A framework for resolving the origin, nature and evolution of the diffuse interstellar band carriers?

Carriers of diffuse interstellar bands (DIBs) still need to be identified. In a recent paper, we reported a correlation between the DIB wavelength and the apparent UV resilience (or boost) of their carriers. We proposed that this might be an indication of the important role of conjugated elongated molecules among the DIB carriers. The aim of this paper is to further understand the origin of this correlation. The analysis of 509 optical DIBs on the lines of sight of HD 183143 and/or HD 204827 reported in the literature shows that this correlation mainly implies the 386 narrow DIBs with a band width <1.1 Å, which include most of the identified DIBs of the C2andζfamilies, while the majority of the 123 broader DIBs, including the identifiedσDIBs, do not display such a correlation. We present a possible origin of this correlation from very strong bands of large conjugated elongated molecules, such as carbon chains, polyacenes, or other catacondensed polycyclic aromatic hydrocarbons. The total amount of carbon contained in all the carriers of these narrow DIBs is a very small fraction of the interstellar carbon if their oscillator strengths are ≥1. The amount of carbon locked in the carriers of the broader DIBs is higher, especially if their oscillator strengths are significantly weaker.

Numerous studies of diffuse interstellar band (DIB) profiles have detected substructures, which in turn suggests that large molecules are acting as their carriers. However, some of the narrowest DIBs generally do not show such substructures, suggesting the possibility of very small carriers. Å and the present detection of weaker side DIBs to each of them in the extensive dataset from the ESO Diffuse Interstellar Bands Large Exploration Survey, we investigated whether they may stem from small linear carrier molecules. This approach can lead to concrete DIB carrier suggestions, which can be tested in laboratory measurements in future studies. We suggest that the DIBs we studied here represent individual rotational transitions of a small molecule. We determined the molecular constants from observations and compared them with data from a large set of quantum-chemical calculations to constrain possible carrier candidates. Furthermore, we determined the rotational temperatures by fitting line ratios using the fitted molecular models. We determined molecular constants for three DIB systems and the corresponding transition types. The fitted rotational temperatures lie within the range of known interstellar diatomic molecules. We identified several DIB carrier candidates, almost all of them molecular ions. Some of them are metastable species, indicating the possibility of collision complexes as DIB carriers. Å DIB, the strongest among the narrow DIBs.

Diffuse interstellar bands (DIBs) are absorption features commonly observed in optical/near-infrared spectra of stars and thought to be associated with polyatomic molecules that comprise a significant reservoir of organic material in the universe. However, the central wavelengths of almost all DIBs do not correspond with electronic transitions of known atomic or molecular species and the specific physical nature of their carriers remains inconclusive despite decades of observational, theoretical, and experimental research. It is well established that DIB carriers are located in the interstellar medium, but the recent discovery of time-varying DIBs in the spectra of the extragalactic supernova SN2012ap suggests that some may be created in massive star environments. Here we report evidence of short time-scale (10-60 d) changes in DIB absorption line substructure toward 3 of 17 massive stars observed as part of a pathfinder survey of variable DIBs conducted with the 1.5-m Tillinghast telescope and Tillinghast Reflector Echelle Spectrograph (TRES) at Fred L. Whipple Observatory. The detections are made in high-resolution optical spectra (R = 44000) having signal-to-noise ratios of 5-15 around the 5797 and 6614 angstrom features, and are considered significant but requiring further investigation. We find that these changes are potentially consistent with interactions between stellar winds and DIB carriers in close proximity. Our findings motivate a larger survey to further characterize these variations and may establish a powerful new method for probing the poorly understood physical characteristics of DIB carriers.

The importance of the ‘‘continuum’’ emission has already been pointed out in several sources of high excitation (such as planetary nebulae) with the quantitative study of the emission bands at 7.7 and 11.3 μm with the IRAS/LRS spectra (Zavagno et al.). We present here a model of the observed continuum emission between 2 μm and 1 mm in the source NGC7027. We find evidences for the existence of a large emission band in the range 5–15 μm. This band is characteristic of amorphous carbon grains. Using the data given by Rouleau and Martin, we have been able to derive the extinction coefficient for three samples of such grains. With our model, we are able to reproduce the observed emission bump using amorphous carbon grains. It is then possible, for the first time, to know the exact contribution of the emission bands with respect to the continuum and, using the derived optical depth, to estimate the mass of carbon contained in amorphous carbon grains.

Small carbon grains, processed by UV radiation and cosmic rays, have been proposed as carriers of the 217.5 nm bump present in the interstellar extinction curves (Hecht 1986; Sorrell 1990). In this paper, we present the results of an experiment aimed at simulating, in a -rst approximation, the cosmic-ray irradiation active in space. We have studied the e†ects induced by 3 keV Heions on the UV spectrum of small cosmic analog carbon grains. Two di†erent kinds of grains have been analyzed. They were produced by vapor conden- sation in hydrogen and argon quenching atmospheres. Spectrophotometric measurements have been carried out on grains as they were produced and after ion irradiation in the spectral range 0.19E2 km. Relevant UV spectral changes are observed after ion irradiation: while the UV absorption band shifts from 203 to 215 nm in hydrogenated amorphous carbon grains, an opposite trend is observed for the samples produced in the argon atmosphere. In this case the UV band moves from 240 to 218 nm. These spectral changes are well correlated with the optical gap variations and are therefore interpreted in terms of grain microstructure changes induced by the interactions with ions. At the highest ion Nuence considered, the two carbons tend to have a similar microstructure, as testi-ed by the UV peak position and optical gap values because of a saturation e†ect of the two competitive processes, amorphization and graphitization, which occur in carbon samples during ion irradiation (Compagnini & Calcagno 1996). The results of the present experiment suggest that hydrogenated amorphous carbon grains cannot be transformed into graphite grains by cosmic-ray irradiation. Moreover, the efficiency of ion irradiation in destroying well-ordered aromatic structures poses the problem of the survival itself of polycrystalline or pure graphite particles in the interstellar medium. Subject headings: cosmic rays E dust, extinction E methods: laboratory E ultraviolet: ISM

Only four of the diffuse interstellar bands (DIBs) are currently accounted for, ascribed to electronic transitions of C$_{60}^+$. Investigations into carriers of other DIBs historically focus on charged and neutral hydrocarbons, and little information is available regarding oxygenated carbon and hydrocarbon species that result from the two most abundant heavy elements in the interstellar medium, C and O. In this study, we assess whether CnO+, HCnO+, and OCnO+ (n = 4–9) cations are viable candidates to account for DIBs using both density-functional theory (DFT) and coupled cluster single-double and perturbative triple theory, CCSD(T). For these species, the linear structures are the most stable isomers with the lowest dissociation threshold corresponding to CO loss. Optical absorptions of the oxygenated carbon chain cations are characterized by calculated vertical excitation wavelengths and their corresponding oscillator strengths using the equation-of-motion CCSD (EOM-CCSD) method. Aside from HC4O+ and HC2n +1O+, all of the species considered in this study have calculated electronic transitions that lie in the visible or near-infrared spectral regions. Minimal column densities necessary for these cations to account for DIBs have been estimated. Based on present results and the known column densities for neutral oxygenated carbon chains in TMC-1, the growth rate of charged O-bearing carbon chains via ion-neutral reaction mechanisms is judged to be too low to form a sufficient population to give rise to DIBs.

Diffuse interstellar bands (DIBs) are an enigmatic set of interstellar absorption features, observed at optical and near infrared wavelengths in the spectra of reddened stars in our Galaxy also in other galaxies. Identifying the carriers of DIBs has been a longstanding challenge and is still very much a work in progress. In recent years, surveys of DIBs have been conducted, but mostly focus on strong bands. In this work, we extracted and measured DIBs recorded in the high-resolution stellar spectra of ε Auriga (ε Aur) and IQ Per observed by using Gunma Astronomical Observatory Echelle Spectrograph (GAOES) (spectral resolution R∼65000). Observations were aimed at obtaining high resolution spectra of double-lined detached eclipsing binaries. For each selected DIB, we performed spectral fitting of a combination of a smooth stellar continuum (polynomial function), an empirical DIB model, and a synthetic telluric transmission. The high-quality data allow us to measure not only strong and well-known DIBs, but also weaker DIBs.

We construct a quasar extinction curve based on the blue and red composite quasar spectra of Richards et al. (2003) prepared from the SDSS survey. This extinction curve does not show any traces of the 2200 A feature characteristic of the Interstellar Medium, and this indicates that graphite grains are likely absent close to quasar nuclei. The extinction is best modeled by AC amorphous carbon grains, assuming a standard distribution of grain sizes (p=3.5) but slightly larger minimum grain size (a_{min} = 0.016 microns) and lower maximum grain size (a_{min} = 0.12 microns) than the respective canonical values for the interstellar medium. The dust composition is thus similar to that of the dust in AGB stars. Since graphite grains form from amorphous carbon exposed to strong UV irradiation the results indicate that either the dust forms surprisingly far from the active nucleus or in a wind that leaves the nucleus quickly enough to avoid crystallization into graphite.

We report mid-infrared (MIR) imaging observations of the carbon star U Ant\nmade with the Infrared Camera (IRC) on board AKARI. Subtraction of the\nartifacts and extended PSF of the central star reveals the detached dust shell\naround the carbon star at MIR wavelengths (15 and 24{\\mu}m) for the first time.\nThe observed radial brightness profiles of the MIR emission are well explained\nby two shells at 43" and 50" from the central star detected in optical\nscattered light observations. Combining Herschel/PACS, AKARI/FIS, and AKARI/IRC\ndata, we obtain the infrared spectral energy distribution (SED) of the thermal\nemission from the detached shell of U Ant in a wide infrared spectral range of\n15- 160 {\\mu}m. Thermal emission of amorphous carbon grains with a single\ntemperature cannot account for the observed SED from 15 to 160 {\\mu}m: it\nunderestimates the emission at 15 {\\mu}m. Alternatively, the observed SED is\nfitted by the model that amorphous carbon grains in the two shells have\ndifferent temperatures of 60 and 104 K, which allocates most dust mass in the\nshell at 50". This supports previous suggestion that the 43" shell is gas-rich\nand the 50" one is dust-rich. We suggest a possibility that the segregation of\nthe gas and dust resulting from the drift motion of submicron-sized dust grains\nrelative to the gas and that the hot dust component associated with the\ngas-rich shell is composed of very small grains that are strongly coupled with\nthe gas.\n

Extinction coefficients of carbon grains in far-infrared

We consider various possible scenarios to explain the recent observation of what has been called a broad Hα absorption in our Galactic halo, with peak optical depth and equivalent width . We show that the absorbed feature cannot arise from the circumgalactic and ISM Hα absorption. As the observed absorption feature is quite broad ( ), we also consider CNO lines that lie close to Hα as possible alternatives to explain the feature. We show that such lines could also not account for the observed feature. Instead, we suggest that it could arise from diffuse interstellar bands (DIBs) carriers or polyaromatic hydrocarbons (PAHs) absorption. While we identify several such lines close to the Hα transition, we are unable to determine the molecule responsible for the observed feature, partly because of selection effects that prevent us from identifying DIBs/PAHs features close to Hα using local observations. Deep integration of a few extragalactic sources with high spectral resolution might allow us to distinguish between different possible explanations.

Polyacenes among other polycyclic aromatic hydrocarbons (PAHs) were suggested to be abundant in the dense regions of the interstellar medium (ISM) and to be responsible for some of the diffuse interstellar bands (DIBs). The identification of DIB carriers requires the laboratory spectra of cold cations for comparison with astrophysical observations. We employed He-tagging spectroscopy to record the spectrum of tetracene cations. In contrast to previous measurements of other polyacene cations, we found very narrow absorption bands of tetracene +. This makes the detection of tetracene+ in the ISM much easier compared to other studied polyacene cations. The four absorption bands of tetracene+ that are the best suited for the detection in observational spectra appeared at 859, 850.9, 838.8, and 827.3 nm. We did not find any DIBs at these positions. However, the observational spectra in this region from ground-based telescopes are heavily contaminated with telluric absorption bands, which greatly complicates the detection of DIBs. Comparison with observations performed by space telescopes is expected to significantly increase the sensitivity of the detection of the DIBs that could potentially be assigned to tetracene+.

view Abstract Citations (58) References (36) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Dehydrogenation Study of Cosmic Carbon Analogue Grains Mennella, V. ; Colangeli, L. ; Blanco, A. ; Bussoletti, E. ; Fonti, S. ; Palumbo, P. ; Mertins, H. C. Abstract We present the results of a systematic study of the dehydrogenation produced by thermal annealing of small (average radius (a) = 5 nm) hydrogenated amorphous carbon grains. The heat treatment of the particles can be a reasonable simulation of the grain processing active in space as modeled by Hecht (1986) and Sorrell (1990). Extinction measurements both on grains as produced and on samples annealed at fixed temperatures between 250 C and 800 C have been performed in the spectral range 190-800 nm. For some samples the measurements have been extended in the extreme-ultraviolet (EUV) down to 40 nm. The extinction spectra of all the samples analyzed in the EUV are characterized by a band falling around 93 nm. On the other hand, all the samples, but the hydrogenated one, show a peak in the UV region. This feature becomes more and more pronounced and shifts toward longer wavelengths as the annealing temperature increases. We assign the high- and low-energy bands, respectively, to sigma-sigma* and pi-pi* electronic transitions of the sp2 hybridized carbon clusters forming the grains. We identify these clusters as the major factor responsible for determining the UV extinction properties of our carbon grains. The shift of the peak is interpreted in terms of a dimensional growth of the sp2 clusters, indicating a progressive graphitization of the samples. The pi-pi* transition is observed at 259 nm in the samples annealed at 800 C, in close agreement with that observed for graphite (260 nm). The results of the annealing experiment offer new hints to interpret the circumstellar and interstellar UV extinction bands. In particular, it appears possible to interpret the origin of these features in terms of pi-pi* electronic transitions in carbonaceous grains. Publication: The Astrophysical Journal Pub Date: May 1995 DOI: 10.1086/175603 Bibcode: 1995ApJ...444..288M Keywords: Carbon; Cosmic Dust; Dehydrogenation; Interstellar Matter; Ultraviolet Astronomy; Ultraviolet Spectra; Annealing; Astrophysics; Electron Transitions; Extreme Ultraviolet Radiation; Interstellar Extinction; Astronomy; ISM: DUST; EXTINCTION; STARS: CARBON; STARS: CIRCUMSTELLAR MATTER; ULTRAVIOLET: ISM full text sources ADS | data products SIMBAD (6)

The correlations deduced by Boulanger et al. from IRAS maps of the Chamaeleon, Taurus and Ursa Major molecular cloud complexes are interpreted in terms of the evolutionary HAC model of interstellar dust. In particular, regions of relatively strong 12–μm emission may be regions where recently accreted carbon is being converted by ambient UV to small PAHs in situ. Regions of weak –μm emission are probably quiescent regions where carbon has been annealed to amorphous carbon. Observational consequences of these inferences are briefly described.

Short-period comet 103P/Hartley 2 (103P) was the flyby target of the Deep Impact eXtended Investigation on 2010 November 4 UT. This comet has a small hyperactive nucleus, i.e., it has a high water production rate for its surface area. The underlying cause of the hyperactivity is unknown; the relative abundances of volatiles in the coma of 103P are not unusual. However, the dust properties of this comet have not been fully explored. We present four epochs of mid-infrared spectra and images of comet 103P observed from Gemini-South +T-ReCS on 2010 November 5, 7, 21 and December 13 UT, near and after the spacecraft encounter. Comet 103P exhibited a weak 10 μm emission feature ≃1.14 ± 0.01 above the underlying local 10 μm continuum. Thermal dust grain modeling of the spectra shows the grain composition (mineralogy) was dominated by amorphous carbon and amorphous pyroxene with evidence for Mg-rich crystalline olivine. The grain size has a peak grain radius range of a peak ∼ 0.5–0.9 μm. On average, the crystalline silicate mass fraction is ≃0.24, fairly typical of other short-period comets. In contrast, the silicate-to-carbon ratio of ≃0.48–0.64 is lower compared to other short-period comets, which indicates that the flux measured in the 10 μm region of 103P was dominated by amorphous carbon grains. We conclude that the hyperactivity in comet 103P is not revealing dust properties similar to the small grains seen with the Deep Impact experiment on comet 9P/Tempel 1 or from comet C/1995 O1 (Hale–Bopp).