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 × 10 ^20 cm ^−2 ) than the theoretical value of the residual atomic gas (∼1 × 10 ^19 cm ^−2 ), suggesting that the H i -to-H _2 transition is still in progress. The timescale of the H i -to-H _2 transition is estimated to be 3 × 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.