Early emission lines in SN 2024ggi revealed by high-resolution spectroscopy

Context. We present an analysis of very early high-resolution spectroscopic observations of the Type II supernova (SN) 2024ggi, a nearby SN that occurred in the galaxy NGC 3621 at a distance of 7.24 Mpc (z ≈ 0.002435). These observations represent the earliest high-resolution spectra of a Type II SN ever made. Aims. We analyzed the very early-phase spectroscopic evolution of SN 2024ggi obtained in a short interval at 20.6 and 27.8 h after its discovery, or 26.6 and 33.8 h after the SN first light. Observations were obtained with the high-resolution spectrograph MIKE (R ≈ 22 600 − 28 000) at the 6.5 m Magellan Clay Telescope, located at the Las Campanas Observatory, on the night of April 12, 2024 UT. Methods. The emission lines were identified and studied in detail during the first hours of SN 2024ggi. We analyzed the evolution of ions of H <ns0:sc>I</ns0:sc>, He <ns0:sc>I</ns0:sc>, He <ns0:sc>II</ns0:sc>, N <ns0:sc>III</ns0:sc>, C <ns0:sc>III</ns0:sc>, Si <ns0:sc>IV</ns0:sc>, N <ns0:sc>IV</ns0:sc>, and C <ns0:sc>IV</ns0:sc> detected across the spectra. We modeled these features with multiple Gaussian and Lorentzian profiles, and estimated their velocities and full widths at half maximum (FWHMs). Results. The spectra show asymmetric emission lines of H <ns0:sc>I</ns0:sc>, He <ns0:sc>II</ns0:sc>, C <ns0:sc>IV</ns0:sc>, and N <ns0:sc>IV</ns0:sc> that can be described by narrow Gaussian cores (FWHM ≤ 200 km s−1) with broader Lorentzian wings, and symmetric narrow emission lines of He <ns0:sc>I</ns0:sc>, N <ns0:sc>III</ns0:sc>, and C <ns0:sc>III</ns0:sc>. The emission lines of He <ns0:sc>I</ns0:sc> are detected only in the first spectrum, indicating the rapid ionization of He <ns0:sc>I</ns0:sc> to He <ns0:sc>II</ns0:sc>. The narrow components of the emission lines show a systematic blueshift relative to their zero-velocity position, with an increase of ∼18 km s−1 in the average velocity between the two epochs. The broad Lorentzian components show a blueshift in velocity relative to the narrow components, and a significant increase in the average velocity of ∼103 km s−1. Such a rapid evolution and significant ionization changes in a short period of time were never observed before, and are probably a consequence of the radiative acceleration generated in the SN explosion.