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

We present ISO-LWS measurements of the main gas cooling lines, (C + ) 158 ma nd (O 0 )6 3m towards a moderate opacity molecular cloud (Av 3), L1721, illuminated by the B2 star Sco ( =5 10). These data are combined with an extinction map and IRAS dust emission images to test our understanding of gas heating and cooling in photodissociation regions (PDRs). This nearby PDR is spatially resolved in the IRAS images; variations in the IRAS colors across the cloud indicate an enhanced abundance of small dust grains within the PDR. A spatial correlation between the gas cooling lines and the infrared emission from small dust grains illustrates the dominant role of small dust grains in the gas heating through the photoelectric eect. The photoelectric eciency, determined from the observations by ratioing the power radiated by gas and small dust grains, is in the range 2 to 3%, in close agreement with recent theoretical estimates (Bakes & Tielens 1994; Weingartner & Draine 2001). The brightness proles across the PDR in the (C + ) 158 ma nd (O 0 )6 3 ml ines are compared with model calculations where the density prole is constrained by the extinction data and where the gas chemical and thermal balances are solved at each position. We show that abundance variations of small dust grains across the PDR must be considered to account for the LWS observations.