Russian astronomers have studied the photodissociation area in the Orion Bridge source in the mid-infrared range. The results, published in the arXiv.org preprint repository, will help scientists understand the evolution of polycyclic aromatic hydrocarbons in space.
The Orion Bar is a region in the interstellar medium at the interface between hot ionized gas and cold molecular clouds, energetically dominated by non-ionizing ultraviolet photons. Thus, this region is a photon-dominated region (PDR) that affects organic molecules present in space, including polycyclic aromatic hydrocarbons (PAHs).
Scientists analyzed archival photometric and spectroscopic observations made with telescopes UKIRT, Spitzer, ISO and SOFIA. They examined the flux density from the emission regions of the Orion Barrier at wavelengths of 3.3, 3.4, 3.6, 6.6, 7.7 and 11.2 micrometers in different places of the photodissociation region and a spectrum of 3-45 micrometers in one place. The researchers also simulated the photodegradation of PAHs due to ultraviolet radiation.
Astronomers were able to calculate the size, hydrogenation state, and charge distribution for PAHs at various locations in the Orion Barrier, and used the simulation results to generate simulated spectra and band intensities. The results obtained satisfactorily describe the change in the flux ratios of different zones with the distance from the ionization source.
The study showed that the loss of carbon during photodissociation of PAHs in the Orion Barrier is limited by the number of carbon atoms, approximately equal to 60. Scientists have come to the conclusion that on the molecular cloud surface is dominated by large cations, but smaller neutral PAHs and anions are widespread in the region deeper.