Droplets. I. Pressure-dominated Coherent Structures in L1688 and B18

Hope How Huan Chen, Jaime E. Pineda, Alyssa A. Goodman, Andreas Burkert, Stella S.R. Offner, Rachel K. Friesen, Philip C. Myers, Felipe Alves, Héctor G. Arce, Paola Caselli, Ana Chacón-Tanarro, Michael Chun Yuan Chen, James Di Francesco, Adam Ginsburg, Jared Keown, Helen Kirk, Peter G. Martin, Christopher Matzner, Anna Punanova, Elena RedaelliErik Rosolowsky, Samantha Scibelli, Youngmin Seo, Yancy Shirley, Ayushi Singh

Research output: Contribution to journalArticlepeer-review

32 Scopus citations


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.

Original languageEnglish (US)
Article number93
JournalAstrophysical Journal
Issue number2
StatePublished - Jun 1 2019


  • ISM: clouds
  • ISM: individual objects (L1688, B18)
  • ISM: kinematics and dynamics
  • magnetohydrodynamics (MHD)
  • radio lines: ISM
  • stars: formation

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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