Spectropolarimetry of the Type II Supernovae 1997ds, 1998A, and 1999gi

We present single‐epoch spectropolarimetry of the Type II supernovae (SNe II) 1997ds, 1998A, and 1999gi. SN 1997ds and SN 1998A were both observed during the early photospheric phase, less than 50 days after explosion, while spectropolarimetry of SN 1999gi was obtained near the start of the transition to the nebular phase, about 110 days after explosion. Uncorrected for interstellar polarization (ISP), SN 1997ds is characterized by pV = 0.85% ± 0.02%, SN 1998A has pV = 0.24% ± 0.05%, and SN 1999gi is polarized at pV = 5.72% ± 0.01%. SN 1997ds and SN 1999gi exhibit distinct polarization modulations (up to Δptot = 1.6% in SN 1997ds and Δptot = 1.0% in SN 1999gi) at the wavelengths of the strongest spectral line features. While no spectral polarization features were observed in SN 1998A, the data are insensitive to polarization features at the levels confirmed in the other two objects. The low continuum polarization inferred for SN 1997ds and SN 1998A and the amplitude of (or limits on) the polarization modulations are consistent with those measured at similar epochs for SN 1987A and the Type II plateau SN 1999em and supports the growing consensus that core‐collapse events with hydrogen envelopes substantially intact at the time of explosion are not significantly aspherical during the early photospheric phase. The spectral shape of the high continuum polarization of SN 1999gi closely resembles a “Serkowski” ISP curve (characterized by pmax = 5.8%, θ = 154°, and λmax = 5300 Å) and is inconsistent with the wavelength‐independent nature of electron scattering expected for an aspherical SN atmosphere. Since Galactic reddening is minimal along this line of sight, the majority of the observed polarization in SN 1999gi is believed to be due to ISP of the host galaxy, although significant (up to p ≈ 2%) intrinsic polarization cannot be ruled out. The potential power of SN spectropolarimetry to study the properties of interstellar dust in external galaxies is described and applied to the SN 1999gi data, where it is shown that if the polarization is indeed predominantly interstellar in origin, then RV = 3.0 ± 0.2 for the dust along this line of sight in NGC 3184.

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