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

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


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


Optical mass function

Invisible companion’s

mass

HD 49798 massS. 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


The companion is a White Dwarf

JD - 2440000

Per

iod

(s)

XMM-Newton results: timing


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


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


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


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


The companion of HD49798 is avery massive white dwarf

Mwd = 1.28 +/- 0.05 Msun

Past evolution


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

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 ?


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


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