H I Self-absorption toward the Cygnus X North: From Atomic Filament to Molecular Filament

Using the H I self-absorption data from the Five-hundred-meter Aperture Spherical radio Telescope, we perform a study of the cold atomic gas in the Cygnus X North region. The most remarkable H I cloud is characterized by a filamentary structure, associated in space and in velocity with the principal molecular filament in the Cygnus X North region. We investigate the transition from atomic filament to molecular filament. We find that the H II regions Cygnus OB2 and G081.920+00.138 play a critical role in compressing and shaping the atomic Cygnus X North filament, where the molecular filament subsequently forms. The cold H I in the DR21 filament has a much larger column density (N(H I) ~1 × 10²⁰ cm^-2) than the theoretical value of the residual atomic gas (~1 × 10¹⁹ cm^-2), suggesting that the H I-to-H₂ transition is still in progress. The timescale of the H I-to-H₂ transition is estimated to be 3 × 10⁵ yr, which approximates the ages of massive protostars in the Cygnus X North region. This implies that the formation of molecular clouds and massive stars may occur almost simultaneously in the DR21 filament, in accord with a picture of rapid and dynamic cloud evolution.