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We present high-cadence optical, ultraviolet (UV), and near-infrared data ofthe nearby ($D\approx23$ Mpc) Type II supernova (SN) 2021yja. Many Type II SNeshow signs of interaction with circumstellar material (CSM) during the firstfew days after explosion, implying that their red supergiant (RSG) progenitorsexperience episodic or eruptive mass loss. However, because it is difficult todiscover SNe early, the diversity of CSM configurations in RSGs has not beenfully mapped. SN 2021yja, first detected within ${\approx}5.4$ hours ofexplosion, shows some signatures of CSM interaction (high UV luminosity, radioand x-ray emission) but without the narrow emission lines or early light curvepeak that can accompany CSM. Here we analyze the densely sampled early lightcurve and spectral series of this nearby SN to infer the properties of itsprogenitor and CSM. We find that the most likely progenitor was an RSG with anextended envelope, encompassed by low-density CSM. We also present archivalHubble Space Telescope imaging of the host galaxy of SN 2021yja, which allowsus to place a stringent upper limit of ${\lesssim}9\ M_\odot$ on the progenitormass. However, this is in tension with some aspects of the SN evolution, whichpoint to a more massive progenitor. Our analysis highlights the need toconsider progenitor structure when making inferences about CSM properties, andthat a comprehensive view of CSM tracers should be made to give a fuller viewof the last years of RSG evolution.

Weak Mass Loss from the Red Supergiant Progenitor of the Type II SN 2021yja