SN 2022acko: The First Early Far-ultraviolet Spectra of a Type IIP Supernova

We present five far- and near-ultraviolet spectra of the Type II plateau supernova SN 2022acko obtained 5, 6, 7, 19, and 21 days after explosion with HST/STIS. The first three epochs are earlier than any Type II plateau supernova previously observed in the far-UV and reveal unprecedented characteristics: strong metal lines dominate these spectra, in contrast to the relatively featureless early optical spectra. The UV flux decreases over the initial time series as the ejecta cool and line blanketing strengthens. Using the NLTE radiative-transfer code CMFGEN, we find a good match to the explosion of a low-mass red supergiant with kinetic energy E_kin = 6 × 10^50 erg and identify, for the first time, the ions that dominate the early UV spectra. Complementary optical photometry and spectroscopy show SN 2022acko has a peak absolute magnitude of V = −15.4 mag and a plateau length of approximately 115 days, with spectra resembling those of SN 2005cs and SN 2012A. Using combined optical and UV spectra, we measure the fraction of flux redward of the uvw2, U, B, and V filters on days 5, 7, and 19 and construct a UV spectral time series for Type II SNe demonstrating the rapid UV flux decline in the first weeks. These results highlight the rich diagnostics available in early UV spectra and motivate future observations to explore the diversity of Type II SNe in the UV.

Megan Newsome, K Azalee Bostroem, Luc Dessart, D John Hillier, Michael Lundquist, Jennifer E Andrews, David J Sand, Yize Dong, Stefano Valenti, Joshua Haislip, Emily T Hoang, Griffin Hosseinzadeh, Daryl Janzen, Jacob E Jencson, Saurabh W Jha, Vladimir Kouprianov, Jeniveve Pearson, Nicolas E Meza Retamal, Daniel E Reichart, Manisha Shrestha, Christopher Ashall, Craig Pellegrino, Jeonghee Rho, Micalyn Rowe, Michaela Schwab, Melissa Shahbandeh, Nathan Smith, Jay Strader, Giacomo Terreran, Schuyler D Van Dyk, Samuel Wyatt, J. I. González Hernández