Protostars at Subsolar Metallicity: First Detection of Large Solid-state Complex Organic Molecules in the Large Magellanic Cloud

TL;DR

This study reports the first detection of complex organic molecules in a protostar within the low-metallicity environment of the Large Magellanic Cloud, revealing insights into star formation chemistry under early universe conditions.

Contribution

It provides the first conclusive detection of certain complex organic ices outside the Milky Way, expanding understanding of astrochemistry in low-metallicity star-forming regions.

Findings

First detection of CH3COOH ice in an astrophysical context

Secure detections of CH3CHO, CH3CH2OH, and HCOOCH3 ices outside the Galaxy

Differences in ice compositions suggest metallicity and UV flux influence chemical abundances.

Abstract

We present the results of James Webb Space Telescope observations of the protostar ST6 in the Large Magellanic Cloud (LMC) with the Medium Resolution Spectrograph of the Mid-Infrared Instrument (4.9-27.9 $\mu$m). Characterized by one-third to half-solar metallicity and strong UV radiation fields, the environment of the LMC allows us to study the physics and chemistry of star-forming regions under the conditions similar to those at earlier cosmological epochs. We detected five icy complex organic molecules (COMs): methanol (CH$_3$OH), acetaldehyde (CH$_3$CHO), ethanol (CH$_3$CH$_2$OH), methyl formate (HCOOCH$_3$), and acetic acid (CH$_3$COOH). This is the first conclusive detection of CH$_3$COOH ice in an astrophysical context, and CH$_3$CHO, CH$_3$CH$_2$OH, and HCOOCH$_3$ ices are the first secure detections outside the Galaxy and in a low-metallicity environment. We address the presence of glycolaldehyde (HOCH$_2$CHO, a precursor of biomolecules), an isomer of HCOOCH$_3$ and CH$_3$COOH, but its detection is inconclusive. ST6's spectrum is also rich in simple ices: H$_2$O, CO$_2$, CH$_4$, SO$_2$, H$_2$CO, HCOOH, OCN$^{-}$, HCOO$^{-}$, NH$_3$, and NH$_4^{+}$. We obtain the composition and molecular abundances in the icy dust mantles by fitting the spectrum in the 6.8-8.4 $\mu$m range with a large sample of laboratory ice spectra using the ENIIGMA fitting tool or the local continuum method. We found differences in the simple and COM ice abundances with respect to H$_2$O ice between ST6 and Galactic protostars that likely reflect differences in metallicity and UV flux. More laboratory ice spectra of COMs are needed to better reconstruct the observed infrared spectra of protostars.