Infrared Dark Clouds as precursors to star clusters

1. Infrared Dark Clouds as precursors to star clusters Jill Rathborne, James Jackson, Edward Chambers, Robert Simon, Qizhou Zhang

2. Infrared Dark Clouds • Clouds that exhibit significant mid-IR opacity • Extreme properties • Cold (<20 K) • Dense (>104 cm-3) • Enormous column densities (>1023 – 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

3. IRDCs in 1.2 mm continuum:dust emission reveals compact cores Sizes < 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

4. 2000 M 1 pc 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)

5. JCMT 450 m Spitzer/ IRAC IRAM/JCMT MIPS 24m 3.6, 4.5, 8.0 m 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 >104 L • Bright (>1 Jy) water maser emission Chambers et al. 2005, Rathborne et al. 2005, 2007, Wang et al. 2006

6. 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

7. SMA 345 GHz VLA 2 Jy 17 M 2’’ (0.02 pc) Water maser W A high-mass protostar MIPS 24m M = 800 M Lbol = 104.5 L 6 ” angular resolution

8. An early stage of high-mass star formation, a hot molecular core

9. SO2 13CO 2’’ (0.02 pc) The “hot core” lines are unresolved;13CO 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

10. 1mm continuum 1.5”, 0.03 pc m m m m 11 M m M = 2 M m m 2 M 4 M A cluster of protostars IRAM PdeBI Spitzer/MIPS 24mm M = 200 M Lbol = 104 L Angular resolution 6’’

11. IRAM PdeBI 1mm continuum M = 19 M M = 3 M 1.8”, 0.04 pc m A high- and low-mass protostar Spitzer/MIPS 24mm M = 230 M Lbol = 103.4 L

12. IRDCs • Size ~ few pc • Mass ~ few 1000 M • IRDC cores • Size ~ 0.5 pc • Mass ~ 100 M • IRDC condensations • Size ~ 0.04 pc • Mass ~ few M Cluster formation in IRDCs

13. IRDC cores are also expected to fragment, however, fewer fragments might be expected  protostars and protostellar condensations At the smallest scales the fragmentation process ends as the condensations reach the sizes and masses of individual protostars IRDCs should fragment into several gravitationally collapsing substructures  IRDC cores 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

14. 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

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