Astronomy and Astrophysics – Astrophysics
Scientific paper
Sep 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...276..473f&link_type=abstract
Astronomy and Astrophysics, Vol. 276, p. 473 (1993)
Astronomy and Astrophysics
Astrophysics
73
Interstellar Medium, Abundances, Molecules, Reflection Nebulae, Stars, Individual, Hd2Oo775, Ism, Individual Objects, Ngc 7023, Radio Lines, Interstellar
Scientific paper
To investigate the effects of the UV radiation on the chemistry of nitrogenated molecules in molecular clouds, we have carried out an observational study of the photodissociation region (PDR) associated with the reflection nebula NGC 7023. We mapped a region of 3' × 3' over the PDR in millimeter transitions of 12CO, 13CO, C18O, HCO+, HCN, HNC and N2H+. Spectra of CS, CN and C2H and of the rarer isotopic species H13CO+, H13CN, and HN13C, were also obtained at selected positions. We find evidences of selective photodissociation in the estimated 13CO/C15O ratio. Furthermore, all molecular abundances, except those of CN and perhaps C2H, decrease towards the star, and significant gradients in the values of some molecular abundance ratios (the HNC/HCN ratio decreases by a factor of 5, the N2H+/HCO+ ratio decreases by a factor of 12, the CN/HCN ratio increases by a factor of 8 and the (CN+HCN+HNC)/NH3 ratio increases by a factor of 30 towards the star position) reveal the existence of important chemical changes in this direction.
Chemical equilibrium model calculations have been also carried out in order to interpret the observed behavior. Our results show that the variations found in molecular abundances cannot be explained by the kinetic temperature and/or the hydrogen density gradients measured in this region. The observed behavior is well explained by the influence of the stellar UV radiation on the chemistry of the molecular gas if the emission arises in a region at a visual extinction between 6 and 10 mag from the star. Molecular destruction in this region (Aυ ˜ 6 mag) is due mainly to reactions with H+, C, H, C+, O, and to electronic recombination. The CN/HCN ratio is suggested as a tracer of enhanced UV fields.
Bachiller Rafael
Cernicharo Jose
Fuente Asuncion
Martín-Pintado Jesús
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