Massive Quiescent Cores in Orion: V. The Physical Structures on Sub-Jeans Scale and the Chemical Properties in Two Extremely Dense Cores

TL;DR

This study uses high-resolution millimeter-wave observations to analyze the physical and chemical structures of two extremely dense star-forming cores in Orion, revealing their fragmentation, density, and chemical properties at sub-Jeans scales.

Contribution

It provides detailed high-resolution insights into the physical and chemical structures of massive dense cores in Orion, highlighting their fragmentation and chemical depletion at unprecedented densities.

Findings

Cores have densities >10^9 cm^-3, among the highest in star-forming regions.

Gas clumps are spatially displaced from dense dust cores and are likely due to thermal Jeans fragmentation.

High [N2H+]/[HCO+] ratios suggest CO depletion in these dense cores.

Abstract

We present a high-resolution $1.5"$ observational study towards two massive dust-and-gas cores, ORI8nw\_2 and ORI2\_6 in Orion Molecular Cloud using the Combined Array for Research in Millimeter-wave Astronomy (CARMA). In both regions the 3.2 mm continuum emission reveals a dense and compact dust core of 1 to 3 solar masses. The cores are estimated to have number densities more than $10^9$ cm$^{-3}$, which are among the highest volume density ever published for the star-forming cores. In both regions the ${\rm N_2H^+}$ shows multiple gas clumps which are spatially displaced from the dense gas traced by the 3.2 mm continuum as well as the ${\rm HCO^+}$. The ${\rm N_2H^+}$ clumps in ORI8nw\_2 are gravitationally unbound. The clumps have masses comparable to the thermal Jeans mass, and are separated with intervals comparable with the Jeans length, thus likely result from thermal Jeans fragmentation. The fragmentation may have prohibited the high-mass star formation in the two cores which originally have larger total masses. In both regions the abundance ratio of ${\rm [N_2H^+]/[HCO^+]}$ appears to be higher than those in infrared dark clouds as a result of CO depletion. The extreme high central density in these Orion cores implies a very small Jeans scale ($<500$ AU). To further reveal the evolution of such dense cores requires higher resolution and sensitivity.