The complexity of Orion: an ALMA view. II. gGg'-Ethylene Glycol and Acetic Acid

TL;DR

This study presents the first high-resolution ALMA imaging of acetic acid and ethylene glycol in Orion-KL, revealing unique spatial distributions and suggesting their formation on icy grain surfaces followed by release during an explosive event.

Contribution

It provides new high-resolution observations of acetic acid and ethylene glycol in Orion-KL, highlighting their distinct spatial distribution and proposing a formation and release mechanism linked to explosive activity.

Findings

Acetic acid and ethylene glycol have different spatial distributions compared to other O-bearing molecules.

The abundance ratios differ significantly from those in low-mass protostars.

The molecules likely form on icy grains and are released during a past explosive event.

Abstract

We report the first detection and high angular resolution (1.8" $\times$ 1.1") imaging of acetic acid (CH$_3$COOH) and gGg$^{\prime}$--ethylene glycol (gGg$^{\prime}$(CH$_2$OH)$_2$) towards the Orion Kleinmann--Low nebula. The observations were carried out at $\sim$1.3mm with ALMA during the Cycle~2. A notable result is that the spatial distribution of the acetic acid and ethylene glycol emission differs from that of the other O-bearing molecules within Orion-KL. Indeed, while the typical emission of O-bearing species harbors a morphology associated with a "V-shape" linking the Hot Core region to the Compact Ridge (with an extension towards the BN object), that of acetic acid and ethylene glycol mainly peaks at about 2" southwest from the hot core region (near sources I and n). We find that the measured CH$_3$COOH:aGg$^{\prime}$(CH$_2$OH)$_2$ and CH$_3$COOH:gGg$^{\prime}$(CH$_2$OH)$_2$ ratios differ from the ones measured towards the low-mass protostar IRAS 16293--2422 by more than one order of magnitude. Our best hypothesis to explain these findings is that CH$_3$COOH, aGg$^{\prime}$(CH$_2$OH)$_2$ and gGg$^{\prime}$(CH$_2$OH)$_2$ are formed on the icy-surface of grains and then released into the gas-phase, via co-desorption with water, due to a bullet of matter ejected during the explosive event that occurred in the heart of the Nebula about 500-700 years ago.