2002ApJ...568L.127F

We observed heads of two molecular pillars in the Eagle Nebula using the Nobeyama Millimeter Array with a spatial resolution of 3", 5", and 3" in the ¹³CO (J=1-0) line, C¹⁸O (J=1-0) line, and 2.7 mm continuum, respectively. We found six ¹³CO subclumps and four C¹⁸O cores. The ¹³CO clouds are elongated so as to have a head-tail structure, with the heads orientated toward the O star exciting M16. The elongation is likely to be due to radiation or wind from the O star. The cloud surface appears to be compressed, as indicated by strong ¹³CO emission at the cloud rim. The shapes of the ¹³CO clouds are quite similar to those of the dark cloud observed in the near-infrared. Three out of the four C¹⁸O cores are located within one of the ¹³CO clouds. One of the three cores, located near the tip of the ¹³CO cloud, is associated with a 2.7 mm continuum emission peak and is most likely a protostar. It is not associated with a known near-infrared source. The other two cores are located farther from the O star and are most likely starless cores. Thus, these C¹⁸O cores are aligned in order of age, with the more evolved objects closer to the O star. This linear sequence suggests the propagation of star formation activity, i.e., sequential star formation, driven by the O star. A similar sequence of a young stellar object and a C¹⁸O core was found in the other head of the molecular pillar.