Molecules and dust in galactic and extragalactic environments
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| DIBs and ISM chemistry | DIBs in the Magellanic Clouds |
Introduction
The Diffuse Interstellar Bands (DIBs) are a large number of absorption lines between 4000 and
10000 Å that are superposed on the interstellar extinction curve (1).
Since the discovery of the first two DIBs in the 1920s, their identification remains one of the
oldest mysteries in stellar spectroscopy (1, 2,
3).
In the last 75 years DIBs have been observed towards more than 100 hot stars
The number of known DIBs is currently about 300 and continuously increases as a result of the higher
sensitivity of detectors (4, 5), see figure 1.
Due to their general correlation with dust extinction, the DIBs have been originally attributed to dust particles At present, no definitive identification of any of the DIB carriers exists. The detection of substructures in the profiles of several DIBs indicates the molecular nature of some DIB carriers (6, 7, 8, 9, 10). Theoretical models of large carbonaceous carrier molecules constrain the formation and destruction rates of such species in the diffuse medium (11, 12). Recent developments in DIB research indicate that most DIB carriers are probably large carbon-bearing molecules that reside ubiquitously in the interstellar gas (e.g. 13).
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Polycyclic aromatic hydrocarbons (PAHs), fullerenes, and carbon chains are among the most promising carrier candidates (7, 14, 15, 16). Astronomical surveys performed over the wavelength range 4000-7000 Å suggest that all measured DIBs do originate from different carriers (17). However, due to technical limitations the NIR and the NUV range has not yet been explored. Strong absorption features are observed in the laboratory in the NIR for large carbonaceous molecular cations (18). Also some DIBs showing similar behaviour in different astronomical environments may arise from structurally related carriers. Recent studies suggest that the environmental dependence of DIBs reflects an interplay of ionization, recombination, dehydrogenation and destruction of chemically stable, carbonaceous species (1, 17, 19). Therefore, investigations of DIBs in regions of different metallicity and UV radiation field allow us to constrain the physico-chemical properties of the different DIB carriers. Furthermore, the identification of the carrier molecules would allow us to determine the fraction of the cosmic carbon (in terms of abundance) they may represent.
Extragalactic Diffuse Interstellar Bands
A handful of DIBs have been observed in extragalactic targets (e.g. 20, 21, 22, 23). Observations of DIBs in the carbon depleted SMC (metallicity Z=0.002) and LMC (Z=0.008) compared to our galaxy (Z=0.02) allow us to test the hypothesis that many DIBs originate from carbonaceous carrier molecules. Observations of DIB carrier molecules in extragalactic environments shed light on the chemical pathways forming large organic molecules in galaxies.
The UV extinction curve
The strongest feature in the interstellar extinction curve is the ultraviolet bump at 2200 Å. It is characterized by a stable position, but its band width changes according to the interstellar environment (24). All recent results point strongly toward a carbonaceous carrier for the UV bump (13) and among those hydrogenated amorphous carbon (HAC) seems to be the most favoured (25). The next figure (From Gordon et al. 2003, http://dirty.as.arizona.edu/~kgordon/ ) shows the average extinction curves toward our own Galaxy and both the Large and Small Magellanic Clouds.
Small Magellanic Cloud
The UV extinction curve for the SMC (see previous figure 3) is in general characterized by the absence of the 2200 Å bump, in contrast to the galactic curve (26). The absence of the bump features in the SMC may therefore be related to the large underabundance of carbon in this galaxy. 27 constructed UV extinction curves for SMC targets and detected the 2200 Å bump only towards AzV 456, which is located in the SMC wing. In exactly this target AzV 456 (Sk143) we have detected the strongest DIBs at 5780 and 5797 Å (28). |
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Examples of our observations for the strongest DIBs (at 5780, 5797 Å and 6284 Å) are given in the previous Figures 4 (28). On the other hand, 29 report the presence of the 4430 Å DIB in four reddened SMC sources which are located in the SMC bar and do not show a UV bump. If those observations are correct this would indicate that some DIB carriers can be formed at low carbon abundances.