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We report detection of quasi-periodic (1.5 day) dimming of HD 240779, thesolar-mass primary in a 5" visual binary (also TIC 284730577), by theTransiting Exoplanet Survey Satellite. This dimming, as has been shown forother "dipper" stars, is likely due to occultation by circumstellar dust. Thebarycentric space motion, lithium abundance, rotation, and chromosphericemission of the stars in this system point to an age of ~125 Myr, and possiblemembership in the AB Doradus moving group. As such it occupies an important butpoorly explored intermediate regime of stars with transient dimming betweenyoung stellar objects in star forming regions and main sequence stars, andbetween UX Orionis-type Ae/Be stars and M-type "dippers". HD 240779, but notits companion BD+10714B, has WISE-detected excess infrared emission at 12 and22 microns indicative of circumstellar dust. We propose that infrared emissionis produced by collisions of planetesimals during clearing of a residual diskat the end of rocky planet formation, and that quasi-periodic dimming isproduced by the rapid disintegration of a 100 km planetesimal near the silicateevaporation radius. Further studies of this and similar systems will illuminatea poorly understood final phase of rocky planet formation like that whichproduced the inner Solar System.

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