The core normal Type Ia supernova 2019np an overall spherical explosion with an aspherical surface layer and an aspherical 56Ni core
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ABSTRACTOptical spectropolarimetry of the normal thermonuclear supernova (SN) 2019np from 14.5 to +14.5d relative to B-band maximum detected an intrinsic continuum polarization (pcont) of 0.21 0.09per cent at the first epoch. Between days 11.5 and +0.5, pcont remained 0 and by day +14.5 was again significant at 0.19 0.10per cent. Not considering the first epoch, the dominant axis of ${ m Si, {small II}}$6355 was roughly constant staying close the continuum until both rotated in opposite directions on day +14.5. Detailed radiation-hydrodynamical simulations produce a very steep density slope in the outermost ejecta so that the low first-epoch pcont 0.2per cent nevertheless suggests a separate structure with an axis ratio 2 in the outer carbon-rich (3.54) 103M. Large-amplitude fluctuations in the polarization profiles and a flocculent appearance of the polar diagram for the ${ m Ca, {small II}}$near-infrared triplet (NIR3) may be related by a common origin. The temporal evolution of the polarization spectra agrees with an off-centre delayed detonation. The late-time increase in polarization and the possible change in position angle are also consistent with an aspherical 56Ni core. The pcont and the absorptions due to ${ m Si, {small II}}$ 6355 and ${ m Ca, {small II}}$NIR3 form in the same region of the extended photosphere, with an interplay between line occultation and thermalization producing p. Small-scale polarization features may be due to small-scale structures, but many could be related to atomic patterns of the quasi-continuum; they hardly have an equivalent in the total-flux spectra. We compare SN2019np to other SNe and develop future objectives and strategies for SNIa spectropolarimetry.
