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 (θ ≈ 160) 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 Hα 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 Höflich, 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. Subject headings: distance scale — polarization — supernovae: individual (SN 1999em) — techniques: polarimetric