Chemistry and excitation of HeH+ and CH+ in the planetary nebula NGC 7027
Milan Sil1,2,3*, Alexandre Faure2, Helmut Wiesemeyer4, Pierre Hily-Blant2, Tomás González-Lezana5, Jérôme Loreau6, K. D. Pérez6, Roman Čurík7, François Lique3
1Institute of Astronomy, National Tsing Hua University, Hsinchu, Taiwan
2Institut de Planétologie et d’Astrophysique de Grenoble, Université Grenoble Alpes, Grenoble, France
3Institut de Physique de Rennes, Université de Rennes, Rennes, France
4Max-Planck-Institut für Radioastronomie, Bonn, Germany
5Instituto de Fı́sica Fundamental, Madrid, Spain
6Department of Chemistry, KU Leuven, Leuven, Belgium
7Academy of Sciences, J. Heyrovský Institute of Physical Chemistry, Prague, Czech Republic
* Presenter:Milan Sil, email:milansil93@gmail.com
The helium hydride cation (HeH+) is believed to be the first molecule formed in the metal-free Universe following the Big Bang. Its first confirmed astronomical detection was made through observation of its pure rotational transition in the young, dense planetary nebula NGC 7027, subsequently validated by the detection of two additional rovibrational emission lines. Multiple rotational and rovibrational transitions of the ionized hydrocarbon methylidynium ion (CH+) have also been identified in NGC 7027. CH+ plays a crucial role in the chemistry of the interstellar medium by driving gas-phase reactions that lead to the formation of complex carbon-chain and organic molecules. Both HeH+ and CH+ are classified as "reactive ions”, characterized by rapid destruction processes that prevent their level populations from reaching collisional equilibrium. For such species, formation and destruction rates can be comparable to inelastic collisional rates and must therefore be incorporated into the statistical equilibrium framework. In this work, I present comprehensive photodissociation and radiative transfer models that successfully reproduce the observed HeH+ and CH+ line fluxes, including detailed treatments of inelastic collisions and chemical pumping processes.
Keywords: astrochemistry, radiative transfer, planetary nebulae, molecular processes