Infant Core-collapse Supernova with Circumstellar Interactions from KMTNet. II. Luminous, Long-lasting Case of KSP-SN-2020f

Abstract We present photometric and spectroscopic observations of a luminous, long-lasting (LLL) Type IIn supernova (SN) KSP-SN-2020f found in a dwarf host galaxy at z = 0.1 from the KMTNet SN Program. Our high-cadence, multicolor ( BVi ) monitoring observations constrain the epoch of first-light to ∼5 hr prior to first detection under the power-law assumption of early light. The source shows fast evolution across the peak of M V ≃ −19.54 mag, with a rise time of 7.9 days followed by a rapid linear decline of 1.9 mag in 50 days. The fast near-peak evolution and large luminosities require excess emission, such as interactions with circumstellar material (CSM), in addition to SN shock cooling emission. Early blue colors and luminous ( M V ≃ −16) late-time evolution (≳200 days) are slower than 56 Co radioactive decay, indicating continuous CSM interactions throughout the SN’s entire evolution. Its H α emission consists of multiple components indicative of substantial CSM interactions with an estimated mass loss rate of M ̇ ≳ 7 × 1 0 − 2 M ⊙ yr −1 . The large wind velocity (∼260 km s −1 ) and mass loss rate are compatible with an luminous blue variable (LBV)-like progenitor for the source. This study provides a first look at the infant-phase evolution of an LLL Type IIn SN, demonstrating that rapid near-peak evolution can coexist with slow late-time evolution, and also suggesting that LBV-like progenitors can produce Type IIn SNe with fast early evolution.