Spectropolarimetry of the type IIP supernova 2021yja: an unusually high continuum polarization during the photospheric phase

TL;DR

This study presents spectropolarimetric observations of supernova 2021yja, revealing an unusually high early polarization indicating an ellipsoidal ejecta shape and possible interaction with aspherical circumstellar matter or an extended hydrogen envelope.

Contribution

First detailed spectropolarimetric analysis of SN 2021yja showing high early polarization and insights into its ejecta geometry and circumstellar environment.

Findings

High early continuum polarization (~0.9%) observed.

Consistent ellipsoidal ejecta geometry inferred.

Potential interaction with aspherical circumstellar matter or hydrogen envelope.

Abstract

We present six epochs of optical spectropolarimetry of the Type IIP supernova (SN) 2021yja ranging from $\sim$ 25 to 95 days after the explosion. An unusually high continuum linear polarization of $p \sim 0.9\%$ is measured during the early photospheric phase, followed by a steady decrease well before the onset of the nebular phase. This behavior has not been observed before in Type IIP supernovae (SNe IIP). The observed continuum polarization angle does not change significantly during the photospheric phase. We find a pronounced axis of symmetry in the global ejecta that is shared in common with the H$\alpha$ and Ca II near-infrared triplet lines. These observations are consistent with an ellipsoidal geometry. The temporal evolution of the continuum polarization is also compatible with the SN ejecta interacting with aspherical circumstellar matter, although no spectroscopic features that may be associated with strong interaction can be identified. Alternatively, we consider the source of the high polarization to be an extended hydrogen envelope that is indistinguishable from low-density circumstellar matter.