Expanding molecular bubble surrounding Tycho's supernova remnant (SN
  1572) observed with IRAM 30 m telescope: evidence for a single-degenerate
  progenitor
  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
the Type Ia Tycho's supernova remnant (SNR), SN 1572, we have carried out a
$^{12}$CO 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 $\sim 61$ km s$^{-1}$ which
expands at a speed of $\sim 4.5$ km s$^{-1}$ and has a mass of $\sim 220$
M$_\odot$ (at the distance of 2.5 kpc). Enhanced $^{12}$CO J=2-1 line emission
relative to $^{12}$CO J=1-0 emission and possible line broadenings (in velocity
range $-64$ to $-60$ km s$^{-1}$) 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$^{-1}$. The most plausible origin for the expanding bubble
is the fast outflow (with velocity of hundreds km s$^{-1}$) 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. 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.
