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:
- Is rapid H2O radiant-cooling a dominant factor keeping gas clouds
compressed - preventing them from bouncing back - once they have been
subjected to shocks. Shock-compression is believed to be one factor that can
trigger protostellar collapse. Determining the role of H2O in this process
is, therefore, important.
- Equally important is the determination of the carbon-to-oxygen ratio
in the interstellar medium. Previous studies have suggested that this ratio
may be far higher in interstellar space than in the solar system or other
regions where it has been unambiguously determined. A substantial fraction of
interstellar oxygen was always assumed to be hidden in unobserved H2O
molecules. With ISO this component is at last coming under scrutiny, and the
results to be gathered during the life of the mission will certainly advance
our understanding of cosmochemical abundances and processes.
Answers to both these questions, concerning the cooling power and abundance of
H
2O, were already coming in, even at this first workshop.
- In the Herbig-Haro source HH 54B, preliminary estimates suggested
that CO emission from this shocked domain out-radiate H2O, by a factor of
10:1. How that emission ratio varies from one shocked region to another, will
tell us the extent to which CO and H2O control shock cooling and potentially
protostellar collapse.
- An answer to the abundance question appeared to have been brought
closer through yet another reported breakthrough reported at the meeting, the
detection of solid CO2. Previously only seen weakly in an IRAS-LRS spectrum
at 15 microns, CO2 was detected by ISO in the interstellar medium, at a
wavelength of 4.27 microns, previously also hidden by telluric absorption. With
this and other detections, it was now becoming possible to see CO2, H2O,
and CO, in both their gaseous and solid forms. Atomic carbon and oxygen
abundances can also be directly observed through those atoms' fine-structure
transitions. And solid carbon, in the form of graphite, has long been observed,
with some confidence. With all these dominant atomic, molecular, and solid
species in which carbon and oxygen occur thus directly observable - with
O2 the only exception - we are a great deal closer to determining the true
abundance ratios of oxygen and carbon in much of the interstellar medium, and
thereby also the universe.
- A particularly valuable investigative tool that emerged as a spin-off
from these considerations, came from a report on the gas-to-solid ratio for CO,
CO2, and H2O observed with ISO. Solid grains composed of these three
species evaporate at quite different characteristic temperatures. The relative
abundances of the solid forms of these molecules in equilibrium with
their vapours, therefore, provide a good estimate of grain temperatures in
interstellar clouds. With ISO we are thereby gaining a thermometer for
measuring the temperatures of grains in interstellar clouds
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:
-
There were beautiful pictures gathered in a 15 micron camera survey, which
exhibited Galactic dust lanes so dense as to be opaque even at these very long
wavelengths. Breath-takingly beautiful camera pictures were also shown of
external galaxies, and dark, extended Galactic clouds. And detailed
investigations of these dark clouds were revealing embedded sources, likely
protostellar objects, in numbers appreciably higher than seen before. ISO's
potential for elucidating star formation in these regions is exceptionally
promising.
-
Most interestingly also, the photometer was detecting regions from which
radiation at 200 microns was the dominant emission. In particular, the
Chameleon-1 region was roughly a hundred times brighter at 200 microns than at 60
microns, and cold dust, responsible for this long-wavelength emission was being
found in many other sources as well, including the galaxies NGC 5266 and 5156.
These two sources had been traversed in a telescope slew during which the
photometer was conducting part of its 200 micron serendipity survey. Such
observations are permitting us to come to a significantly better understanding
than we had to date of the total energy budget of different types of Galactic
and extragalactic sources. Their emissions can now be observed all the way from
the gamma ray region, through the X-ray, EUV, UV, optical, IR, submillimeter and
radio range, with this closure of one of the last remaining energy gaps
around 200 microns.
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.