m ISO-Observations of L1457
Ralf Timmermann , Benedikt Köster & Jürgen Stutzki
Universität zu Köln, I. Physikalisches Institut, Zülpicher Straße 77, 50937 Köln, Germany
We report the results of [C II] 158 and [O I] 63 m fine-structure
line observations along two cuts across a molecular region of L1457
(MBM 12) carried out with ISO-LWS. The source is the nearest known
molecular cloud at a distance of 65 pc. [C II] emission was
detected at all positions observed. The [C II] emission that
originates from far off the molecular clumps is assigned to the
cooling of the diffuse atomic interstellar medium. We deduce a cooling
efficiency
erg s-1 H-atom-1. Our value is about 2 times
larger than those found in a rocket-borne observation (Matsuhara et
al. 1997) and by COBE (Dwek et al. 1997) at high latitude. The kinetic
temperature of the atomic gas is
K for a volume
density
cm-3(Moriarty-Schieven et al. 1997),
quite lower than that commonly adopted (
K).
We observed excess [C II] emission from the dense clumpy
molecular component. It is likely to originate from molecular gas that
is irradiated by an interstellar FUV-radiation field of order
.
This is on the same order of the
far-IR radiation field (
)
derived from
IRAS observations. Using PDR models of spherical symmetry we find the
molecular gas of L1457 consists of clumps with a density
cm-3. Their masses range between 10-4 and
10-3 M
consistent with the clump distribution derived
from high resolution CO observations. [O I] 63 m emission was not
detected toward L1457. The upper limit on the [O I] 63 m flux sets a
limit of
for the FUV-radiation field.