The Transitional Stripped-Envelope SN 2008ax: Spectral Evolution and
  Evidence for Large Asphericity
  Supernova (SN) 2008ax in NGC 4490 was discovered within hours after shock
breakout, presenting the rare opportunity to study a core-collapse SN beginning
with the initial envelope-cooling phase immediately following shock breakout.
We present an extensive sequence of optical and near-infrared spectra, as well
as three epochs of optical spectropolarimetry. Our initial spectra, taken two
days after shock breakout, are dominated by hydrogen Balmer lines at high
velocity. However, by maximum light, He I lines dominated the optical and
near-infrared spectra, which closely resembled those of normal Type Ib
supernovae (SNe Ib) such as SN 1999ex. This spectroscopic transition defines
Type IIb supernovae, but the strong similarity of SN 2008ax to normal SNe Ib
beginning near maximum light, including an absorption feature near 6270A due to
H-alpha at high velocities, suggests that many objects classified as SNe Ib in
the literature may have ejected similar amounts of hydrogen as SN 2008ax,
roughly a few x 0.01 M_sun. Early-time spectropolarimetry (6 and 9 days after
shock breakout) revealed strong line polarization modulations of 3.4% across
H-alpha, indicating the presence of large asphericities in the outer ejecta.
The continuum shares a common polarization angle with the hydrogen, helium, and
oxygen lines, while the calcium and iron absorptions are oriented at different
angles. This is clear evidence of deviations from axisymmetry even in the outer
ejecta. Intrinsic continuum polarization of 0.64% only nine days after shock
breakout shows that the outer layers of the ejecta were quite aspherical. A
single epoch of late-time spectropolarimetry, as well as the shapes of the
nebular line profiles, demonstrate that asphericities extended from the
outermost layers all the way down to the center of this SN. [Abridged]
