Revealing a main-sequence star that consumed a planet with JWST

The subluminous red nova (SLRN) ZTF SLRN-2020 is the most compelling direct detection of a planet being consumed by its host star, a scenario known as a planetary engulfment event. We present JWST spectroscopy of ZTF SLRN-2020 taken +830 d after its optical emission peak using the NIRSpec fixed-slit $3-5$ $\mu$m high-resolution grating and the MIRI $5-12$ $\mu$m low-resolution spectrometer. NIRSpec reveals the ${}^{12}$CO fundamental band ($\nu =1-0$) in emission at $\sim 4.7$ $\mu$m, Brackett-$\alpha$ emission, and the potential detection of PH${}_3$ in emission at $\sim 4.3$ $\mu$m. The JWST spectra are consistent with the claim that ZTF SLRN-2020 arose from a planetary engulfment event. We utilize DUSTY to model the late-time $\sim 1-12$ $\mu$m spectral energy distribution (SED) of ZTF SLRN-2020, where the best-fit parameters indicate the presence of warm, $720_{-50}^{+80}$ K, circumstellar dust with a total dust mass of Log$\left(\frac{M_{\mathrm{d}}}{{\mathrm{M}}_{\odot }}\right)=-10.61_{-0.16}^{+0.08}$ M${}_{\odot }$. We also fit a DUSTY model to archival photometry taken +320 d after peak that suggested the presence of a cooler, T${}_{\mathrm{d}}=280_{-20}^{+450}$ K, and more massive, Log$\left(\frac{M_{\mathrm{d}}}{{\mathrm{M}}_{\odot }}\right)=-5.89_{-3.21}^{+0.29}$, circumstellar dust component. Assuming the cool component originates from the ZTF SLRN-2020 ejecta, we interpret the warm component as fallback from the ejecta. From the late-time SED model we measure a luminosity of L${}_{\ast }=0.29_{-0.06}^{+0.03}$ L${}_{\odot }$ for the remnant host star, which is consistent with a $\sim 0.7$ M${}_{\odot }$ K-type star that should not yet have evolved off the main sequence. If ZTF SLRN-2020 was not triggered by stellar evolution, we suggest that the planetary engulfment was due to orbital decay from tidal interactions between the planet and the host star.