2025ApJ...983...87L

The subluminous red nova (SLRN) Zwicky Transient Facility (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 days after its optical emission peak using the NIRSpec fixed-slit 3-5 μm high-resolution grating and the MIRI 5-12 μm low-resolution spectrometer. NIRSpec reveals the ¹²CO fundamental band (ν = 1-0) in emission at ∼4.7 μm, Brackett-α emission, and the potential detection of PH₃ in emission at ∼4.3 μm. 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 ∼1-12 μm spectral energy distribution (SED) of ZTF SLRN-2020, where the best-fit parameters indicate the presence of warm, $72{0}_{-50}^{+80}$ K, circumstellar dust with a total dust mass of Log $\left(\tfrac{{M}_{{\rm{d}}}}{{M}_{\odot }}\right)=-10.6{1}_{-0.16}^{+0.08}$M_⊙. We also fit a DUSTY model to archival photometry taken +320 days after the peak that suggested the presence of a cooler, ${T}_{{\rm{d}}}=28{0}_{-20}^{+450}$ K, and more massive, Log $\left(\tfrac{{M}_{{\rm{d}}}}{{M}_{\odot }}\right)=-5.8{9}_{-3.21}^{+0.29}$, 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 ${L}_{* }=0.2{9}_{-0.06}^{+0.03}$L_⊙ for the remnant host star, which is consistent with a ∼0.7 M_⊙ 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.