Microlensing of circumstellar envelopes - I. Simplified considerations fordiagnosing radial and azimuthal flow

This paper presents first results on the line profile shapes from a circums tellar envelope in bulk motion as modified by a microlensing event. Only ge ometrically and optically thin spherical shells in uniform expansion or rot ation are considered here so as to emphasise the information content availa ble in the profile shapes. In particular it is demonstrated that for the ca se of expansion, the line emission can increase by significant factors and the time variation of the profile shape is symmetric about line centre. For uniform rotation the line emission also increases significantly, but the t ime evolution of the profile shape is distinctly asymmetric. Thus, microlen sing is seen to yield information about the velocity field in the extended envelope. We elaborate on (a) the observational advantages of tailoring mic rolensing programs toward detecting extended circumstellar envelopes, (b) t he use of multiline observations to infer other properties of the envelopes , such as the ionization stratification, (c) the use of the continuum exces s emission at infrared wavelengths as a means of probing the envelope struc ture, and (d) the use of polarisation for constraining the properties of "c lumpy" winds.