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

Using the HI self-absorption data from the Five-hundred-meter Aperture Spherical radio Telescope (FAST), we perform a study of the cold atomic gas in the Cygnus-X North region. The most remarkable HI cloud is characterized by a filamentary structure, associated in space and in velocity with the principle molecular filament in the Cygnus-X North region. We investigate the transition from the atomic filament to the molecular filament. We find that the HII 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 HI in the DR21 filament has a much larger column density (N(HI) \(\sim\) 1 \(\times\) 10\(^{20}\) cm\(^{-2}\)) than the theoretical value of the residual atomic gas (\(\sim\) 1 \(\times\) 10\(^{19}\) cm\(^{-2}\)), suggesting that the HI-to-H\(_2\) transition is still in progress. The timescale of the HI-to-H\(_2\) transition is estimated to be 3 \(\times\) 10\(^{5}\) 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.