Overview of complex organic molecule observations in protostellar systems

TL;DR

This review addresses how complex organic molecules (COMs) form and vary in protostellar systems by synthesizing extensive gas-phase surveys from ALMA/SMA/NOEMA and ice-phase measurements enabled by JWST. It highlights that ice chemistry plays a crucial role in COM formation, with many gas–ice ratios showing agreement and a general abundance of icy COMs relative to methanol, though notable scatter exists due to physical structures like disks and shocks. The analysis reveals that N-bearing COMs tend to correlate with luminosity and temperature more strongly than O-bearing species, implying different formation pathways and desorption histories, while some molecules show clear chemical links and others reflect local environmental conditions. The review emphasizes the need for high-angular-resolution, large-sample gas–ice studies and predicts that upcoming surveys COMPASS and NASCENT-stars, together with continued JWST observations, will significantly advance understanding of chemical versus physical drivers in protostellar COM chemistry and its connection to cometary and planetary material.

Abstract

Complex organic molecules (COMs) have been detected abundantly at various stages of star formation, particularly in the warm protostellar phase. The progress in gas-phase measurements has been accelerated by the advent of the Atacama Large Millimeter/submillimeter Array and in ice measurements by the James Webb Space Telescope. Particularly, the community has moved from single-source studies of COMs to statistical analyses because of these powerful instruments. In this article, I review surveys that consider COMs in the gas and ice. The two takeaways from this review include; 1. Gas-phase abundance ratios for some COMs show a small difference across many objects and the ice abundance ratios show similar or higher values to the gas, both pointing to the importance of ice chemistry in COM formation, 2. Some COM ratios show larger differences across many objects which could be due to either chemical or physical effects, thus both factors need to be considered when interpreting the data.

Figures (5)

• Figure 1: Example spectra of ALMA (top) and JWST (bottom) to showcase the typical molecular observations in the gas and ice. The ALMA data are taken from Chen2024 and extracted at the peak of the continuum. The JWST data are taken from the JOYS+ program (Sect. \ref{['sec:JOYS']}) and the spectrum is extracted from a cone aperture centered on the continuum at 5.25 $\mu$m.
• Figure 2: Gas-phase surveys
• Figure 3: Ratios of various O- and N-bearing COMs with respect to methanol as a function of luminosity. The black solid line is a simple fit obtained using $\chi^2$ method, while the shaded black areas use bootstrapping to find the uncertainty of the fit. The inner darker areas show the 68% confidence intervals (1$\sigma$) of the bootstrapped predicted y-values at each x-value, while the more extended lighter areas show the 95% confidence intervals (2$\sigma$). For the NH$_2$CHO and CH$_3$CN panels DIHCA values are not included in the black fit (see text) while the slope is printed in olive color if DIHCA values were included. The PRODIGE object, B1-bS (lowest luminosity), and EMoCA/ReMoCA values, SgrB2(N1-N2), are removed from the black and olive fits in the NH$_2$CHO and CH$_3$CN panels. The highlighted orange regions indicate the range of values of ices for protostellar systems from Slavicinska2023, Chen2024, Rocha2024, and Nazari2024_ices. These values include the error bars on the ice measurements and considering the small statistics they are presented as highlighted regions rather than scatter points. The tentative detection of CH$_3$CN in ices is indicated by the dashed arrow and the lowest available upper limit on NH$_2$CHO with a solid arrow. The individual objects and the references for COM measurements are given in Table \ref{['tab:refs']}.
• Figure 4: Approximate measure of N/O ratio in COMs as a function of galactic distance, luminosity, envelope mass, and luminosity over mass. The black solid line is a simple fit obtained using $\chi^2$ method, while the shaded areas use bootstrapping to find the uncertainty in the fit. The inner darker and extended lighter areas show the same as Fig. \ref{['fig:ratios']}. Table \ref{['tab:refs']} presents the references for these measurements. The relation of N/O as a function of distance is mainly explained by the relation with luminosity and mass, although more deep data with optically thin line for objects in the range of 1-4 kpc from the Galactic center are needed for confirmation.
• Figure 5: Temperature structure of a Class 0 and a Class I protostellar system. These are taken from Nazari2024_gap models with a final stellar mass of 0.5 M$_{\odot}$ at $t=10^4$ yr and $t=3\times 10^5$ yr. The disk radius is assumed as 50 au for both models, while the disk mass is 0.02 M$_{\odot}$ and 0.01 M$_{\odot}$ for the Class 0 and Class I cases, respectively. Other parameters of the model are described in full in Table B.2 of Nazari2024_gap.