SUBMILLIMETER ARRAY OBSERVATIONS OF INFRARED DARK CLOUDS: A TALE OF TWO CORES

We present high angular resolution submillimeter continuum images and molecular line spectra obtained with the Submillimeter Array toward two massive cores that lie within infrared dark clouds (IRDCs), one actively star-forming (G034.43+00.24 MM1) and the other more quiescent (G028.53-00.25 MM1). The high angular resolution submillimeter continuum image of G034.43+00.24MM1 reveals a compact (similar to 0.03 pc) and massive (similar to 29M(circle dot)) structure, while the molecular line spectrum shows emission from numerous complex molecules. Such a rich molecular line spectrum from a compact region indicates that G034.43+00.24 MM1 contains a hot molecular core, an early stage in the formation of a high-mass protostar. Moreover, the velocity structure of its (13)CO (3-2) emission indicates that this B0 protostar may be surrounded by a rotating circumstellar envelope. In contrast, the submillimeter continuum image of G028.53-00.25 MM1 reveals three compact (less than or similar to 0.06 pc), massive (9 -21M(circle dot)) condensations, but there are no lines detected in its spectrum. We suggest that the core G028.53-00.25 MM1 is in a very early stage in the high-mass star formation process; its size and mass are sufficient to form at least one high-mass star, yet it shows no signs of localized heating. Because the combination of high-velocity line wings with a large IR-to-millimeter bolometric luminosity (similar to 10(2) L(circle dot)) indicates that this core has already begun to form accreting protostars, we speculate that the condensations may be in the early phase of accretion and may eventually become high-mass protostars. Therefore, we have found the possible existence of two high-mass star-forming cores: one in a very early phase of star formation and one in the later hot-core phase. Together, the properties of these two cores support the idea that the earliest stages of high-mass star formation occur within IRDCs.