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An ultra-massive, fast-spinning white dwarf in a peculiar binary system

An ultra-massive, fast-spinning white dwarf in a peculiar binary system. S. Mereghetti (1) , A.Tiengo (1) , P. Esposito (1) , N. La Palombara (1) , G.L.Israel (2) , L.Stella (2). 1) INAF – IASF MILANO 2) INAF Oss. Astron. ROMA. HD 49798 : a single-lined spectroscopic binary.

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An ultra-massive, fast-spinning white dwarf in a peculiar binary system

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  1. An ultra-massive, fast-spinning white dwarf in a peculiar binary system S. Mereghetti(1), A.Tiengo(1), P. Esposito (1), N. La Palombara (1), G.L.Israel (2), L.Stella (2) 1) INAF – IASF MILANO 2) INAF Oss. Astron. ROMA

  2. HD 49798 : a single-lined spectroscopic binary + ? HD 49798 Companion invisible in optical / UV data Hot subdwarf (sdO type ) B = 8 mag S. MEREGHETTI - Tuebingen - Aug 18, 2010

  3. The brightest sdO Heber 2009, ARAA HD 49798 T = 47,500 K L ~ 104 L sun Log g = 4.25 S. MEREGHETTI - Tuebingen - Aug 18, 2010

  4. HD 49798 • the brightest sdO  well studied single-lined spectroscopic binary • Hydrogen poor, Helium rich • Porb = 1.5476632(32) days • circular orbit • Optical mass function 0.263 +/- 0.004 Msun Thackeray 1970 Kudritzky et al. 1978 Hamann et al. 1981 Bruhweiler et al. 1981 Stickland & Lloyd 1984 Bisscheroux et al. 1997 Hamann 2010 Przybilla et al 2010 S. MEREGHETTI - Tuebingen - Aug 18, 2010

  5. Optical mass function Invisible companion’s mass HD 49798 mass S. MEREGHETTI - Tuebingen - Aug 18, 2010

  6. Discovery of X-ray periodicity The companion is either a Neutron Star or a White Dwarf ROSAT  Soft X-ray source P = 13.2 s (Israel et al. 1997, ApJ) S. MEREGHETTI - Tuebingen - Aug 18, 2010

  7. XMM-Newton observation • 44 ks observation on May 10, 2008 • Soft BB + hard component spectrum • Lx = 2 1031 erg/s @ 650 pc (0.2-10 keV) kT=39 eV The companion is a White Dwarf G ~ 2 NH=1.7 1019 cm-2 S. MEREGHETTI - Tuebingen - Aug 18, 2010

  8. XMM-Newton results: timing 0.1-0.5 keV P = 13.18425(4) s Pulsed fraction 62% dP/dt < 4 10-13 s/s Period (s) 0.5-10 keV JD - 2440000 S. MEREGHETTI - Tuebingen - Aug 18, 2010

  9. Time delays in X-ray pulses • X-ray projected semi major-axis : Ax sini = 9.78 +/- 0.06 light-sec + opt. mass funct.  q = MHD/Mx = 1.17 +/- 0.01 S. MEREGHETTI - Tuebingen - Aug 18, 2010

  10. Optical + X-ray mass functions Mx MHD/Mx= 1.17 M HD S. MEREGHETTI - Tuebingen - Aug 18, 2010

  11. Discovery of X-ray eclipse • Eclipse duration = 1.3 hours • Radius of HD 49798 = 1.45+/-0.25 Rsun •  inclination 79—84 degrees S. MEREGHETTI - Tuebingen - Aug 18, 2010

  12. Opt. and X-ray m.f. + inclination Mx = 1.28 Msun MHD = 1.5 Msun S. MEREGHETTI - Tuebingen - Aug 18, 2010

  13. MHD = 1.50 +/- 0.05 MsunMwd = 1.28 +/- 0.05 MsunThe companion of HD49798 is avery massive white dwarf S. MEREGHETTI - Tuebingen - Aug 18, 2010

  14. The companion of HD49798 is avery massive white dwarfMwd = 1.28 +/- 0.05 Msun • Dynamical measurement  Robust, ‘’assumptions-free” result •  not dependent on Mass-Radius relation • Most WD masses are derived indirectly: • spectroscopic and photometric methods  L, T, log g  M/R2 • gravitational redshift  M/R • and rely on M-R relation to finally get M S. MEREGHETTI - Tuebingen - Aug 18, 2010

  15. Past evolution S. MEREGHETTI - Tuebingen - Aug 18, 2010

  16. Futureevolution The WD is currently accreting in the wind of HD49798 When HD49798 evolves it will fill its Roche-lobe again Stable mass transfer of a few tenths of Msun onto the WD The WD will reach the Chandra limit S. MEREGHETTI - Tuebingen - Aug 18, 2010

  17. The fastest spinning white dwarf • P=13.2 s (only ~5 times slower than break-up) • Fast spin gives a limit on RWD < 7000 km • Accretion implies low magnetic field (to avoid the propeller effect)  B < few kG • Origin of fast rotation: • spun-up by accretion ?? when ? why not to an even shorter P ? • rapidly rotating at birth ? S. MEREGHETTI - Tuebingen - Aug 18, 2010

  18. Conclusions • Robust, dynamical determination of M > 1.2 Msun for a white dwarf • Post Common Envelope binary with well determined masses  optimal test-bench for evolutionary models • The fastest spinning WD (P=13.2 s)  low B to avoid propeller ( < few kG) • Possible future evolution: - SN Iawith short delay time or - non-recycled millisecond pulsar S. MEREGHETTI - Tuebingen - Aug 18, 2010

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