The Early Phases of Supernova 2020pni: Shock Ionization of the Nitrogen-enriched Circumstellar Material

Abstract

We present multiwavelength observations of the Type II SN 2020pni. Classified at ∼1.3 days after explosion, the object showed narrow (FWHM intensity <250 km s⁻¹) recombination lines of ionized helium, nitrogen, and carbon, as typically seen in flash-spectroscopy events. Using the non-LTE radiative transfer code CMFGEN to model our first high-resolution spectrum, we infer a progenitor mass-loss rate of Ṁ = (3.5-5.3) × 10⁻³ M_⊙ yr⁻¹ (assuming a wind velocity of v_w = 200 km s⁻¹), estimated at a radius of Rᵢₙ = 2.5 × 10¹⁴ cm. In addition, we find that the progenitor of SN 2020pni was enriched in helium and nitrogen (relative abundances in mass fractions of 0.30–0.40 and 8.2 × 10⁻³, respectively). Radio upper limits are also consistent with dense circumstellar material (CSM) and a mass-loss rate of Ṁ > 5 × 10⁻⁴ Ṁ yr⁻¹. During the initial 4 days after first light, we also observe an increase in velocity of the hydrogen lines (from ∼250 to ∼1000 km s⁻¹), suggesting complex CSM. The presence of dense and confined CSM, as well as its inhomogeneous structure, indicates a phase of enhanced mass loss of the progenitor of SN 2020pni during the last year before explosion. Finally, we compare SN 2020pni to a sample of other shock-photoionization events. We find no evidence of correlations among the physical parameters of the explosions and the characteristics of the CSM surrounding the progenitors of these events. This favors the idea that the mass loss experienced by massive stars during their final years could be governed by stochastic phenomena and that, at the same time, the physical mechanisms responsible for this mass loss must be common to a variety of different progenitors.

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