Infrared Dark Clouds are the birthplaces of both high-mass stars and star clusters. Dr. James Jackson (Boston University) and his team will analyze a large suite of observations at centimeter, millimeter, sub-millimeter, and infrared wavelengths, supplemented by new observations at the Very Large Telescope, the Australian Telescope National Facility 22-m telescope, Plateau de Bure, and Institut de Radio Astronomie Millimétrique 30-m telescopes to explore the poorly understood processes at play in high-mass protostellar evolution. The team will complete six key science projects in this funding period: (1) Measuring the spectral energy distributions, and hence, bolometric luminosities, of ~100 high-mass protostars, (2) Searching for high-mass "pre-stellar" cores, (3) Testing the suggestion of "competitive accretion" by searching for circumstellar disks surrounding high mass stars, (4) Testing the idea that young O stars actually enter the main-sequence as B stars and then continue to accrete material on their way to becoming main-sequence O stars, (5) Measuring the clump mass function in high-mass star forming clouds, and (6) Studying the chemical evolution of high-mass protostellar cores. When this project is complete, the team will have made a significant contribution to the understanding of the poorly-known processes at play in the earliest stages of high-mass star formation and will have also tested two key theoretical ideas: "competitive accretion" and the idea that O stars first enter the main-sequence as accreting B stars.
Dr. Jackson's group has led the way in identifying and characterizing Infrared Dark Clouds (IRDCs). Their characteristic sizes (~few parsecs) and masses (~few 1,000 solar-masses) resemble those of cluster-forming molecular clumps such as those in Orion and Ophichus. Their cold temperatures, however, suggest that they are in an earlier evolutionary state. Imaging of the mm/sub-mm dust continuum reveals that IRDCs contain compact cores, whose sizes (<0.5 parsecs) and masses (~100 solar masses) match those of "hot cores" associated with high-mass protostars. An examination of these cores reveals that about 1/3 contain unambiguous evidence of star formation. The large bolometric luminosities of the embedded protostars within several IRDC cores show that they are currently forming high-mass stars. Interferometric observations reveal that these high-mass stars are accompanied by a number of lower mass protostars. All of the initial evidence, therefore, suggests that IRDCs host the earliest stages of the formation of high-mass stars and star clusters. In addition to the research that will be carried out under this award, this proposal will fund the training and education of two female graduate students and one female post-doctoral student. Moreover, the Boston University Astronomy Department has entered into a collaborative agreement with Fisk University, a historically black college, and Vanderbilt University to participate in the Fisk-Vanderbilt Masters-Ph.D. Bridge Program which allows Fisk students to pursue their Master's degrees at Fisk with guaranteed admission into Ph.D. programs at Vanderbilt and Boston University.
are giant gas clouds that look like dark patches in infrared images of our home galaxy, the Milky Way. When Infrared Dark Clouds were first identified, they were viewed mainly as curiosities, and early speculation suggested that they played no role in the process of star formation. Our work showed that IRDCs are in fact host the early stages of high-mass star formation. They are dark simply because they are cold. As the cold molecular cores collapse and form stars, internal heating begins, and eventually the IRDCs are warm and bright. Work supported by this grant first demonstrated that IRDCs are indeed the precursors to high mass stars. They contain high-mass cores that are in the early stages of star-formation. We found hundreds of such cores, and showed that they smoothly evolve from cold quiescent cores, to warmer protostellar cores, and finally to "hot cores" associated with H II regions (Chambers et al. 2009; Battersby et al. 2010; Rathborne et al. 2010, 2011). Thus, we have unambiguously established that high-mass stars begin their lives within IRDCs. We have also found a technique to classify the evolutionary state based on their infrared characteristics (Chambers et al. 2009; Rathborne et al. 2010) and have shown that the properties of the cores are exactly those expected for an evolving population internally heated by newly formed stars (Rathborne et al. 2010). One of the biggest surprises of this study was the realization that the infrared dark clouds typically have a very filamentary shape. The most extreme examples is the Nessie Nebula (Jackson et al. 2010), which has an aspect ratio (length/width) of over 300:1. The filamentary nature of IRDCs may provide a clue to the link between them and star formation. In the Nessie Nebula, we noticed that the star-forming clumps are spaced regularly along the filament, like beads on a string. This regular spacing may suggest a formation mechanism. We speculate that a fluid instability called the "sausage instability" leads to regular spaced clumps, and that such clumps would have exactly the right properties to make star clusters. If confirmed, this result would explain why all star clusters have their characteristic mass. Our work has also succeeded in determining the location of IRDCs within the Milky Way. We showed that IRDCs are almost exclusively found in spiral arms (Jackson et al. 2008, Finn et al. 2013). Since high-mass stars are born and live their lives in spiral arms, this results further strengthens the link between high-mass star-formation and IRDCs.
