recent models of the hydrogen recombination line emission from hii regions show two effects due to dust internal to the nebula. these are (a) reduction in the lyman optical depth due to dust absorption and consequent modification of the spectrum from case b towards case a, and (b) excitation and stimulated emission of upper levels via re-cycled thermal ir photons. the departure of the emitted spectrum from case b should be measureable and, since the quantitative effect depends on the dust content and its optical properties, would provide the first opportunity to study the internal dust. the combined deviation in line flux due to the two processes described above occurs over a range of principle quantum numbers (n, depending on dust properties), but is most significant for n = 8 to 40. other processes also modify the line emission from case b, , principally dust extinction at short wavelengths and self-stimulated emission at long wavelengths. these latter processes must first be characterised in order to study the deviations due to internal dust. consequently we have embarked on a campaign of multi-wavelength measurements of hydrogen recombination line fluxes, recently obtaining near-ir (ukirt), sub-mm (n = 26 - 30; jcmt) and mm (n = 40 - 70; nobeyama) data. we now wish to complete this data set by obtaining measurements in the mid-ir with iso/sws of five/six lines with n = 4 - 9. this data will allow us to quantify the dust extinction and intrinsic ionization rate and , in conjunction with the existing data, investigate the dust content and its properties via the physical mechanisms described.
Instrument
SWS02 , SWS07
Temporal Coverage
1996-09-20T03:16:33Z/1996-11-07T20:52:28Z
Version
1.0
Mission Description
The Infrared Space Observatory (ISO) was the worlds 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.
