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The Diffuse Interstellar Bands Journal of Physics: Conference Series PAPER • OPEN ACCESS Recent citations The diffuse interstellar bands - a brief review - Low energy electron impact resonances of anthracene probed by 2D photoelectron To cite this article: T R Geballe 2016 J. Phys.: Conf. Ser. 728 062005 imaging of its radical anion Golda Mensa-Bonsu et al - Hydrogenated fullerenes (fulleranes) in space Yong Zhang et al View the article online for updates and enhancements. - Cracking an interstellar mystery Michael McCabe This content was downloaded from IP address 170.106.33.22 on 29/09/2021 at 09:17 11th Pacific Rim Conference on Stellar Astrophysics IOP Publishing Journal of Physics: Conference Series 728 (2016) 062005 doi:10.1088/1742-6596/728/6/062005 The diffuse interstellar bands - a brief review T R Geballe Gemini Observatory, 670 N. A‘ohoku Place, Hilo, HI 96720 USA E-mail: [email protected] Abstract. The diffuse interstellar bands, or DIBs, are a large set of absorption features, mostly at optical and near infrared wavelengths, that are found in the spectra of reddened stars and other objects. They arise in interstellar gas and are observed toward numerous objects in our galaxy as well as in other galaxies. Although long thought to be associated with carbon-bearing molecules, none of them had been conclusively identified until last year, when several near- infrared DIBs were matched to the laboratory spectrum of singly ionized buckminsterfullerene + (C60). This development appears to have begun to solve what is perhaps the greatest unsolved mystery in astronomical spectroscopy. Also recently, new DIBs have been discovered at infrared wavelengths and are the longest wavelength DIBs ever found. I present the general characteristics of the DIBs and their history, emphasizing recent developments. 1. Introduction The diffuse interstellar bands, which are commonly known as DIBs, are a class of absorption features that are observed in the spectra of objects in our galaxy and other galaxies. They arise in the interstellar medium, but are not due to atoms or simple molecules. When the first bands, at 5780 A˚ and 5797 A,˚ were discovered it was suspected, but not definitively shown that the absorptions were physically associated with interstellar medium; that became clear later. Originally the absorptions were only seen in optical spectra, but subsequently DIBs have been found at both longer and shorter wavelengths. They are most easily seen in the spectra of hot stars, because spectra of such stars contain fewer interfering photospheric lines than spectra of cooler stars. Recent reviews of DIBs [1,2] are available and are considerably more comprehensive than the present paper. Readers are referred to them and references therein for more detailed information and discussion. 2. Description and Early history The discovery of the first diffuse interstellar bands was made by a graduate student at Lick Observatory in California, Mary Lea Heger, almost a century ago during her search for ”stationary” sodium D lines toward orbiting pairs of hot stars [3], whose intrinsic spectra shifted due to the changing orbital velocities. An excellent recounting of the discovery by McCall and Griffin is available [4]. The bands that Heger observed, as well as several additional ones discovered later by Merrill and his collaborators were shown by the latter investigators to arise in interstellar gas [5], not due to their stationary behavior, but because their strengths increased with increasing reddening of the stellar spectra. Although as discussed below the bands are Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd 1 11th Pacific Rim Conference on Stellar Astrophysics IOP Publishing Journal of Physics: Conference Series 728 (2016) 062005 doi:10.1088/1742-6596/728/6/062005 Figure 1. Synthetic DIB spectrum of Lan et al. [9] based on work by Jenniskens and D´esert [10], at a resolution of 2 A˚ and for a reddening E(B − V ) of 0.1 mag. Note the small range of the vertical scale. found predominantly in the diffuse (low density) interstellar medium, in the name DIB the term “diffuse” applies to their appearance on photographic plates where the first absorptions discovered are broader (more “diffuse”) than interstellar absorption lines of atomic and molecular species. In 1975 Herbig [6] listed 39 DIBs between 4400 A˚ and 6850 A.˚ With the advent of modern instrumentation and advances in spectroscopy, particularly in the near-infrared, the number has skyrocketed to the present value of ∼500, through the work of many but in particular Hobbs and his collaborators [7,8]. However, as of the beginning of 2015, not a single DIB had been unequivocally identified with a specific carrier. Many have referred to the unknown identities of the DIBs as the greatest unsolved mystery in astronomical spectroscopy. Figure 1 from Lan et al. [9] is a synthetic pure DIB spectrum for a reddening E(B − V )of one-tenth mag, based on observations and analysis by Jenniskens and D´esert [10] who averaged a large number of stellar spectra after removing the emission and/or absorption lines intrinsic to the stars themselves. For this reddening the strongest DIBs have optical depths less than 0.02. Most of the optical DIBs detected in the last decade are considerably weaker than those shown in Fig. 1 and are narrow (sub-Angstrom); detection and characterization of them requires high spectral resolution and high sensitivity [7,8]. 3. Interstellar environment Convincing evidence that most DIBs are formed predominantly in the diffuse interstellar medium comes from the strong correlations of their strengths vs E(B − V ) for small values of that parameter. Figure 2 from Lan et al. [9] shows this behavior for a number of DIBs, using as background sources objects both inside and outside of the Galaxy. Low values of E(B − V ) generally imply lines of sight passing through diffuse clouds rather than than dense clouds. As shown in these figures the correlation with E(B − V ) disappears at higher values of reddening, which correspond to sightlines that are more likely to include regions of denser and/or UV- shielded gas. Thus the flattening at these higher values suggests that most DIBs do not exist in the denser interstellar medium. Indeed it has also been found that on average DIB strengths are either uncorrelated with or anti-correlated with N(H2)[9]. However, there is convincing evidence that some DIBs are formed in denser or more shielded environments. The best known examples of this are the so-called C2 DIBs, whose strengths roughly scale with N(C2)/E(B − V ) [11]. Their carriers presumably are more susceptible to destruction by UV radiation than other DIBs carriers, and thus they exist mainly in regions where the molecular fraction is high, in either diffuse cloud cores or in dense clouds. 2 11th Pacific Rim Conference on Stellar Astrophysics IOP Publishing Journal of Physics: Conference Series 728 (2016) 062005 doi:10.1088/1742-6596/728/6/062005 Figure 2. Dependence of DIBs equivalent widths on reddening for sightlines to Galactic stars, quasars, and galaxies (color-coded in original figure), from Lan et al. [9]. Wavelengths of DIBs are given in Angstroms. 4. DIB - DIB correlations The large numbers of DIBs coupled with the absence of convincing identifications for them has led many to search for correlations between the strengths of individual DIBs as a way to constrain the numbers of carriers and possibly identify bands produced by the same carrier. Quoting Ad´amkovics, Blake and McCall [12], “Any group of features arising from a particular carrier, or set of chemically related carriers, must maintain the same relative intensities in all lines of sight.” Although there are noteworthy counterexamples of this even among the stronger DIBs (a glaring instance is the aforementioned 5780 and 5797 A˚ bands [13], in most cases band strengths show a rough correlation from direction to direction, suggesting that most DIBs carriers tend to form under similar physical conditions . A very tight correlation between the strengths of two DIBs might suggest that each arises from the same carrier, but such correlations are very rarely the case. McCall [14] reported that in a study of 58 DIBs observed on 40 sightlines only 1.5% of the pairs had strength correlation coefficients of 0.95 or higher. This suggests that the DIBs must be produced by many species. 5. Free molecules or solids? The strengths of many DIBs scale roughly with the visual interstellar extinction for small values of the latter. Because extinction is produced by dust particles, this behavior suggested to early investigators that DIBs carriers are physically associated with the dust particles. However, there are a number of arguments strongly suggesting otherwise and indicating that the carriers are free molecules rather than molecules bound to grains. • Solid state absorption features tend to be much broader than the DIBs [15]. • The central wavelengths of the DIBs do not vary from sightline to sightline and the absorption profiles have been found to vary only in a very few cases [15]. In solids the 3 11th Pacific Rim Conference on Stellar Astrophysics IOP Publishing Journal of Physics: Conference Series 728 (2016) 062005 doi:10.1088/1742-6596/728/6/062005 Figure 3. High resolution spectra of two DIBs toward two stars, showing resolved structure [16]. The interstellar lines of K I are shown to the right of each DIB spectrum central wavelengths and shapes of absorption bands are influenced by the interaction of the absorbing molecules with their neighboring molecules, and significant differences are observed on different sightlines.
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