Physical Properties of Dense Cores in the Rho Ophiuchi Main Cloud and A
  Significant Role of External Pressures in Clustered Star Formation
  Using the archive data of the H13CO+ (J=1-0) line emission taken with the
Nobeyama 45 m radio telescope with a spatial resolution of about 0.01pc, we
have identified 68 dense cores in the central dense region of the rho Ophiuchi
main cloud. The H13CO+ data also indicates that the fractional abundance of
H13CO+ relative to H2 is roughly inversely proportional to the square root of
the H2 column density with a mean of 1.72 x 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 +- 3.6 Msolar, 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 rho 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 rho Oph cores appear to be nearly independent of the core
radii over the range of 0.01 - 0.1 pc and deviate upwards from the Heyer &
Brunt relation. This may be evidence that turbulent motions are driven by
protostellar outflows in the cluster environment.
