Physical Properties of Dense Cores in the ρ Ophiuchi Main Cloud and A Significant Role of External Pressures in Clustered Star Formation

Using the archive data of the HCO (J = 1 − 0) line emission taken with the Nobeyama 45 m radio telescope with a spatial resolution of ∼ 0.01pc, we have identified 68 dense cores in the central dense region of the ρ Ophiuchi main cloud. The HCO data also indicates that the fractional abundance of HCO relative to H2 is roughly inversely proportional to the square root of the H2 column density with a mean of 1.72×10−11. The mean radius, FWHM line width, and LTE mass of the identified cores are estimated to be 0.045 ± 0.011 pc, 0.49 ± 0.14 km s, and 3.4 ± 3.6M⊙, respectively. The majority of the identified cores have subsonic internal motions. The virial ratio, the ratio of the virial mass to the LTE mass, tends to decrease with increasing the LTE mass and about 60 percent of the cores have virial ratios smaller than 2, indicating that these cores are not transient structures but self-gravitating. The detailed virial analysis suggests that the surface pressure often dominates over the self-gravity and thus plays a crucial role in regulating core formation and evolution. By comparing the ρ Oph cores with those in the Orion A molecular cloud observed with the same telescope, we found that the statistical properties of the core physical quantities are similar between the two clouds if the effect of the different spatial resolutions is corrected. The line widths of the ρ Oph cores appear to be nearly independent of the core radii over the range of 0.01 − 0.1 pc and have significant excess from the Heyer & Brunt relation. This excess may be evidence that turbulent motions are driven by protostellar outflows in the cluster environment. Subject headings: ISM: clouds — ISM: individual (ρ Ophiuchi) — ISM: structure — stars: formation — turbulence