The Immediate, Exemplary, and Fleeting Echelle Spectroscopy of SN 2023ixf: Monitoring Acceleration of Slow Progenitor Circumstellar Material Driven by Shock Interaction

We present high-resolution WIYN/NEID echelle spectroscopy (R ≈ 70,000) of the supernova (SN) 2023ixf in M101, obtained 1.51 to 18.51 days after explosion over nine epochs. Daily monitoring for the first 4 days after explosion shows narrow emission features (≤200 km s⁻¹), exhibiting predominantly blueshifted velocities that rapidly weaken, broaden, and vanish in a manner consistent with radiative acceleration and the SN shock eventually overrunning or enveloping the full extent of the dense circumstellar medium (CSM). The most rapid evolution is in the He i emission, which is visible on day 1.51 but disappears by day 2.62. We measure the maximum pre-SN speed of He i to be 25 − 5 + 0 ± 2 km s⁻¹, where the error is attributable to the uncertainty in how much the He i had already been radiatively accelerated and to measurement of the emission-line profile. The radiative acceleration of CSM is likely driven by the shock-CSM interaction, and the CSM is accelerated to ≥200 km s⁻¹ before being completely swept up by the SN shock to ∼2000 km s⁻¹. We compare the observed spectra with spherically symmetric r1w6b HERACLES/CMFGEN model spectra and find the line evolution to generally be consistent with radiative acceleration, optical depth effects, and evolving ionization state. The progenitor of SN 2023ixf underwent an enhanced mass-loss phase ≳4 yr prior to core collapse, creating a dense, asymmetric CSM region extending out to approximately r_CSM = 3.7 × 10¹⁴ (v_shock/9500 km s⁻¹) cm.