SN 2019nyk: A rapidly declining Type II supernova with early interaction signatures

We present an optical photometric and spectroscopic analysis of the fast-declining hydrogen-rich Type II supernova (SN) 2019nyk. The light curve properties of SN 2019nyk align well with those of other fast-declining Type II SNe, such as SNe 2013by and 2014G. SN 2019nyk exhibits a peak absolute magnitude of −18.09 ± 0.17 mag in the V band, followed by a rapid decline at 2.84 ± 0.03 mag (100 d)⁻¹ during the recombination phase. The early spectra of SN 2019nyk exhibit high-ionisation emission features as well as narrow H Balmer lines, persisting until 4.1 d since explosion, indicating the presence of circumstellar material (CSM) in close proximity. A comparison of these features with other Type II SNe displaying an early interaction reveals similarities between these features and those observed in SNe 2014G and 2023ixf. We also compared the early spectra to literature models, estimating a mass-loss rate of the order of 10⁻³ M_⊙ yr⁻¹. Radiation hydrodynamical modelling of the light curve also suggests the mass loss from the progenitor within a short period prior to explosion, totalling 0.16 M_⊙ of material within 2900 R_⊙ of the progenitor. Furthermore, light curve modelling infers a zero-age main sequence mass of 15 M_⊙ for the progenitor, a progenitor radius of 1031 R_⊙, and an explosion energy of 1.1 × 10⁵¹ erg.