  The gas kinetic temperature in the centres of starless, high-density cores is
predicted to fall as low as 5-6 K. The aim of this study was to determine the
kinetic temperature distribution in the low-mass prestellar core Oph D where
previous observations suggest a very low central temperature. The densest part
of the Oph D core was mapped in the NH3(1,1) and (2,2) inversion lines using
the Very Large Array (VLA). The physical quantities were derived from the
observed spectra by fitting the hyperfine structure of the lines, and
subsequently the temperature distribution of Oph D was calculated using the
standard rotational temperature techniques. A physical model of the cores was
constructed, and the simulated spectra after radiative transfer calculations
with a 3D Monte Carlo code were compared with the observed spectra.
Temperature, density, and ammonia abundance of the core model were tuned until
a satisfactory match with the observation was obtained. The high resolution of
the interferometric data reveals that the southern part of Oph D comprises of
two small cores. The gas kinetic temperatures, as derived from ammonia towards
the centres of the southern and northern core are 7.4 and 8.9 K, respectively.
The observed masses of the cores are only 0.2 M_Sun. Their potential collapse
could lead to formation of brown dwarfs or low-mass stars.
