The Per-Tau Shell: A Giant Star-Forming Spherical Shell Revealed by 3D
  Dust Observations
  A major question in the field of star formation is how molecular clouds form
out of the diffuse Interstellar Medium (ISM). Recent advances in 3D dust
mapping are revolutionizing our view of the structure of the ISM. Using the
highest-resolution 3D dust map to date, we explore the structure of a nearby
star-forming region, which includes the well-known Perseus and Taurus molecular
clouds. We reveal an extended near-spherical shell, 156 pc in diameter,
hereafter the "Per-Tau Shell", in which the Perseus and Taurus clouds are
embedded. We also find a large ring structure at the location of Taurus,
hereafter, the "Tau Ring". We discuss a formation scenario for the Per-Tau
Shell, in which previous stellar and supernova (SN) feedback events formed a
large expanding shell, where the swept-up ISM has condensed to form both the
shell and the Perseus and Taurus molecular clouds within it. We present
auxiliary observations of HI, H$\alpha$, $^{26}$Al, and X-rays that further
support this scenario, and estimate Per-Tau Shell's age to be $\approx 6-22$
Myrs. The Per-Tau shell offers the first three-dimensional observational view
of a phenomenon long-hypothesized theoretically, molecular cloud formation and
star formation triggered by previous stellar and SN feedback.
