HST Far-ultraviolet Transit Observations of Two Neptune Progenitors Younger than 30 Myr

Abstract Photoevaporation is believed to dominate the removal of planetary atmospheres when they are young (<100 Myr). Signatures of atmospheric mass-loss can be observed in the ultraviolet (UV) through the near-infrared. We present Far-UV transit observations of AU Mic b (∼22 Myr) and V1298 Tau c (∼28 Myr) with the Hubble Space Telescope. We search for evidence of escaping metals in the C ii, Si ii, and Si iii emission lines and compare their behavior to tracers of stellar activity. We detect no evidence of atmospheric mass loss, and place upper limits on the radii of AU Mic b and V1298 Tau c.