Sei Saito (Tohoku University): On the Formation of the W-shaped O II lines in Spectra of H-poor Superluminous Supernovae

Monday 11 Mar 2024, 14:00 → 15:00 Europe/Stockholm

Title: On the Formation of the W-shaped O II lines in Spectra of H-poor Superluminous Supernovae
Speaker: Sei Saito
Affiliation: Tohoku University
Time: Monday 11th March 2024, 14:00 to 15:00
Location: 101190 Å

Abstract:  Superluminous supernovae (SLSNe) are extremely luminous transients, which are about 10 - 100 times more luminous than normal supernovae (SNe). The power source for SLSNe is still a big mystery. SLSNe (H-poor SLSNe) are characterized by O II lines around ∼ 4000 Å and ∼ 4500 Å in pre-/near-maximum spectra, so called the W-shaped O II lines. However, the condition where these lines appear has been unclear, and it has been considered as a sign of non- thermal processes. As He I lines in spectra of Type Ib supernovae (SNe Ib) provide information on γ-ray deposition from 56Ni, investigating departure from local thermodynamic equilibrium (LTE) for the O II lines may give a hint of power sources of SLSNe-I. In this work, we calculate synthetic spectra to reproduce observed spectra of SLSNe-I, parameterizing departure coefficients for the O II (expressed as b). We find that most of the observed spectra can be reproduced by b <∼ 10, which means that no strong departure from LTE is needed for the formation of the W-shaped O II lines. We also show that the appearance of the W-shaped O II lines is very sensitive to temperature and that only spectra with T ∼ 14, 000 − 16, 000 K produce the W-shaped O II lines without large departures from LTE. Using the results that population of O II does not largely deviate from population in LTE, we constrain the non-thermal ionization rate at photospheres.