Abstract
Abstract We present an analysis of the internal velocity structures of the newly identified sub-0.1 pc coherent structures, droplets, in L1688 and B18. By fitting 2D linear velocity fields to the observed maps of velocity centroids, we determine the magnitudes of linear velocity gradients and examine the potential rotational motions that could lead to the observed velocity gradients. The results show that the droplets follow the same power-law relation between the velocity gradient and size found for larger-scale dense cores. Assuming that rotational motion giving rise to the observed velocity gradient in each core is a solid-body rotation of a rotating body with a uniform density, we derive the “net rotational motions” of the droplets. We find a ratio between rotational and gravitational energies, β, of ∼0.046 for the droplets, and when including both droplets and larger-scale dense cores, we find β ∼ 0.039. We then examine the alignment between the velocity gradient and the major axis of each droplet, using methods adapted from the histogram of relative orientations introduced by Soler et al. We find no definitive correlation between the directions of velocity gradients and the elongations of the cores. Lastly, we discuss physical processes other than rotation that may give rise to the observed velocity field.
Highlights
Shu et al (1987) examined analytical star formation models and summarized an evolutionary sequence of a slowly rotating core with accretion initiated by an inside-out gravitational collapse
By fitting 2D linear velocity fields to the observed maps of velocity centroids, we determine the magnitudes of linear velocity gradients and examine the potential rotational motions that could lead to the observed velocity gradients
We examine the alignment between the velocity gradient and the major axis of each droplet, using methods adapted from the histogram of relative orientations (HRO) introduced by Soler et al (2013)
Summary
Shu et al (1987) examined analytical star formation models and summarized an evolutionary sequence of a slowly rotating core with accretion initiated by an inside-out gravitational collapse. At the time when the paper by Shu et al (1987) was first published, most systematic observational attempts to measure rotational motions in molecular clouds were made primarily based on analyses of 13CO observations, focusing on the more extended and less dense cloud material surrounding potentially star-forming cores (Arquilla 1984; Arquilla & Goldsmith 1985; Goldsmith & Arquilla 1985; Arquilla & Goldsmith 1986). Extended analyses of observations and simulations have shown that cores are situated in the densest parts of a network of filamentary structures, often seen at an intersection of filaments (McKee & Ostriker 2007; Myers 2009; Arzoumanian et al 2013). Filamentary structures are shown to host most of the star forming cores (Andre et al 2014; Padoan et al 2014; Hacar et al 2013; Tafalla & Hacar 2015; Monsch et al 2018)
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