Abstract
Context. Star formation takes place in cold dense cores in molecular clouds. Earlier observations have found that dense cores exhibit subsonic non-thermal velocity dispersions. In contrast, CO observations show that the ambient large-scale cloud is warmer and has supersonic velocity dispersions. Aims. We aim to study the ammonia (NH3) molecular line profiles with exquisite sensitivity towards the coherent cores in L1688 in order to study their kinematical properties in unprecedented detail. Methods. We used NH3 (1,1) and (2,2) data from the first data release (DR1) in the Green Bank Ammonia Survey (GAS). We first smoothed the data to a larger beam of 1′ to obtain substantially more extended maps of velocity dispersion and kinetic temperature, compared to the DR1 maps. We then identified the coherent cores in the cloud and analysed the averaged line profiles towards the cores. Results. For the first time, we detected a faint (mean NH3(1,1) peak brightness < 0.25 K in TMB), supersonic component towards all the coherent cores in L1688. We fitted two components, one broad and one narrow, and derived the kinetic temperature and velocity dispersion of each component. The broad components towards all cores have supersonic linewidths (ℳS ≥ 1). This component biases the estimate of the narrow dense core component’s velocity dispersion by ≈28% and the kinetic temperature by ≈10%, on average, as compared to the results from single-component fits. Conclusions. Neglecting this ubiquitous presence of a broad component towards all coherent cores causes the typical single-component fit to overestimate the temperature and velocity dispersion. This affects the derived detailed physical structure and stability of the cores estimated from NH3 observations.
Highlights
Star formation takes place in dense cores, which are embedded in molecular clouds
We focus on L1688, which is part of the Ophiuchus molecular cloud because it is one of the nearest star-forming regions and with the most extended NH3 emission beyond cores among the Green Bank Ammonia Survey (GAS) DR1 regions
A faint, broad component has been detected towards coherent dense cores
Summary
Star formation takes place in dense cores, which are embedded in molecular clouds These cores in various molecular clouds are studied in detail in relation to their physical and chemical properties. When studied using molecular transitions tracing higher densities (n(H2) > 104 cm−3), the cores are revealed to show subsonic levels of turbulence (Barranco & Goodman 1998; Kirk et al 2007; Rosolowsky et al 2008). This is in contrast to lower density gas surrounding the cores, which show supersonic linewidths (Goodman et al 1998). Thanks to the numerous hyperfine components, NH3 remains optically thin in the individual components even at high column densities (see Caselli et al 2017), thereby making it an important and useful tracer of cold and dense gas. Barranco & Goodman (1998) studied four cores in NH3 (1,1) emission, and found that the linewidths within the cores were roughly constant
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