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Due to instrumental limitations and a lack of disk detections, the structurebetween the envelope and the rotationally supported disk has been poorlystudied. This is now possible with ALMA through observations of CO isotopologsand tracers of freezeout. Class 0 sources are ideal for such studies giventheir almost intact envelope and young disk. The structure of the disk-envelopeinterface of the prototypical Class 0 source, VLA1623A which has a confirmedKeplerian disk, is constrained from ALMA observations of DCO+ 3-2 and C18O 2-1.The physical structure of VLA1623 is obtained from the large-scale SED andcontinuum radiative transfer. An analytic model using a simple network coupledwith radial density and temperature profiles is used as input for a 2D lineradiative transfer calculation for comparison with the ALMA Cycle 0 12m arrayand Cycle 2 ACA observations of VLA1623. DCO+ emission shows a clumpy structurebordering VLA1623A's Keplerian disk, suggesting a cold ring-like structure atthe disk-envelope interface. The radial position of the observed DCO+ peak isreproduced in our model only if the region's temperature is between 11-16K,lower than expected from models constrained by continuum and SED. Altering thedensity has little effect on the DCO+ position, but increased density is neededto reproduce the disk traced in C18O. The DCO+ emission around VLA1623A is theproduct of shadowing of the envelope by the disk. Disk-shadowing causes a dropin the gas temperature outside of the disk on >200AU scales, encouragingdeuterated molecule production. This indicates that the physical structure ofthe disk-envelope interface differs from the rest of the envelope, highlightingthe drastic impact that the disk has on the envelope and temperature structure.The results presented here show that DCO+ is an excellent cold temperaturetracer.

A low-mass protostar's disk-envelope interface: disk-shadowing evidence from ALMA DCO+ observations of VLA1623