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We present six epochs of optical spectropolarimetry of the Type IIP supernova(SN) 2021yja ranging from $\sim$ 25 to 95 days after the explosion. Anunusually high continuum linear polarization of $p \sim 0.9\%$ is measuredduring the early photospheric phase, followed by a steady decrease well beforethe onset of the nebular phase. This behavior has not been observed before inType IIP supernovae (SNe IIP). The observed continuum polarization angle doesnot change significantly during the photospheric phase. We find a pronouncedaxis of symmetry in the global ejecta that is shared in common with theH$\alpha$ and Ca II near-infrared triplet lines. These observations areconsistent with an ellipsoidal geometry. The temporal evolution of thecontinuum polarization is also compatible with the SN ejecta interacting withaspherical circumstellar matter, although no spectroscopic features that may beassociated with strong interaction can be identified. Alternatively, weconsider the source of the high polarization to be an extended hydrogenenvelope that is indistinguishable from low-density circumstellar matter.

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