Millimeter Observations of the Type II SN 2023ixf: Constraints on the Proximate Circumstellar Medium

We present 1.3 mm (230 GHz) observations of the recent and nearby Type II supernova, SN 2023ixf, obtained with the Submillimeter Array (SMA) at 2.6–18.6 days after explosion. The observations were obtained as part the SMA Large Program, POETS (Pursuit of Extragalactic Transients with the SMA). We do not detect any emission at the location of SN 2023ixf, with the deepest limits of L ν (230 GHz) ≲ 8.6 × 1025 erg s−1 Hz−1 at 2.7 and 7.7 days, and L ν (230 GHz) ≲ 3.4 × 1025 erg s−1 Hz−1 at 18.6 days. These limits are about a factor of 2 times dimmer than the millimeter emission from SN 2011dh (IIb), about 1 order of magnitude dimmer compared to SN 1993J (IIb) and SN 2018ivc (IIL), and about 30 times dimmer than the most luminous nonrelativistic SNe in the millimeter band (Type IIb/Ib/Ic). Using these limits in the context of analytical models that include synchrotron self-absorption and free–free absorption, we place constraints on the proximate circumstellar medium around the progenitor star, to a scale of ∼2 × 1015 cm, excluding the range Ṁ∼few×10−6−10−2 M ⊙ yr−1 (for a wind velocity, v w = 115 km s−1, and ejecta velocity, v ej ∼ (1 − 2) × 104 km s−1). These results are consistent with an inference of the mass-loss rate based on optical spectroscopy (∼2 × 10−2 M ⊙ yr−1 for v w = 115 km s−1), but are in tension with the inference from hard X-rays (∼7 × 10−4 M ⊙ yr−1 for v w = 115 km s−1). This tension may be alleviated by a nonhomogeneous and confined CSM, consistent with results from high-resolution optical spectroscopy.