Contribution Contributed Talk
Envelope infall in the youngest protostars
Protostars and disks are formed within infalling envelopes of molecular gas and dust. The mass infall rate sets many of the conditions for early disk evolution. High infall rates may increase the mass of the protostellar disk enough to force gravitationally instability leading to the formation of binaries or planets and to episodic accretion from the disk to the protostar. Until recently, our ability to directly measure mass infall rates was restricted by severe observational challenges. Today the next generation of interferometers (ALMA and NOEMA) can obtain observations with the required sensitivity and spatial resolution to observe envelope infall on scales down to the expected sizes of protostellar disks (100 AU). With wide-field Spitzer and Herschel surveys of Orion A we have identified a class of protostars, the PACS Bright Red Sources (PBRS) characteristic of the earliest stage (< 25,000 yrs) of Class 0 evolution. Our CARMA observations found compact (~ 2000 AU) envelopes with high column densities. Our more recent NOEMA data exhibit self-absorption profiles in the H2CO line at 225 GHz, precisely measuring infall velocities (at 0.1 kms) down to a spatial resolution of a few hundred 900 AU, approaching the threshold for disk formation. I will discuss our methods for using wide-field IR surveys to find the youngest protostars and our methods for measuring the infall rates in their envelopes. I will discuss what our models of infall and disk formation indicate for our ALMA observations proposed for the current cycle, which are capable of resolving the transition region between the envelope and the protostellar disk. This is the region where the predominantly radial velocities of infall are turned by angular momentum conservation into the rotational velocities of the disk. These observations have the potential to resolve long-standing ambiguities in the interpretation of the protostellar luminosity distribution and our understanding of how the stellar and disk masses are assembled.