Transcript Infrared Dark Clouds in the GRS

Infrared Dark Clouds:
precursors to massive star clusters
Jill Rathborne1, Robert Simon2, James Jackson1 & Ronak Shah1
1 Institute
for Astrophysical Research, Boston University, 2 I.Physikal. Institut, Universitat zu Koln, 50937 Koln, Germany
Abstract
Infrared Dark Clouds (IRDCs) have recently been discovered in the Galactic surveys of MSX and ISO (Carey et al. 1998, Hennebelle et al. 2001). IRDCs are seen as
extinction features at mid-IR wavelengths, are very dense (> 105 cm-3), cold (< 25 K), and have very high column densities (1023-1025 cm-2; Egan et al. 1998, Carey et
al. 1998 ). We find the sizes and masses of IRDCs are typical of clumps that have recently formed stellar clusters. The dust masses of the many cold, compact sources
within IRDCs are typical of cores that will form individual massive stars. This is exactly what we expect if IRDCs are the precursors to massive stellar clusters.
Molecular properties
Dust Properties
Morphological similarities between the IR dark features and
13CO(1-0) emission allow us to assign distances to each
IRDC. Using the BU-FCRAO Galactic Ring Survey (Simon
et al. 2001), we have determined distances to ~300 IRDCs.
In addition, the molecular data reveal IRDCs have typical
sizes of a few parsecs, masses of a few 1000 Msun, and are
associated with larger molecular clouds. Such properties are
typical of clumps that have recently formed stellar clusters.
Observations with SIMBA on the SEST reveal 1.2 mm
continuum emission associated with each IRDC observed. The
morphology of the extended 1.2 mm emission matches the IR
extinction extremely well (left). In many cases multiple
1.2mm compact sources are also found within the IRDCs. The
compact sources have a median mass of 120 Msun, with a range
of 15-450 Msun (histogram below). These are presumably the
cores that will form the individual stars, of masses ~10 Msun.
The observed multiplicity suggests that the fragmentation
process has already begun. These results lend strong support to
recent ideas that suggest the stellar IMF is determined by the
fragmentation of clumps into cores, well before star formation
has begun (e.g. Williams et al. 2000).
A histogram of the masses of the
compact sources. We see a range
in masses from 15-450 Msun,
with a median of 120 Msun.
Galactic Distribution
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Face-on distribution for ~300 IRDCs (above). The inset shows
the number distribution with Galactocentric radius, weighted
by the area of the corresponding face-on segment. We see a
clear concentration of IRDCs at a radius of ~5 kpc, at precisely
the location of the Galaxy’s most massive and active star
forming structure, the so-called 5 kpc ring. This suggests a
close connection between IRDCs and star formation.
Images of nine IRDCs. The colour-scale is 8mm emission from MSX with
contours of 1.2 mm emission obtained with SIMBA on the SEST (levels
are 0.12 (3s), 0.24, 0.36, 0.72 Jy/beam). The morphology of the 1.2 mm
continuum emission matches the extinction features well. In some cases we
see extended filamentary features, while others show compact sources.
These are presumably the cores that will form the individual massive stars.
References
Carey, et al., 1998, ApJ, 508, 721
Egan, et al. 1998, ApJ, 494, L199
Hennebelle, et al., 2001, A&A,365,598
Simon, et al., 2001, ApJ, 551, 747
Williams, et al., 2000, Protostars & Planets IV, 97