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

Abstract

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.

Published in ApJ on Apr 10, 2025; 22 pages, 9 figures, 3 tables

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• The fate of the progenitors of luminous red novae: Infrared detection of LRNe years after the outburst
• Observational Signatures of Planetary Tidal Disruption Events Around Solar-Mass Stars
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