Abstract
Abstract We present the observation and analysis of newly discovered coherent structures in the L1688 region of Ophiuchus and the B18 region of Taurus. Using data from the Green Bank Ammonia Survey, we identify regions of high density and near-constant, almost-thermal velocity dispersion. We reveal 18 coherent structures are revealed, 12 in L1688 and 6 in B18, each of which shows a sharp “transition to coherence” in velocity dispersion around its periphery. The identification of these structures provides a chance to statistically study the coherent structures in molecular clouds. The identified coherent structures have a typical radius of 0.04 pc and a typical mass of 0.4 M ☉, generally smaller than previously known coherent cores identified by Goodman et al., Caselli et al., and Pineda et al. We call these structures “droplets.” We find that, unlike previously known coherent cores, these structures are not virially bound by self-gravity and are instead predominantly confined by ambient pressure. The droplets have density profiles shallower than a critical Bonnor–Ebert sphere, and they have a velocity (V LSR) distribution consistent with the dense gas motions traced by NH3 emission. These results point to a potential formation mechanism through pressure compression and turbulent processes in the dense gas. We present a comparison with a magnetohydrodynamic simulation of a star-forming region, and we speculate on the relationship of droplets with larger, gravitationally bound coherent cores, as well as on the role that droplets and other coherent structures play in the star formation process.
Highlights
In the early 1980s, NH3 was identified as an excellent tracer of the cold, dense gas associated with highly extinguished compact regions
We present a comparison with a magnetohydrodynamic simulation of a star-forming region, and we speculate on the relationship of droplets with larger, gravitationally bound coherent cores, as well as on the role that droplets and other coherent structures play in the star formation process
We look for coherent structures defined by abrupt drops in NH3 linewidth and an interior with uniform, nearly thermal velocity dispersion5, reminiscent of previously known coherent cores examined by Goodman et al (1998), Caselli et al (2002), and Pineda et al (2010)
Summary
In the early 1980s, NH3 was identified as an excellent tracer of the cold, dense gas associated with highly extinguished compact regions. In search of other coherent structures in nearby molecular clouds, we follow the same procedure adopted by Pineda et al (2010) and identify a total of 18 coherent structures, 12 in the L1688 region in Ophiuchus and 6 in the B18 region in Perseus, using data from the Green Bank Ammonia Survey (GAS; Friesen et al 2017) Many of these structures may be associated with previously known cores or density features, this is the first time “transitions to coherence” are captured using a single tracer. Gas motions, in spite of the subsonic velocity dispersions found in these structures2 We term this newly discovered population of gravitationally unbound and pressure confined coherent structures “droplets” and examine their relation to the known gravitationally bound and likely star-forming coherent cores and other dense cores. The GBT observations have a spectral resolution of 5.7 kHz, or ∼ 0.07 km s−1 at 23 GHz
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