  The low-mass condensations (LMCs) have been observed within the molecular
cloud cores. In this research, we investigate the effect of isobaric thermal
instability (TI) applied for forming these LMCs. For this purpose, at first we
investigate the occurrence of TI in the molecular clouds. Then, for studying
the significance of linear isobaric TI, we use a contracting axisymmetric
cylindrical core with axial magnetic field. Consideration to cooling and
heating mechanisms in the molecular clouds shows that including the heating due
to ambipolar diffusion can lead to the occurrence of TI in a time-scale smaller
than dynamical time-scale. Application of linear perturbation analysis shows
that isobaric TI can take place in outer region of the molecular cloud cores.
Furthermore, the results showthat perturbations with wavelengths greater than
few astronomical units are protected from destabilization property of thermal
conduction, so they can grow to form LMCs. Thus, the results show that the
mechanism of TI can be used to explain the formation of LMCs as the progenitors
of collapsing proto-stellar entities, brown dwarfs, or proto-planets.
