# Planetesimals Around Stars with TESS (PAST): I. Transient Dimming of a Binary Solar Analog at the End of the Planet Accretion Era

## TL;DR

This paper reports the discovery of quasi-periodic dimming in a 125-million-year-old binary star system, likely caused by disintegrating planetesimals, providing insights into the final stages of rocky planet formation.

## Contribution

It presents the first detection of transient dimming in a star of this age, linking it to planetesimal disintegration and disk clearing at the end of planet formation.

## Key findings

- Quasi-periodic dimming observed every 1.5 days.
- Infrared excess indicates circumstellar dust presence.
- Dimming likely caused by disintegrating planetesimal near evaporation radius.

## Abstract

We report detection of quasi-periodic (1.5 day) dimming of HD 240779, the solar-mass primary in a 5" visual binary (also TIC 284730577), by the Transiting Exoplanet Survey Satellite. This dimming, as has been shown for other "dipper" stars, is likely due to occultation by circumstellar dust. The barycentric space motion, lithium abundance, rotation, and chromospheric emission of the stars in this system point to an age of ~125 Myr, and possible membership in the AB Doradus moving group. As such it occupies an important but poorly explored intermediate regime of stars with transient dimming between young stellar objects in star forming regions and main sequence stars, and between UX Orionis-type Ae/Be stars and M-type "dippers". HD 240779, but not its companion BD+10714B, has WISE-detected excess infrared emission at 12 and 22 microns indicative of circumstellar dust. We propose that infrared emission is produced by collisions of planetesimals during clearing of a residual disk at the end of rocky planet formation, and that quasi-periodic dimming is produced by the rapid disintegration of a 100 km planetesimal near the silicate evaporation radius. Further studies of this and similar systems will illuminate a poorly understood final phase of rocky planet formation like that which produced the inner Solar System.