摘要
Context . Methanol (CH 3 OH) masers that are collisionally pumped, referred to as ‘class I’, commonly trace shock interfaces in star-forming regions, but their incidence in embedded low-mass protostars remains poorly quantified. Aims . We aim to (i) establish the occurrence, spatial origin, and excitation of a newly detected class I CH 3 OH maser at 302.913 GHz (12 0,12 –11 1,11 A, E u = 180.9 K) towards low-mass protostars and (ii) test whether the environments that pump this maser exhibit signatures of shock-driven complex chemistry. Methods . We utilised data from the COMPASS ALMA Large Program to search for maser activity in the 302.9 GHz line in a sample of 11 low-mass protostellar sources. The COMPASS programme provides unbiased surveys of these sources between 279.0 and 311.7 GHz covering several transitions of CH 3 OH as well as other complex organic molecules (COMs) at typical angular resolutions of 0.″3 − −0.″5. Measuring departures from local thermodynamic equilibrium (LTE) in the 302.9 GHz CH 3 OH excitation via rotation diagram analysis robustly revealed masing. Results . The 302.9 GHz CH 3 OH line is detected as a non-thermal excess (maser) in seven out of 11 COMPASS sources, with peak brightness temperatures ranging from a few to a thousand kelvin. One of these detections is towards HOPS 108, which hosts the brightest maser, and it is likely associated with the powerful outflow driven by the nearby intermediate-mass protostar HOPS 370. Excluding this case, the maser is detected in six of the ten low-mass protostars (60%). All six low-mass protostars in which maser emission is detected are Class 0 sources interacting with dense ambient material, whereas non-detections correspond to three Class I objects and one isolated Class 0 core. This indicates a preference for early evolutionary stages and environments rich in gas for outflow–ambient medium interactions. The maser emission is spatially coincident with thermal CH 3 OH emission along the outflow cavity walls and bow shocks, but it has more compact knot-like morphologies. Lastly, among the surveyed COMs, only acetaldehyde (CH 3 CHO) closely traces the shocked morphology of the 302.9 GHz CH 3 OH maser. However, this CH 3 CHO feature is observed only towards the brightest maser in the region around HOPS 108, which is influenced by the intermediate-mass protostellar outflow. Conclusions . Our study reveals that class I CH 3 OH masers in low-mass protostellar systems are more prevalent than previously recognised and preferentially occur in younger, more embedded systems. Although this has only been observed in an intermediate-mass protostellar outflow, CH 3 CHO is enhanced in a manner consistent with shock-driven chemistry along the outflow cavities. Testing whether a similar relationship applies in the low-mass regime will require more sensitive CH 3 CHO observations towards regions where the class I CH 3 OH masers are detected.