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We present ISO-LWS measurements of the main gas cooling lines, C+ 158 mum andO 63 mum towards a moderate opacity molecular cloud (Av=3), L1721, illuminatedby the B2 star nu Sco (X = 5-10). These data are combined with an extinctionmap and IRAS dust emission images to test our understanding of gas heating andcooling in photo-dissociation regions (PDRs). This nearby PDR is spatiallyresolved in the IRAS images; variations in the IRAS colors across the cloudindicate an enhanced abundance of small dust grains within the PDR. A spatialcorrelation between the gas cooling lines and the infrared emission from smalldust grains illustrates the dominant role of small dust grains in the gasheating through the photoelectric effect. The photoelectric efficiency,determined from the observations by ratioing the power radiated by gas andsmall dust grains, is in the range 2 to 3% in close agreement with recenttheoretical estimates. The brightness profiles across the PDR in the C+ 158 mumand O 63 mum lines are compared with model calculations where the densityprofile is constrained by the extinction data and where the gas chemical andthermal balances are solved at each position. We show that abundance variationsof small dust grains across the PDR must be considered to account for the LWSobservations.Comment: 10 pages, 15 figure

Photoelectric effect on dust grains across the L1721 cloud in theρOphiuchi molecular complex