Transition from coherent cores to surrounding cloud in L1688

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
e-mail: spandan@mpe.mpg.de
² Department of Astronomy, The University of Texas at Austin, Austin, TX 78712, USA
³ Department of Physics, 4-181 CCIS, University of Alberta, Edmonton, AB T6G 2E1, Canada
⁴ Department of Astronomy & Astrophysics, University of Toronto, 50 St. George St., Toronto, ON M5S 3H4, Canada
⁵ Observatorio Astronómico Nacional (OAN-IGN), Alfonso XII 3, 28014, Madrid, Spain
⁶ Steward Observatory, 933 North Cherry Ave., Tucson, AZ 85721, USA
⁷ Ural Federal University, 620002 Mira st. 19, Yekaterinburg, Russia
⁸ Department of Physics and Astronomy, University of Victoria, 3800 Finnerty Rd., Victoria, BC V8P 5C2, Canada
⁹ Herzberg Astronomy and Astrophysics, National Research Council of Canada, 5071 West Saanich Rd., Victoria, BC V9E 2E7, Canada

Received: 12 November 2020
Accepted: 9 February 2021

Abstract

Context. Stars form in cold dense cores showing subsonic velocity dispersions. The parental molecular clouds display higher temperatures and supersonic velocity dispersions. The transition from core to cloud has been observed in velocity dispersion, but temperature and abundance variations are unknown.

Aims. We aim to measure the temperature and velocity dispersion across cores and ambient cloud in a single tracer to study the transition between the two regions.

Methods. We use NH₃ (1,1) and (2,2) maps in L1688 from the Green Bank Ammonia Survey, smoothed to 1′, and determine the physical properties by fitting the spectra. We identify the coherent cores and study the changes in temperature and velocity dispersion from the cores to the surrounding cloud.

Results. We obtain a kinetic temperature map extending beyond dense cores and tracing the cloud, improving from previous maps tracing mostly the cores. The cloud is 4–6 K warmer than the cores, and shows a larger velocity dispersion (Δσ_v = 0.15–0.25 km s⁻¹). Comparing to Herschel-based dust temperatures, we find that cores show kinetic temperatures that are ≈1.8 K lower than the dust temperature, while the gas temperature is higher than the dust temperature in the cloud. We find an average p-NH₃ fractional abundance (with respect to H₂) of (4.2 ± 0.2) × 10⁻⁹ towards the coherent cores, and (1.4 ± 0.1) × 10⁻⁹ outside the core boundaries. Using stacked spectra, we detect two components, one narrow and one broad, towards cores and their neighbourhoods. We find the turbulence in the narrow component to be correlated with the size of the structure (Pearson-r = 0.54). With these unresolved regional measurements, we obtain a turbulence–size relation of σ_v,NT ∝ r^0.5, which is similar to previous findings using multiple tracers.

Conclusions. We discover that the subsonic component extends up to 0.15 pc beyond the typical coherent boundaries, unveiling larger extents of the coherent cores and showing gradual transition to coherence over ~0.2 pc.