1. Scientific Case
 
G78.2+2.1 is a 62' diameter shell-like radio SNR in Cygnus whose 1 GHz flux of
340 Jy is exceeded only by Cas A, the Crab and Vela. Most of the flux comes
from the SE part where the SNR is supposed to be colliding with a dense
molecular cloud and this also coincides with a 90um source detected in a 1981
survey of the Cygnus X region that made G78.2+2.1 brighter than any other SNR.
The remnant is also of particular interest because it was proposed by Pollock
in 1985 as the identification of the COS-B gamma-ray source 2CG078+01, with
the high-energy emission powered by the interaction of relativistic particles
generated by the SNR with the dense material of the molecular cloud. More
recent gamma-ray observations made with EGRET aboard the Compton Observatory,
as well as confirming the reality of the 2CG078+01,  have led to a revival of
interest in the possibility that remnant-cloud interactions form a class of
gamma-ray sources. The importance of high spatial resolution ISO IR
observations is to map in detail the morphology of the interaction between
remnant and cloud in a filter sensitive to dust continuum emission. Pollock
suggested that the IR emission is powered by the absorption by dust in and
immediately outside the remnant of optical, UV and X-radiation. The CAM
observations of Cas A have shown the feasibility of observations of this type
and in particular that dust is to found in SNRs. One of the reasons that
G78.2+2.1 has not received more attention in the literature is the high
absorption at optical and X-ray frequencies that obscure the view in the
direction of Cygnus X. The clarity of the IR maps is thus likely to make a
significant impact on the understanding of one of the brightest unidentified
sources of high-energy gamma-radiation.