The robust detection and spatial distribution of acetaldehyde in Orion KL: ALMA observations and chemical modeling

Abstract

Despite the organic molecule inventory detected in Orion KL, acetaldehyde (CH3CHO) -- one of the most ubiquitous interstellar aldehydes -- has not been firmly identified with mm-wave interferometry. We analyze extensive ALMA archival datasets (142-355 GHz) to search for acetaldehyde, revealing two distinct acetaldehyde emission peaks and one component with more complex kinematic structures. One peak aligns with MF10/IRc2, where emissions of other O-bearing complex organic molecules are rarely reported, while the other is coincident with the ethanol peak in the hot core-SW. The MF10/IRc2 detection suggests unique chemistry, possibly influenced by repeated heating events. In contrast, co-detection with ethanol indicates an ice origin and suggests a potential chemical relationship between the two species. We determined acetaldehyde column densities and its kinetic temperatures toward these two peaks under LTE assumptions and compare its distribution with ethanol and other molecules that have an aldehyde (HCO) group, such as methyl formate, glycolaldehyde, and formic acid. Toward the ethanol peak, observed abundance ratios between HCO-containing species are analyzed using a chemical model. The model suggests two key points: 1) the destruction of ethanol to form acetaldehyde in the ice may contribute to the observed correlation between the two species, 2) a long cold-collapse timescale and a methyl formate binding energy similar to or lower than water are needed to explain the observations. The relative abundance ratios obtained from the model are highly sensitive to the assumed kinetic temperature, which accounts for the high spatial variability of the aldehyde ratios observed toward Orion KL.