Massive Quiescent Cores in Orion. V. The Internal Structures and Physical and Chemical Properties of Two Extremely Dense Cores

We present a high-resolution (~ 1.''5) observational study of two massive dust-gas cores, ORI8nw_2 and ORI2_6, in the Orion molecular cloud using the Combined Array for Research in Millimeter-wave Astronomy. In each region the 3.2 mm continuum emission exhibits a dense and compact dust core at the center with 1-3 solar masses. The cores have number densities exceeding 10⁹ cm^-3, which are among the highest volume densities observed in star-forming cores. In both regions the N₂H⁺ shows clumpy structures that are spatially displaced from the densest gas. In OIR8nw_2 in particular, the N₂H⁺ shows a noticeable filament structure with a central cavity shell. The calculation for the dynamical state shows that this core can be potentially supported by the magnetic field against its gravitational instability, but the fragmentation might still occur and produce the observed N₂H⁺ clumps if the gas density exceeds 5 × 10⁷ cm^-3 and this value is available within the observed density range. Also, the extremely high density at the core center suggests super-Jeans condition and the possibility for further fragmentation. For the chemical properties, the N₂H⁺-to-HCO⁺ abundance ratios are shown to be different than those observed in infrared dark clouds. A combined analysis with the other Orion cores and the chemical model suggests that the different abundance ratios can be explained by the low CO abundances in our cores. To further reveal the evolution of such dense cores, higher resolution and sensitivity are required.