DISCOVERY OF THE ECLIPSING DETACHED DOUBLE WHITE DWARF BINARY NLTT 11748

We report the discovery of the first eclipsing detached double white dwarf (WD) binary. In a pulsation search, the low-mass helium core WD NLTT 11748 was targeted for fast (1 minute) differential photometry with the Las Cumbres Observatory's Faulkes Telescope North. Rather than pulsations, we discovered 180?s 3%-6% dips in the photometry. Subsequent radial velocity measurements of the primary white dwarf from the Keck telescope found variations with a semi-amplitude K 1 = 271 ? 3 km s?1 and confirmed the dips as eclipses caused by an orbiting WD with a mass M 2 = 0.648-0.771 M ? for M 1 = 0.1-0.2 M ?. We detect both the primary and secondary eclipses during the P orb = 5.64?hr orbit and measure the secondary's brightness to be 3.5% ? 0.3% of the primary at SDSS-g'. Assuming that the secondary follows the mass-radius relation of a cold C/O WD and including the effects of microlensing in the binary, the primary eclipse yields a primary radius of R 1 = 0.043-0.039 R ? for M 1 = 0.1-0.2 M ?, consistent with the theoretically expected values for a helium core WD with a thick, stably burning hydrogen envelope. Though nearby (at 150?pc), the gravitational wave strain from NLTT 11748 is likely not adequate for direct detection by the Laser Interferometer Space Antenna. Future observational efforts will determine M 1, yielding accurate WD mass-radius measurement of both components, as well as a clearer indication of the binary's fate once contact is reached.

[1]  Harry L. Shipman,et al.  Procyon B: Outside the Iron Box , 2002 .

[2]  Simon F. Portegies Zwart,et al.  Population synthesis for double white dwarfs. I. Close detached systems. , 2001 .

[3]  S. Vennes,et al.  The double degenerate system NLTT 11748 , 2010, 1005.3938.

[4]  The ultra-cool white dwarf companion of PSR J0751+1807 , 2006, astro-ph/0601205.

[5]  The Lowest Mass White Dwarf , 2006, astro-ph/0611498.

[6]  A. V. Tutukov,et al.  Supernovae of type I as end products of the evolution of binaries with components of moderate initial mass (M< or approx. =9 M/sub sun/) , 1984 .

[7]  E. Ofek,et al.  Pulsations of the Low-Mass ZZ Ceti Star HS 1824+6000 , 2008, 0808.3957.

[8]  M. Bailes,et al.  A test of general relativity from the three-dimensional orbital geometry of a binary pulsar , 2001, Nature.

[9]  B. Gaensicke,et al.  ULTRACAM observations of SDSS J170213.26+322954.1 - an eclipsing cataclysmic variable in the period gap , 2006, astro-ph/0607056.

[10]  Lars Bildsten,et al.  A search for rapid photometric variability in symbiotic binaries , 2001 .

[11]  D. Kaplan.  MASS CONSTRAINTS FROM ECLIPSE TIMING IN DOUBLE WHITE DWARF BINARIES , 2010, 1006.3772.

[12]  E. Agol.  Occultation and Microlensing , 2002, astro-ph/0207228.

[13]  Modeling complete distributions with incomplete observations : The velocity ellipsoid from Hipparcos data , 2005, astro-ph/0505057.

[14]  W. Van Hamme,et al.  New limb-darkening coefficients for modeling binary star light curves , 1993 .

[15]  M. V. van Kerkwijk,et al.  THE (DOUBLE) WHITE DWARF BINARY SDSS 1257+5428 , 2010, 1003.2169.

[16]  L. Althaus,et al.  Full evolution of low-mass white dwarfs with helium and oxygen cores , 2007 .

[17]  R. Lupton,et al.  TWINS: THE TWO SHORTEST PERIOD NON-INTERACTING DOUBLE DEGENERATE WHITE DWARF STARS , 2009, 0911.1798.

[18]  S. Anderson,et al.  PSR J1909–3744: A Binary Millisecond Pulsar with a Very Small Duty Cycle , 2003, astro-ph/0311415.

[19]  M. Couture,et al.  HIRES: the high-resolution echelle spectrometer on the Keck 10-m Telescope , 1994, Astronomical Telescopes and Instrumentation.

[20]  P. Garnavich,et al.  The mass of the neutron star in the binary millisecond pulsar PSR J1012 + 5307 , 1998 .

[21]  Characterizing the galactic gravitational wave background with LISA , 2005, gr-qc/0504071.

[22]  C. Copperwheat,et al.  Precise mass and radius values for the white dwarf and low mass M dwarf in the pre‐cataclysmic binary NN Serpentis , 2009, 0909.4307.

[23]  V. Kaspi,et al.  High - precision timing of millisecond pulsars. 3: Long - term monitoring of PSRs B1855+09 and B1937+21 , 1994 .

[24]  E. Agol,et al.  Analytic Light Curves for Planetary Transit Searches , 2002, astro-ph/0210099.

[25]  G. Nelemans,et al.  Hubble Space Telescope Parallaxes of AM CVn Stars and Astrophysical Consequences , 2007, 0705.3855.

[26]  S. Vennes,et al.  A new extremely low-mass white dwarf in the NLTT catalogue , 2009, 0909.3249.

[27]  Steve B. Howell,et al.  Discovery of the Partially Eclipsing White Dwarf Binary SDSS J143547.87+373338.5 , 2008, 0803.0751.

[28]  David M. Russell,et al.  Robotic Astronomy with the Faulkes Telescopes and Las Cumbres Observatory Global Telescope , 2010 .

[29]  R. Webbink.  Double white dwarfs as progenitors of R Coronae Borealis stars and type I supernovae , 1984 .

[30]  LP 400-22, A VERY LOW-MASS AND HIGH-VELOCITY WHITE DWARF , 2006, astro-ph/0604470.

[31]  D. Steeghs,et al.  DETECTION OF A WHITE DWARF COMPANION TO THE WHITE DWARF SDSSJ125733.63+542850.5 , 2010, 1002.4677.

[32]  M. Bailes,et al.  The Mass of a Millisecond Pulsar , 2005 .

[33]  G. Nelemans.  The Galactic gravitational wave foreground , 2009, 0901.1778.

[34]  R. Lupton,et al.  FIRST RESULTS FROM THE SWARMS SURVEY. SDSS 1257+5428: A NEARBY, MASSIVE WHITE DWARF BINARY WITH A LIKELY NEUTRON STAR OR BLACK HOLE COMPANION , 2009, 0910.2709.

[35]  A. Serenelli,et al.  Evolution and colours of helium-core white dwarf stars: the case of low-metallicity progenitors , 2002, astro-ph/0208408.

[36]  D. Koester,et al.  The masses of PSR J1911-5958A and its white dwarf companion , 2006, astro-ph/0603267.

[37]  Warren R. Brown,et al.  ACCURATE MASSES FOR THE PRIMARY AND SECONDARY IN THE ECLIPSING WHITE DWARF BINARY NLTT 11748 , 2010, 1008.1069.

[38]  B. Gaensicke,et al.  SDSS J150722.30+523039.8: a cataclysmic variable formed directly from a detached white dwarf/brown dwarf binary? , 2007, 0708.0097.

[39]  Harry L. Shipman,et al.  Testing the White Dwarf Mass-Radius Relation with Hipparcos , 1997 .

[40]  Warren R. Brown,et al.  THE DISCOVERY OF BINARY WHITE DWARFS THAT WILL MERGE WITHIN 500 Myr , 2009, 0911.1781.

[41]  Pierre Bergeron,et al.  Calibration of Synthetic Photometry Using DA White Dwarfs , 2005 .

[42]  South Africa,et al.  The mass and radius of the M-dwarf in the short-period eclipsing binary RR Caeli , 2007, astro-ph/0702005.

[43]  G. Nelemans,et al.  Mass transfer between double white dwarfs , 2003, astro-ph/0312577.

[44]  R. F. Jameson,et al.  High-resolution optical spectroscopy of Praesepe white dwarfs , 2009, 0901.4464.

[45]  COOLING SEQUENCES AND COLOR-MAGNITUDE DIAGRAMS FOR COOL WHITE DWARFS WITH HYDROGEN ATMOSPHERES , 2000, astro-ph/0006363.