The SN 2023ixf Progenitor in M101. I. Infrared Variability

Observational evidence points to a red supergiant (RSG) progenitor for SN 2023ixf. The progenitor candidate has been detected in archival images at wavelengths (>= 0.6 mu m) where RSGs typically emit profusely. This object is distinctly variable in the infrared (IR). We characterize the variability using pre-explosion mid-IR (3.6 and 4.5 mu m) Spitzer and ground-based near-IR (JHK s ) archival data jointly covering 19 yr. The IR light curves exhibit significant variability with rms amplitudes in the range 0.2-0.4 mag, increasing with decreasing wavelength. From a robust period analysis of the more densely sampled Spitzer data, we measure a period of 1091 +/- 71 days. We demonstrate using Gaussian process modeling that this periodicity is also present in the near-IR light curves, thus indicating a common physical origin, which is likely pulsational instability. We use a period-luminosity relation for RSGs to derive a value of M K = -11.58 +/- 0.31 mag. Assuming a late M spectral type, this corresponds to log(L/L circle dot)=5.27 +/- 0.12 at T eff = 3200 K and to log(L/L circle dot)=5.37 +/- 0.12 at T eff = 3500 K. This gives an independent estimate of the progenitor's luminosity, unaffected by uncertainties in extinction and distance. Assuming the progenitor candidate underwent enhanced dust-driven mass loss during the time of these archival observations, and using an empirical period-luminosity-based mass-loss prescription, we obtain a mass-loss rate of around (2-4) x 10-4 M circle dot yr-1. Comparing the above luminosity with stellar evolution models, we infer an initial mass for the progenitor candidate of 20 +/- 4 M circle dot, making this one of the most massive progenitors for a Type II SN detected to date.