The C18O core mass function toward Orion A: Single-dish observations

Abstract

Abstract We have performed an unbiased dense core survey toward the Orion A Giant Molecular Cloud in the C18O (J = 1–0) emission line taken with the Nobeyama Radio Observatory (NRO) 45 m telescope. The effective angular resolution of the map is 26″, which corresponds to ∼0.05 pc at a distance of 414 pc. By using the Herschel–Planck H2 column density map, we calculate the C18O fractional abundance and find that it is roughly constant over the column density range of ≲5 × 1022 cm−3, although a trend of C18O depletion is determined toward higher column density. Therefore, C18O intensity can follow the cloud structure reasonably well. The mean C18O abundance in Orion A is estimated to be 5.7 × 10−7, which is about three times larger than the fiducial value. We identified 746 C18O cores with astrodendro and classified 709 cores as starless cores. We compute the core masses by decomposing the Herschel–Planck dust column density using the relative proportions of the C18O integrated intensities of line-of-sight components. Applying this procedure, we attempt to remove the contribution of the background emission, i.e., the ambient gas outside the cores. Then, we derived mass function for starless cores and found that it resembles the stellar initial mass function (IMF). The CMF for starless cores, dN/dM, is fitted with a power-law relation of Mα with a power index of α = −2.25 ± 0.16 at the high-mass slope (≳0.44 M⊙). We also found that the ratio of each core mass to the total mass integrated along the line of sight is significantly large. Therefore, in the previous studies, the core masses derived from the dust image are likely to be overestimated by at least a factor of a few. Accordingly, such previous studies may underestimate the star formation efficiency of individual cores.