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Astron. Astrophys. 336, L53-L56 (1998)

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4. Discussion

We have observed the [C ii] 158 µm fine-structure line along two cuts across a dense molecular clump of L1457. [C ii] emission is detected at all positions. A fraction can be assigned to the cooling of the CNM. We deduced a cooling efficiency [FORMULA] erg s-1 H- atom-1, consistent with previous measurements. Moreover, we find excess [C ii] emission that arises from clumpy molecular gas with [FORMULA][FORMULA][FORMULA]=[FORMULA] cm-3. This is actually larger than that determined by Pound et al. (1990) by a factor of [FORMULA]10. Utilizing PDR models for spherical symmetric clumps we deduce an interstellar FUV-radiation field [FORMULA].

The PDR models that fit the observations best are clumps with masses M=[FORMULA] [FORMULA] ([FORMULA]=[FORMULA]) and [FORMULA] [FORMULA] ([FORMULA]=[FORMULA]). Their sizes are about 0.0044 and 0.0095 pc, corresponding to [FORMULA] and [FORMULA]. The average molecular column densities of such clumps are [FORMULA] and [FORMULA] cm-2, respectively, somewhat smaller than those deduced from observations of clump CO03 (Pound et al. 1990, Ingalls et al. 1994).

Ingalls et al. (1994) revealed a clumpiness in their 13CO(2-1) maps that continues down to scales of [FORMULA], whilst Zimmermann (1993; see also Kramer et al. 1998) reached a spatial resolution of [FORMULA] and found substructures down to masses [FORMULA] [FORMULA] (0.0086 pc). If clumps are indeed this small 13CO observations favor the evidence of high density clumps [FORMULA][FORMULA][FORMULA] cm-3 owing to the smaller clump sizes required in our PDR model computations. We have further evidence that the molecular gas of L1457 consists mainly of clumps of small masses. Kramer et al. (1998) exhibited that the clump mass spectrum for L1457 is compatible with a power law, [FORMULA], in the mass range [FORMULA] [FORMULA] when a Gaussian clump decomposition algorithm is employed on CO maps.

It turns out that PDR model computations at lower densities [FORMULA][FORMULA][FORMULA]=[FORMULA] cm-3 do not fit the CO observations very well, whereas the computed [C i] 492 GHz emission generally matches better at this density. Models with [FORMULA][FORMULA][FORMULA]=[FORMULA] cm-3, however, require larger clump masses and their sizes are 0.095 (5[FORMULA]0) and 0.20 pc (10[FORMULA]6) for M=[FORMULA] [FORMULA] ([FORMULA]=[FORMULA]) and [FORMULA] [FORMULA] ([FORMULA]=[FORMULA]), even much larger than ISO's beam size. This is also not consistent with high resolution CO observations (see above).

No [O i] 63 µm emission was detected in the molecular gas of L1457. This may demonstrate the absence of warm gas that would be present in the vicinity of commencing star-formation.

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© European Southern Observatory (ESO) 1998

Online publication: July 27, 1998
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