we are carrying out a research programme to measure the transitions of molecular hydrogen at wavelengths between 0.7 and 2.5 microns in photodissociation regions (pdr). we have found that the ortho to para (o/p) ratio (measured from emission lines with v>0) appears to be ubiquitously below 3, ie. non-lte. this may be due to the dynamics of the hot pdr gas (which escapes under its own pressure allowing cooler gas behind it to be excited), or the formation mechanism of the molecule on grains, where the o/p ratio is first set, or to spin exchange processes. uncertainties in the current models are the collision cross-sections of the h2 molecule and the contribution to the spectrum from molecules formed in an excited state. collisional processes thermalise the lower levels and may also redistribute the higher energy levels, but the cross sections are uncertain by an order of magnitude. it is therefore imperative to have knowledge of the density of the gas in the objects we have already observed in order to be able to quantify this value. this will be done by measuring the emission lines whose ratios can be used as density tracers, eg. feii 26 and 35 microns; oi 63 and 146 microns. furthermore, we require to measure the fundamental pure rotational lines of h2 - s(0) to s(3) - as the ratios of these lines provide the temperature (an important parameter in pdr models) and to complete short wavelength studies by measuring the transitions from v=0. we would also measure the high j co lines (j>13) at 150 - 200 microns as they can also be used as high density tracers. it may also prove to be possible to measure the h2/co abundance ratio by comparing line emission, arising from similair levels, from the two species (eg. 0-0 s(0) and co j=14-13 at virgul500k, and also s(1) and j=17-16 at virgul900k).
Instrument
LWS02 , SWS02
Temporal Coverage
1996-12-11T01:50:12Z/1997-04-14T11:50:57Z
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.
European Space Agency, A.Chrysostomou, 1999, 'LINE EMISSION FROM PHOTODISSOCIATION REGIONS', 1.0, European Space Agency, https://doi.org/10.5270/esa-c20p31f
