Gaia-GIC-1: An Evolving Catastrophic Planetesimal Collision Candidate

Abstract

We report the discovery of the optical dipper and low-luminosity infrared stellar transient Gaia20ehk (hereafter, Gaia-GIC-1), which is currently undergoing high-amplitude variability due to transiting dusty material. In this work, we identify Gaia-GIC-1 as a likely young F-type star based on the spectral energy distribution before the onset of the high-amplitude optical variability. We detect a significant periodic modulation of 380.5 days in Gaia-G band before the onset of the infrared brightening, consistent with a $\sim$1.1 AU orbit assuming circular orbits and a 1.3 M$_{\odot}$ star. The system has remained in an infrared bright state for $>$4 years since the last near-infrared detection, confirmed by recent SPHEREx observations, while continuing to undergo large amplitude irregular optical dimming. We measure the dust temperature from the freshly generated debris to be $\sim$900 Kelvin based on available WISE photometry, and the dust clump size to have a minimum cross-sectional area of 0.13 AU$^{2}$, and the dust mass $4\times 10^{20}$ kg. Currently, optical follow-up spectroscopy has not revealed any prominent features in the system, likely due to its highly variable nature. We hypothesize that Gaia-GIC-1 represents debris recently formed in a planetary collision, which produced a clumpy dust cloud on a bound orbit, producing the observed dimming events. The ongoing collisional activity in this system presents a unique opportunity for understanding terrestrial planet formation.

Figures (8)

• Figure 1: Multi–wavelength view of Gaia-GIC-1. Top‑left: DECaPS2 three‑color ($g\,r\,z$) mosaic, $40\hbox{$^{\prime\prime}$}\times40\hbox{$^{\prime\prime}$}$, with the identified star marked by yellow cross‑hairs. Top‑middle and top‑right: WISE/NEOWISE $W2$ ($4.6\,\mu$m) cut‑outs of identical size from 2018 and 2024, respectively, showing an increase in mid‑IR brightening and a subtle centroid shift relative to the neighboring field. Bottom: unWISE $W1/W2$ NEO7 coadd. The dashed yellow box indicates a 20$'$x20$'$ region around Gaia-GIC-1.
• Figure 2: Compiled optical and infrared light curve of Gaia-GIC-1. (Top) Optical Gaia-$G$ and $i$ band light curves. The dashed horizontal line is the median Gaia-$G$ magnitude in quiescence. (Bottom) WISE photometry in the W$_{1}$ and W$_{2}$, where upside-down triangles indicate the 3-$\sigma$ upper limits. The dashed vertical gray lines mark a 380.5-day interval, and the highlighted solid gray line is the epoch of the Gaia alert. The data are available as a machine-readable table.
• Figure 3: Optical spectra of Gaia-GIC-1 obtained with SOAR (purple) and SALT (black) telescopes. Gray lines show the unbinned spectra, while the colored lines show binned versions for clarity. Flux is given in arbitrary units.
• Figure 4: Corner plot showing posterior distributions from MIST fitting to pre-eclipse photometry of Gaia-GIC-1. Diagonal panels show marginalized posteriors for Teff, [Fe/H], $A_V$, radius, mass, distance, and log g. The off-diagonal panels show parameter correlations with 68% and 95% confidence contours.
• Figure 5: (Top) Spectral energy distribution of Gaia-GIC-1 spanning optical to mid-infrared wavelengths. The black dashed line shows the stellar photosphere model derived from MIST isochrone fitting, with the light gray shaded region indicating the associated uncertainty, and the upside-down triangles denote the $3\sigma$ upper limits. The solid black line represents a modified blackbody fit to the infrared excess emission after subtracting the photospheric contribution. (Bottom) Dust temperature evolution after subtracting the stellar photospheric contribution. The data are available as a machine-readable table.