Molecular absorptions originated in interstellar clouds

Jacek Krelowski
Center for  Astronomy, N. Copernicus University
Gagarina 11, Pl-87-100 Torun,
and
Institute of Theoretical Physics and Asytrophysics, Gdansk University
Ul. Wita Stwosza 57, Pl-80-952 Gdansk
 Poland
www.astri.uni.torun.pl/~jacek, email: jacek@astri.uni.torun.pl

Interstellar HI clouds, composed of neutral hydrogen, contain about 90% of
the total mass of interstellar medium. Their absorption spectra are composed
of: continuous extinction, atomic lines, molecular features and the
unidentified diffuse interstellar bands (DIBs). The latter are also
believed to be carried by some complex molecules. This motivates
investigations of inter-relations between molecular features and DIBs.
A vast majority of DIBs is characterized by small central depths and thus
they are observable only since solid state detectors are widely applied in
astrophysics. The interstellar medium is composed of clouds and thus
interstellar absorptions, seen along the same line of sight, may be in fact
ill-defined averages over different environments. The extensive database of
echelle spectra, collected in a large part at SAO and Terskol Observatories,
allowed to prove that the CaII column density correlates with parallaxes of
OB-3 stars in contrast to other interstellar species. Thus CaII is quite
evenly distributed in the interstellar medium while other species (NaI, KI,
CH, CN, DIB carriers) are not. The observed spectra cannot be physically
interpreted if being mixtures of features originated in different clouds,
i.e. in different environments. The abundance ratios of interstellar
molecules (identified and DIB carriers) differ from cloud to cloud due to
different physical processes which govern their formation. Also DIB profiles
vary from cloud to cloud - this fact strongly supports a molecular origin
of these, still unidentified, features and motivates investigation of their
relations to other molecules; they can reveal physical conditions which
facilitate formation of the DIB carriers and lead to their identification.