, J.Martín-Pintado
, R.Neri
, C.Rogers
,
G.Moriarty-Schieven
Centro Astronómico de Yebes (IGN), Apdo. 148, E-19080 Guadalajara,
Spain
Institut de Radioastronomie Millimétrique, 300 rue de la Piscine,
38406 St-Martin-d'Heères, France
Dominion Radio Astrophysical Observatory, Herzberg Institue of Astrophysics,
National Research Council of Canada, P.O. box 248, Penticton, BC, V2A 6K3,
Canada
Figure: An HCO
J=1
0 mosaic of the line emission towards the
reflection nebula NGC 7023. The filamentary structures are regions of
enhanced hydrogen density located in a narrow ridge (
) 
North-West from HD 200775 at the interface between
the molecular and the atomic regions. Contours are from 100 mJy to 700 mJy
in steps of 100 mJy. The velocity is marked on the top-left corner of
the panels.
The combination of interferometric and single-dish data allow us, for the
first time, to determine the intimate structure of the photodissociation
region associated with the prototypical reflection nebula NGC 7023. We have
mapped the J=1
0 HCO
line using the IRAM millimeter
interferometer and the 30-m single dish telescope, and the 21cm HI line
using the Very Large Array of NRAO. The interferometric HCO
data
show the existence of four high density molecular filaments with different
velocities (1.9 kms
, 2.4 kms
, 2.9 kms
, 4.0 kms
)
located at the interface between the atomic and the molecular gas. An
additional filament has been tentatively detected at 5.8 kms
. From the
comparison between the single-dish and the interferometric HCO
data
with HI emission, we derive that the filament at 4.0 kms
and, very
likely, that at 5.8 kms
are located on the inner walls of the HI
clump, immersed in an atomic medium, while those at 1.9 and 2.4 kms
are immersed in the molecular cloud. The filament at 2.9 kms
would
be located in an intermediate zone. The thickness of the filaments are
and the derived densities vary between 
and a few 
cm
. The molecular filaments match perfectly the infrared filaments
found at 2.1 
m, suggesting that infrared and millimeter data are probing
regions of enhanced hydrogen density.
