Subarcsecond Multi-line Observations of NH$_3$ with VLA toward the Class 0 Source IRAS 16293-2422

Abstract

Ammonia (NH$_3$) is one of the key volatiles that plays a central role in nitrogen chemistry and its evolution during the epoch of star and planet formation. We present subarcsecond ($\sim0.\!\!^{\prime\prime}5$) resolution observations of NH$_3$ molecular emission lines with Karl G. Jansky Very Large Array (VLA) toward the Class 0 multiple system IRAS 16293-2422 including source A and source B as major components. This comprises the most comprehensive set of NH$_3$ line observations in protostellar sources to date, which includes 17 inversion transitions with a wide range of upper state energies ($E_\mathrm{u}$) spanning from $\sim$23 K to $\sim$1,580 K. We detect spatially resolved emission of a number of transitions, and find that the high-$E_\mathrm{u}$ ($\gtrsim$1,000 K) lines show compact distributions in the vicinity of protostars while low-$E_\mathrm{u}$ ($\lesssim$150 K) lines exhibit more extended emission. Utilizing a two-component model, we constrain the rotation temperature and NH$_3$ column density for both source A and source B. The rotation temperature of the warmer component reaches $\sim$200-300 K, indicating that the high-$E_\mathrm{u}$ lines selectively trace the inner hot region. We suggest that this hot NH$_3$ gas in source A is originated from the local shock heating based on the comparison with the previous high-resolution ALMA observations, while that in source B could be explained by the mass accretion heating in the innermost hot region. We also briefly discuss the chemistry related to NH$_3$ based on the abundance ratios relative to major icy molecules derived using literature values.