Low-J CO line emission from PDRs: a natural explanation for the narrow range of observed line ratios

We argue that the surprisingly narrow range of low-J (CO)-C-12, (CO)-C-13 a nd (CO)-O-18 line ratios observed in massive star-formation regions is natu rally explained if the CO emission is understood to arise in an ensemble of dense clumps which are embedded in a lower-density interclump medium which is pervaded by stellar radiation. We demonstrate this by presenting PDR co mputations which focus explicitly on the (CO)-C-12, (CO)-C-13 and (CO)-O-18 J = 1 --> 0, J = 2 --> 1 and J = 3 --> 2 rotational line emission for a wi de range of cloud conditions. We consider spherical clouds which are illumi nated by isotropic far-ultraviolet (FUV) radiation fields. Our models provi de a self-consistent treatment of the chemical and thermal balance together with the radiative transfer of the CO line emission. We present results fo r clouds with power-law density gradients with average hydrogen particle de nsities [n] ranging from 10(4) to 10(7)cm(-3) and total average hydrogen co lumn densities [N] between 2.5 x 10(21) and 4.0 x 10(22)cm(-2). We consider clouds exposed to FUV fields (chi) 10(2) to 10(4) times more intense than the mean interstellar radiation field. We find that the resulting line rati os are insensitive to the cloud conditions and reproduce the observed value s of the relative CO line strengths.