UNVEILING SOURCES OF HEATING IN THE VICINITY OF THE ORION BN/KL HOT CORE AS TRACED BY HIGHLY EXCITED INVERSION TRANSITIONS OF AMMONIA

Abstract

Using the Expanded Very Large Array, we have mapped the vicinity of the Orion BN/KL Hot Core with subarcsecond angular resolution in seven metastable inversion transitions of ammonia (NH[subscript 3]): (J, K) = (6,6) to (12,12). This emission comes from levels up to 1500 K above the ground state, enabling identification of source(s) responsible for heating the region. We used this multi-transition data set to produce images of the rotational/kinetic temperature (T rot/T kin) and the column density N col of NH[subscript 3] for ortho and para species separately and on a position-by-position basis. We find T rot and N col in the range 160-490 K and (1-4) × 10[superscript 17] cm[superscript –2], respectively. Our spatially resolved images show that the highest (column) density and hottest gas is found in a northeast-southwest elongated ridge to the southeast of Source I. We have also measured the ortho-para ratio of ammonia, estimated to vary in the range 0.9-1.6. Enhancement of ortho with respect to para and the offset of hot NH3 emission peaks from known (proto)stellar sources provide evidence that the NH3 molecules have been released from dust grains into the gas phase through the passage of shocks and not by stellar radiation. We propose that the combined effect of Source I's proper motion and its low-velocity outflow impinging on a pre-existing dense medium is responsible for the excitation of NH3 and the Orion Hot Core. Finally, we found for the first time evidence of a slow (~5 km s[superscript –1]) and compact (~1000 AU) outflow toward IRc7.