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

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 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 4 , where 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 0.3-0.7, and thus an O⁰ abundance of 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.

© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999
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