Gravitationally unstable condensations revealed by ALMA in the TUKH122
  prestellar core in the Orion A cloud
  We have investigated the TUKH122 prestellar core in the Orion A cloud using
ALMA 3 mm dust continuum, N$_2$H$^+$ ($J=1-0$), and CH$_3$OH ($J_K=2_K-1_K$)
molecular line observations. Previous studies showed that TUKH122 is likely on
the verge of star formation because the turbulence is almost dissipated and
chemically evolved among other starless cores in the Orion A cloud. By
combining ALMA 12-m and ACA data, we recover extended emission with a
resolution of $\sim5"$ corresponding to 0.01 pc and identify 6 condensations
with a mass range of $0.1-0.4$ $M_\odot$ and a radius of $\lesssim0.01$ pc.
These condensations are gravitationally bound following a virial analysis and
are embedded in the filament including the elongated core with a mass of
$\sim29$ $M_\odot$ and a radial density profile of $r^{-1.6}$ derived by {\it
Herschel}. The separation of these condensations is $\sim0.035$ pc, consistent
with the thermal jeans length at a density of $4.4\times10^5$ cm$^{-3}$. This
density is similar to the central part of the core. We also find a tendency
that the N$_2$H$^+$ molecule seems to deplete at the dust peak condensation.
This condensation may be beginning to collapse because the linewidth becomes
broader. Therefore, the fragmentation still occurs in the prestellar core by
thermal Jeans instability and multiple stars are formed within the TUKH122
prestellar core. The CH$_3$OH emission shows a large shell-like distribution
and surrounds these condensations, suggesting that the CH$_3$OH molecule formed
on dust grains is released into gas phase by non-thermal desorption such as
photoevaporation caused by cosmic-ray induced UV radiation.
