Near-Infrared Spectroscopy and Detection of Carbon Monoxide in the Type II Supernova SN 2023ixf

TL;DR

SN 2023ixf is analyzed with a 9–307 day near-infrared time series to trace CO formation and dust production in a nearby Type II SN. The authors model the CO first overtone emission under LTE, deriving $T_{ m CO}$, $V_{ m CO}$, and $M_{ m CO}$, and find CO velocities of $V_{ m CO}\, ext{in the range }$3000–3500 km s$^{-1}$ with temperatures near $2000$–$2300$ K, alongside a warm dust component with $T_{ m d}\, ext{around }$900–1050 K and $M_{ m d}\, ext{≈ }(0.7–2.3) imes10^{-5}$ M$_{igodot}$. The early NIR spectra reveal intermediate-width and narrow lines indicative of persistent ejecta-CSM interaction, while optical spectra show Cachitos high-velocity features associated with CSM interaction. Comparisons with SN 2017eaw and SN 1987A reveal higher CO velocities in SN 2023ixf, suggesting diversity in CO and dust formation among CCSNe. The combined CO and dust signatures, together with JWST mid-IR hints, imply active dust formation in the ejecta, with implications for the role of CCSNe in contributing dust to the early universe.

Abstract

Core-collapse supernovae (CCSNe) may contribute 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 explosion, allowing the study of their evolution with time series observations. In 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). 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. 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 model the CO emission and find the CO to have a higher velocity (3000-3500 km/s) than that in Type II-pec SN 1987A (1800-2000 km/s) during similar phases (t=199-307 d) and a comparable CO temperature to SN 1987A. A flattened continuum at wavelengths greater than 1.5 $μ$m 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$^{-5} M_{\odot}$.}

Figures (9)

• Figure 1: Gemini/GNIRS (17, 37, 67, 249, and 277 d; blue), Keck/NIRES (9, 19, 47, 71, 259, and 275 d; black), IRTF/SpeX (39, 278, and 307 d; green), and MMT/MMIRS (199 and 257 d; red) spectra of SN 2023ixf, in time order (top to bottom). Each spectrum is labeled with the days elapsed after the explosion. The wavelengths of known strong lines meikle89 are marked with dotted lines. Regions with significant noise due to strong telluric absorption are shaded in grey. The wavelength region of the first overtone CO emission is shaded in light blue. We also include Paschen $\alpha$ (1.875 $\mu$m), which falls in a region of poor telluric transmission, marked by a black dotted line. The Brackett $\delta$ and $\epsilon$ lines, which also fall in the region, are artifacts due to incomplete removal of the same lines in the standard stars, despite appearing to be present in some of the spectra.
• Figure 2: (a) Velocity profiles of He i 1.083 $\mu$m from 9 to 71 d. Lines identified from intermediate-width (IW) features (FWHM$\sim$1000 km s$^{-1}$) are marked with black dotted vertical lines. The inset at the bottom right corner shows narrow (FWHM$\sim$100 km s$^{-1}$) features at 47, 67, and 71 d, near the zero velocity of He i 1.083 $\mu$m. (b) Velocity profiles of Pa $\beta$ 1.282 $\mu$m and Br $\gamma$ 2.166 $\mu$m from selected epochs. The Pa $\beta$ line shows an IW feature at 9 d, and the Br $\gamma$ line shows IW features at 17 and 39 d. See Table \ref{['Tlinefluxes']} for the FWHMs and the shifts from zero velocity of the IW and narrow lines under the columns labeled $'$Narrow line', and also show the velocity shifts of broad absorption, and the velocity interval between minimum and maximum of P Cygni profile.
• Figure 3: H $\alpha$(a) and H $\beta$(b) line profiles of SN 2023ixf; Of particular interest are a “dip” ('B' marked in red) on the blue side of the profile with velocities of $\sim$-6000 km s$^{-1}$ at 57, 67, and 71 d, and a moderately strong absorption feature ('A' marked in black) at high velocity (-12,000 km s$^{-1}$) between 27 and 57 d leonard22. These features (indicated by arrows and labeled 'A' and 'B') are called $'$Cahitos' gutierrez17. These feature are caused by interactions, geometry, and viewing angles between the reverse shock, the CSM and ejecta chugai04. The epochs are labeled on the right. The absorption-like features at 14,000 km s$^{-1}$ in H $\alpha$ are due to telluric absorption.
• Figure 4: Multi-component line fitting of He i 1.083 $\mu$m (a) and Pa $\beta$ 1.282 $\mu$m (b) at 9 d with with Gaussian and Lorentzian components. The He i 1.083 $\mu$m line profile (a) was fitted with two broad Gaussian profiles with FWHMs of 5250 and 6010 km s$^{-1}$ for absorption and emission, along with two intermediate-width (IW) Gaussian profiles with FWHMs of 1000 and 670 km s$^{-1}$ for absorption and emission. The Pa $\gamma$ 1.094 $\mu$m that overlaps the He i 1.083 $\mu$m was simultaneously fitted with an intermediate-width Gaussian profile with FWHM of 980 km s$^{-1}$. The Pa $\beta$ 1.282 $\mu$m line profile (b) was fitted with two broad Gaussian profiles with FWHMs of 4790 and 5150 km s$^{-1}$ for absorption and emission, respectively, and an intermediate-width Lorentzian emission profile with FWHM of 720 km s$^{-1}$ (red).
• Figure 5: Observed Line first-overtone bands in SN 2023ixf from 249 to 307 d are superposed with the best LTE model fits (black dashed). A linear continuum was adopted for each spectrum. More details are provided in the text. The best-fit parameter values are presented in Table \ref{['Tobs']}.