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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. - PowerPoint PPT Presentation
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An ultra-massive,fast-spinning white dwarf in a peculiar binary systemS. Mereghetti(1), A.Tiengo(1), P. Esposito (1), N. La Palombara (1),G.L.Israel (2), L.Stella (2)
1) INAF – IASF MILANO2) INAF Oss. Astron. ROMA
HD 49798 : a single-lined spectroscopic binary
Hot subdwarf (sdO type ) B = 8 mag
HD 49798
S. MEREGHETTI - Tuebingen - Aug 18, 2010
+ ?Companion invisible in optical / UV data
The brightest sdOHeber 2009, ARAA
HD 49798
T = 47,500 K
L ~ 104 L sun
Log g = 4.25
S. MEREGHETTI - Tuebingen - Aug 18, 2010
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. 1978Hamann et al. 1981 Bruhweiler et al. 1981Stickland & Lloyd 1984 Bisscheroux et al. 1997Hamann 2010 Przybilla et al 2010
S. MEREGHETTI - Tuebingen - Aug 18, 2010
Optical mass function
Invisible companion’s
mass
HD 49798 massS. MEREGHETTI - Tuebingen - Aug 18, 2010
S. MEREGHETTI - Tuebingen - Aug 18, 2010
Discovery of X-ray periodicity
ROSAT Soft X-ray sourceP = 13.2 s(Israel et al. 1997, ApJ)
The companion is either a Neutron Star or a White Dwarf
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
~ 2
NH=1.7 1019 cm-2
S. MEREGHETTI - Tuebingen - Aug 18, 2010
The companion is a White Dwarf
JD - 2440000
Per
iod
(s)
XMM-Newton results: timing
S. MEREGHETTI - Tuebingen - Aug 18, 2010
dP/dt < 4 10-13 s/s
P = 13.18425(4) s0.1-0.5 keV
0.5-10 keV
Pulsed fraction 62%
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
Optical + X-ray mass functions
MHD/Mx = 1.17
Mx
M HDS. MEREGHETTI - Tuebingen - Aug 18, 2010
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
Opt. and X-ray m.f. + inclination
Mx = 1.28 Msun
MHD = 1.5 MsunS. MEREGHETTI - Tuebingen - Aug 18, 2010
MHD = 1.50 +/- 0.05 Msun
Mwd = 1.28 +/- 0.05 Msun
The companion of HD49798 is a
very massive white dwarf
S. MEREGHETTI - Tuebingen - Aug 18, 2010
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/Rand rely on M-R relation to finally get M
S. MEREGHETTI - Tuebingen - Aug 18, 2010
The companion of HD49798 is avery massive white dwarf
Mwd = 1.28 +/- 0.05 Msun
Past evolution
S. MEREGHETTI - Tuebingen - Aug 18, 2010
Futureevolution
S. MEREGHETTI - Tuebingen - Aug 18, 2010
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
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
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 Ia with short delay time or - non-recycled millisecond pulsar
S. MEREGHETTI - Tuebingen - Aug 18, 2010