Infrared Spectropolarimetric Detection of Intrinsic Polarization from a
  Core-Collapse Supernova
  Massive stars die an explosive death as a core-collapse supernova (CCSN). The
exact physical processes that cause the collapsing star to rebound into an
explosion are not well-understood, and the key in resolving this issue may lie
in the measurement of the shape of CCSNe ejecta. Spectropolarimetry is the only
way to perform this measurement for CCSNe outside of the Milky Way and
Magellanic Clouds. We present an infrared (IR) spectropolarimetric detection of
a CCSN, enabled by the new highly sensitive WIRC+Pol instrument at Palomar
Observatory, that can observe CCSNe (M = -17 mags) out to 20 Mpc to ~0.1%
polarimetric precision. IR spectropolarimetry is less affected than optical by
dust scattering in the circumstellar and interstellar media, thereby providing
a more unbiased probe of the intrinsic geometry of the SN ejecta. SN 2018hna, a
SN 1987A-like explosion, shows 2.0+-0.3% continuum polarization in the J band
oriented at ~160 degree on-sky at 182 d after the explosion. Assuming prolate
geometry like in SN 1987A, we infer an ejecta axis ratio of <0.48 with the axis
of symmetry pointing at 70 degree position angle. The axis ratio is similar to
that of SN 1987A suggesting that they may share intrinsic geometry and
inclination angle. Our data do not rule out oblate ejecta. We also observe one
other core-collapse and two thermonuclear SNe in the J band. SN 2020oi, a
stripped-envelope Type Ic SN in Messier 100 has p = 0.37+-0.09% at peak light,
indicative of either a 10% asymmetry or host interstellar polarization. The SNe
Ia, 2019ein and 2020ue have p < 0.33% and < 1.08% near peak light, indicative
of asymmetries of less than 10% and 20%, respectively.
