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 (Δσv = 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 σv,NT ∝ r0.5, 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.
Highlights
Star formation takes place in dense cores in molecular clouds
The starforming cores are characterised by higher density and lower temperatures compared to the ambient cloud
Out of the four regions in the GAS DR1, we focus on L1688 in this paper, as we were able to obtain an extended kinetic temperature map of the cloud for this region
Summary
Star formation takes place in dense cores in molecular clouds. Detailed studies of dense cores unveil their physical and chemical properties, which provide the initial conditions in the process of star formation. Using NH3 (1,1) line emission, Barranco & Goodman (1998) found that the line width inside the four cores studied was roughly constant, and slightly greater than the pure thermal value. These latter authors reported that at the edge of the cores, the line widths begin to increase. It is important to study these transition regions, as this could give us clues on how dense cores form and accrete material from the surrounding cloud
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