McCall Research Group Illinois

Diffuse Interstellar Bands (DIBs)

Diffuse insterstellar bands
A simulated spectrum of the diffuse interstellar bands.

Our group is working to solve the mystery of the Diffuse Interstellar Bands, a series of visible absorption lines that has been unidentified for 90 years.

Apache Point Observatory
The Apache Point Observatory 3.5-meter telescope.

What are the Diffuse Interstellar Bands?

In 1919, Mary Lea Heger discovered the first of what would come to be known as the "Diffuse Interstellar Bands." The DIBs are a series of absorption lines that are observed toward just about every star in the galaxy that has interstellar material in front of it. These lines are generally quite broad and unresolved (although a few show substructure that looks tantalizingly like a rotational band contour), and they appeared "diffuse," or fuzzy, on early photographic plates. They are known to be interstellar, because they do not suffer the periodic Doppler shifts associated with stellar lines in binary star systems. Hence the term diffuse interstellar bands.

To this date, not a single one of the hundreds of known DIBs has been identified with a molecular carrier. The constancy of the absorption wavelengths implies that the carriers are in the gas phase, and the fact that they are broad implies that their carriers are molecular rather than atomic. Many laboratory spectroscopists have measured the spectra of countless candidate molecules, and none of these spectra have matched the astronomical spectra. This problem has often been called the longest-standing problem in all of spectroscopy!

What are we doing about it?

Our group, as part of a large collaboration of astronomers, has been conducting a survey of the DIBs of unprecedented extent. We have been using the Apache Point Observatory 3.5-meter telescope in New Mexico to measure the entire visible spectrum (from 3600-10200 Angstroms) of 160 stars, of which 115 lie behind interstellar clouds. The spectra are obtained at fairly high resolution (R~37,500) and high signal-to-noise ratio (>500). This dataset represents the largest survey ever of these enigmatic features.

An echelle spectrograph from APO
A raw image of a cross-dispersed echelle spectrum taken at APO. This image contains the entire visible spectrum of a star lying behind an interstellar cloud. The dark bands at the top are Fraunhofer's H and K lines of calcium, the doublet toward the bottom is the sodium D lines, and complex bands near the bottom are the A bands of atmospheric oxygen.

Spectral Atlases

One aim of our work is to compile definitive spectral atlases of the DIBs. These atlases will be useful in interpreting future observations, but also for comparison with new laboratory spectra obtained by spectroscopists around the world. So far, we have published our spectral atlas toward the binary star HD 204827 (see paper #35), which sports 380 diffuse bands. We have also recently published our atlas of HD 183143 (paper #43), which is unique in that it lies behind a large interstellar cloud that contains an undetectably small quantity of C2 . This sightline also contains hundreds of DIBs, but amazingly many of them are different from those seen in HD 204827.

Correlations

Another aim of our work is to search for correlations between the different DIBs, in the hopes that this will give us some observational constraints on the nature of their carriers. For example, if two different DIBs are due to the same molecular carrier, and all of these molecules are in their ground state, then the relative intensities of these two DIBs should simply be the ratio of the intrinsic strengths of the two vibronic transitions. No change in temperature, density, or chemistry should alter that ratio. So we have been searching for pairs of DIBs that are very well correlated, in that they have the same intensity ratio from one line of sight to the next. We have already detected the first pair of DIBs that seems to be perfectly correlated, within observational uncertainties (see paper #44). These studies could point the way to pulling out the spectra of individual molecules from the complex set of DIBs, which could greatly aid laboratory studies.