An HST Survey of Protostellar Outflow Cavities: Does Feedback Clear Envelopes?

Abstract

We study protostellar envelope and outflow evolution using Hubble Space Telescope NICMOS or WFC3 images of 304 protostars in the Orion molecular clouds. These near-IR images resolve structures in the envelopes delineated by the scattered light of the central protostars with 80 au resolution, and they complement the 1.2 μm to 870 μm spectral energy distributions (SEDs) obtained with the Herschel Orion Protostar Survey program. Based on their 1.60 μm morphologies, we classify the protostars into five categories: nondetections, point sources without nebulosity, bipolar cavity sources, unipolar cavity sources, and irregulars. We find point sources without associated nebulosity are the most numerous, and show through monochromatic Monte Carlo radiative transfer modeling that this morphology occurs when protostars are observed at low inclinations or have low envelope densities. We also find that the morphology is correlated with the SED-determined evolutionary class, with Class 0 protostars more likely to be nondetections, Class I protostars to show cavities, and flat-spectrum protostars to be point sources. Using an edge detection algorithm to trace the projected edges of the cavities, we fit power laws to the resulting cavity shapes, thereby measuring the cavity half-opening angles and power-law exponents. We find no evidence for the growth of outflow cavities as protostars evolve through the Class I protostar phase, in contradiction with previous studies of smaller samples. We conclude that the decline of mass infall with time cannot be explained by the progressive clearing of envelopes by growing outflow cavities. Furthermore, the low star formation efficiency inferred for molecular cores cannot be explained by envelope clearing alone.

Wide-angle protostellar outflows driven by narrow jets in stratified cores

“…This is also in good agreement with cavity widths observed at the same (projected) distance in scattered light, lying in the range 1100-8500 au in 75% of cases (cf. semi-opening angles reported in Habel et al 2021). Therefore, a jet driven into a flattened singular core seems able to reproduce typical observed outflow widths on both small and large scales for realistic long ages of ≥ 10 000 yrs.…”

“…In an ALMA survey of the widths of 22 (mostly Class 0) CO outflows (Dutta et al 2020), 50% subtend projected full-opening angles in the range α = 25 • -65 • at a projected altitude z proj = 800 au. In a sample of 29 older outflow cavities (mostly Class 1) imaged in scattered light with HST (Habel et al 2021), we see that 50% are in the range α = 8 • -46 • at z proj = 8000 au, and the fraction of point sources (viewed down the cavity interior) suggests a maximum deprojected opening angle α deproj ≤ 70 • . Therefore, CO outflows seem to be more collimated on average than previously believed and it is necessary to investigate whether a pure jet (in a stratified core) could reproduce typical observed widths, before drawing any conclusions on a dominant contribution from a wider angle wind.…”

A potential new phase of massive star formation

“…The opening angle of an outflow is often considered an indicator of how evolved the protostellar system is (e.g Arce et al 2007;Kuiper et al 2016), even though recent Hubble observations have questioned this idea (Habel et al 2021). To determine the opening angle of the outflow emanating from the H1 fragment, we used both the 12 CO(2-1) emission and the cavities seen with the 70 µm emission.…”

The VLA/ALMA Nascent Disk and Multiplicity (VANDAM) Survey of Orion Protostars V. A Characterization of Protostellar Multiplicity

“…The large sample of HOPS protostars observed with HST NICMOS and WFC3 (Kounkel et al 2016;Habel et al 2021) enables us to compare protostellar multiples that are detected from their direct stellar emission to those detected via their circumstellar dust emission. This allows us to determine how much incompleteness there might be in a particular type of observation.…”

“…The infrared-only incompleteness is 31% if Class 0 protostars are included in the counting, but drops to ∼11% if only Class I and Flat spectrum protostars are included. Much of this incompleteness in the infrared is due to extinction; Class 0 protostars are rarely detected in the HST 1.6 µm data, while many Class Is are only detected in scattered light (Habel et al 2021). The detection of companions in the HST data is primarily toward the ∼30% of the protostars that are visible as point sources (Kounkel et al 2016).…”