High-resolution Observations of HI in the IC 63 Reflection Nebula

Abstract

Photodissociation regions (PDRs), where the (far-)ultraviolet light from hot young stars interact with the gas in surrounding molecular clouds, provide laboratories for understanding the nature and role of feedback by star formation on the interstellar medium. While the general nature of PDRs is well understood—at least under simplified conditions—the detailed dynamics and chemistry of these regions, including gas clumping, evolution over time, etc., can be very complex. We present interferometric observations of the 21 cm atomic hydrogen line, combined with [C II] 158 μm observations, toward the nearby reflection nebula IC 63. We find a clumpy H I structure in the PDR, and a ring morphology for the H I emission at the tip of IC 63. We further unveil kinematic substructure, of the order of 1 km s−1 , in the PDR layers and several legs that will disperse IC 63 in <0.5 Myr. We find that the dynamics in the PDR explain the observed clumpy H I distribution and lack of a well-defined H I/H2 transition front. However, it is currently not possible to conclude whether H I self-absorption and nonequilibrium chemistry also contribute to this clumpy morphology and missing H I/H2 transition front.