An Observational View of Structure in Protostellar Systems
The envelopes and disks that surround protostars reflect the initial conditions of star and planet formation and govern the assembly of stellar masses. Characterizing these structures requires observations that span the near-infrared to centimeter wavelengths. Consequently, the past two decades have seen progress driven by numerous advances in observational facilities across this spectrum, including the \textit{Spitzer Space Telescope}, \textit{Herschel Space Observatory}, the Atacama Large Millimeter/submillimeter Array, and a host of other ground-based interferometers and single-dish radio telescopes. Nearly all protostars appear to have well-formed circumstellar disks that are likely to be rotationally-supported; the ability to detect a disk around a protostar is more a question of spatial resolution than whether or not a disk is present. The disks around protostars have inherently higher millimeter/submillimeter luminosities as compared to disks around more-evolved pre-main sequence stars, though there may be systematic variations between star forming regions. The envelopes around protostars are inherently asymmetric and streamers emphasize that mass flow through the envelopes to the disks may not be homogeneous. The current mass distribution of protostars may be impacted by selection bias given that it is skewed toward solar-mass protostars, inconsistent with the stellar initial mass function.