An ACA 870 Micron Continuum Survey of Orion Protostars: A New Window on the Evolution of Protostellar Envelopes

We present the results from an 870 μm continuum survey of 300 protostars from the Herschel Orion Protostar Survey (HOPS) using the Atacama Compact Array (ACA). These data measure flux densities out to envelope scales ~8000 AU, and can resolve the structure of envelopes with 1600 AU resolution. We classify protostars by their ACA morphologies and present their flux densities. We then compare the ACA observations to ALMA 12m array snapshot observations at ~0.1” (40 AU) resolution that detect the disks within the envelopes. Over 90% of the ACA sources have disk(s) near their center. We calculate 12m to ACA flux ratios, essentially disk to combined disk and envelope flux ratios, for the protostars. Our sample shows a clear evolution in this ratio with SED-based class. Class 0 protostars are mostly envelope dominated with flux ratios less than 0.5. In contrast, Flat Spectrum protostars are disk dominated with ratios near one. Class I protostars can be either disk or envelope-dominated. As the ratios increase, the envelope fluxes and inferred mass infall rates decrease. We propose the 12m/ACA ratio, i.e. disk to combined flux ratio, is an evolutionary indicator that is less sensitive to inclination.