The Type II SN 2025pht in NGC 1637: A Red Supergiant with Carbon-rich Circumstellar Dust as the First JWST Detection of a Supernova Progenitor Star

Abstract We present follow-up imaging and spectroscopy and pre-explosion imaging of SN 2025pht located in NGC 1637 at 12 Mpc. Our spectroscopy shows that SN 2025pht is a Type II supernova (SN) with broad lines of hydrogen and with minimal line-of-sight extinction inferred from Na i D absorption. NGC 1637 was the target of several epochs of Hubble Space Telescope (HST) and James Webb Space Telescope (JWST) imaging covering the site of SN 2025pht from 31 to 0.7 yr prior to discovery. Using a follow-up HST image of SN 2025pht aligned to these data, we demonstrate that there is a credible progenitor candidate detected in multiple epochs of HST imaging and in JWST imaging from 1.3 to 8.7 μ m, the first JWST counterpart to an SN and the longest-wavelength detection of an SN progenitor star. Fitting this source to red supergiant (RSG) spectral energy distributions (SEDs), we show that it is consistent with a <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi>L</mml:mi> <mml:mo>/</mml:mo> <mml:msub> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> <mml:mo>=</mml:mo> <mml:mn>5.0</mml:mn> </mml:math> RSG heavily reddened by circumstellar dust. The JWST photometry enables strong constraints on the nature of the circumstellar medium, and we find that the SED favors graphite-rich dust as opposed to silicate-rich dust and an optical circumstellar extinction of A V = 5.3 mag. We discuss the implications of a pre-SN RSG enshrouded in carbon-rich dust and this finding for the overall population of progenitor stars to Type II SNe, as well as the future of SN progenitor star discovery with JWST.