The 11.2 μm emission of PAHs in astrophysical objects

Article / Letter to editorSterrewach

Catalytic conversion of methanol to larger organic molecules over crystalline forsterite: laboratory study and astrophysical implications

Laboratory catalytic reactions of methanol over heated crystalline silicates (forsterite) lead to the formation of gas-phase olefinic and polycyclic aromatic hydrocarbon (PAH) molecules, and are of potential importance in astrophysical environments including hot molecular cores, protoplanetary disks and shocks. In our experiments the methanol reagent, together with intermediate and product gas-phase molecular species were detected using time-of-flight mass-spectrometry (TOF-MS). A solid deposited on the crystalline forsterite surface was examined subsequently using high-resolution transmission electron microscopy and thermal gravimetric techniques and found to comprise amorphous and graphitic carbon. The chemical players in this work – gas-phase methanol, crystalline silicates and PAHs, have been identified spectroscopically in a range of astrophysical environments including young and evolved stars, protoplanetary disks, comets, captured dust particles and meteorites. It is envisaged that reactions on bare dust grains as studied here both experimentally and theoretically through DFT calculations, can have implications for chemical transformations and conversions, in forming PAH molecules and potentially in the synthesis of prebiotic molecules

The EDIBLES survey:VI. Searching for time variations of interstellar absorption features

Context. Interstellar absorption observed toward stellar targets changes slowly over long timescales, mainly due to the proper motion of the background target relative to the intervening clouds, such that over time, different parts of the intervening cloud are probed. On longer timescales, the slowly changing physical and chemical conditions in the cloud can also cause variation. Detecting such time variations thus provides an opportunity to study cloud structure.Aims. We searched for systematic variations in the absorption profiles of the diffuse interstellar bands (DIBs) and interstellar atomic and molecular lines by comparing the high-quality data set from the recent ESO diffuse interstellar bands large exploration survey (EDIBLES) to older archival observations, bridging typical timescales of ~10 yr with a maximum timescale of 22 yr.Methods. For 64 EDIBLES targets, we found adequate archival observations. We selected 31 strong DIBs, seven atomic lines, and five molecular lines to focus our search on. We carefully considered various systematic effects and used a robust Bayesian quantitative test to establish which of these absorption features could display significant variations.Results. While systematic effects greatly complicate our search, we find evidence for variations in the profiles of the λλ4727 and 5780 DIBs in a few sightlines. Toward HD 167264, we find a new Ca I cloud component that appears and becomes stronger after 2008. The same sightline furthermore displays marginal, but systematic changes in the column densities of the atomic lines originating from the main cloud component in the sightline. Similar variations are seen toward HD 147933.Conclusions. Our high-quality spectroscopic observations in combination with archival data show that it is possible to probe interstellar time variations on time scales of typically a decade. Despite the fact that systematic uncertainties as well as the generally somewhat lower quality of older data complicate matters, we can conclude that time variations can be made visible, both in atomic lines and DIB profiles for a few targets, but that generally, these features are stable along many lines of sight. We present this study as an archival baseline for future comparisons, bridging longer periods.<br/

Current assessment of the Red Rectangle band problem

In this paper we discuss our insights into several key problems in the identification of the Red Rectangle Bands (RRBs). We have combined three independent sets of observations in order to try to define the constraints guiding the bands. We provide a summary of the general behavior of the bands and review the evidence for a molecular origin of the bands. The extent, composition, and possible absorption effects of the bands are discussed. Comparison spectra of the strongest band obtained at three different spectral resolutions suggests that an intrinsic line width of individual rotational lines can be deduced. Spectroscopic models of several relatively simple molecules were examined in order to investigate where the current data are weak. Suggestions are made for future studies to enhance our understanding of these enigmatic bands

Radiative cooling of carbon cluster anions C2n+1− (n = 3–5)

Radiative cooling of carbon cluster anions C2n+1− (n = 3–5) is investigated using the cryogenic electrostatic ion storage ring DESIREE. Two different strategies are applied to infer infrared emission on slow (milliseconds to seconds) and ultraslow (seconds to minutes) timescales. Initial cooling of the ions over the millisecond timescale is probed indirectly by monitoring the decay in the yield of spontaneous neutralization by thermionic emission. The observed cooling rates are consistent with a statistical model of thermionic electron emission in competition with infrared photon emission due to vibrational de-excitation. Slower cooling over the seconds to minutes timescale associated with infrared emission from low-frequency vibrational modes is probed using time-dependent action spectroscopy. For C9− and C11−, cooling is evidenced by the time-evolution of the yield of photo-induced neutralization following resonant excitation of electronic transitions near the detachment threshold. The cross-section for resonant photo-excitation is at least two orders of magnitude greater than for direct photodetachment. In contrast, C7− lacks electronic transitions near the detachment threshold

On the possible detection of negative ions in the interstellar medium

The possible detection of negative molecular ions by radioastronomy is discussed, with reference to the ions C2H—, [Math] and CN—, Approximate transition frequencies are predicted, and the physical and chemical characteristics of these ions are considered

SUB-DOPPLER LASER SPECTROSCOPY OF MOLECULAR IONS IN ION BEAMS

Les spectres d'absorption électronique à haute résolution de CO+, 13CO+, O2+, CH+ et H2O+ ont été obtenus en utilisant un laser et un faisceau d'ions. Un faisceau d'ions rapides de masse donnée est irradié coaxiallement à l'aide d'un laser à argon et les transitions moléculaires sont mises en résonance avec la fréquence laser par l'effect Doppler en changeant la vitesse des ions. Les transitions sont détectées par l'intermédiaire de la prédissociation moléculaire, des collisions ou des réactions avec les molécules d'un gaz neutre. La résolution est généralement de l'ordre de 100MHz.Sub-Doppler electronic absorption spectra of 12CO+, 13CO+, O2+, CH+ and H2O+ have been obtained by laser/ion beam methods. A fast beam of mass-selected ions is irradiated coaxially with an argon ion laser, and rotational lines in the electronic transition are Doppler-tuned into resonance with the laser by changing the ion velocity. The transitions are detected through predissociation, collisions or ion-molecule reactions, and the linewidths are normally of order 100MHz

Spectroscopy of Protonated and Deprotonated PAHs

The spectroscopic properties of protonated and deprotonated PAHs are investigated through Density Functional Theory (DFT) calculations, with reference to their potential astrophysical significance. Attention is focussed on electronic and rotational spectra