An Expanding Shell of Neutral Hydrogen Associated with SN 1006: Hints for the Single-degenerate Origin and Faint Hadronic Gamma-Rays

We report new H I observations of the Type Ia supernova remnant (SNR) SN 1006 using the Australia Telescope Compact Array with an angular resolution of $4\buildrel{\,\prime}\over{.} 5\times 1\buildrel{\,\prime}\over{.} 4$ 4.'5×1.'4 (~2 pc at the assumed SNR distance of 2.2 kpc). We find an expanding gas motion in position-velocity diagrams of H I with an expansion velocity of ~4 km s^-1 and a mass of ~1000 M _⊙. The spatial extent of the expanding shell is roughly the same as that of SN 1006. We here propose a hypothesis that SN 1006 exploded inside the wind-blown bubble formed by accretion winds from the progenitor system consisting of a white dwarf and a companion star, and then the forward shock has already reached the wind wall. This scenario is consistent with the single-degenerate model. We also derived the total energy of cosmic-ray protons W _p to be only ~1.2-2.0 × 10⁴⁷ erg by adopting the averaged interstellar proton density of ~25 cm^-3. The small value is compatible with the relation between the age and W _p of other gamma-ray SNRs with ages below ~6 kyr. The W _p value in SN 1006 will possibly increase up to several 10⁴⁹ erg in the next ~5 kyr via the cosmic-ray diffusion into the H I wind shell.