Infrared Dark Clouds as precursors to star clusters

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Infrared Dark Clouds as precursors to star
clusters

• Jill Rathborne, James Jackson, Edward Chambers,
  Robert Simon, Qizhou Zhang

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Infrared Dark Clouds

• Clouds that exhibit significant mid-IR opacity
• Extreme properties
• Cold (lt20 K)
• Dense (gt104 cm-3)
• Enormous column densities (gt1023 1025 cm-2)
• Dark at 100 ?m
• Sizes (few pc) and masses (few 1000 M?)
  comparable to warm, cluster-forming molecular
  clumps
• Colder and with little obvious star formation

GLIMPSE 8 mm image
Perault et al. 1996 Egan et al. 1998 Carey et
al. 1998, 2000 Hennebelle et al. 2001 Simon et
al. 2006 Rathborne et al. 2006
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IRDCs in 1.2 mm continuumdust emission reveals
compact cores
Sizes lt 0.5 pc Masses 75 M ? These
properties are what we expect for cores that will
give rise to high-mass protostars
Rathborne et al. 2006
Contours 1.2 mm colour 8 ?m MSX
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Are the IRDC cores forming stars?

• Detected 140 compact, cold cores
• 2/3 of the cores show no evidence for star
  formation
• Massive starless cores?

GLIMPSE 3-color image (3.6, 4.5, 8.0 ?m)
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Active star formation in IRDC cores

• Strong mm/sub-mm continuum
• Broad linewidths in CS, HCN
• Strong SiO emission
• Extended, enhanced 4.5 ?m emission
• Compact, bright 24 ?m emission
• High bolometric luminosities gt104 L?
• Bright (gt1 Jy) water maser emission

Chambers et al. 2005, Rathborne et al. 2005,
2007, Wang et al. 2006
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Are IRDC cores forming single stars or star
clusters?

• Some IRDCs are forming high-mass stars
• High-mass stars form in clusters with many
  lower-mass stars
• If IRDCs are forming clusters, then they should
  also contain many lower-mass protostars
• Because IRDCs are distant, most observations have
  lacked the sensitivity and angular resolution to
  detect lower-mass protostars and separate
  multiple protostars

Sensitive, high angular resolution mm/submm
observations can distinguish between these two
scenarios
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A high-mass protostar
MIPS 24?m
M 800 M? Lbol 104.5 L?
6 angular resolution
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An early stage of high-mass star formation, a hot
molecular core
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The hot core lines are unresolved13CO 3-2
reveals circumstellar structure
Color SO2 (116,6-125,7) Contours 13CO 3-2 Red
61 km s-1 Blue 55 km s-1
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A cluster of protostars
IRAM PdeBI
Spitzer/MIPS 24mm
M 200 M? Lbol 104 L?
Angular resolution
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A high- and low-mass protostar
Spitzer/MIPS 24mm
M 230 M? Lbol 103.4 L?
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Cluster formation in IRDCs
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Jeans masses, Jeans lengths, and hierarchical
fragmentation
These results support the idea that gravitational
collapse is occurring within the IRDCs and that
fragmentation process has stopped at these
protostellar scales
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Summary

• IRDCs have sizes and masses comparable to warm,
  cluster-forming molecular clouds
• High-mass and multiple protostars found within
  IRDC cores
• The properties of IRDCs, their cores and
  protostellar condensations provide broad support
  for hierarchical fragmentation within IRDCs

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IRDCs are important laboratories to study the
very earliest stage in the formation of star
clusters