Evidence of Planetesimal infall on to the very young Herbig Be star LkH$_α$234

Abstract

We report here the first evidence for planetesimal infall onto the very young Herbig Be star LkH$_α$234. These results are based on observations acquired over 31 days using spectroscopy of the sodium D lines, the He I 5876Å, and hydrogen H$_α$ lines. We find Redshifted Absorption Components (RAC) with velocities up to 200 km/s and very mild Blueshifted Absorption Components (BEC) up to 100 km/s in the Na I lines. No correlation is observed between the appearance of the Na I RAC & BEC and the H$_α$ and He I line variability, which suggests that these (Na I RAC & BEC) are formed in a process unrelated to the circumstellar gas accretion. We interpret the Na I RAC as 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. The dramatic appearance of the sodium RAC and mild BEC is readily explained by the dynamics of this infalling body making LkH$_α$234 the youngest (age $\sim$ 0.1 Myr) system with evidence for the presence of planetesimals.