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

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1. Introduction

L1457 (MBM 12) is the nearest known molecular cloud at a distance of 65 pc (Hobbs et al. 1986). It has been studied in different tracers, such as CO, [C i], H i and through optical absorption. 13CO(2-1) line observations at the CSO (Ingalls et al. 1994) and the IRAM 30m-telescope (Zimmermann 1993) provide evidence that L1457 consists of dense molecular clumps down to masses of [FORMULA] [FORMULA] and sizes [FORMULA]2000 AU.

It is assumed that L1457 is not associated with sources of FUV-radiation. When such radiation impinges onto a molecular clump a PDR is formed on its outer layer. At low FUV-intensities the [C ii] 158 µm emission is a strong function of the FUV-field, since it maintains an equilibrium in C + h[FORMULA] [FORMULA] C+ + e- and CO + h[FORMULA] [FORMULA] C + O. Thus, C+ column densities depend strongly on the FUV-radiation field [FORMULA] in the PDR. Moreover, [C ii] cooling is also a function of [FORMULA] owing to the gas temperature. The emission is, however, expected to be faint at [FORMULA], where [FORMULA][FORMULA] erg s-1cm- 2sr-1 (Draine 1978). Therefore, appropriate spectrometers on board the Kuiper Airborne Observatory could not detect such emission easily due to the large thermal background. It has been discovered to date only from the high-latitude molecular clouds in Ursa Major through a rocket-borne observation (Matsuhara et al. 1997).

In order to test the FUV-field in the vicinity of L1457 we performed [C ii] 158 µm and [O i] 63 µm observations with the Infrared Space Observatory (ISO). The [C ii] together with previous CO and [C i] 492 GHz observations were also utilized as tracers to constrain the mass and density of the molecular clumps.

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© European Southern Observatory (ESO) 1998

Online publication: July 27, 1998