Prof James Jackson
(Director, Institute for Astrophysical Research, Boston University)
Infrared Dark Clouds: Precursors to Massive Stars and Clusters - Prof James Jackson Colloquium
Infrared Dark Clouds (IRDCs) are a new class of objects discovered
as extinction features at mid-infrared wavelengths. Preliminary
studies show they are dense (>10^5 cm^-3), cold (<25 K), and have
very high column densities (~10^23 to 10^25 cm^-2). The absence of
embedded mid-IR/far-IR sources, together with the low temperatures,
indicate that IRDCs lack the massive young stars usually associated
with dense molecular gas clouds.
By matching morphologies of 13CO emission to the mid-infrared extinction,
we have determined the kinematic distances to ~300 IRDCs. We find that
their sizes (a few pc) and masses (a few thousand M_sun) are typical of
molecular hot cores associated with massive star forming regions.
Their Galactic distribution peaks in the 5 kpc molecular ring, the
Galaxy's dominant star-forming structure. Each IRDC contains one to
a few compact cores, with sizes >0.5 pc and masses ~70 M_sun. These
cores are the likely sites of future massive star formation. Spitzer
data show that a few of these cores contain bright 8 - 24 um sources
with L~10,000 Lsun, indicating the presence of massive protostars.
IRDCs, therefore, are the precursors to massive stars and clusters.
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