Near-infrared spectroscopy and detection of carbon monoxide in the Type II supernova SN 2023ixf

¹ Department of Physics and Astronomy, Seoul National University, Gwanak-ro 1, Gwanak-gu, Seoul 08826, South Korea
² SETI Institute, 339 Bernardo Ave., Ste. 200, Mountain View, CA 94043, USA
³ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065, USA
⁴ National Astronomical Research Institute of Thailand, 260 Moo 4, Donkaew, Maerim, Chiang Mai 50180, Thailand
⁵ Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA
⁶ Gemini Observatory/NSF’s National Optical-Infrared Astronomy Research Laboratory, 670 N. Aohoku Place, Hilo, HI 96720, USA
⁷ Department of Physics and Astronomy, University of California, 1 Shields Avenue, Davis, CA 95616-5270, USA
⁸ Department of Physics, Virginia Tech, Blacksburg, VA 24061, USA
⁹ Florida State University, Tallahassee, FL 32309, USA
¹⁰ Las Cumbres Observatory, 6740 Cortona Drive, Suite 102, Goleta, CA 93117-5575, USA
¹¹ Department of Physics, University of California, Santa Barbara, CA 93106-9530, USA
¹² Adler Planetarium, 1300 S. DuSable Lake Shore Dr., Chicago, IL 60605, USA
¹³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218-2410, USA
¹⁴ California Institute of Technology, Pasadena, CA 91125, USA
¹⁵ George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, Department of Physics and Astronomy, Texas A&M University, College Station, TX 77843, USA
¹⁶ Planetary Science Institute, 1700 East Fort Lowell Road, Suite 106, Tucson, AZ 85719-2395, USA
¹⁷ Hamburger Sternwarte, Gojenbergsweg 112, D-21029 Hamburg, Germany
¹⁸ Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, Norman, OK 73019-2061, USA
¹⁹ Rutgers Department of Physics and Astronomy, 136 Frelinghuysen Rd, Piscataway, NJ 08854, USA
²⁰ Institute of Space Sciences (ICE-CSIC), Campus UAB, Carrer de Can Magrans, s/n, E-08193 Barcelona, Spain
²¹ Institut d’Estudis Espacials de Catalunya (IEEC), 08860 Castelldefels (Barcelona), Spain

^⋆ Corresponding authors: rogersh0125@snu.ac.kr, jrho@seti.org, scyoon@snu.ac.kr

Received: 22 April 2025
Accepted: 24 September 2025

Abstract

Context. Core-collapse supernovae (CCSNe) may have contributed a significant amount of dust in the early Universe. Freshly formed coolant molecules (e.g., CO) and warm dust can be found in CCSNe as early as ∼100 d after the SN explosion, allowing the study of their evolution with time series observations.

Aims. Through study of the Type II SN 2023ixf, we aim to investigate the temporal evolution of the temperature, velocity, and mass of CO and compare them with other CCSNe, exploring their implications for the dust formation in CCSNe. From observations of velocity profiles of lines of other species (e.g., H and He), we also aim to characterize and understand the interaction of the SN ejecta with preexisting circumstellar material (CSM).

Methods. We present a time series of 16 near-infrared spectra of SN 2023ixf from 9 to 307 d, taken with multiple instruments: Gemini/GNIRS, Keck/NIRES, IRTF/SpeX, and MMT/MMIRS.

Results. The early (t ≲ 70 d) spectra indicate interaction between the expanding ejecta and nearby CSM. At t ≲ 20 d, intermediate-width line profiles corresponding to the ejecta-wind interaction are superposed on evolving broad P Cygni profiles. We find intermediate-width and narrow lines in the spectra until t ≲ 70 d, which suggest continued CSM interaction. We also observe and discuss high-velocity absorption features in H α and H β line profiles formed by CSM interaction. The spectra contain CO first overtone emission between 199 and 307 d after the explosion. We modeled the CO emission and found the CO to have a higher velocity (3000–3500 km s⁻¹) than that in Type II-pec SN 1987A (1800–2000 km s⁻¹) during similar phases (t = 199 − 307 d) and a comparable CO temperature to SN 1987A. A flattened continuum at wavelengths greater than 1.5 μm accompanies the CO emission, suggesting that the warm dust is likely formed in the ejecta. The warm dust masses are estimated to be on the order of ∼10⁻⁵ M_⊙.