H i Narrow-line Self-absorptions toward the High-mass Star-forming Region G176.51+00.20

Using the Five-hundred-meter Aperture Spherical radio Telescope (FAST) 19-beam tracking observational mode, high-sensitivity and high-velocity-resolution H i spectral lines have been observed toward the high-mass star-forming region G176.51+00.20. This is a pilot study searching for H i narrow-line self-absorption (HINSA) toward high-mass star-forming regions where bipolar molecular outflows have been detected. This work is confined to the central seven beams of FAST. Two HINSA components are detected in all seven beams, which correspond to a strong CO emission region (SCER; with a velocity of ∼−18 km s−1) and a weak CO emission region (WCER; with a velocity of ∼−3 km s−1). The SCER detected in Beam 3 is probably more suitably classified as a WCER. In the SCER, the HINSA is probably associated with the molecular material traced by the CO. The fractional abundance of HINSA ranges from ∼1.1 × 10−3 to ∼2.6 × 10−2. Moreover, the abundance of HINSA in Beam 1 is lower than that in the surrounding beams (i.e., Beams 2 and 4–7). This possible ring could be caused by the ionization of H i or the relatively rapid conversion from H i to H2 in the higher-density inner region. In the WCER (including Beam 3 in the SCER), the HINSA is probably not associated with CO clouds, but with CO-dark or CO-faint gas.