McCall Research Group Illinois

Searches for New Interstellar Molecules

Keck telescopes
The twin 10-meter Keck telescopes on the summit of Mauna Kea, in Hawaii.

One of the major goals of our group's astronomical (and, in fact, laboratory) research is the detection of new interstellar molecules -- not just for the sake of detecting them, but because of the incredible information that molecules can provide tracers of interstellar chemical and physical conditions. Our work spans the entire range of the electromagnetic spectrum accessible from Earth, from the optical to the mid-infrared to the millimeter wave.

IC 1396
The IC 1396 H II region, with our C4/C5 sightline HD 204827 circled. Credit:

Optical Spectroscopy

We have performed high-resolution spectroscopic searches for interstellar molecules at observatories such as McDonald Observatory, Lick Observatory, and Keck Observatory. As an example, we have used the huge light-collecting capability of Keck to obtain a very high signal-to-noise ratio of a diffuse interstellar cloud (towards HD 204827) that has huge amounts of the small carbon chains C2 and C3. In this spectrum, we have searched for the spectra of C4 and C5, but without success. However, we are able to place an upper limit on the amount of these longer carbon chains which is very significant, in that it dramatically disagrees with current chemical models of diffuse clouds.

The TEXES (Texas Echelon Cross Echelle Spectrograph) at IRTF.
NASA's IRTF 3.0-meter telescope on Mauna Kea, Hawaii (the mountain in the background is Maui).
CRL 618/ortho-benzyne spectrum
Spectrum toward CRL 618 (gray) and simulated spectrum of ortho-benzyne (black).

Mid-Infrared Spectroscopy

To complement our laboratory spectroscopy of C60, we have begun an observational campaign to search for the spectrum of C60 in dense interstellar clouds and circumstellar envelopes. This work has been done in collaboration with Matt Richter, using the TEXES spectrograph at NASA's InfraRed Telescope Facility (IRTF). TEXES operates in the atmospheric windows between 5-25 microns, and has a very high resolution (R~100,000). We have some data already in hand, but we are awaiting the results of our laboratory experiment so we can tell whether or not C60 is there in the data!

Millimeter-Wave Spectroscopy

Many large molecules and molecular ions are most profitably studied using pure rotational spectroscopy. This will be the case for most of the molecular ions we observe with SCRIBES, and consequently we are collaborating with Leslie Looney and Lew Snyder in the Illinois Astronomy department to search for these ions using the recently-commissioned CARMA array. While we are awaiting the arrival of SCRIBES data, we are testing large molecule detection with CARMA by searching for the important pre-biotic molecule urea.

We have also completed a search for ortho-benzyne (o-C6H4) in the envelope of the proto-planetary nebula CRL 618 using the Green Bank Telescope. While we did not detect ortho-benzyne, we derived a sensitive upper limit, and this work raised some very interesting questions about the mechanisms for forming benzene and polycyclic aromatic hydrocarbons (PAHs) in proto-planetary nebulae.

The Combined Array for Research in Millimeter-wave Astronomy (CARMA), operated by the University of Illinois in a consortium with the University of California at Berkeley, the University of Maryland, the California Institute of Technology, and the University of Chicago.

Related Content


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