Expanding the C₃H₆O₂ isomeric interstellar inventory: Discovery of lactaldehyde and methoxyacetaldehyde in G+0.693-0.027

¹ Centro de Astrobiología (CAB), INTA-CSIC, Carretera de Ajalvir km 4, Torrejón de Ardoz, 28850 Madrid, Spain
² Star and Planet Formation Laboratory, Pioneering Research Institute, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
³ Departamento de Física de la Tierra y Astrofísica, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
⁴ Massachusetts Institute of Technology, Cambridge, MA 02139, USA
⁵ National Radio Astronomy Observatory, Charlottesville, VA 22903, USA
⁶ European Southern Observatory, Alonso de Córdova 3107, Vitacura 763 0355, Santiago, Chile
⁷ Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura 763 0355, Santiago, Chile
⁸ Department of Physics, Astronomy and Geosciences, Towson University, Towson, MD 21252, USA
⁹ Observatorio Astronómico Nacional (OAN-IGN), Calle Alfonso XII 3, 28014 Madrid, Spain
¹⁰ Observatorio de Yebes (OY-IGN), Cerro de la Palera SN, Yebes, Guadalajara, Spain
¹¹ Department of Analytical Chemistry, University of Chemistry and Technology, Technická 5, 166 28 Prague 6, Czechia
¹² Grupo de Espectroscopía Molecular, Edificio Quifima, Parque Científico UVa, Universidad de Valladolid, Valladolid, 47005 Spain
¹³ Departamento de Química Física, Facultad de Ciencia y Tecnología, Universidad del País Vasco (EHU), 48940 Leioa, Spain
¹⁴ Instituto Biofisika (CSIC, EHU), 48940 Leioa, Spain
¹⁵ Univ Rennes, Ecole Nationale Supérieure de Chimie de Rennes, CNRS, ISCR-UMR6226, 35000 Rennes, France
¹⁶ Université Paris Cité and Univ Paris Est Creteil, CNRS, LISA, 75013 Paris, France

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Received: 28 November 2025
Accepted: 9 January 2026

Abstract

Aims. The tentative detection of 3-hydroxypropanal (HO(CH₂)₂C(O)H) toward the Galactic center molecular cloud G+0.693-0.027 prompts a systematic survey in this source aimed at detecting all C₃H₆O₂ isomers with available spectroscopy.

Methods. We used 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.

Results. We report the first interstellar detection of lactaldehyde (CH₃CH(OH)C(O)H) and methoxyacetaldehyde (CH₃OCH₂C(O)H), together with the second detections (i.e., confirmation) of methyl acetate (CH₃C(O)OCH₃) and hydroxyacetone (CH₃C(O)CH₂OH), and new detections in this source of both anti - and g auche - conformers of ethyl formate (CH₃CH₂OC(O)H; the latter being tentative). For these species, we derived a fractional abundance relative to H₂ of ~(0.81, 0.24, 16, 1.6, 1.3, 1.4) × 10⁻¹⁰, respectively. In contrast, neither propionic acid, CH₃CH₂C(O)OH, nor glycidol, c-CH₂OCHCH₂OH (i.e., the most and the least stable species within the C₃H₆O₂ family, respectively) were detected, and we provide upper limits on their fractional abundances of ≤1.5 × 10⁻¹⁰ and ≤3.7 × 10⁻¹¹. Interestingly, all C₃H₆O₂ 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 CH₃OH and CH₃CH₂OH 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.