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Astron. Astrophys. 350, 529-540 (1999)

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4. O0 and C+ emission

C+ is not usually abundant in shocked regions; dissociative shocks can produce C+, but its 158 µm emission never exceeds more than [FORMULA]10% of the [OI ] 63 µm emission (Hollenbach & McKee 1989) while our observed [CII ] 158 µm /[OI ] 63 µm ratio is 0.3. On the other hand, even very weak far UV fields, comparable to the average intertellar field, can produce C+ fluxes which are detectable with LWS.

Indeed, the flux observed towards L1448-mm, which corresponds to a surface brightness of 4.8 10-6 erg s-1 cm-2 sr-1 assuming unit beam filling, is comparable to the value observed at other locations in the cloud (Paper II) and can be accounted for by an average FUV field of only [FORMULA] 4 [FORMULA], where [FORMULA] is the FUV flux measured in units of the local interstellar FUV flux (1.6 10-3 erg s-1 cm-2, Habing 1968). Such a weak field would however be unable to excite the observed [OI ] 63 µm emission, which is more likely associated with shock excitation. If we assume that the [OI ] 63 µm originates from the same gas which is responsible for the CO, H2O and OH emission, we derive a column density of O0 of about 6 1016 cm-2 (for the given conditions, the 63 µm line becomes optically thick at column densities larger than 1020 cm-2); this implies an O0/CO abundance ratio of [FORMULA]0.3-0.7, and thus an O0 abundance of [FORMULA]5 10-5, which is about a factor of ten lower than the oxygen interstellar abundance (Meyer et al. 1998). This is indeed what it is expected if a significant fraction of the available oxygen is tied up in H2O.

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© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999