Expanding the C$_3$H$_6$O$_2$ Isomeric Interstellar Inventory: Discovery of Lactaldehyde and Methoxyacetaldehyde in G+0.693-0.027

TL;DR

This work expands the interstellar inventory of C$_3$H$_6$O$_2$ isomers by reporting the first interstellar detections of lactaldehyde and methoxyacetaldehyde, the confirmation of methyl acetate and hydroxyacetone, and tentative detections of anti- and gauche-ethyl formate in G+0.693-0.027. Using an ultra-deep broadband spectral survey with the Yebes 40 m and IRAM 30 m telescopes and LTE modeling via SLIM/Madcuba Autofit, the authors derive column densities and fractional abundances for six isomers and establish meaningful upper limits for two non-detected isomers. The results show a clear abundance ranking and suggest that all detected isomers form predominantly through grain-surface radical–radical chemistry starting from CO, driven by CO hydrogenation to CH$_3$OH and CH$_3$CH$_2$OH and enhanced by shocks and elevated cosmic-ray ionization. The findings highlight the growing role of complex O-bearing chemistry in the ISM and provide constraints on formation pathways, while underscoring how isomeric stability alone cannot predict interstellar abundances.

Abstract

The tentative detection of 3-hydroxypropanal (HO(CH$_2$)$_2$C(O)H) toward the Galactic center molecular cloud G+0.693-0.027 prompts a systematic survey in this source aimed at detecting all C$_3$H$_6$O$_2$ isomers with available spectroscopy. We use an ultra-deep broadband spectral survey of G+0.693-0.027, carried out with the Yebes 40 m and IRAM 30 m telescopes, to conduct the astronomical search. We report the first interstellar detection of lactaldehyde (CH$_3$CH(OH)C(O)H) and methoxyacetaldehyde (CH$_3$OCH$_2$C(O)H), together with the second detections (i.e., confirmation) of methyl acetate (CH$_3$C(O)OCH$_3$) and hydroxyacetone (CH$_3$C(O)CH$_2$OH), and new detections in this source of both $anti$- and $gauche$- conformers of ethyl formate (CH$_3$CH$_2$OC(O)H), the latter tentatively. In contrast, neither propionic acid, CH$_3$CH$_2$C(O)OH, nor glycidol, c-CH$_2$OCHCH$_2$OH (i.e., the most and the least stable species within the C$_3$H$_6$O$_2$ family, respectively) were detected, and we provide upper limits on their fractional abundances of $\leq$1.5 $\times$ 10$^{-10}$ and $\leq$3.7 $\times$ 10$^{-11}$. Interestingly, all C$_3$H$_6$O$_2$ isomers can be synthesized through radical-radical reactions on the surface of dust grains, ultimately tracing back to CO as the parent molecule. We suggest that formation of the detected isomers is mainly driven by successive hydrogenation of CO, producing CH$_3$OH and CH$_3$CH$_2$OH as the primary parent species. Conversely, propionic acid is thought to originate from the oxygenation of CO via the HOCO intermediate, which help us rationalize its non-detection. Overall, our findings notably expand the known chemical inventory of the interstellar medium and provide direct observational evidence that increasingly complex chemistry involving O-bearing species occurs in space.

Figures (11)

• Figure 1: Relative energies (including zero-point energy, ZPE, corrections) plotted as a function of the total dipole moment for the targeted C$_3$H$_6$O$_2$ isomers, computed at the B2PLYPD3/aug-cc-pVTZ level of theory Grimme2011 using the Gaussian 16 program package Frisch2016. Optimized 3D structures at the same level of theory are also shown (carbon atoms in gray, oxygen atoms in red and hydrogen atoms in white). The status of detections in G+0.693 is indicated using the following color code in the molecule name: new first detections in the ISM are shown in red, tentative detections in orange, non-detections in purple and previous detections toward different sources in black. Previous detections toward other sources of methyl acetate methyl_acetate_Orion, anti and gauche ethyl formate ethyl_formate_Bellochemethyl_acetate_Orionrivilla_chemical_2017peng_alma_2019, hydroxyacetone hydroxyacetone_IRAS and 3-hydroxypropanal are also included. Also, new detections based in this work are highlighted in boldface. The 3D representations of all molecules have been visualized with IQmol ( https://www.iqmol.org/).
• Figure 2: Selected unblended or slightly blended transitions of methyl acetate, CH$_{3}$C(O)OCH$_3$, identified toward G+0.693, which were used to derive the LTE physical parameters of the molecule (see text; listed in Table \ref{['tab:LTEMA']}). The red line shows the best LTE fit of CH$_{3}$C(O)OCH$_3$, while the blue line represents the combined emission of all molecules identified in the survey (observed spectra are shown as gray histograms). The symmetry state of the detected transitions is shown at the bottom of each panel (full list of quantum numbers listed in Table \ref{['tab:LTEMA']}). The * symbol indicates that there are various transitions in the same panel, each comprising the same E$_3$/E$_4$/E$_1$ symmetry states. The transitions are sorted by decreasing intensity. The molecular structure of CH$_{3}$C(O)OCH$_3$ is also shown (C atoms in gray, O atoms in red and H atoms in white).
• Figure 3: Transitions of a-ethyl formate, $a$-CH3CH2OC(O)H, identified toward G+0.693, which were used to derive the LTE physical parameters of the molecule (see text; listed in Table \ref{['tab:aethylformate']}). The red line shows the best LTE fit of $a$-CH3CH2OC(O)H, while the blue line represents the combined emission of all molecules identified in the survey, including $a$-CH3CH2OC(O)H (observed spectra are shown as gray histograms). The quantum numbers for each transition are shown at the bottom of each panel. The transitions are sorted by decreasing intensity. The molecular structure of $a$-CH3CH2OC(O)H is also shown (C atoms in gray, O atoms in red and H atoms in white).
• Figure 4: Transitions of hydroxyacetone, CH$_{3}$C(O)CH$_2$OH, detected toward G+0.693 (see text; listed in Table \ref{['tab:hydroxyacetone']}). The red line shows the best LTE fit of CH$_{3}$C(O)CH$_2$OH, while the blue line represents the combined emission of all molecules identified in the survey, including CH$_{3}$C(O)CH$_2$OH (observed spectra are shown as gray histograms). The quantum numbers for each transition are shown at the bottom of each panel (A state lines unless stated otherwise). The transitions are sorted by increasing frequency. The molecular structure of CH$_{3}$C(O)CH$_2$OH is also shown (C atoms in gray, O atoms in red and H atoms in white).
• Figure 5: Transitions of lactaldehyde, CH3CH(OH)C(O)H, identified toward G+0.693 (see text; listed in Table \ref{['tab:lact']}). The red line shows the best LTE fit of CH3CH(OH)C(O)H, while the blue line represents the combined emission of all molecules identified in the survey, including CH3CH(OH)C(O)H (observed spectra are shown as gray histograms). The quantum numbers for each transition are shown at the bottom of each panel. For panels with multiple lines, we show the quantum number of the lowest-frequency transition. The transitions are sorted by increasing frequency. The molecular structure of CH3CH(OH)C(O)H is also shown (C atoms in gray, O atoms in red and H atoms in white).