  The internal structure of the prestellar core G208.68-19.02-N2 (G208-N2) in
the Orion Molecular Cloud 3 (OMC-3) region has been studied with the Atacama
Large Millimeter/submillimeter Array (ALMA). The dust continuum emission
revealed a filamentary structure with a length of $\sim$5000 au and an average
H$_2$ volume density of $\sim$6 $\times$ 10$^7$ cm$^{-3}$. At the tip of this
filamentary structure, there is a compact object, which we call a ``nucleus",
with a radius of $\sim$150--200 au and a mass of $\sim$0.1 M$_{\odot}$. The
nucleus has a central density of $\sim$2 $\times$ 10$^9$ cm$^{-3}$ with a
radial density profile of $r^{-1.87{\pm}0.11}$. The density scaling of the
nucleus is $\sim$3.7 times higher than that of the singular isothermal sphere.
This as well as the very low virial parameter of 0.39 suggest that the gravity
is dominant over the pressure everywhere in the nucleus. However, there is no
sign of CO outflow localized to this nucleus. The filamentary structure is
traced by the N$_2$D$^+$ 3--2 emission, but not by the C$^{18}$O 2--1 emission,
implying the significant CO depletion due to high density and cold temperature.
Toward the nucleus, the N$_2$D$^+$ also shows the signature of depletion. This
could imply either the depletion of the parent molecule, N$_2$, or the presence
of the embedded very-low luminosity central source that could sublimate the CO
in the very small area. The nucleus in G208-N2 is considered to be a prestellar
core on the verge of first hydrostatic core (FHSC) formation or a candidate for
the FHSC.
