A molecular cloud complex above the galactic plane.: I. Extended CO observations of the NGC 281 region

Abstract

We have mapped the entire extent of the molecular cloud complex associated with NGC 281, both in 12 CO ( J=1−0) and 13 CO ( J=1−0), using the Taeduk Radio Astronomy Observatory (TRAO) 14 m telescope. We also observed a few selected fields with bright 13 CO emission, in CS ( J=2−1) and HCO + ( J=1−0). This region is distinguished by its vertical height of ∼320 pc above the midplane of the Perseus spiral arm, at a distance of 2.9 kpc. The molecular gas in the mapped region is found to extend far beyond the H ii region NGC 281 (Sh 184). The southern boundary of the CO emission closely follows the southern boundary of the 100 μm dust emission. The molecular cloud complex consists of two main clouds bisected by the H ii region, as well as several other clouds distributed over a range of 90 pc along the galactic longitude direction. Except for one cloud at V LSR=−43.9 km s −1, all these clouds occupy a small, contiguous velocity range centered at V LSR=−30 km s −1. Although these clouds are weakly connected, they clearly show distinct, knotted features along the galactic longitude direction. We assigned names to each cloud (A–H), derive their various physical parameters, and discuss their physical properties. These clouds do not show any specific, systematic behavior in their velocity field, except for a small gradient along galactic longitude (for A–E). The brightest 12 CO and 13 CO emissions are detected toward cloud A, which is located farthest from the two main clouds. We found star formation to be ongoing in clouds A and B, though these are not as active as D and E. The associated newborn stars have FIR properties similar to those of Herbig Ae/Be stars, and several outflows are also seen. We estimated the cloud masses in three different ways, and discuss the discrepancies between the resulting values. We estimate the total mass of the mapped region to be ∼3.7×10 4 M ⊙, using a conversion factor from CO luminosity to gas mass. About half of the clouds seem to be gravitationally bound.