摘要
Context. Complex organic molecules (COMs) are detected in the interstellar medium, both in cold molecular clouds and in the warm inner regions of protostars. Whether these COMs are eventually directly inherited by protostars and planets or whether their chemistry is reset by in situ reactions remains unclear. Recent (sub)millimeter wavelength observations have spatially resolved the emission of gas-phase COMs around low-mass protostars, not only in compact hot corinos but also in the extended structures beyond them. Understanding the physical properties and desorption mechanisms of these molecules is essential for tracing their chemical evolution during the protostellar evolutionary sequence. Methanol (CH3OH), the most abundant COM in star-forming regions, serves as an ideal tracer for investigating these processes. Aims. As part of the ALMA Cycle 9 Large Program Complex Organic Molecules in Protostars with ALMA Spectral Surveys (COMPASS), we investigate the CH3OH emission that extends beyond the hot corino of the low-mass protostar BHR71-IRS1. Methods. We identified spatially extended emission of CH3OH around BHR71-IRS1 and subsequently explored plausible desorption scenarios. We assessed characteristic emission structures by applying a principal component analysis (PCA) to the identified emission lines. The physical properties of their line-emitting gas were derived under the local thermodynamic equilibrium (LTE) assumption through both rotational diagram and spectral modeling analyses. Results. In total, 26 spatially extended CH3OH lines have been detected around BHR71-IRS1. These emission lines are distributed across three distinct structures: (1) X-shaped cavity walls, (2) compact northern and southern shocked knots, and (3) additional distant eastern and southeastern components. The cavity walls exhibit excitation temperatures of ∼25 K and column densities of ∼1014 cm−2, while the knots show relatively higher excitation temperatures (70–120 K) and column densities (1014–1016 cm−2). The eastern and southeastern components appear unrelated to BHR71-IRS1, and their physical properties are not well constrained under LTE conditions, suggesting that a non-LTE analysis may be required. Conclusions. Plausible desorption mechanisms are evaluated by comparing the physical parameters with model predictions that simulated the effects of far-ultraviolet (FUV) photons and C-type shocks. The low excitation temperatures and narrow line widths (∼0.7 km s−1) in the cavity walls favor a reactive desorption scenario driven by FUV irradiation from IRS1. In contrast, the knots with higher excitation temperatures and column densities support the dust sputtering scenario induced by C-type shocks.