Multiple low-turbulence starless cores associated with intermediate- to high-mass star formation

Abstract

To characterize the initial conditions for intermediate- to high-mass star formation, we observed two Infrared Dark Clouds (IRDCs) that remain absorption features up to 70mum wavelength, with the PdBI in the 3.23mm dust continuum as well as the N2H+(1--0) and 13CS(2-1) line emission. While IRDC19175-4 is clearly detected in the 3.23mm continuum, the second source in the field, IRDC19175-5, is only barely observable above the 3sigma continuum detection threshold. However, the N2H+(1-0) observations reveal 17 separate sub-sources in the vicinity of the two IRDCs. Most of them exhibit low levels of turbulence (dv \leq 1km/s), indicating that the fragmentation process in these cores may be dominated by the interplay of thermal pressure and gravity, but not so much by turbulence. Combining the small line widths with the non-detection up to 70mum and the absence of other signs of star formation activity, most of these 17 cores with masses between sub-solar to ~10M_sun are likely still in a starless phase. Furthermore, we find a large CS depletion factor of the order 100. Although the strongest line and continuum peak is close to virial equilibrium, its slightly broader line width compared to the other cores is consistent with it being in a contraction phase potentially at the verge of star formation. The relative peak velocities between neighboring cores are usually below 1km/s, and we do not identify streaming motions along the filamentary structures. Average densities are between 10^5 and 10^6cm^{-3} (one to two orders of magnitude larger than for example in the Pipe nebula) implying relatively small Jeans-lengths that are consistent with the observed core separations of the order 5000AU. The quest for high-mass starless cores prior to any star formation activity remains open.