Observing the gas temperature drop in the high-density nucleus of L 1544

Abridged: The thermal structure of a starless core is crucial for our understanding of the physics in these objects and hence for our understanding of star formation. Theory predicts a gas temperature drop in the inner 5000 AU of these objects, but there has been little observational proof of this. We performed VLA observations of the NH3 (1,1) and (2,2) transitions towards the pre-stellar core L 1544 in order to measure the temperature gradient between the high density core nucleus and the surrounding core envelope. Our VLA observation for the first time provide measurements of gas temperature in a core with a resolution smaller than 1000 AU. We have also obtained high resolution Plateau de Bure observations of the 110 GHz 111-101 para-NH2D line in order to further constrain the physical parameters of the high density nucleus. We have estimated the temperature gradient using a model of the source to fit our data in the u,v plane. We find that indeed the temperature decreases toward the core nucleus from 12 K down to 5.5 K resulting in an increase of a factor of 50% in the estimated density of the core from the dust continuum if compared with the estimates done with constant temperature of 8.75 K. We also found a remarkably high abundance of deuterated ammonia with respect to the ammonia abundance (50%+-20%), which proves the persistence of nitrogen bearing molecules at very high densities (2e6 cm-3) and shows that high-resolution observations yield higher deuteration values than single-dish observations. Our analysis of the NH3 and NH2D kinematic fields shows a decrease of specific angular momentum from the large scales to the small scales.