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

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.