2013A&A...559A.113X

We investigate the triggered star formation occurring in the infrared dark clouds (IRDC) G38.95-0.47 between HII regions G38.91-0.44 and G39.30-1.04, and study the detailed morphology, distribution, and physical parameters of the molecular gas and dust in this region. We present the radio continuum, infrared, and CO molecular observations of IRDC G38.95-0.47 and its adjacent HII regions G38.91-0.44 (N74), G38.93-0.39 (N75), and G39.30-1.04. The Purple Mountain Observation (PMO) 13.7 m radio telescope was used to detect ¹²CO J=1-0, ¹³CO J=1-0 and C¹⁸O J=1-0 lines. The carbon monoxide (CO) molecular observations can ensure the real association between the ionized gas and the neutral material observed nearby. To select young stellar objects (YSOs) associated this region, we used the GLIMPSE I catalog. The ¹³CO J=1-0 emission presents two large cloud clumps. The clump consistent with IRDC G38.95-0.47 shows a triangle-like shape, and has a steep integrated-intensity gradient toward HII regions G38.91-0.44 and G39.30-1.04, suggesting that the two HII regions have expanded into the IRDC. Four submillmeter continuum sources have been detected in the IRDC G38.95-0.47. Only the G038.95-00.47-M1 source with a mass of 117M_☉ has outflow and infall motions, indicating a newly forming massive star. We detected a new collimated outflow in the clump compressed by G38.93-0.39. The derived ages of the three HII regions are 6.1x10⁵yr, 2.5x10⁵yr, and 9.0x10⁵yr, respectively. In the IRDC G38.95-0.47, the significant enhancement of several Class I YSOs indicates the presence of some recently formed stars. Comparing the ages of these HII regions with YSOs (Class I sources and massive G038.95-00.47-M1 source), we suggest that YSOs may be triggered by G38.91-0.44 and G39.30-1.04 together, which supports the radiatively driven implosion model. It may be the first time that the triggered star formation has occurred in the IRDC compressed by two HII regions. The new detected outflow may be driven by a star cluster.