Abstract

Using observations of molecular line emission from species including NH3, OH, and C18O, Goodman et al. (1998) proposed that there exists a characteristic size scale of ~0.1 pc where the internal velocity dispersion of a core becomes subsonic and uniform, and called such subsonic cores the “coherent cores.”  Goodman et al. further proposed that star formation takes place in the “calm” subsonic environment inside the coherent cores, where gas does not follow the “turbulent” scaling laws observed on larger scales.  Since then, multiple coherent cores have been identified (Goodman et al., 1998; Caselli et al., 2002), and Pineda et al. (2010) further observed the spatial change in velocity dispersion from supersonic to subsonic values, occurring near the edge of a coherent core in the B5 region in Perseus.  Following upon the work of Pineda et al., we have recently identified a group of sub-0.1 pc coherent structures in L1688 in Ophiuchus and in B18 in Taurus, using data from the Green Bank Ammonia Survey (GAS; Friesen & Pineda et al., 2017).  The typical size and mass of these structures are 0.04 pc and 0.3 Msun, and we termed this subset of coherent structures the “droplets” owing to their small sizes.  We find that, unlike the larger-scale coherent cores, the droplets are not gravitationally bound.  The droplets are instead confined by the ambient pressure due to the thermal and non-thermal motions of the gas in the turbulent environment.  In this talk, I will give a brief overview of the physical properties of these droplets, in the context of other coherent cores and dense cores, and then look into the nature of the pressure confinement.  I will then discuss the mechanism which might direct the formation and (potential) growth and/or destruction of these pressure-dominated sub-0.1 pc coherent structures—the droplets.