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

Infrared Dark Clouds GLIMPSE 8 mm image 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 Infrared dark clouds are molecular clouds that exhibit significant mid IR opacity such that they are seen as extinction features against the bright mid IR galactic background. They have extreme properties in that they are cold, very dense, with enormous column densities. Some are dark up to 100um. Molecular line and mm continuum data show their sizes and masses are comparable to warm, cluster-forming molecular clumps (like Orion and Ophiucus). The noticeable difference, however, is that IRDCs are much colder with little evidence of 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

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 Because IRDCs are so cold, they are strong emitters at millimeter and submillimeter wavelengths. An advantage of using the millimeter continuum is that the emission is optically thin, so we can trace their internal structure. We recently conducted a survey of a sample of IRDCs in the milllimeter continuum using the iRAM 30 m. We found that the millimeter continuum emission matched the dark extinction features very well and that all IRDCs contain compact, cold cores with typical sizes of 0.5 pc and masses of 75 Msun. These properties are what one would expect for cores that will give rise to high-mass protostars. Contours 1.2 mm; colour 8 m MSX Rathborne et al. 2006

Are the IRDC cores forming stars? 1 pc Detected 140 compact, cold cores 2/3 of the cores show no evidence for star formation Massive starless cores? What we’d like to determine is if these cores are forming stars. From the 140 compact cores we identified from our mm continuum survey, we found that 2/3 of the cores show no evidence of star formation (no infrared emission, no shocked gas, little molecular line emission). These are likely the earliest stages in the star formation process - the massive starless cores. GLIMPSE 3-color image (3.6, 4.5, 8.0 m)

Active star formation in IRDC cores Spitzer/ IRAC MIPS 24m 3.6, 4.5, 8.0 m IRAM/JCMT JCMT 450 m 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 In contrast, the remaining one third of our sample shows evidence for active star formation. In particular, the cores show (1) broad linewidths in the high-density tracing molecular lines, CS and HCN indicating protostellar outflows (2) Strong SiO emission, indicating shocks induced by outflows (3) extended, diffuse 3--8 um emission, with an apparent enhancement in the 4.5um band
which may arise from a shock-excited H2 (4) compact ($<$6''), bright 24\,\um\, emission, indicating the presence of warm dust and an embedded protostar. In some cases we see high bolometric luminosities and very bright water maser emission. What is unclear at the moment is if these high luminosities arise from a singe high-mass protostar or a number of lower-mass protostars. Chambers et al. 2005, Rathborne et al. 2005, 2007, Wang et al. 2006

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 There is growing evidence that some IRDCs are in fact forming high-mass protostars. IR studies of young embedded clusters reveal that high-mass stars form within clusters accompanied by many stars of differing mass. If this is the case and high-mass star formation is occurring within IRDCs, then they ought to also be forming an associated cluster of lower-mass stars. Given the typical distances to the IRDCs, most data have lacked the sensitivity to detect any low-mass protostars. To begin to address this question, we have observed a sample of high- mass cores using the IRAM and SMA interferometers.These data will have improved sensitivity and angular resolution that will allow us to distinguish between these scenarios. Sensitive, high angular resolution mm/submm observations can distinguish between these two scenarios

A high-mass protostar 2 Jy W Water maser M = 800 M Lbol = 104.5 L SMA 345 GHz 2’’ (0.02 pc) 2 Jy 17 M MIPS 24m Water maser W VLA This is an example of an IRDC core that is forming a high-mass protostar. It shows strong molecular line emission from the high-density tracing molecular lines HCN and CS and very broad SiO emission. It has extended, green emission in IRAC 3 colour images (I.e. an enhancement at 4.5um which most likely arises from shocked H2). While the cloud itself remains dark at 24um, very bright 24um emission is found coincident with it. The mm to IR spectral energy distribution for this core reveals that its bolometric luminosity is high. The high-angular resolution image obtained with the SMA reveals a single, unresolved object suggesting that the majority of the luminosity probably arises from a single high-mass protostar. Moreover, we see very bright water maser emission exactly coincident with this unresolved source. M = 800 M Lbol = 104.5 L 6 ” angular resolution

An early stage of high-mass star formation, a hot molecular core The presence of a variety of complex spectral features confirms this as a hot molecular core and, thus, in an early stage of forming a high-mass protostar. Dimethyl ether CH3OCH3 Methanol CH3OH Acetonitrile CH3CN

The “hot core” lines are unresolved; 13CO 3-2 reveals circumstellar structure 2’’ (0.02 pc) Color: SO2 (116,6-125,7) Contours: 13CO 3-2 Red: 61 km s-1 Blue: 55 km s-1 Indeed, we find that the complex hot core lines are unresolved, while the 13CO Emission traces the circumstellar structure. Which either arises from an outflow or disk.

A cluster of protostars IRAM PdeBI Spitzer/MIPS 24mm 1mm continuum 1.5”, 0.03 pc m M = 2 M 11 M 2 M 4 M The situation that is more typical for the IRDC cores is that, at higher angular resolution, the mm cores break up into multiple protostellar condensations. For this particular IRDC core, we see a 24um point source coincident with the mm core. At high-angular resolution, we see emission also coincident with the peak in the lower-angular resolution continuum emission and if we zoom in, we find that in fact the high-angular resolution images reveal at least four unresolved condensations with masses ranging from 2 to 11 solar masses. M = 200 M Lbol = 104 L Angular resolution 6’’

A high- and low-mass protostar 1mm continuum 1.8”, 0.04 pc IRAM PdeBI Spitzer/MIPS 24mm M = 19 M M = 3 M m Again, in this particular case, we see the core is coincident with 24um emission and that there is bright, compact emission coincident with the lower-angular resolution mm core. If we zoom in on this emission, we find that the core contains two unresolved condensations, a high-mass and low-mass protostar perhaps. M = 230 M Lbol = 103.4 L

Cluster formation in IRDCs Size ~ few pc Mass ~ few 1000 M IRDC cores Size ~ 0.5 pc Mass ~ 100 M The emerging picture of IRDCs is one in which they are the birthplaces of high-mass protostars and star clusters. Each IRDC contains many cores, and in turn, these cores contain condensations that will likely give rise to the individual stars. Notice that we see structure on all size scales, from large to small and from the diffuse to the very dense. This suggests that hierarchical fragmentation may be occurring within IRDCs. Because we can estimate the densities and masses of the IRDCs, cores and condensations, we can directly test the idea that IRDCs represent a later stage in the fragmentation of a molecular cloud by comparing their gas masses to the Jeans mass. IRDC condensations Size ~ 0.04 pc Mass ~ few M

Jeans masses, Jeans lengths, and hierarchical fragmentation Object Gas Mass (M ) Jeans (M) MG/MJ Radius (pc) J IRDCs ~1800 ~280 ~5 ~1.5 ~0.6 IRDC cores ~150 ~140 ~1.3 ~0.1 ~0.07 IRDC condensations ~3 ~30 ~0.2 ~0.02 IRDCs should fragment into several gravitationally collapsing substructures  IRDC cores IRDC cores are also expected to fragment, however, fewer fragments might be expected  protostars and protostellar condensations For the IRDCs, the gas masses exceed the jeans masses to within a factor of a few. This, one would expect IRDCs to fragment into several gravitationally collapsing substructures. Indeed, IRDCs typically contain many compact cores, which are presumably these gravitationally unstable, collapsing fragments. In turn, the IRDC core themselves also exceed their Jeans masses, with typical ratios slightly larger than unity. Further gravitational collapse then occurs, although fewer fragments might be expected. The unambiguous presence of protostars within IRDC cores and the direct detection of small stellar-mass condensations demonstrate that further fragmentation and gravitational collapse is indeed occurring within IRDC cores. At the smallest scales of the individual condensations, the gas mass is typically smaller than the Jeans mass. Ratios less than unity could arise because of the influence of magnetic fields at these small scales or because the condensations are unresolved in the current data. The steady fall in the gas to Jeans mass ratio from several for the IRDCs to approximately unity for the IRDC cores, to 0.2 for protostellar condensations suggests that the fragmentation process if most efficient at larger scales and less efficient at smaller scales, eventually reaching a natural halt to the scale of typical protostars, perhaps reflecting the importance of magnetic fields or turbulence at small size scales. This can also be seen if we look at the Jeans lengths of the IRDCs, cores and condensations. Interestingly, the Jeans length for the IRDCs are a factor of a few larger than the radii calculated for the IRDC cores. Similarly, the Jeans lengths for the IRDC cores are also a factor of a few larger than the radii of the IRDC condensations. On the smallest scales of these individual condensations, however, the Jeans lengths are comparable to the radii suggesting these structures are unlikely to fragment further. These results support the idea that gravitational collapse is occurring within the IRDCs and that fragmentation process has stopped at these protostellar scales. At the smallest scales the fragmentation process ends as the condensations reach the sizes and masses of individual protostars These results support the idea that gravitational collapse is occurring within the IRDCs and that fragmentation process has stopped at these protostellar scales

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

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