Is It Round? Spectropolarimetry of the Type II‐P Supernova 1999em

We present the first multi-epoch spectropolarimetry of a type II plateau supernova (SN II-P), with optical observations of SN 1999em on days 7, 40, 49, 159, and 163 after discovery. These data are used to probe the geometry of the electron-scattering atmosphere before, during, and after the plateau phase, which ended roughly 90 days after discovery. Weak continuum polarization with an unchanging polarization angle (theta ~ 160 deg) is detected at all epochs, with p ~ 0.2% on day 7, p ~ 0.3% on days 40 and 49, and p ~ 0.5% in the final observations. Interstellar polarization is shown to likely be less than 0.1%. Distinct polarization modulations across strong line features are present on days 40, 49, 159, and 163, with polarization peaks associated with strong P Cygni absorption troughs and nearly complete depolarization across the H-alpha emission profile. The temporal evolution of the continuum polarization and sharp changes across lines indicate polarization intrinsic to SN 1999em. When modeled in terms of the oblate, electron-scattering atmospheres of Hoeflich, the observed polarization implies an asphericity of at least 7% during the period studied. The temporal polarization increase may indicate greater asphericity deeper into the ejecta. We discuss the implications of asphericity on the use of type II-P supernovae as primary extragalactic distance indicators through the Expanding Photosphere Method (EPM), and suggest that the agreement of previous EPM measurements with a linear Hubble law restricts the mean SN II-P asphericity to values less than 30% during the photospheric phase.