Mapping the prestellar core Ophiuchus D (L1696A) in ammonia

TL;DR

This study maps the temperature distribution in the prestellar core Oph D using ammonia observations, revealing very low temperatures and small core masses, which may lead to brown dwarf or low-mass star formation.

Contribution

It provides high-resolution ammonia mapping and a physical model of Oph D, demonstrating detailed temperature and density structures in a prestellar core.

Findings

Gas kinetic temperatures are 7.4 K and 8.9 K in the cores.

The cores have masses around 0.2 solar masses.

The cores may collapse to form brown dwarfs or low-mass stars.

Abstract

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.