Dense shell around a young intermediate-mass star: NGC-2264 IRS 1

The results of (HCO+)-C-13 J = 1-0 line and 93-GHz continuum observations of NGC 2264 IRS 1, a luminous infrared source known as Allen's source, are reported. IRS I is a young intermediate-mass star with a surrounding YSO cluster, and is associated with a molecular outflow. High angular resolution interferometric observations were performed using the Nobeyama Millimeter Array, and mapping observations were conducted using a 45-m telescope. The continuum map reveals four sources, three of which correspond to a submillimeter source detected by Ward-Thompson et al. (2000, AAA 73.137.062). Four compact clumps are also observed in the (HCO+)-C-13, map, two of which are associated with these millimeter/ submillimeter continuum sources without bipolar outflow. These (HCO+)-C-13, clumps as a whole form an incomplete dense shell of 0.12 pc in diameter around IRS 1, and the outer part of the shell is clearly traced on the single-dish map. The shell, or cavity, structure suggests that the outflow from IRS I has evacuated part of the surrounding envelope in similar to0.1 Myr. The kinematic properties can be interpreted as being two clumps of entrained or compressed material derived from steady outflow. It is concluded that the activity of IRS I may have triggered the formation of cluster members in the dense surrounding shell.