Diffuse Interstellar Bands in the High-Resolution GAOES Data

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.

Context. The limited number of high-resolution spectra of hot stars is inadequate for statistical studies of diffuse interstellar bands (DIBs). In contrast, the vast quantity of low-resolution spectroscopic surveys on cool stars holds great potential for investigating the relationship between DIBs and the known interstellar medium (ISM), as well as the spatial distribution of their unidentified carriers. Aims. We attempt to measure the DIBs λ5780, λ5797, and λ6614 in over two million low-resolution spectra of cool stars from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). Based on these DIB measurements, we reviewed and investigated the correlation between DIBs and extinction; the kinematics of DIBs; and the Galactic distribution of DIBs from a statistical perspective. Methods. We developed a pipeline to measure the DIBs λ5780, λ5797, and λ6614 in the LAMOST low-resolution spectra. Four modules in the pipeline consist of building the target and reference dataset; extracting the ISM residual spectra from the target spectra; measuring the DIBs in the residual spectra; and quality control of the measurements. Results. We obtained DIB measurements of spectra of late-type stars from LAMOST, and selected 176 831, 13 473, and 110 152 high-quality (HQ) measurements of the DIBs λ5780, λ5797, and λ6614, respectively, corresponding to 142074, 11 480, and 85 301 unique sources. Using these HQ measurements, we present Galactic maps of the DIBs λ5780 and λ6614 in the northern sky for the first time. The central wavelengths of the DIBs λ5780, λ5797, and λ6614 in air are determined to be 5780.48 ± 0.01, 5796.94 ± 0.02, and 6613.64 ± 0.01 Å, respectively, based on their kinematics. A statistical fit of the equivalent widths of these three DIBs per unit extinction provides values of 0.565, 0.176, and 0.256 Å mag−1. As a result of this work, three catalogs of the HQ measurements for the DIBs λ5780, λ5797, and λ6614 are provided via https://nadc.china-vo.org/res/r181484/. Conclusions. To the best of our knowledge, this is the largest number of measurements of these three DIBs to date. It is also the first time that Galactic maps of the DIBs λ5780 and λ6614 in the northern hemisphere are presented, and that the central wavelengths of the DIBs λ5780, λ5797, and λ6614 are estimated from kinematics.

Polycyclic aromatic hydrocarbon ions and the diffuse interstellar bands

A theory of interaction between fast ions and solids

Context. The diffuse interstellar bands (DIBs) are absorption features seen in the spectra of astronomical objects that arise in the interstellar medium. Today, more than 500 DIBs have been observed, mostly in the optical and near-infrared wavelengths. The origin of the DIBs is unclear; only ionised buckminsterfullerene, C60+, has been identified as a viable candidate for two strong and three weaker DIBs. Aims. We investigate the correlations between the strengths of the two strongest C60+ DIBs as well as their environmental behaviour. Methods. We analysed measurements of the strengths of the two C60+ DIBs at 9577 and 9633 Å for 26 lines of sight. We used two different methods, including Monte Carlo simulations, to study their correlations and the influence of measurement errors on the correlation coefficients. We examined how the strength of the C60+ DIBs changes as a result of different environmental conditions, as measured by the concentration of H/H2 and the strength of the ambient UV radiation. Results. In contrast to results recently reported by Galazutdinov et al. (2021, AJ, 161, 127), we find a high correlation between the strengths of the C60+ DIBs. We also discovered that the behaviour of the correlated C60+ bands is quite distinct from other DIBs at 5780, 5797, and 6203 Å in different environments.

Profiles of the λλ5797 and 6614 diffuse interstellar bands (DIBs) in our high-resolution spectra (R = 120,000-60,000) of five stars provide strong support for the model proposed by Webster, in which 13C atoms are randomly substituted for 12C in highly symmetric molecules. These stars have narrow interstellar Na D lines, and the DIB profiles can be reproduced, as predicted, by a band of equally spaced, equally wide Gaussians with a Poisson intensity distribution. Other good fits are possible with components of unequal FWHM, but the widths, positions, and relative amplitudes of the components then differ from star to star, which is less credible for a common absorber. The average numbers of 13C atoms per molecule derived from the model are about 0.95 and 1.41 for the λλ5797 and 6614 DIBs, respectively, which correspond to between 40 and 100 carbon atoms per molecule. Sharp features in the cores of the λ5797 DIB are not exactly matched by the Gaussians, which implies a more complex intrinsic structure. For HD 37061 in Orion, both DIBs are weak and have a component on the short-wavelength side of the normal profile, possibly indicating higher rotational temperatures.

The paper presents a review of observational data concerning the unidentified diffuse interstellar bands (DIBs). The history of the investigation of these fascinating spectral features is presented together with very recent results obtained with the aid of solid state detectors. The high signal‐to‐noise spectra allowed to divide diffuse bands into at least 3 families defined as sets of these features characterized by constant intensity ratios in every observed spectrum. “Intrinsic” profiles of DIBs, i.e. profiles that originated in single interstellar clouds free of any Doppler splitting were derived from high resolution spectra. The same observations revealed convincingly the Doppler structures inside DIB profiles observed in distant objects. It is shown that DIBs may originate in clouds of very low opacity — their agent(s) being thus resistant against diffuse UV radiation. The agent(s) seems not to be the dust causing the visual extinction as the bands may be absent in some of reddened spectra. Relations between DIBs and other interstellar absorptions are also briefly reviewed.

We discuss the absorption due to various constituents of the interstellar medium of M82 seen in moderately high resolution, high signal-to-noise ratio optical spectra of SN 2014J. Complex absorption from M82 is seen, at velocities 45 $\le$ $v_{\rm LSR}$ $\le$ 260 km s$^{-1}$, for Na I, K I, Ca I, Ca II, CH, CH$^+$, and CN; many of the diffuse interstellar bands (DIBs) are also detected. Comparisons of the column densities of the atomic and molecular species and the equivalent widths of the DIBs reveal both similarities and differences in relative abundances, compared to trends seen in the ISM of our Galaxy and the Magellanic Clouds. Of the ten relatively strong DIBs considered here, six (including $\lambda$5780.5) have strengths within $\pm$20% of the mean values seen in the local Galactic ISM, for comparable N(K I); two are weaker by 20--45% and two (including $\lambda$5797.1) are stronger by 25--40%. Weaker than "expected" DIBs [relative to N(K I), N(Na I), and E(B-V)] in some Galactic sight lines and toward several other extragalactic supernovae appear to be associated with strong CN absorption and/or significant molecular fractions. While the N(CH)/N(K I) and N(CN)/N(CH) ratios seen toward SN 2014J are similar to those found in the local Galactic ISM, the combination of high N(CH$^+$)/N(CH) and high W(5797.1)/W(5780.5) ratios has not been seen elsewhere. The centroids of many of the M82 DIBs are shifted, relative to the envelope of the K I profile -- likely due to component-to-component variations in W(DIB)/N(K I) that may reflect the molecular content of the individual components. We compare estimates for the host galaxy reddening E(B-V) and visual extinction A$_{\rm V}$ derived from the various interstellar species with the values estimated from optical and near-IR photometry of SN 2014J.

Considered here is the interrelation between five diffuse interstellar bands (DIBs), λλ 5545, 6113, 6196, 6445 and 6614 Å. Two DIBs (λλ 6196 and 6614 Å) are already known as being well correlated with each other; their relation with three other weaker bands is investigated for the first time. To accomplish this task, high-resolution spectra (λ/δλ ≈100 000) with high signal-to-noise ratios (S/N) of 54 hot O–B stars with reddening 0.12–1.45 mag were used. Analysis of measured equivalent widths has allowed us to establish linear dependences and evaluate linear correlation coefficients as high as 0.968–0.988 between the intensities of these five DIBs. Such a degree of correlation may indicate their common origin. Several spacings in wavenumbers found between these DIBs correspond to the energies of vibrational transitions in some polycyclic aromatic hydrocarbons resulting in IR emissions at λλ 16.4, 11.3, 7.7, 6.2 and 3.3 μm.

This paper describes the specific profile of the diffuse interstellar band (DIB) 6196 Å in objects belonging to the Sco OB2 association. The investigated feature is the narrowest known strong diffuse band. Reddened Sco OB2 association objects show slightly broadened and split (W-shaped) 6196 DIB profiles. The split cannot be explained by the presence of two (or more) clouds of different radial velocities along the line of sight because it is not observed in the much narrower identified atomic/molecular features. The observed DIB profile may (in some cases) be a composition of the band centre and its head of a still unknown molecule. Our estimation of the rotational temperature of C2 exhibits systematically higher values for objects with the observed split effect (Sco OB2 members). These split profiles are observed in both ζ and σ types of clouds, although in Sco OB2 objects only, suggesting that the phenomenon originates in some specific and currently unclear physical/chemical conditions inside the Sco OB2 association. It could be that a lower amount of small dust particles in Sco OB2 affects the flux of ultraviolet photons, providing a higher rotational temperature of DIB 6196 carriers, which is seen as a profile split.

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

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.

Carriers of the diffuse interstellar bands (DIBs) cannot be definitively identified without laboratory spectra. Several techniques, including matrix isolation, cavity ringdown spectroscopy, resonance enhanced multiphoton ionization, and ion trapping, have been used to measure the electronic spectra of carbon chains and their derivatives. The gas-phase laboratory spectra could then be compared to the astronomical data of known DIBs. The choice of molecules studied in the gas phase depends on the presence of strong electronic transitions at optical wavelengths, the lifetimes of excited electronic states, and chemical feasibility in diffuse astrophysical environments. Collisional-radiative rate models have also be used in conjunction with laboratory spectra to predict absorption profiles under interstellar conditions.