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.′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.