JJ Hermes University of Warwick

Mean Earth--Moon

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,

•  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 

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 

•  The Galaxy is not old enough for a single star to evolve into a

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

•  ELM Survey: SDSS u-g, g-r color selection •  Discovery spectroscopy from

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 

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 

•  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

•  Ellipsoidal variations: Changing projected area of a tidally distorted star •  Tidal bulge rotates once per orbit –  We see its oblique (fat) side

J0751-0141: A New Eclipsing WD+WD Binary Kilic  et  al.  2014,   MNRAS,  438,  L26 P orb  =  115.22  min 3.2% rel. amplitude

Yellow: Our eight new tidally distorted ELM WDs Black: Eclipsing WD+WD binaries Pink: Eclipsing

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 

•  Lowest-mass WDs (≤0.18 M¤ ) have

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 

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: 

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 

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,

Pulsating Hydrogen-Atmosphere (DA) WDs •  Global g-mode pulsations driven by a hydrogen partial ionization zone 

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 

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 

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 

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 

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 

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 

Lowest-Gravity WDs All Show Metals Hermes  et  al.  2014,  MNRAS,  444,  1674 

•  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

•  Ca II lines phase with the ~288 km/s RV of the Balmer lines –  Metals are photospheric, not interstellar •  We obtained an HST/COS

•  This ELM WD is carbon deficient, just like planetary debris •  BUT: Oxygen abundance inconsistent with rocky accretion:

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 

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 

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 

J0651+2844: A 12.75-min WD+WD Binary Average distance between the Earth and the Moon: 384,400 km 

(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 

Orbital Decay in J0651+2844 After just 13 months we confirmed orbital decay from gravitational radiation. Hermes  et  al.  2012,  ApJ,  757,  L21 

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 

•  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 

•  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

•  J0651+2844 is an excellent verification sourcefor direct detection of

•  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 

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! 

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Hermes  et  al.  2014,  ApJ,  792,  39 

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

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