Interstellar turbulence is supposed to be one of the most important factors to regulate star formation activities. Shocks induced by supersonic turbulence dramatically increase the density and temperature in the post-shock layer and promote the structure formation, such as dense cores and protostars (e.g., Padoan 1995; Padoan & Nordlund 2002; Klessen et al. 2005; Matsumoto et al. 2015a; Inoue et al. 2018). Observational studies have been attempting to search for dense cores that originated from turbulent phenomena. Recent systematic surveys with ALMA found that there are no or a few starless cores with their internal substructures originating from turbulent fragmentation (cf., Padoan & Nordlund 2002; Offner et al. 2010) in each star-forming region (Dunham et al. 2016; Kirk et al. 2017). On the contrary, complex spatial/velocity gas structures in the protostellar envelopes with the spatial scale from 0.1 pc down to a few tens of astronomical units are also revealed both by the single-dish observations and the interferometric observations (e.g., Tobin et al. 2011; Tokuda et al. 2014, 2016; Maureira et al. 2017). Some multiscale polarization observations and numerical simulations suggest that turbulent motions, rather than magnetic fields, are dynamically important to form complex gas morphologies of protostellar envelopes (Hull et al. 2017a, 2017b). To understand such diversities/complexities at early stages of star formation, it is important to investigate physical properties of structures originating from the turbulent gas kinematics. In early phases of star formation, the shock waves can be formed by interactions among different density gas with the different velocities because the gas motions, such as infalling/outflowing gas, are supersonic. Detections of high-J transitions of molecular lines (e.g., CO) excited by magnetohydrodynamic (MHD) shocks have been expected by theoretical modelings (e.g., Pon et al. 2012; Lehmann & Wardle 2016). Recent submillimeter observations (e.g., Shinnaga et al. 2009; van Kempen et al. 2009a, 2009b; Kristensen et al. 2013) have detected warm envelopes seen in high-J CO lines from protostellar sources. However, the origin and heating mechanisms of warm gas in low-mass star-forming dense cores are still under debate. For example, van Kempen et al. (2009b) suggested several origins to produce the high-J CO lines; (1) inner envelope heated by protostellar luminosity, (2) shocked gas in the outflow, and (3) quiescent gas heated by UV photons. Shinnaga et al. (2009) also detected the extended CO (J = 6–5, 7–6) emission with the size scale of a few thousand astronomical units in our current target, MC27/L1521F (see also later descriptions in this section). They argued that the warm gas may be coming from shock regions created by interactions between the collapsing envelopes and the internal disk-like materials around the protostar. Although their observations were not able to resolve the internal substructures of the dense core due to the lack of the angular resolution, they pointed out the importance for investigating such warm gas formed in an early phase of star formation to understand the evolution from dense cores to protostars.
Citation: Hull et al. 2017a
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