A molecular cloud forming in the disk-halo interface
A. Weiss, A. Heithausen, U. Herbstmeier, U. Mebold;
AaA, 1999, 344, 955
ABSTRACT:We present medium and high angular resolution H i and CO observations of the galactic cirrus cloud IVC 135+54-45 and compare the line emission with
intensities of the IRAS 100 mu m and 60 mu m bands.
Its distance above the galactic
plane is about 220 pc, thus well outside the scale height of normal molecular
gas (~ 74 pc).
Thus IVC 135+54-45 offers a unique possibility to study the
cold interstellar medium in the transition zone between the galactic plane
and the halo.
We establish a linear relation between the IRAS 100 mu m
intensities and the proton column densities in the outer part of the cloud where the
H_2 content is negligible.
In the centre of the cloud we find two clumps
which have more FIR-emission than can be accounted for by a linear NHi/I_100
correlation.
The clumps are connected to H i filaments.
A detailed analysis of one of the
clumps reveals that the FIR-excess is associated mainly with the filament and
to a lesser degree with the clump.
This result is in contradiction to our
CO observations which show strong emission in the clump and only weak
emission in the filament.
We have evidence that the FIR-excess in the filament
is partly caused by an increased dust temperature and not due to the
existence of H_2 alone.
We conclude that the filament and the clump represent
different epochs in the transition from atomic to molecular gas.
The clump is
more evolved and might be close to chemical equilibrium while formation of
molecular gas has not reached equilibrium in the filament.
We ascribe the
increased temperature in the filament to chemical energy released by H_2
formation on dust grains.
KEYWORDS: ism: abundances, ism: clouds, ism: individual objects: ivc 135+54-45, ism: molecules, infrared: ism: continuum, radio lines: ism
PERSOKEY:cirrus, h_i, co, 21 cm, h2, fir, iras, dust, ,
CODE: weiss99