First ISO Science Workshop

(ESTEC, Noordwijk, The Netherlands, 29-31 May 1996)

After a construction phase of more than a decade, one can hardly blame a spacecraft observatory for wanting to show off a little. ISO did that with a vengeance at a workshop on first results, held at ESTEC on May 29 -31 and attended by 260 scientists from around the world.

So numerous were the interesting findings, that hardly anyone noticed that infrared astronomy had just reached two of its holy grails and was hurtling past them at great speed.

The first of these is the detection, with the Short Wavelength Spectrometer, SWS, of the lowest rotational transition of molecular hydrogen, the S(0) transition of H2, whose spectral line at 28 microns had never before been detected. The other was the detection of thermal water vapour emission, believed to be a prime coolant of cold shocked regions and, therefore, an essential factor in rapidly cooling shock-compressed gas on its way to protostellar collapse.

The S(0) transition was too faint to be previously observed even with the Kuiper Airborne Observatory, the only other far-infrared observatory equipped with powerful infrared spectrometers. The H2 molecule is symmetric and therefore radiates only through quadrupole emission, which is orders of magnitude weaker than normal, dipole emission from asymmetric (polar) molecules like carbon monoxide, CO.

Because this H2 line had never been detected, current estimates of the molecular hydrogen content of galaxies has been quite uncertain, though cold, molecular hydrogen is often a galaxy's dominant gaseous constituent. To date, estimates of the H2 content of molecular clouds and galaxies has rested on determining the abundance of CO, and then inferring an H2 content on the basis of presumed CO/H2 ratios. Complicating the issue has been the C12O self-absorption, which meant that only the far lower, C13O abundances could be considered reliably established. But then, one required a double extrapolation to determine H2 content, involving not only a guess at the CO/H2 ratios but an additional uncertainty in the C13/C12 fractions expected in molecular clouds.

To a significant extent these indirect determinations of molecular hydrogen abundance may now become a thing of the past, as molecular hydrogen detections were reported not only in Galactic sources, but also in several actively star-forming galaxies, such as M82 and NGC 253. In the ultraluminous galaxy NGC 6240, H2 transitions as energetic as the S(7) line were detected in emission. In the galaxy NGC 6946, a first quantitative estimate of the H2 content indicated as much as 80 Msolar pc-2 of surface area, for the galaxy seen face-on.

These detections will not solve all our problems, since temperatures have to be as high as a few hundred degrees Kelvin before even the S(0) transition of H2 can be excited, whereas temperatures deep inside molecular clouds generally are far lower. But at least it should now become possible to establish the ratio of H2 to C12O or C13O in warmer regions of clouds and then extrapolate these ratios to colder domains to obtain the total mass of molecular hydrogen - the gaseous component responsible for star formation.

Equally exciting as these reports, were talks on the detection of water vapour emission, observed in any number of sources. The water vapour content of Earth's atmosphere is so great, even above aircraft and balloon altitudes, that the spectral regions in which cosmic H2O strongly emits, are totally blocked. It took ISO, the first spacecraft to take infrared-sensing spectrometers totally beyond the atmosphere, to make observations of thermally-emitting H2O. This radiation was detected in stellar winds streaming out from evolved oxygen-rich stars, in shocks emanating from Herbig-Haro objects, and in a variety of shock-compressed regions. Water vapour was also observed in absorption in dense clouds. Previously H2O emission had been observed only in circumstellar and interstellar masers - rare, peculiar objects that offered no guide to the overall H2O contents of typical Galactic clouds or external galaxies.

With the new water vapour observations in hand, some obtained with SWS, others with the Long Wavelength Spectrometer LWS, we should be able to answer a number of questions that have long remained open:

Answers to both these questions, concerning the cooling power and abundance of H2O, were already coming in, even at this first workshop.

At this first ISO workshop there was far more, of course - enough to bring a gleam to the eye of each attendee. Among the many reports, totaling about eighty if one counts oral as well as poster sessions, were astonishing results of all kinds:

This was an exciting workshop. ISO is fulfilling its promises to the astronomical community and then some!

Martin Harwit
12 June 1996

First ISO Results have been published in the A&A Special Issue, Volume 315, Number 2, pp L27-L400, 1996.