THE UNITED KINGDOM INFRARED TELESCOPE
ANNUAL REPORT
1995 AND 1996

3. The Service Observing Program

3.2. Selected Scientific Result: Detection of H

T.R. Geballe (JAC)

Soon after the discovery of polyatomic molecules in space, the ion-molecule reaction scheme was developed as a plausible way of creating complex molecules through gas-phase reactions in gas too cold for neutral-neutral reactions to overcome their energy barriers. The only competing model involves reactions on the surfaces of dust grains. However, this scheme has the difficulty that the molecules that form on cold grains should stay there, and not be observed in the gas phase, except where they are heated and ejected from the grains in the vicinity of embedded stars. H is the most important ion in the ion-molecule reaction scheme; it is essentially the starting point for all ion-molecule chemistry. Hence detecting H and measuring its abundances in dark clouds are key tests of our understanding of interstellar chemistry. Last year the UKIRT Service Programme provided the vehicle by which this molecule was finally observed in interstellar space. The history of how this happened is the subject of this article. The scientific significance of the detection has been discussed elsewhere (Geballe and Oka 1996; also see the June 1997 UKIRT Newsletter on the UKIRT World Wide Web pages).

A few years after the fundamental vibration-rotation band of H (at about 4m) was first measured in the laboratory by Dr. Takeshi Oka (Oka 1980), now of the University of Chicago, Dr. Oka contacted me about the possibility of searching for it in interstellar space from UKIRT. We concluded that the most likely way to detect the molecular ion in molecular clouds was to search for its absorption lines in infrared sources embedded in the clouds or in infrared-bright stars lying behind the clouds.

There followed successful proposals to PATT for time in 1985 and 1987 to search for H using CGS2 and a Fabry-Perot interferometer. Although the proposals succeeded in winning time, the observations did not succeed in detecting the species (Geballe and Oka 1989), although in retrospect an unclaimed 2 sigma ``detection'' in W33A is consistent with our later clear detection in this object.

The H absorption lines were expected to be very weak and very narrow so, in addition to the observational issues of which lines to seek and which objects to observe, there were two major technical difficulties: sensitivity and resolution. CGS2+FP, although near the state-of-the-art in performance at the time, was highly strained to obtain the needed sensitivity in a reasonable amount of observing time. Moreover, the highest obtainable resolution with the UKIRT Fabry-Perot interferometers was about 9 km/s, which meant that the absorption lines likely would be unresolved and appear even less deep than they actually were. The replacement of CGS2+FP in 1991 by CGS4, which was considerably more sensitive but had a usual maximum resolution of about 40 km/sec (although around 20 km/s onoccasion), led us to consider that the chances for a successful detection of H at UKIRT were not much improved. During the next few years, several other groups continued the search for H at UKIRT and elsewhere, looking for line emission (rather than absorption) in heated regions of molecular clouds and in other objects, but with no convincing detections.

At about the time that our ``search'' paper (Geballe and Oka 1989) was published, emission lines from the overtone vibration-rotation band of H at 2m were detected in the polar regions of Jupiter (Maillard and Drossart 1989, Trafton, Lester, and Thompson 1989), marking the first time the species had been detected outside of the laboratory. As a result of this we temporarily shelved our search for interstellar H in order to follow up on the solar system discovery. Emission from H in the fundamental and overtone bands now has been found in Jupiter, Saturn, and Uranus. UKIRT has figured prominently in these discoveries and in several subsequent detailed studies (e.g. Oka and Geballe 1990, Trafton et al. 1993, Geballe et al. 1993, Ballester et al. 1994, Lam et al. 1997).

Meanwhile, our neighbors at the NASA Infrared Telescope Facility (IRTF) had completed CSHELL, a new infrared echelle spectrograph with a maximum resolution of 7 km/s and with a sensitivity which rivalled that of CGS4. To Oka and me CSHELL appeared to be the most appropriate instrument for continuing the search for interstellar H. We applied for observing time at the IRTF in 1995 and 1996, but were turned down; the oversubscription ratio at the IRTF is roughly the same as at UKIRT.

By this time CGS4 had been upgraded to the current 256256 array, with an accompanying further significant increase in sensitivity and, in its echelle mode, a resolution of 15 km/s. Since we had not applied for normal PATT time in summer 1996 for the interstellar project (expecting that our IRTF proposal for the same time frame would be approved), we decided to submit a Service application to observe what we considered to be the two most promising sources, W33A and GL2136 primarily in one narrow wavelength interval where two lines might be expected. This proposal was awarded a high marking and, on April 29 1996 measurements were obtained of both objects. These appeared to detect the lines in both objects. On July 15 a second set of Service observations were obtained of the same line pair and the same objects, and the detections were repeated. The shift in wavelength of the lines between April and July, due to the Earth's orbital motion (Fig. 10), clearly demonstrated that the lines were formed in the molecular clouds. This result prompted a Letter to Nature (Geballe & Oka 1996) reporting the discovery.

  
Figure 10: CGS4 echelle spectra of the embedded infrared source AFGL 2136 in the vicinity of the H ortho-para doublet near 3.668m, in April and July 1996. The wavelength shift between the two dates is due to the Earth's orbital motion. The gaps in the spectra near the H lines is at the location of a strong absorption by telluric methane.

We wish to thank the wisdom of the UKIRT Time Allocation Group for continuing to make UKIRT Service time available to the astronomical community. Via a subsequent PATT programme Oka, McCall and I have now detected H in several molecular clouds. At least two additional papers are in preparation. We look forward to the availability of the echelle with CGS4's long camera, which will provide yet higher spectral resolution and make possible more detailed studies of H and detections of other important interstellar molecules.

References

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Geballe, T.R. and Oka, T. 1996, Nature, 384, 334
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