The Double-Degenerate Nucleus of the Planetary Nebula TS 01. A Close
  Binary Evolution Showcase
  We present a detailed investigation of SBS1150+599A, a close binary star
hosted by the planetary nebula PN G135.9+55.9 (TS01, Stasinska et al, 2009).
The nebula, located in the Galactic halo, is the most oxygen-poor one known to
date and is the only one known to harbor a double degenerate core. We present
XMM-Newton observations of this object, which allowed the detection of the
previously invisible component of the binary core, whose existence was inferred
so far only from radial velocity and photometric variations. The parameters of
the binary system were deduced from a wealth of information via three
independent routes using the spectral energy distribution (from the infrared to
X-rays), the light and radial velocity curves, and a detailed model atmosphere
fitting of the stellar absorption features of the optical/UV component. We find
that the cool component must have a mass of 0.54+/-0.2 Msun, an average
effective temperature, Teff, of 58000+/-3000 K, a mean radius of 0.43+/-0.3
Rsun, a gravity log g=5.0+/-0.3, and that it nearly fills its Roche lobe. Its
surface elemental abundances are found to be: 12 + log He/H = 10.95+/-0.04 dex,
12 + log C/H = 7.20+/-0.3 dex, 12 + log N/H < 6.92 and 12 + log O/H < 6.80, in
overall agreement with the chemical composition of the planetary nebula. The
hot component has Teff = 160-180 kK, a luminosity of about ~10e4 Lsun and a
radius slightly larger than that of a white dwarf. It is probably bloated and
heated as a result of intense accretion and nuclear burning on its surface in
the past. The total mass of the binary system is very close to Chandrasekhar
limit. This makes TS01 one of the best type Ia supernova progenitor candidates.
We propose two possible scenarios for the evolution of the system up to its
present stage.
