High-resolution Spectroscopy of SN 2023ixf’s First Week: Engulfing the Asymmetric Circumstellar Material

We present a series of high-resolution echelle spectra of SN 2023ixf in M101, obtained nightly during the first week or so after discovery using PEPSI on the Large Binocular Telescope. Na i D absorption in these spectra indicates a host reddening of E ( B − V ) = 0.031 mag and a systemic velocity of +7 km s −1 relative to the average redshift of M101. Dramatic changes are seen in the strength and shape of strong emission lines emitted by circumstellar material (CSM), including He ii λ 4686, C iv λλ 5801,5811, H α , and N iv λλ 7109,7123. In general, these narrow lines broaden to become intermediate-width lines before disappearing from the spectrum within a few days, indicating a limited extent to the dense CSM of around 20–30 au (or ≲10 14.7 cm). H α persists in the spectrum for about a week as an intermediate-width emission line with P Cyg absorption at 700–1300 km s −1 arising in the post-shock shell of swept-up CSM. Early narrow emission lines are blueshifted and indicate an expansion speed in the pre-shock CSM of about 115 km s −1 , but with even broader emission in higher-ionization lines. This is faster than the normal winds of red supergiants, suggesting some mode of eruptive mass loss from the progenitor or radiative acceleration of the CSM. A lack of narrow blueshifted absorption suggests that most of the CSM is not along our line of sight. This and several other clues indicate that the CSM of SN 2023ixf is significantly aspherical. We find that CSM lines disappear after a few days because the asymmetric CSM is engulfed by the supernova photosphere.