DIB Carriers Research Articles

The diffuse interstellar band around 8620 Å

Context.Diffuse interstellar bands (DIBs) are important interstellar absorption features of which the origin is still debated. With the large data sets from modern spectroscopic surveys, background stars are widely used to show how the integrated columns of DIB carriers accumulate from the Sun to great distances. To date, studies on the kinematics of the DIB carriers are still rare.Aims.We aim to make use of the measurements from the Giraffe Inner Bulge Survey (GIBS) and theGaia–ESO survey (GES) to study the kinematics and distance of the carrier of DIBλ8620, as well as other properties.Methods.The DIBs were detected and measured following the same procedures as in Zhao et al. (2021, A&A, 645, A14; hereafter Paper I), assuming a Gaussian profile. The median radial velocities of the DIB carriers in 38 GIBS and GES fields were used to trace their kinematics, and the median distances of the carriers in each field were estimated by the median radial velocities and two applied Galactic rotation models.Results.We successfully detected and measured DIBλ8620 in 760 of 4117 GES spectra with |b| ≤ 10° and signal-to-noise ratio (S∕N) > 50. Combined with the DIBs measured in GIBS spectra (Paper I), we confirmed a tight relation between EW andE(J−KS) as well asAV, with similar fitting coefficients to those found by previous works. With a more accurate sample and the consideration of the solar motion, the rest-frame wavelength of DIBλ8620 was redetermined as 8620.83 Å, with a mean fit error of 0.36 Å. We studied the kinematics of the DIB carriers by tracing their median radial velocities in each field in the local standard of rest (VLSR) and into the galactocentric frame (VGC), respectively, as a function of the Galactic longitudes. Based on the medianVLSRand two Galactic rotation models, we obtained valid kinematic distances of the DIB carriers for nine GIBS and ten GES fields. We also found a linear relation between the DIBλ8620 measured in this work and the near-infrared DIB in APOGEE spectra at 1.5273 μm, and we estimated the carrier abundance to be slightly lower compared to the DIBλ15273.Conclusions.We demonstrate that the DIB carriers can be located much closer to the observer than the background stars based on the following arguments: (i) qualitatively, the carriers occupy in the Galactic longitude–velocity diagram typical rotation velocities of stars in the local Galactic disk, while the background stars in the GIBS survey are mainly located in the Galactic bulge; (ii) quantitatively, all the derived kinematic distances of the DIB carriers are smaller than the median distances to background stars in each field. A linear correlation between DIBλ8620 and DIBλ15273 has been established, showing similar carrier abundances and making them both attractive for future studies of the interstellar environments.

A Critical Review of PAHs as DIB Carriers - Progress and Open Questions

AbstractPAHs are among the most commonly proposed and popular candidates for DIB carriers. We present a critical assessment of the PAH-DIB model in view of the progress and the advances that have recently been achieved through a series of complementary studies involving astronomical observations of DIBs, laboratory simulation of interstellar analogs for PAHs (neutrals and ions), space exposure experiments of PAHs, theoretical calculations of PAH spectra and the modeling of diffuse and translucent interstellar clouds. What have we learned from these complementary studies? What are the constraints that can now be derived for the PAHs as DIB carriers? What are the strengths and the weaknesses of the PAH model to account for the DIBs?

Exploring the Limits to Observational Diffuse Interstellar Band Studies

AbstractThe status of DIB research (Herbig 1995) has strongly advanced since the DIB conference in Boulder in 1994. In the same year we reported the discovery of two near IR diffuse bands coincident with C60+, that was confirmed in subsequent years. Since then a number of DIB observational studies have been published such as DIB surveys, measurements of DIB families, correlations and environment dependences as well as DIBs in extra-galactic sources. Resolved substructures were measured and compared to predicted rotational contours of large molecules. Polarisation studies provided constraints on possible carrier molecules and upper limits. DIBs carriers have been linked with several classes of organic molecules observed in the interstellar medium, in particular to the UIR bands (assigned to PAHs), the Extended Red Emission (ERE) or the recently detected Anomalous Microwave Emission (AME, assigned to spinning dust). In particular fullerenes and PAHs have been proposed to explain some DIBs and specific molecules were searched for in DIB spectra. DIB carriers could be present in various dehydrogenation and ionization states. Experiments in the laboratory and in space contribute to our understanding of the photo-stability of possible DIB carriers. In summary, the status of DIB research in the last 20 years has strongly advanced. We review DIB observational results and their interpretation and introduce the relevant plenary discussion.

Solid State DIBs

AbstractThe diffuse interstellar bands are not due to solid state species. However, under the explicit assumption that DIB carriers survive the transfer from translucent to dark clouds, it is expected that for the low temperatures in the dense interstellar medium also DIB carriers accrete onto dust grains. Like all other molecules, apart from molecular hydrogen, they will get embedded in an ice matrix that largely consists of amorphous solid water. This offers - in principle - a tool to search for DIBs in complete different environments, both in space (i.e., towards embedded young stellar objects) and in the laboratory, namely in the solid state simulating interstellar ice analogues. Currently experiments are ongoing in the Sackler Laboratory for Astrophysics at Leiden Observatory to record optical ice spectra of potential DIB carriers. For this a new experimental approach has been developed. Its performance and potential are discussed.

Blue Luminescence and Extended Red Emission: Possible Connections to the Diffuse Interstellar Bands

AbstractBlue luminescence (BL) and extended red emission (ERE) are observed as diffuse, optical-wavelength emissions in interstellar space, resulting from photoluminescence by ultraviolet(UV)-illuminated interstellar grains. Faintness and the challenge of separating the BL and ERE from the frequently much brighter dust-scattered continuum present major observational hurdles, which have permitted only slow progress in testing the numerous models that have been advanced to explain these two phenomena. Both the ERE, peaking near 680 nm (FWHM ~ 60 - 120 nm) and the BL, asymmetrically peaking at ~ 378 nm (FWHM ~ 45 nm), were first discovered in the Red Rectangle nebula. Subsequently, ERE and BL have been observed in other reflection nebulae, and in the case of the ERE, in carbon-rich planetary nebulae, H II regions, high-latitude cirrus clouds, the galactic diffuse ISM, and in external galaxies. BL exhibits a close spatial and intensity correlation with emission in the aromatic emission feature at 3.3 micron, most likely arising from small, neutral polycyclic aromatic hydrocarbon (PAH) molecules. The spectral characteristics of the BL also agree with those of fluorescence by PAH molecules with 13 to 19 carbon atoms. The BL phenomenon is thus most readily understood as the optical fluorescence of small, UV-excited aromatic molecules. The ERE, by contrast, though co-existent with mid-IR PAH emissions, does not correlate with emissions from either neutral or ionized PAHs. Instead, the spatial ERE morphology appears to be strictly governed by the density of far-UV (E ≥ 10.5 eV) photons, which are required for the ERE excitation. The most restrictive observational constraint for the ERE process is its exceptionally high quantum efficiency. If the ERE results from photo-excitation of a nano-particle carrier by photons with E ≥ 10.5 eV in a single-step process, the quantum efficiency exceeds 100%. Such a process, in which one to three low-energy optical photons may be emitted following a single far-UV excitation, is possible in highly isolated small clusters, e.g. small, dehydrogenated carbon clusters with about 20 to 28 carbon atoms. A possible connection between the ERE carriers and the carriers of DIBs may exist in that both are ubiquitous throughout the diffuse interstellar medium and both have an abundance of low-lying electronic levels with E ≤ 2.3 eV above the ground state.

Analysis of Anomalous Diffuse Interstellar Bands in the Spectrum of Herschel 36

AbstractThe extraordinary DIBs observed toward Herschel 36 (Dahlstrom et al. 2013) have been analyzed (Oka et al. 2013). The analysis led us to a new way to classify the carriers of DIBs depending on whether the molecules are polar or non-polar. The pronounced Extended Tails toward Red (ETR) observed for DIBs λ5780.5, λ5797.1, and λ6613.6 are explained as due to radiative excitation of high rotational levels of polar carrier molecules in an environment with high radiative temperature ~90 K. Other DIBs (e.g., λ5849.8, λ6196.0, and λ6379.3) which do not show ETR are likely due to non-polar molecules. Model calculations taking into account the interplay of radiative and collisional effects reproduce the observed ETR using realistic molecular parameters if the radiative temperature is sufficiently high (~90 K). The calculation suggests that the carriers of DIBs with ETR are likely medium size molecules with 3 - 6 heavy atoms unless the radiative temperature is much higher.

DIBs Broadening with Increased Abundance of Vibrationally Excited H2 Molecules

AbstractThe analysis of DIB profiles was used in various attempts to discover the DIBs carriers. The broadening of the 6196 Å DIB with increasing C2 rotational temperature was reported by Kaźmierczak et al. (2009). We present the broadening of the 4763, 5780 and 6614 Å DIBs connected with higher abundance of vibrationally excited hydrogen molecule. The DIBs broadening is also correlated with increasing H2 rotational temperature on the ν=2 vibrational level. The broadening may be caused by growing population of higher rotational levels in the DIB carrier. Some DIBs, like 4964 Å, do not show any width changes with higher population of vibrationally excited H2.

The Stability of Cosmic Fullerenes and Fullerenic Aggregates

AbstractEstablishing the stability of cosmic fullerenes and fullerenic aggregates is extremely relevant for a variety of reasons. For instance, the emission features of C60 and C70 fall in the same spectral region as the Un-identified InfraRed (UIR) bands, which they could contribute to. To be able to contribute to the UIR emission, however, fullerenes must be able to survive long enough against the destruction mechanisms operating in the interstellar medium. In this study we focus on the effects of collisional processing, i.e., the bombardment by energetic ions and electrons. A recent experimental/theoretical study has shown that ion collisions with C60 clusters result in the dissociation of the cluster with the simultaneous formation of covalent fullerene dimers, which could play a role as DIBs carriers. We present here our first results about the collisional processing of C60 molecules and clusters by H, He and C ions in interstellar shocks. We have adapted the models that have previously been developed to successfully treat the collisional processing of PAHs in space. The nature of the interaction and the similarities between PAHs and fullerenes make this approach appropriate. In addition, our study shows that the formation of covalent dimers following ion collisions with C60 clusters is compatible with the astrophysical conditions under consideration.

Diffuse Interstellar Bands: Families and Correlations

AbstractThe term “families of diffuse bands” (DIBs) appeared in 1986/87 when my collaborators: Gordon A.H. Walker, Bengt E. Westerlund and I found that the strength ratio of the major DIBs 5780 and 5797 is heavily variable. We proved that at the same E(B-V) the DIB intensities may vary by as much as a factor of three or more. A similar result was published by Karl Josafatsson and Ted Snow soon after. A decade later, we proved (with Chris Sneden) that certain DIB strength ratios seem to be related to intensities of the known features of simple molecular species; this led to the introduction of the so called σ and ζ type interstellar clouds. The former are characterized by very weak molecular features (but broad DIBs – very strong) while the latter by rather strong bands of simple radicals and weak broad DIBs. Currently we face a bunch of questions: are the DIB intensities related to those of certain molecular species, e.g. C2 as suggested by Lew Hobbs' and Ted Snow's group? Do the DIB profiles, found to be complex by Peter Sarre, depend on e.g. the rotational temperatures of simple, linear carbon species? Do the DIB profiles depend on the irradiation of interstellar clouds by nearby stars? The relative DIB strengths as well as those of the simple radicals seem to be related to the shapes of interstellar extinction curves. We thus face three players in the interstellar translucent clouds: dust particles, simple radicals and the DIB carriers. Apparently, their mutual relations depend on local physical parameters of intervening clouds; these relations are not clear yet.

Diffuse Interstellar Bands: How are they related to known Gas-Phase Constituents of the ISM?

AbstractIn this brief review of recent work relating the DIBs to other gas-phase constituents of the ISM, we explore correlations between DIB equivalent widths and the column densities of various atomic and molecular species, drawn from a large database constructed for that purpose. The tightness and slopes of the correlations can provide information on how the DIBs might be related to those species (physically, chemically, spatially) and on various properties of the DIB carriers. Deviations from the mean relationships can reveal dependences of DIB strengths on other parameters, regional variations in DIB behavior, and individual sight lines where unusual environmental conditions affect the DIBs. Variations in DIB profiles (e.g., wings, substructure) and relative strengths may be related to differences in physical conditions inferred from atomic and/or molecular absorption lines.

A Possible New Sequence of DIBs

AbstractOne of the key factors in explaining nature of DIB carriers is to find regularities in their spectrum. Such a regular set of DIBs exists within interval λλ 6770–6865 Å. A possible new sequence of weak DIBs between λλ 5910–5990 Å is presented here. The new sequence is clearly visible in high resolution spectra (R ≳ 100,000) of some hot, reddened stars. Wavenumbers' differences, or spacings, between the most strong bands have values ~35, ~40 cm−1, which are very similar to those, found for DIBs near 6800 Å.

Infrared diffuse interstellar bands in the Galactic Centre region

The spectrum of any star viewed through a sufficient quantity of diffuse interstellar material reveals a number of absorption features collectively called 'diffuse interstellar bands' (DIBs). The first DIBs were reported about 90 years ago, and currently well over 500 are known. None of them has been convincingly identified with any specific element or molecule, although recent studies suggest that the DIB carriers are polyatomic molecules containing carbon. Most of the DIBs currently known are at visible and very near-infrared wavelengths, with only two previously known at wavelengths beyond one micrometre (10,000 ångströms), the longer of which is at 1.318 micrometres (ref. 6). Here we report 13 diffuse interstellar bands in the 1.5-1.8 micrometre interval on high-extinction sightlines towards stars in the Galactic Centre. We argue that they originate almost entirely in the Galactic Centre region, a considerably warmer and harsher environment than where DIBs have been observed previously. The relative strengths of these DIBs towards the Galactic Centre and the Cygnus OB2 diffuse cloud are consistent with their strengths scaling mainly with the extinction by diffuse material.

The PAH-DIB Hypothesis

In this contribution I review the PAH-DIB hypothesis. Firstly, I list several properties of PAHs and their behaviour and expected signatures in space. Next, I give an overview of the current observational insights on the DIB carriers. I conclude with a brief description of recent results on the search for the DIB carriers and the prospects we have for identification.

Electron density and carriers of the diffuse interstellar bands

We have used the ionisation equilibrium equation to derive the electron density in interstellar clouds in the direction to 13 stars. A linear relation was found, that allows the determination of the electron density from the Mg I and Mg II column densities in diffuse clouds. The comparison of normalised equivalent width of 12 DIBs with the electron density shows that the DIBs equivalent width do not change with electron density varying in the range ne=0.01-2.5 cm^-3. Therefore the DIBs carriers (1) can be observed only in one ionisation stage, or (2) the DIBs are arising in cloud regions (eg. cores or cloud coronas) for which we can not determine the electron density.

On the relation between diffuse bands and column densities of H$\mathsf{_{2}}$, CH and CO molecules

Mutual relations between column densities of H 2 , CH and CO molecules as well as between the latter and strengths of the major 5780 and 5797 diffuse bands are presented and discussed. The CH radical seems to be a good H 2 tracer, possibly better than CO. It is also demonstrated that the molecular fraction of the H 2 molecule is correlated with an intensity ratio of 5797 and 5780 DIBs, suggesting the possible formation of narrow DIB carriers in denser clouds, dominated by molecular hydrogen and reasonably shielded from ionizing UV radiation by small dust grains.

Small scale structure in molecular gas from multi-epoch observations of HD 34078

We present spectroscopic observations of the runaway reddened star HD 34078 acquired during the last three years at Observatoire de Haute Provence and McDonald Observatory as well as other spectra obtained since 1990. Thedrift of the line of sight through the foreground cloud due to the large transverse velocity of HD 34078 allows us to probe the spatial distribution or CH, CH + , CN and DIBs carriers at scales ranging from about I AU up to 150 AU. In particular, time variations in the equivalent width of absorption lines are examined. A few past and recent high resolution observations of CH and CH + absorption are used to search for line profile variations and to convert equivalent widths into column densities. The data set reveals a 20% increase in CH column density over the past 10 years with no corresponding variation in the column density of CH + or in the strengths of the 5780 and 5797 A DIBs. CN observations indicate that its excitation temperature has significantly increased from <3.1 K in 1993 to 3.6 ′ 0.17 K in 1998 while the CN column shows only a modest rise of 12′6%. The data also strongly suggest the existence of weak correlated variations in CH and CH + columns over periods of 6-12 months (or 10 AU). These results are discussed in relation to CH + production mechanisms. A dense newly intervening clump is considered in order to explain the long-term increase in the column density of CH, but such a scenario does not account for all observational constraints. Instead, the observations are best described by CH + production in a photodissociation region, like that suggested for the Pleiades and IC 348.

A survey of weak diffuse interstellar bands in selected ranges between 5600 and 7000 Å

The paper describes briefly the history of the discovery of diffuse interstellar bands (D] Bs) and their “younger relatives”, weak unidentified interstellar features, recently, discovered and described in scientific literature. The features of both kinds form a verv rich spectrum of interstellar clouds, the spectrum varying, however, from object to object. The high quality echetle spectra acquired at McDonald Observatory (R = 60,000 ; S/N = 500) indicate that both strong and weak features form several subsets (“families”) which can be considered as spectra of single species populating the interstellar clouds. This fact is important as any two of the well-known. strong DlBs hardly belong to one such “family” which makes the identification of the DIB carriers so difficult. The paper also discusses the difficulties of the detection of the weak features, especially when one deals with very shallow features. Hundreds of prospectively observed weak unidentified features can give us an important insight into the nature of interstellar clouds, especially their apparently very complicated chemistry.