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We model the radiatively driven flow from IM Lup -- a large protoplanetarydisc expected to be irradiated by only a weak external radiation field (atleast 10$^4$ times lower than the UV field irradiating the Orion Nebula Clusterproplyds). We find that material at large radii ($>400$AU) in this disc issufficiently weakly gravitationally bound that significant mass loss can beinduced. Given the estimated values of the disc mass and accretion rate, theviscous timescale is long ($\sim10$Myr) so the main evolutionary behaviour forthe first Myr of the disc's lifetime is truncation of the disc byphotoevaporation, with only modest changes effected by viscosity. We alsoproduce approximate synthetic observations of our models, finding substantialemission from the flow which can explain the CO halo observed about IM Lup outto $\geq1000$AU. Solutions that are consistent with the extent of the observedCO emission generally imply that IM Lup is still in the process of having itsdisc outer radius truncated. We conclude that IM Lup is subject to substantialexternal photoevaporation, which raises the more general possibility thatexternal irradiation of the largest discs can be of significant importance evenin low mass star forming regions.

First evidence of external disc photoevaporation in a low mass star forming region: the case of IM Lup