tokens: from isolated cold globules to rich clusters. There are two common observational measures of environment. First, there are the environmental conditions set by the properties of the natal molecular gas, particularly the dense gas. Second, there are environmental variations traced by the protostars, including the densities of young stars and the systematic changes in their properties. These protostars and their properties have been quantified with Spitzer data (Megeath et al. 2012 , 2016 ) and Herschel data from HOPS (Furlan et al. 2016 ). With Herschel observations at longer wavelengths and of wider fields than included in HOPS, Stutz Kainulainen ( 2015 ) measured the column density ( N H ) and mass distributions across Orion A, quantifying environmental differences. They found that the column density probability distribution functions vary with location. Furthermore, Stutz Gould ( 2016 ) and Stutz ( 2018 ) found that the mass per unit length ( M / L ) varies significantly across Orion A, with the Orion Nebula Cluster (ONC) having a higher M / L than the Integral-Shaped Filament (ISF), and the ISF having a higher M / L than LDN 1641. These column density and mass variations imply variations in the volume density and gravitational potential across the cloud. Environmental variations are also found in the radial velocities of the gas. González Lobos Stutz ( 2019 ) showed that gas radial velocities in both high- and low-density tracers show significant variations within the ISF. In particular, the northern portion has more centrally concentrated gas with stronger overall velocity gradients compared to the southern portion, which transitions to the LDN 1641 region. Systematic variations of the temperatures, line widths, and densities of cores across Orion A were also found by Wilson et al. ( 1999 ). Turning to the stellar content, the observed properties of the protostars vary with environment. Kryukova et al. ( 2012 ) found variations in the Orion clouds, where protostars are more luminous in dense regions. Stutz Kainulainen ( 2015 ) found that the fraction of protostars in the young Class 0 phase varies systematically with changes in environment as indicated by variations in the slope of the dust column density probability distribution function ( N -PDF) across the region. The variation in this fraction may result from differences in the star formation history between these regions, as also suggested by Fischer et al. ( 2017 ), the influence of feedback (Sadavoy et al. 2014 ), or variations in infall rates of the protostars due to variations in the densities of their birth environments (Kryukova et al. 2012 , 2014 ; Dunham et al. 2014 ). This paper is part of a series describing results from HOPS. These papers include evolutionary studies of protostars via modeling of their SEDs (Furlan et al. 2016 ; Fischer et al. 2017 ), the analysis of far-IR protostellar spectra (Manoj et al. 2013 , 2016 ), the discovery and characterization of the
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