: Oxygen abundance inconsistent with rocky accretion:
  32. GALEX J1717: A 5.9-hr, Metal-Rich He-WD+WD R 1  =  0.093

     ±  0.013  R¤ i  =  86.9  ±  0.4  deg P orb  =  5.90724895(41)  hr -­‐‑20 v rot  =  50+30  km  s-­‐‑1 P rot  =  2.3+2.0  hr Hermes  et  al.  2014,  MNRAS,  444,  1674 •  Prot < Porb but not yet formally significant •  Direct test of tidal synchronization! -­‐‑1.0
  33. Some Open Questions Regarding ELM WDs – CNO flashing episodes and

    HR-diagram loops – Hydrogen-layer masses in He-core WDs – The ubiquity of metals in the lowest-gravity WDs – Tidal torques on binary inspiral
  34. phase = 0 •  We detected eclipses in April 2011

    •  This is the most compact detached binary system currently known! J0651+2844: A 12.75-min WD+WD Binary Brown  et  al.  2011,  ApJ,  737,  L23
  35. J0651+2844: A 12.75-min WD+WD Binary Average distance between the Earth

    and the Moon: 384,400 km
  36. (from Phase 0 to Phase 1 is 12.75 minutes) Hermes

     et  al.  2012,  ApJ,  757,  L21 P orb  =  765.20644(95)  s K 1  =  616.9  ±  5.0  km  s-­‐‑1 i  =  86.3  ±  1.0  deg T eff,1  =  16,340  ±  260  K M 1  =  0.252  ±  0.04  M¤ T eff,2  =  10,370  ±  360  K M 2  =  0.50  ±  0.04  M¤ J0651+2844: A 12.75-min WD+WD Binary
  37. Orbital Decay in J0651+2844 After just 13 months we confirmed

    orbital decay from gravitational radiation. Hermes  et  al.  2012,  ApJ,  757,  L21
  38. We expect dPorb /dt = (-0.26 ± 0.05) ms/yr and

    observe (-0.2834 ± 0.0039) ms/yr! – a 1.4% measurement! Orbital Decay in J0651+2844
  39. •  This 12.75-min WD+WD binary is decaying > 3.5 times

    faster than the 7.75-hr Hulse-Taylor binary pulsar, which was the first indirect detection of gravitational radiation (1993 Nobel prize in physics) Weisberg  et  al.  2010,  ApJ,  722,  1030 J0651+2844 PSR B1913+16 dP/dt = -0.283 ms/yr dP/dt = -0.076 ms/yr Orbital Decay in J0651+2844
  40. •  Gravity bends space; it effectively determines geometry of space

    •  General Relativity: Any mass in nonuniform, nonspherical motion emits gravitational radiation •  Ripples in space-time caused by gravitational radiation carry away energy •  This is an energy leak and acts
  41. •  J0651+2844 is an excellent verification sourcefor direct detection of

  42. •  Tidal torques should increase the rate of orbital decay

    in J0651+2844 –  Additional angular momentum is lost from the orbit to spin-up the WDs to remain synchronized, leading to >5% faster rate of orbital decay (e.g.,  Piro  2011,  ApJ,  740,  L53;  Fuller  &   Lai  2012,  MNRAS,  421,  426) The Fate of the WDs in J0651+2844
  43. Conclusions •  Extremely Low-Mass (ELM, <0.3 M¤ ) White Dwarfs

    (WDs) constrain the endpoints of stellar and binary evolution •  “Low-Mass White Dwarfs Need Friends” –  Close companions provide many ways to observationally constrain systems •  Pulsations allow us a new way to explore He-Core, ELM WD Interiors •  ELM WDs provide a unique test for tidal effects on binary inspiral •  The first directly detected gravitational waves and confirmed EM counterpart systems will likely be ELM WDs D. Berry, GSFC!
  44. None
  45. Hermes  et  al.  2014,  ApJ,  792,  39

  46. Corsico  &  Althaus.  2014,  A&A,  569,  106

  47. None