since a decade, polycyclic aromatic hydrocarbons (pahs) have been proposed to be an important dust component of the interstellar medium. this assumption is mostly supported by the comparison between the observational data in the ir and the laboratory spectra. however, the exact chemical structure of the interstellar molecules is still poorly known and no individual species have been yet identified. part of the difficulty is probably related to the chemical alteration of these compounds when submitted to the uv interstellar field. more specifically, the molecules can be ionized and photodissociated. evidence of photochemical evolution of pahs have been reported from ground-based studies in the 3um window for a few reflection nebulae and the orion bar photodissociation region. these spatial studies show strong variations of the relative intensities of the 3.3um band and its satellite band at 3.4um. detailed photochemical models predict similar variations in the modes in the 5-8um region. iso will provide the opportunity of studying the spatial distribution of the pah bands at 5.2, 5.6, 6.2, 6.9 and 7.7um which measure c-h, ch3, triple-bonded cc, and c=c vibrational modes. we propose to study these bands in a number of hii regions and reflection nebulae. due to the geometrical dilution and dust extinction, the fuv intensity varies considerably over these nebulae. thus, the relative spatial distribution of these bands will reflect the varying chemical nature of the pahs caused by the radiation field.
Instrument
CAM01 , CAM04 , SWS06
Temporal Coverage
1996-11-24T14:10:53Z/1996-12-01T15:38:08Z
Version
1.0
Mission Description
The Infrared Space Observatory (ISO) was the world's first true orbiting infrared observatory. Equipped with four highly-sophisticated and versatile scientific instruments, it was launched by Ariane in November 1995 and provided astronomers world-wide with a facility of unprecedented sensitivity and capabilities for a detailed exploration of the Universe at infrared wavelengths.
European Space Agency, 1999, Photochemical Evolution Of Polycyclic Aromatic Hydrocarbons In The Interstellar Medium, 1.0, European Space Agency, https://doi.org/10.5270/esa-e69d8i6
