2009A&A...500.1119N

We examined the physical properties of molecular clouds (morphology, column density, number density, mass) to investigate the mechanism of triggered star formation by UV radiation from a massive star. We made extensive, high-resolution maps of molecular clouds associated and interacting with the W5-East HII region using the 45-m telescope at the Nobeyama Radio Observatory (HPBW=15.6") in ¹³CO (J=1-0) and C¹⁸O (J=1-0) to reveal details in the high-density regions of the molecular clouds. In addition, to investigate the spatial distributions of young stellar objects (YSOs) in the W5-East HII region, we mapped the spatial distributions of Class I and II candidates. We identified eight ¹³CO molecular clouds (three of them are known bright-rimmed clouds) and nine C¹⁸O clumps. The masses of the clouds and clumps range from 460 to 36000 M_☉ and from 55 to 740 M_☉, respectively. The peak ¹³CO column densities of the clouds facing the HII region are twice as large as the others. They have steep density gradients toward the HII region, indicating interactions with the HII region. We selected 55 Class I candidates and 778 Class II candidates associated with the W5-East HII region from the previous Spitzer IRAC/MIPS survey. Most Class I candidates are located around integrated intensity peaks of ¹³CO, whereas most Class II candidates are distributed along the front sides of the BRC arcs close to the exciting star. The alignments of the YSO candidates and the molecular clouds in order of age indicate that triggered star formation occurs in the W5-East HII region as a result of gas compression by strong UV radiation. Based on the column densities of ¹³CO and the spatial distribution of YSO candidates, we identified a new bright-rimmed cloud candidate on the west side of the W5-East HII region.