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H13CO+ and CH3OH Line Observations of Prestellar Dense Cores in the TMC-1C Region. II. Internal Structure

We have carried out detailed H13CO+ (J = 1-0) and CH3OH (JK = 20-10 A+) observations of two dense cores in the TMC-1C region with the 45 m telescope at Nobeyama Radio Observatory and the Nobeyama Millimeter Array (NMA). These two cores, H13CO+ core 2 and CH3OH core 6, were identified previously by the H13CO+ and CH3OH mapping observations of the entire TMC-1C region and show no sign of protostar formation. We found that the internal structure of H13CO+ core 2 is different from that of cores with class 0 protostars; this core consists of three velocity components and shows the radial column density distribution of ~r-0.43±0.08, which is flatter than that in the protostellar cores (~r-1.0). The internal structure of CH3OH core 6 is different from that of H13CO+ core 2; the combined images of the 45 m and NMA data have revealed that there are three smaller scale CH3OH "clumps" ~2500 AU in size inside this core. The mass of the small-scale clumps, ~0.02 M☉, is much smaller than the virial mass (~0.3 M☉), suggesting that these clumps are gravitationally unbound. The velocity dispersion observed in these small-scale clumps is 0.45-0.5 km s-1, which is comparable to that of clumps with 0.02-0.03 pc in radius. A comparison of the velocity dispersion of the small-scale clumps and that of larger scale cores suggests that the velocity dispersion that follows the power law in the radius larger than ~0.02-0.03 pc turns into a constant value of ~0.45-0.5 km s-1 in the smaller size scale. The mass-size relation of the prestellar gas structures suggests that they follow the trend of constant gas density (~105 cm-3) in the radius from 0.006 to ~0.3 pc and that the smaller scale structures tend to be gravitationally unbound. The presence of the small clumps with substellar masses in prestellar cores implies that coalescence processes are likely to play an important role in forming protostars inside the cores. The observational difference of the internal structures among CH3OH core 6, H13CO+ core 2, and protostellar cores might represent the difference of the evolutionary stages.