Mapping the prestellar core Ophiuchus D (L1696A) in ammonia

Context. The gas kinetic temperature in the centres of starless, high-density cores is predicted to fall as low as 5 − 6 K. The temperature gradient, which affects the dynamics and chemistry of these objects, should be discernible with radio interferometers reaching a spatial resolution of 1000 AU or better. Aims. 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. Methods. 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. Results. The high resolution of the interferometric data reveals that the southern part of Oph D comprises of two small cores in consistence with the 1.3 mm dust continuum map. 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. These values represent line-of-sight averages using the LTE assumption. A model using modified Bonnor-Ebert spheres, in which the temperature decreases to 6.1 K and 8.9 K in the centres of southern and northern core, matched the observed values satisfactorily. The southern core, which has more steep temperature gradient, has central density of nc = 4 × 106 cm-3, and the data suggests depletion of ammonia within 700 AU from the centre. The northern core, which is almost isothermal, seems to be less dense. The radial velocity gradients in these cores are almost opposite in direction, which may be an indication that turbulent fragmentation has a role in the formation of these cores. The observed masses of the cores are only ~0.2 M⊙. Their potential collapse could lead to formation of brown dwarfs or low-mass stars