A C18O STUDY OF THE ORIGIN OF THE POWER-LAW NATURE IN THE INITIAL MASS FUNCTION

We have performed C18O (J = 1-0) mapping observations of a 20' x 20' area of the OMC-1 region in the Orion A cloud. We identified 65 C18O cores, which have a mean radius, a velocity width in FWHM, and an LTE mass of 0.18 ± 0.03 pc, 0.40 ± 0.15 km s-1, and 7.2 ± 4.5 M sun, respectively. All the cores are most likely to be gravitationally bound by considering the uncertainty in the C18O abundance. We derived a C18O core mass function, which shows a power-law-like behavior above 5 M sun. The best-fit power-law index of -2.3 ± 0.3 is consistent with those of the dense core mass functions and the stellar initial mass function (IMF) previously derived in the OMC-1 region. This agreement strongly suggests that the power-law form of the IMF has been already determined at the density of ∼103 cm-3, traced by the C18O (J = 1-0) line. Consequently, we propose that the origin of the IMF should be searched in tenuous cloud structures with densities of less than 103 cm-3.