An infrared transient from a star engulfing a planet

Abstract

It is well known that planets with short orbital periods (≲ 10 days) are common around stars like the Sun 1-3 . Stars expand as they evolve, and thus we expect their close planetary companions to be engulfed [4][5][6] . However, this phase has never been directly observed. Here, we present the discovery of ZTF SLRN-2020, a short-lived optical outburst in the Galactic disk accompanied by bright and long-lived infrared emission. The resulting light curve and spectra share striking similarities with those of red novae 7, 8 -a class of eruptions now confirmed 9 to arise from mergers of binary stars. Its exceptionally low optical luminosity (≈ 10 35 erg s −1 ) and radiated energy (≈ 6.5 × 10 41 erg) point to the engulfment of a planet (of 1 − 10 Jupiter masses) by its Sun-like host star. We estimate the Galactic rate of such Sub-luminous Red Novae (SLRNe) to be ∼ 0.1−few yr −1 . Future Galactic plane surveys are well-poised to routinely identify them, revealing the demographics of planetary engulfment and the ultimate fate of planets in the inner Solar System.Using data from the Zwicky Transient Facility (ZTF) time domain survey 10 , we searched 2 for slowly evolving outbursts near the Galactic plane (Methods). We identified a transient optical source named ZTF 20aazusyv (hereafter ZTF SLRN-2020) that exhibited a fast rise from quiescence to peak outburst flux in ≈ 10 days, subsequently fading by ≈ 10× over six months (Figure 1).The long optical outburst duration together with its faint peak flux distinguishes it from common Galactic plane transients resulting from white dwarfs with close binary companions (the 'dwarf' and 'classical' novae 11 ). The transient also exhibits a mid-IR brightening starting ≈ 7 months prior to the optical outburst, together with bright mid-infrared (IR) emission (> 50× brighter than the optical r-band at ≈ 4 months after optical peak) that lasted for ≳ 15 months. No X-ray emission was detected in follow-up observations during the outburst using the Swift telescope 12 , ruling out an unstable disk accretion episode around a neutron star or black hole 13 (Methods).The bright mid-IR emission during the outburst is suggestive of emission from a warm dust shell surrounding the stellar photosphere. We model the optical to mid-IR spectral energy distribution (SED; Methods) at ≈ 120 days after the optical peak. The analysis reveal a relatively hot inner photosphere (≈ 9000 K) surrounded by a warm dust shell with temperature ≈ 1000 K, located behind a dust visual extinction column of A V ≈ 3.6 mag (Figure 2). Using the 90% confidence interval on the foreground extinction together with three-dimensional Galactic dust distribution maps (Methods), we infer the source to be located at a distance of 2 to 7 kpc. A joint analysis of the overlap between the different dust maps suggests a likely distance of ≈ 4 kpc (Methods). Performing the same analysis at ≈ 320 days after peak, we find the SED to have predominantly shifted into the IR bands caused by an increase in the dust optical depth, that can be att...

RZ Piscium Hosts a Compact and Highly Perturbed Debris Disk

“…Punzi et al (2018) suggested that the hot gas revealed from the optical spectroscopy could originate from the accretion of material from a hot Jupiter-like planet swallowed by the star. The aftermath of such an engulfment might have been observed recently in ZTF SLRN-2020 (De et al 2023). If true, the detailed evolution of the hot gas in RZ Psc might provide an additional test bed for such a phenomenon.…”

Orbital Decay of Hot Jupiters due to Weakly Nonlinear Tidal Dissipation

“…McQuillan et al (2013) find a dearth of close-in planets orbiting rapidly rotating stars, which Teitler & Königl (2014) attribute to tidal ingestion of giant planets. The recently reported infrared transient ZTF SLRN-2020 appears to capture the moments of a planet's ingestion by a main sequence or early subgiant branch star with mass around 0.8-1.5M e , and could be the culmination of tide-induced orbital decay (De et al 2023).…”

Evidence That the Occurrence Rate of Hot Jupiters around Sun-like Stars Decreases with Stellar Age

“…The rapid rotation reported for HJ hosts may also be a signature of stellar spin-up associated with orbital decay, i.e., angular momentum transfer from the orbit to spin (e.g., Penev et al 2018;Tejada Arevalo et al 2021). Even the detection of an infrared transient associated with the engulfment of a shortperiod planet has been reported (De et al 2023). Of particular relevance to our paper is the work by Hamer & Schlaufman (2019), who showed that Sun-like stars known to host HJs have a smaller Galactic velocity dispersion than their control stars without known planets, and those with known cold Jovian planets.…”