# ALMA Detection of Vibrationally Excited ($v\mathrm{_t} = 1,2$) Acetic Acid toward NGC 6334I

## TL;DR

This paper reports the first interstellar detection of vibrationally excited acetic acid states in NGC 6334I using ALMA, revealing compact emission regions and providing excitation temperature and column density estimates.

## Contribution

It presents the first detection of vibrationally excited acetic acid in space, expanding understanding of interstellar complex organic molecules and their excitation conditions.

## Key findings

- Detected vibrationally excited acetic acid states in NGC 6334I.
- Estimated excitation temperature as 142 K and column density as 1.12e17 cm^-2.
- Found emission concentrated toward MM1 and MM2 regions, with variations explained by continuum absorption or outflows.

## Abstract

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 ($v_\mathrm{t} = 1, 2$) 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 $1.12(7) \times 10^{17} \mathrm{cm^{-2}}$ respectively. Based on the intensity maps of the vibrationally excited CH$_3$COOH transitions, we found that the CH$_3$COOH emissions are compact and concentrated toward the MM1 and MM2 regions with a source size smaller than 2 arcsec. After locating the emission from different CH$_3$COOH transitions, which cover a large range of excitation energies, we are able to explain the variation of the CH$_3$COOH emission peak within the MM2 core by invoking continuum absorption or outflows.