Photospheric-Phase Spectropolarimetry and Nebular-Phase Flux Spectra of the Peculiar Type Ic Supernova 2002ap

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  • 20 June 2002
Abstract
We present optical spectropolarimetry of SN 2002ap from 16 and 37 days after the estimated date of explosion. We find evidence for a high level of intrinsic continuum polarization at both epochs: p >~ 1.3% on day 16 and p >~ 1.0% on day 37. Prominent line polarization is also seen, especially in the Ca II infrared triplet during the second epoch. When interpreted in terms of the oblate, electron-scattering model atmospheres of Hoeflich (1991), our results imply an asymmetry of at least 20% (day 16) and 17% (day 37). The data suggest a fair degree of axisymmetric asymmetry in both epochs; however, the polarization angle is different by about 55 deg. between the two epochs, implying a complex morphology for the thinning ejecta. We also present flux spectra of SN 2002ap taken 131 and 140 days after the explosion. The spectra are characterized by a very weak continuum and broad emission lines, indicating that SN 2002ap has entered the nebular phase. The spectral features are substantially similar to those of ``normal'' SNe Ic in the nebular phase, and the emission lines are not significantly broader. However, some of the broad lines are very sharply peaked, and likely possess a distinct narrow component (probably unresolved by our spectra, FWHM <~ 400 km/s that is redshifted by ~580 km/s with respect to the systemic velocity of the host galaxy. This narrow component may indicate the presence of a circumstellar wind from the progenitor star.

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