A Centimeter-wave Study of Methanol and Ammonia Isotopologues in Sgr B2(N): Physical and Chemical Differentiation between Two Hot Cores

Abstract

We present new radio-frequency interferometric maps of emission from the 14 NH 3 , 15 NH 3 , and NH 2 D isotopologues of ammonia and the 12 CH 3 OH and 13 CH 3 OH isotopologues of methanol toward Sgr B2(N). With a resolution of ∼3″ (0.1 pc), we are able to spatially resolve emission from two hot cores in this source and separate it from absorption against the compact H ii regions in this area. The first (N1) is the well-known v  = 64 km s −1 core, and the second (N2) is a core 6″ to the north at v  = 73 km s −1 . Using emission from 15 NH 3 and hyperfine satellites of 14 NH 3 metastable transitions, we estimate the 14 NH 3 column densities of these sources and compare them to those of NH 2 D. We find that the ammonia deuteration fraction of N2 is roughly 10–20 times higher than that in N1. We also measure an [ 15 NH 3 / 14 NH 3 ] abundance ratio that is apparently 2–3 times higher in N2 than in N1, which could indicate a correspondingly higher degree of nitrogen fractionation in N2. In addition, we find that N2 has a factor of 7 higher methanol abundance than N1. Together, these abundance signatures suggest that N2 is a younger source, for which species characteristic of grain chemistry at low temperatures are currently being actively liberated from ice mantles and have not yet reached chemical equilibrium in the warm gas phase. The high D abundance and possible high 15 N abundance in NH 3 found in N2 are interesting for studying the potential interstellar origin of abundances in primitive solar system material.