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Register a new userDiffuse interstellar bands in M33
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Keith T. Smith
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We present the first sample of diffuse interstellar bands (DIBs) in the nearby galaxy M33. Studying DIBs in other galaxies allows the behaviour of the carriers to be examined under interstellar conditions which can be quite different from those of the Milky Way, and to determine which DIB properties can be used as reliable probes of extragalactic interstellar media. Multi-object spectroscopy of 43 stars in M33 has been performed using Keck/DEIMOS. The stellar spectral types were determined and combined with literature photometry to determine the M33 reddenings E(B-V)_M33. Equivalent widths or upper limits have been measured for the {lambda}5780 DIB towards each star. DIBs were detected towards 20 stars, demonstrating that their carriers are abundant in M33. The relationship with reddening is found to be at the upper end of the range observed in the Milky Way. The line of sight towards one star has an unusually strong ratio of DIB equivalent width to E(B-V)_M33, and a total of seven DIBs were detected towards this star.
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Recently, the presence of fullerenes in the interstellar medium (ISM) has been confirmed especially with the first confirmed identification of two strong diffuse interstellar bands (DIBs) with C60+. This justifies reassesing the importance of interstellar fullerenes of various sizes with endohedral or exohedral inclusions and heterofullerenes (EEHFs). The phenomenology of fullerenes is complex. In addition to fullerene formation in shock shattering, fully dehydrogenated PAHs in diffuse interstellar (IS) clouds could perhaps efficiently transform into fullerenes including EEHFs. But it is extremely difficult to assess their expected abundance, composition and size distribution, except for C60+. EEHFs share many properties with C60, as regards stability, formation/destruction and chemical processes, and many basic spectral features. We address the interstellar importance of various EEHFs as possible DIB carriers. Specifically, we discuss IS properties and the contributions of fullerenes of various sizes and charge such as C60+, metallofullerenes, heterofullerenes, fulleranes, fullerene-PAH compounds, H2@C60. We conclude that the landscape of interstellar fullerenes is probably much richer than heretofore realized. EEHFs, together with pure fullerenes of various sizes, have properties necessary to be suitably carriers of DIBs: carbonaceous nature; stability and resilience in the ISM; various heteroatoms and ionization states; relatively easy formation; few stable isomers; right spectral range; energy internal conversion; Jahn-Teller fine structure. This is supported by the C60+ DIBs. But, the lack of information about optical spectra other than C60 and IS abundances still precludes definitive assessment of the importance of fullerenes as DIB carriers. Their compounds could significantly contribute to DIBs, but it still seems difficult that they are the only important DIB carriers.
The identification of the carriers of the diffuse interstellar bands (DIBs) remains to be established, with the exception of five bands attributed to C60+, although it is generally agreed that DIB carriers should be large carbon-based molecules (with ~10-100 atoms) in the gas phase, such as polycyclic aromatic hydrocarbons (PAHs), long carbon chains or fullerenes. More specific possible carriers among PAHs are investigated, namely elongated molecules, which could explain a correlation between the DIB wavelength and the apparent UV resilience of their carriers. We address the case of polyacenes, C4N+2-H2N+4, with N~10-18 fused rectilinear aligned hexagons. Polyacenes are attractive DIB carrier candidates because their high symmetry and large linear size allow them to form regular series of bands in the visible range with strengths larger than most other PAHs, as confirmed by recent laboratory results up to undecacene (C46H26). Those with very strong bands in the DIB spectral domain are just at the limit of stability against UV photodissociation. They are part of the prominent PAH family of interstellar carbon compounds, meaning that only ~10-5 of the total PAH abundance is enough to account for a medium-strength DIB. After summarizing the current knowledge about the properties of polyacenes and recent laboratory results, the likelihood that they might meet the criteria for being carriers of some DIBs is addressed by reviewing the following properties: wavelength and strength of their series of visible bands; interstellar stability and abundances, charge state and hydrogenation; and DIB rotation profiles. No definite inconsistency has been identified that precludes polyacenes from being the carriers of some DIBs with medium or weak strength, including the so-called C2 DIBs. But additional experimental data about long acenes and their visible bands are needed to make robust conclusions
With the use of the data from archives, we studied the correlations between the equivalent widths of four diffuse interstellar bands (4430$r{A}$, 5780$r{A}$, 5797$r{A}$, 6284$r{A}$) and properties of the target stars (colour excess values, distances and Galactic coordinates). Many different plots of the diffuse interstellar bands and their maps were produced and further analysed. There appears to be a structure in the plot of equivalent widths of 5780$r{A}$ DIB (and 6284$r{A}$ DIB) against the Galactic $x$-coordinate. The structure is well defined below $sim150$ m$r{A}$ and within $|x|<250$ pc, peaking around $x=170$ pc. We argue that the origin of this structure is not a statistical fluctuation. Splitting the data in the Galactic longitude into several subregions improves or lowers the well known linear relation between the equivalent widths and the colour excess, which was expected. However, some of the lines of sight display drastically different behaviour. The region within $150^circ<l<200^circ$ shows scatter in the correlation plots with the colour excess for all of the four bands with correlation coefficients $textrm{R}<0.58$. We suspect that the variation of physical conditions in the nearby molecular clouds could be responsible. Finally, the area $250^circ<l<300^circ$ displays (from the statistical point of view) significantly lower values of equivalent widths than the other regions -- this tells us that there is either a significant underabundance of carriers (when compared with the other regions) or that this has to be a result of an observational bias.
The Solar System is located within a low-density cavity, known as the Local Bubble, which appears to be filled with an X-ray emitting gas at a temperature of 10$^6$ K. Such conditions are too harsh for typical interstellar atoms and molecules to survive. There exists an enigmatic tracer of interstellar gas, known as Diffuse Interstellar Bands (DIB), which often appears as absorption features in stellar spectra. The carriers of these bands remain largely unidentified. Here we report the three-dimensional structure of the Local Bubble using two different DIB tracers ($lambda$5780 and $lambda$5797), which reveals that DIB carriers are present within the Bubble. The map shows low ratios of $lambda$5797/$lambda$5780 inside the Bubble compared to the outside. This finding proves that the carrier of the $lambda$5780 DIB can withstand X-ray photo-dissociation and sputtering by fast ions, where the carrier of the $lambda$5797 DIB succumbs. This would mean that DIB carriers can be more stable than hitherto thought and that the carrier of the $lambda$5780 DIB must be larger than that of the $lambda$5797 DIB. Alternatively, small-scale denser (and cooler) structures that shield some of the DIB carriers must be prevalent within the Bubble, implying that such structures may be an intrinsic feature of supernova-driven bubbles.
The wavelength displacement of the Diffuse Interstellar Bands at 4502, 5705, 5780, 6284, and 7224 AA with respect to the well known, narrow atomic/molecular interstellar lines (of Ca{sc ii} and Na{sc i}) have been measured in the spectra of the 2 Orion Trapezium stars HD 37022 and HD 37020, using the HARPStextendash N spectrograph, fed with the 3.5 m Telescopio Nazionale Galileo, and the BOES spectrograph, fed with the 1.8m Korean telescope. The red shift is $sim$25 km/s for all these DIBs. We discuss the various possible origins of this very peculiar wavelength shift in the light of the particular physical conditions in the Orion Trapezium. The above mentioned shift is seemingly absent in the DIBs at 6196 and 6993 AA.
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