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

TL;DR

This study uses JWST/MIRI MRS to detect and characterize solid-state complex organic molecules in the icy mantles of ST6, a protostar in the low-metallicity Large Magellanic Cloud. Through careful continuum and silicate subtraction, the authors identify CH3OH, CH3CHO, CH3CH2OH, HCOOCH3, and CH3COOH ices in ST6, with CH3COOH representing the first conclusive ice detection of this molecule and CH3CHO, CH3CH2OH, and HCOOCH3 detected securely outside the Galaxy in a low-metallicity environment. The results reveal differences in simple and COM ice abundances relative to H2O when compared to Galactic protostars, likely driven by metallicity and UV flux, and underscore the need for more laboratory ice spectra to improve spectral reconstructions. Overall, the work demonstrates that solid-state chemistry under subsolar metallicity can yield complex, potentially prebiotic, molecular inventories in protostellar envelopes, with implications for the chemical evolution of star-forming regions in the early universe.

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 $μ$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 $μ$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.

Figures (2)

• Figure 1: The left panel shows the three-color mosaic of the LMC combining Herschel/HERITAGE SPIRE 500 $\mu$m (red) and 250 $\mu$m (green), and PACS 160 $\mu$m (blue; meixner2013) images with the positions of star-forming regions N 158 (hosting ST6) and 30 Dor indicated. The yellow rectangle outlines the field of view of the image shown in the right panel: a three color mosaic of N 158, combining Spitzer/SAGE IRAC 8.0 $\mu$m (red), 4.5 $\mu$m (green; meixner2006), and MCELS H$\alpha$ (blue; smith1998) images. The position of ST6 is indicated with a white circle and labeled. The inset shows a zoom in on ST6 and its surroundings. The contours represent the $^{13}$CO (2--1) emission with contour levels of (10, 20, 40, 80)% of the emission peak of 12.4 Jy beam$^{-1}$ km s$^{-1}$; the synthesized beam size is $7"\times7"$. See Appendix \ref{['app:location']} for more details.
• Figure 2: The JWST/MIRI MRS spectrum of the massive protostar ST6 in the LMC. The gas-phase emission lines (shown in gray) are masked for the ice analysis. Gas-phase absorption lines remain in the spectrum. In this work, we concentrate on the spectral region between 6.8 and 8.4 $\mu$m (indicated in purple) covering the main ice features associated with COMs. Major ice and silicate (9.7 and 18 $\mu$m) features are indicated.