We investigate the nucleation of carbon and hydrogen atoms in the gas phase to form large carbon chains, clusters and cages by reactive molecular dynamics simulations. We study how temperature, particle density, presence of hydrogen, and carbon inflow affect the nucleation of molecular moieties with different characteristics.

The 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.

PAHs 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?

Fullerenes and their derivatives are amongst the most stable carbonaceous species known and have therefore been proposed as ideal candidates to carry some of the diffuse interstellar bands (DIBs). Evidence for these species in space first came from a few DIBs that have been tentatively identified with electronic transitions of C60+; in recent years, infrared observations have furthermore revealed fullerenes in a variety of circumstellar and interstellar environments. With the presence of the fullerene family in space established, we review what is known about cosmic fullerenes and their derivatives and what role they could play as potential DIB carriers.

In the 1990s the hydrogen molecule, by far the most abundant molecular species in the interstellar medium, has been proposed as a possible carrier of the diffuse interstellar bands. While some remarkable coincidences were found in the rich spectrum of inter-Rydberg transitions of this molecule with DIB-features, both in frequency position as in linewidth, some open issues remained on a required non-linear optical pumping scheme that should explain the population of certain intermediate levels and act as a selection mechanism. Recently a similar scheme has been proposed relating the occurrence of the UV-bump (the ubiquitous 2170 Å extinction feature) to the spectrum of H2, therewith reviving the H2 hypothesis.

We propose that the diffuse interstellar bands (DIBs) arise from absorption lines of electronic transitions in molecular clusters primarily composed of a single molecule, atom, or ion (“seed”), embedded in a single-layer shell of H2 molecules (Bernstein et al. 2013). Less abundant variants of the cluster, including two seed molecules and/or a two-layer shell of H2 molecules may also occur. The lines are broadened, blended, and wavelength-shifted by interactions between the seed and surrounding H2 shell. We refer to these clusters as CHCs (Contaminated H2 Clusters). CHC spectroscopy matches the diversity of observed DIB spectral profiles, and provides good fits to several DIB profiles based on a rotational temperature of 10 K. CHCs arise from ~cm-sized, dirty H2 ice balls, called CHIMPs (Contaminated H2 Ice Macro-Particles), formed in cold, dense, Giant Molecular Clouds (GMCs), and later released into the interstellar medium (ISM) upon GMC disruption. Attractive interactions, arising from Van der Waals and ion-induced dipole potentials, between the seeds and H2 molecules enable CHIMPs to attain cm-sized dimensions. When an ultraviolet (UV) photon is absorbed in the outer layer of a CHIMP, it heats the icy matrix and expels CHCs into the ISM. While CHCs are quickly destroyed by absorbing UV photons, they are replenished by the slowly eroding CHIMPs. Since CHCs require UV photons for their release, they are most abundant at, but not limited to, the edges of UV-opaque molecular clouds, consistent with the observed, preferred location of DIBs. An inherent property of CHCs, which can be characterized as nanometer size, spinning, dipolar dust grains, is that they emit in the radio-frequency region. Thus, CHCs offer a natural explanation to the anomalous microwave emission (AME) feature in the ~10-100 GHz spectral region.

We suggest that the diffuse interstellar bands (DIBs) are absorption lines arising from electronic transitions in molecular clusters primarily composed of a single molecule, atom, or ion (“seed”), embedded in a single-layer shell of H2 molecules (Bernstein et al. 2013). We refer to these clusters as CHCs (Contaminated H2 Clusters). CHCs arise from cm-sized, dirty H2 ice balls, called CHIMPs (Contaminated H2 Ice Macro-Particles), formed in cold, dense, Giant Molecular Clouds (GMCs), and later released into the interstellar medium (ISM) upon GMC disruption. Absorption by the CHIMP of a UV photon releases CHCs. CHCs produce DIBs when they absorb optical photons. When this occurs, the absorbed photon energy disrupts the CHC.

The dust grains flowing out of the stars are most likely non-spherical and inhomogeneous; viz. porous, fluffy and composites of many very small particles glued together. We calculate the scattering, absorption and extinction efficiencies of such composite grains. The composite grains consist of host silicate spheroid and inclusions of PAHs. We study the extinction as a function of the volume fraction of the inclusions. In particular, we study the variation in the absorption peak, at the wavelength ‘4430Å’ with the volume fraction of the PAH inclusions. We present the composite grain models and discuss the results.

The identification of the carrier(s) of diffuse interstellar bands (DIBs) is one of the oldest mysteries in stellar spectroscopy. With the advent of 8-10m-class telescopes substantial progress has been made in measuring the properties of DIBs in the optical and near-infrared wavelength domain, not only in the Galaxy, but also in different environments encountered in Local Group galaxies and beyond. Still, the DIB carriers have remained unidentified. The coming decade will witness the development of extremely large telescopes (GMT, TMT and E-ELT) and their instrumentation. In this overview I will highlight the current instrumentation plan of these future observatories, emphasizing their potential role in solving the enigma of the DIBs.

Rather than a summary of the conference, I present here an overview of the status of the field and our progress over the last two decades from the points of view of astronomy, molecular physics, spectroscopy, and astrochemistry. While at first sight, progress may seem slow, actually, we have made an important stride forward. We have recognized now that the problem is very complex and identifying the carriers of the Diffuse Interstellar Bands will require a concerted effort of astronomers, molecular physicists, spectroscopists, and astrochemists. While this is a daunting prospect, we have identified the tools that we need to make this happen.

We present a road map of several research avenues leading to the identification of the diffuse interstellar band carriers. The proposed programs represent some consensus among the DIB community, and will certainly take many years to complete. However, the scientific payoff will be huge, and will ultimately lead to the solution of the DIB problem.