An ALMA search for substructure and fragmentation in starless cores in Orion B North




Fielder, Samuel Dumaresq

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We present Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 3 observations of 73 starless and protostellar cores in the Orion B North molecular cloud. We detect a total of 34 continuum sources at 106 GHz, and after comparisons with other data, 4 of these sources appear to be starless. Three of these starless core detections are found at the edges of target fields, while the final one detection matches the peak of a starless core. We use synthetic observations of starless core collapse under the turbulent fragmentation model, to compute the expected number of starless cores that should be detectable with our ALMA observations and find one starless core should be detectable, consistent with our data. Comparable ALMA analyses have now been performed on three nearby molecular clouds; the number of detections in Orion B North and Ophiuchus are consistent with the turbulent fragmentation predictions, while the lack of detections in Chamaeleon I is inconsistent. We perform a virial analysis of the starless core population in all three clouds to test whether or not core boundedness can explain the differences in the number of ALMA observations. Using a simple alpha parameter analysis, we find that the starless core population in Chamaeleon I is systematically more unbound compared to Ophiuchus and Orion B North. With the addition of external pressure binding terms in our analysis, we conclude that the Chamaeleon I dense core population is still less bounded than the other two clouds. Furthermore, the pressure binding in Chamaeleon I contributes typically an order of magnitude more to the overall core boundedness than the other two regions. These differences may explain why the Chamaeleon I cores do not follow the turbulent fragmentation model predictions, while the Ophiuchus and Orion B North cores are consistent with the model.



star formation, radio astronomy, interferometry, starless cores, fragmentation, collapse, molecular clouds