tokens: in general (see e.g. Viti et al. 2001 ; and Hollenbach et al. 2009 , for a discussion of this specific aspect). Meaningful constraints on the oxygen chemistry could then be obtained through the observation of HO and O. Unfortunately, no velocity-resolved observations of water have been performed in Cep E by Herschel , and no instrument can allow them at the moment. O is best observed from space observatories (the transition at 487.249 GHz is close to an atmospheric absorption feature), but it has not been observed in Cep E, and its symmetry makes its detection difficult from the outset. Another possibility is that the shock that we observed is illuminated by the protostar that drives the outflow. In such irradiated shocks (e.g. Lesaffre et al. 2013 ; Melnick Kaufman 2015 ), both HO and CO can be photo-dissociated, increasing the gas-phase abundance of O i . This effect is not taken into account in the Paris-Durham code. However, the possibility that shocks at position BII could be affected by the distant protostar is relatively low: the BII and mm position are pc away. The Cep E-mm protostar has a luminosity of about 100 , and a T-Tauri star typically emits between 10 and 10 of its total luminosity in the UV range between 3200 and 5200 Å ( Gullbring et al. 1998 ). Conservatively assuming that Cep E-mm is radiating 10% of its total luminosity in this UV range and taking dilution into account, we found that the radiation field in BII is about 4.8 in units of Draine’s ( Draine 1978 ) or Mathis’s ( Mathis et al. 1983 ) interstellar radiation field (ISRF). This value has then to be corrected for the absorption by dust grains. We assumed an average local density in the outflow cavity of 10 cm, which translates into a column density of cm and into a visual extinction of mag ( Lefloch et al. 1996 provided local estimates for this value: 61 mag around the driving protostar, and 3.2–3.4 mag in the H knots along the flow). Using the exponential attenuation approximation , where , and is the effective attenuation cross section from Draine Bertoldi (1996) , we found that the radiation field really felt in BII is about in Draine’s or Mathis’s ISRF units. Another explanation is that several shock structures are present in our beam. In this case, the missing O i emission could be accounted for by the presence of another shock structure. This additional shock layer must present different physical characteristics, as simply combining shock layers similar to those we used would simultaneously increase the [OI] emission insufficiently and increase the CO emission too much, producing another type of discrepancy between observations and models. One such shock structure could be a stationary dissociative -type shock. In particular, a -type shock with cm, magnetic field parameter , and shock velocity km s ( Flower Pineau des Forêts 2015 ) could be combined with our current best-fit solution, providing a satisfying fit for the O i and CO emission. The additional
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