There are several outstanding mysteries in interstellar medium spectroscopy which have remained unsolved after decades of effort. The diffuse interstellar bands (DIBs) have been known for almost a century (Heger 1922). Although more than 400 bands from the near UV to near infrared have been detected, none of them has been identified. In the Milky Way Galaxy, DIBs have been seen towards over one hundred stars. In the Magellanic Clouds, DIBs have been seen in the spectrum of SN 1987A as well as in the spectra of reddened stars (Ehrenfreund et al. 2002). DIB carriers in the interstellar medium of external galaxies can be probed by supernovae (Sollerman et al. 2005), and DIBs have been detected in external galaxies with redshifts up to 0.5 (Sarre 2006).

The 217.5 nm extinction feature has been known for about 45 years (Stecher1965). It was extensively observed by the International Ultraviolet Explorer (IUE) satellite and is found to have remarkable constancy in its peak wavelength of 217.5 nm, corresponding to 5.7 eV. This is not just a local phenomenon as the feature has been detected in galaxies as distant as redshift z>2 (Eliasdottir et al. 2009).

A family of unidentified infrared emission (UIE) features was discovered over 30 years ago (Russell et al. 1977) and the number of features has been expanding as the result of infrared spectroscopic observations from ISO and Spitzer. The UIE phenomenon include aromatic bands at 3.3, 6.2, 7.7, 8.6, and 11.3 micron, aliphatic features at 3.4 and 6.9 micron, broad emission plateaus at 8, 12, and 17 micron, as well as a host of weaker features that are too broad to be atomic or molecular lines. The UIE features are seen in very different radiation environments. The energy source responsible for the excitation of the features ranges from tens of thousands of degrees in planetary nebulae, ~30000 K in HII regions, to thousands of degrees in reflection nebulae and proto-planetary nebulae. Although the UIE phenomena have been widely suggested to be due to polycyclic aromatic hydrocarbon molecules, other forms of carbonaceous materials have also been under discussion.

The observation of the Extended Red Emission (ERE) also goes back 30 years. ERE is commonly seen in reflection nebulae (Witt & Schild, 1988, Witt & Boroson, 1990). It has also been detected in dark nebulae, cirrus clouds, planetary nebulae, HII regions, the diffuse interstellar medium, and in haloes of galaxies. The central wavelength of the emission shifts from object to object, or even between locations within the same object. Other than the fact that it is likely due to photoluminescence, exact nature of its carrier is still unknown. Other unidentified optical emissions include a set of bright visible bands seen alongside ERE in the Red Rectangle nebula (Schmidt & Witt 1991) and Blue Luminescence which is seen both in this object and a number of other sources (Vijh, Witt, & Gordon 2004).

The 21 and 30 mm unidentified infrared features are generally associated with objects in the late stages of stellar evolution (Kwok et al. 1989, Forrrest et al. 1981), and can be responsible for a significant fraction of the energy output of the stellar sources (Hrivnak et al. 2000). High resolution ISO observations have found that the 21 micron features have the same intrinsic profile and peak wavelength (20.1 micron) (Volk et al.1999). There is no evidence for any discrete sub-structure due to molecular bands in the observed spectra, suggesting that the 21-micron feature is either due to a solid substance or a mixture of many similarly structured large molecules.

It is interesting to note that these phenomena have been observed not only in the diffuse ISM, but also in circumstellar environments, in the galactic halo, and in external galaxies. In some cases they have been observed in galaxies with redshifts > 2 suggesting that the carriers responsible for these features were already present in the early Universe.

We now recognize that all these UV, optical and infrared spectral features are ubiquitous in the Universe implying that their carriers must be a substance made of common elements, most likely carbon There is increasing consensus that the carriers of are of organic composition but the relevant materials have yet to be produced in a terrestrial environment. The recent detections of C60 and C70 in planetary nebulae (Cami et al. 2010, Garcia-Hernandez et al. 2010) and reflection nebulae (Sellgren et al. 2010) have also raised interest in other carbon allotropes in the ISM. Although specialized conferences have been held on these topics in the past, we hope that by bringing people with different expertise together will enable us to achieve synergy and explore common ground in discovering the origin of these mysterious spectral features.

In this Special Session, we hope to bring together observers who can report on the latest measurements of these features, modelers who use molecular data to interpret the observations, and laboratory spectroscopists who can help in their identification. A Special Session in 2012 would be particularly timely given new and forthcoming observational data from Herschel, Spitzer and Diffuse Band Surveys, together with strong increasing interest among laboratory researchers and theoretical chemists.


  • Cami, J., Bernard-Salas, J., Peeters, E. & Malek, S. E. Detection of C60 and C70 in a Young Planetary Nebula. Science, 329, 1180 (2010).
  • Ehrenfreund, P. et al. Detection of Diffuse Interstellar Bands in the Magellanic Clouds. Astrophys. J., 576, L117-L120 (2002).
  • Elíasdóttir, A. et al. Dust Extinction in High-z Galaxies with Gamma-Ray Burst Afterglow Spectroscopy: The 2175 Å Feature at z = 2.45. Astrophys. J., 697, 1725-1740 (2009).
  • Forrest, W. J., Houck, J. R. & McCarthy, J. F. A far-infrared emission feature in carbon-rich stars and planetary nebulae. Astrophys. J., 248, 195-200 (1981).
  • García-Hernández, D. A. et al. Formation of Fullerenes in H-containing Planetary Nebulae. Astrophys. J., 724, L39-L43 (2010).
  • Heger, M.L., Further study of the sodium lines in class {B} stars, Lick Observatory Bulletin, 337, 141-145 (1922)
  • Hrivnak, B. J., Volk, K. & Kwok, S. 2-45 Micron Infrared Spectroscopy of Carbon-rich Proto-Planetary Nebulae. Astrophys. J., 535, 275-292 (2000).
  • Kwok, S., Volk, K. M. & Hrivnak, B. J. A 21 micron emission feature in four proto-planetary nebulae. Astrophys. J., 345, L51-L54 (1989).
  • Russell, R. W., Soifer, B. T. & Willner, S. P. The 4 to 8 micron spectrum of NGC 7027. Astrophys. J., 217, L149-L153 (1977).
  • Sarre, P. J. The diffuse interstellar bands: A major problem in astronomical spectroscopy. J. Mol. Spectrosc., 238, 1-10 (2006).
  • Schmidt, G. D. & Witt, A. N., Distribution and excitation of unidentified molecules in the Red Rectangle, Astrophys. J., 383, 698-704 (1991).
  • Sellgren, K. et al. C60 in Reflection Nebulae. Astrophys. J., 722, L54-L57 (2010).
  • Sollerman, J. et al. Diffuse Interstellar Bands in NGC 1448. Astron. Astrophys., 429, 559-567 (2005).
  • Stecher, T. P. Interstellar Extinction in the Ultraviolet. Astrophys. J., 142, 1683 (1965).
  • Tielens, A.G.G.M. & Snow, T.P. The Diffuse Interstellar Bands, Dordrecht: Kluwer Academic Publishers, 1995, Astrophysics and Space Science Library
  • Vijh, U.P. et al. Discovery of Blue Luminescence in the Red Rectangle. Astrophys. J., 606, L65-L68 (2004).
  • Volk, K., Kwok, S. & Hrivnak, B. J. High-Resolution Infrared Space Observatory Spectroscopy of the Unidentified 21 Micron Feature. Astrophys. J., 516, L99-L102 (1999).
  • Witt, A. N. & Boroson, T. A. Spectroscopy of extended red emission in reflection nebulae. Astrophys. J., 355, 182-189 (1990).
  • Witt, A. N. & Schild, R. E. Hydrogenated amorphous carbon grains in reflection nebulae. Astrophys. J., 325, 837-845 (1988).