300: An ACA 870 $μ$m Continuum Survey of Orion Protostars and their Evolution

Samuel Federman, S. Thomas Megeath, John J. Tobin, Patrick D. Sheehan, Riwaj Pokhrel, Nolan Habel, Amelia M. Stutz, William J. Fischer, Lee Hartmann, Thomas Stanke, Mayank Narang, Mayra Osorio, Prabhani Atnagulov, Rohan Rahatgaonkar

Published 2022-10-14Version 1

We present an 870 $\mu$m continuum survey of 300 protostars from the Herschel Orion Protostar Survey (HOPS) using the Atacama Compact Array (ACA). These data measure protostellar flux densities on envelope scales $\le$ 8000 AU (20") and resolve the structure of envelopes with 1600 AU (4") resolution, a factor of 3-5 improvement in angular resolution over existing single dish 870 $\mu$m observations. We compare the ACA observations to Atacama Large Millimeter Array (ALMA) 12m array observations at 870 $\mu$m with $\sim$0.1" (40 AU) resolution. Using the 12 meter data to measure the fluxes from disks and the ACA fluxes within 2500 au to measure the combined disk plus envelope fluxes, we calculate the 12 m/ACA 870 $\mu$m flux ratios. Our sample shows a clear evolution in this ratio. Class 0 protostars are mostly envelope-dominated with ratios $<$ 0.5. In contrast, Flat Spectrum protostars are primarily disk-dominated with ratios near one, although with a number of face-on protostars with large envelopes. Class I protostars span the range from envelope to disk dominated. The increase in ratio is accompanied by a decrease in the envelope fluxes and estimated mass infall rates. We estimate that 80% of the mass is accreted during the envelope-dominated phase. We find that the 12m/ACA flux ratio is an evolutionary indicator that largely avoids the inclination and foreground extinction dependence of SED-based indicators.