Publications
of the
MPIfR
Optical & Infrared
Interferometry Group
B. Larsson, R. Liseau, and A.
Men'shchikov:
The ISO-LWS map of the Serpens cloud core.
II. The line spectra
Astronomy & Astrophysics 386, 1055-1073
(2002)
Abstract.
We present spectrophotometric ISO imaging with the LWS and the CAM-CVF
of the
Serpens molecular cloud core. The LWS map is centred on the far
infrared and
submillimetre source FIRS 1 SMM 1 and its size is 8' x 8'.
The fine structure line emission in OI 63µm and CII 157µm is
extended on the arcminute scale and can be successfully modelled to
originate
in a PDR with G_0 = 15 +/- 10 and n(H2) in the range of 10^4 -
10^5 cm^-3. Extended emission might also be observed in the rotational
line emission of H2O and high-J CO. However, lack of sufficient angular
resolution prevents us from excluding the possibility that the emission
regions
of these lines are point like, which could be linked to the embedded
objects
SMM9 S68 and SMM4. Toward the Class 0 source SMM1, the LWS
observations reveal, in addition to fine structure line emission, a
rich
spectrum of molecular lines, superposed onto a strong, optically thick
dust
continuum (Larsson et al. 2000). The sub-thermally excited and
optically thick
CO, H2O and OH lines are tracing an about 10^3 AU source with
temperatures higher than 300K and densities above 10^6 cm-3
(M=0.01Msun). The molecular abundances, X=N(mol)/N(H2), are
X=(1,0.1,0.02,>0.025) x 10^-4 for CO, H2O, OH and
13CO, respectively. Our data are consistent with an ortho-to-para ratio
of
3 for H2O. OH appears highly overabundant, which we tentatively ascribe
to
an enhanced (X-ray) ionisation rate in the Serpens cloud core (zeta
>
10^-18 s^-1. We show that geometry is of concern for the correct
interpretation of the data and based on 2D-radiative transfer modelling
of the
disk/torus around SMM1, which successfully reproduces the entire
observed SED
and the observed line profiles of low-to-mid-J CO isotopomers, we can
exclude
the disk to be the source of the LWS-molecular line emission. The same
conclusion applies to models of dynamical collapse (`inside-out'
infall). The
6" pixel resolution of the CAM-CVF permits us to see that the region of
rotational H2 emission is offset from SMM1 by 30", at position angle
340 degree, which is along the known jet flow from the Class 0 object.
This
H2 gas is extinguished by Av=4.5mag and at a temperature of
10^3K, which suggests that the heating of the gas is achieved through
relatively slow shocks. This is also in agreement with the deduced low
ortho-to-para ratio of H2-o/p=1. Although we are not able to establish
any firm conclusion regarding the detailed nature of the shock waves,
our
observations of the molecular line emission from SMM1 are to a limited
extent
explainable in terms of an admixture of J-shocks and of C-shocks, the
latter
with speeds of about (15--20) km/s, whereas dynamical infall is not
directly revealed by our data.
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