2019ApJ...872..120K

High-resolution spectropolarimetric observations of V339 Del were performed in the optical wavelength region every night from UT 2013 August 15.49 to 21.23 (corresponding to the pre- and near-maximum phases of the nova) to reveal the asymmetry of the nova ejecta shortly after the explosion. Observed polarization spectra showed intrinsic line polarization for Hα, O I λ777.3 nm, Si II λλ634.7, 637.1 nm, in addition to intrinsic continuum polarization. The line polarization of Hα on August 15.49 (+0.91 day post-discovery) indicates an expanding nova ejecta, which contains a torus surrounding the nova photosphere. The line polarization of O I λ777.3 nm is also consistent with this hypothesis. The line polarizations of Si II λλ634.7, 637.1 nm might indicate a variation in the photospheric temperature of the nova on the same date. Based on the day-by-day changes of the intrinsic continuum and line polarization found in the observations, it can be concluded that the effective geometry of V339 Del had changed from a torus-dominant to a bipolar-shaped geometry between +0.91 and +2.15 days post-discovery. The intrinsic continuum polarization became weaker after +2.93 days due to the larger size of the expanding nova ejecta. This is consistent with high-angular observations of V339 Del with a near-infrared interferometer and with a recent nova ejecta model that can explain the γ-ray emission and dust formation found in V339 Del.