ALMA Detection of Vibrationally Excited (v t = 1, 2) Acetic Acid toward NGC 6334I

Vibrationally excited states of detected interstellar molecules have been shown to account for a large portion of unidentified spectral lines in observed interstellar spectra toward chemically rich sources. Here, we present the first interstellar detection of the first and second vibrationally excited torsional states of acetic acid (v _t = 1, 2) toward the high-mass star-forming region NGC 6334I. The observations presented were taken with the Atacama Large Millimeter/submillimeter Array in bands 4, 6, and 7 covering a frequency range of 130-352 GHz. By comparing a single-excitation temperature model to the observations, the best-fit excitation temperature and column density are obtained to be 142(25) K and 1.12(7) × 10¹⁷ cm^-2, respectively. Based on the intensity maps of the vibrationally excited CH₃COOH transitions, we found that the CH₃COOH emissions are compact and concentrated toward the MM1 and MM2 regions with a source size smaller than 2″. After locating the emission from different CH₃COOH transitions, which cover a large range of excitation energies, we are able to explain the variation of the CH₃COOH emission peak within the MM2 core by invoking continuum absorption or outflows.