Evidence of Planetesimal Infall onto the Very Young Herbig Be Star LkH 234

We report here the first evidence for planetesimal infall onto the very youngHerbig Be star LkH$_\alpha$234. These results are based on observationsacquired over 31 days using spectroscopy of the sodium D lines, the He I5876\AA, and hydrogen H$_\alpha$ lines. We find Redshifted AbsorptionComponents (RAC) with velocities up to 200 km/s and very mild BlueshiftedAbsorption Components (BEC) up to 100 km/s in the Na I lines. No correlation isobserved between the appearance of the Na I RAC & BEC and the H$_\alpha$ and HeI line variability, which suggests that these (Na I RAC & BEC) are formed in aprocess unrelated to the circumstellar gas accretion. We interpret the Na I RACas evidence for an infalling evaporating body, greater than 100 km in diameter,which is able to survive at distances between 2.0 to 0.1 AU from the star. Thedramatic appearance of the sodium RAC and mild BEC is readily explained by thedynamics of this infalling body making LkH$_\alpha$234 the youngest (age $\sim$0.1 Myr) system with evidence for the presence of planetesimals.Comment: Accepted for publications in ApJLetter