Spatially resolving dusty outflows with JWST/MIRI

Dusty outflows may be an avenue for grain transport in protostellar systems. Such outflows from protostellar star-disk systems have been predicted to explain the crystallinity fractions of long-period comets by lifting thermally processed grains from the hot inner disk into the outflow. Sub-millimetre observations have been interpreted as supporting this view, revealing dust opacity indices β<1, possibly implying large grains in protostellar outflow cavities. Yet, the interpretation of sub-millimetre observations is complicated by temperature variations in the outflow, and β is relatively insensitive to dust composition. Another difficulty with understanding dusty outflows is that theoretical work has mostly focused on the late stages of star formation (Class II), where the dusty envelope has already been cleared. Sub-arcsecond resolution mid-infrared observations with the James Webb Space Telescope are opening a new window into dusty outflows in the earliest stages of star formation. In this talk, based on the JWST Observations of Young Stars (JOYS) survey, we will discuss the detection of dust emission in the Class 0 systems L1448-mm, HH211, and BHR71-IRS1. We argue that the emission is explained by dust lifted from the disk during episodic accretion and heated envelope dust from the outflow cavity. We will also introduce an analytic radiative transfer model that allows us to investigate dust properties (temperature, composition and grain size) in outflow cavities with MIRI MRS. We found that the 5-27 μm mid-infrared emission in the  L1448-mm outflow cavity can be well explained with scattering and two emission temperatures, at 90 K and 400 K. The best-fit models require >90% 0.1 μm grains. We argue that this is difficult to reconcile with the millimetre-sized grain interpretation of β = 0.41±0.16 measured at ~500 au in sub-mm surveys. We will discuss radiative transfer modelling that combines mid-infrared JWST observations with sub-mm ALMA observations of dusty outflows. Looking ahead, we will show preliminary results of a continuum survey of Class 0 sources, to move the mid-infrared study of dusty outflows to significant sample sizes. JWST and ALMA have the resolution and sensitivity to test the role of dusty outflows in embedded Class 0 systems, but we need to develop the analytical tools to interpret these observations.