2016ApJ...826...34Z

Whether the progenitors of SNe Ia are single-degenerate or double-degenerate white dwarf (WD) systems is a highly debated topic. To address the origin of Tycho's Type Ia supernova remnant (SNR), SN 1572, we have carried out a ¹²CO J=2-1 mapping and a 3 mm line survey toward the remnant using the IRAM 30 m telescope. We show that Tycho is surrounded by a clumpy molecular bubble at a local standard of rest velocity of ∼61 km {s}^{-1} , which expands at a speed of ∼4.5 km {s}^{-1} and has a mass of ∼220 {M}_☉ (at the distance of 2.5 kpc). Enhanced ¹²CO J=2-1 line emission relative to ¹²CO J=1-0 emission and possible line broadenings (in velocity range -64 to -60 km/s) are found at the northeastern boundary of the SNR, where the shell is deformed and decelerated. These features, combined with the morphological correspondence between the expanding molecular bubble and Tycho, suggest that the SNR is associated with the bubble at the velocity range -66 to -57 km/s. The most plausible origin for the expanding bubble is the fast outflow (with velocity of hundreds km/s) driven from the vicinity of a WD as it accreted matter from a nondegenerate companion star. The SNR has been expanding in the low-density wind-blown bubble, and the shock wave has just reached the molecular cavity wall. This is the first unambiguous detection of an expanding bubble driven by the progenitor of a Type Ia SNR, which constitutes evidence for a single-degenerate progenitor for this SN Ia.