Carriers Of Diffuse Interstellar Bands Research Articles

The diffuse interstellar band carriers in interstellar space: all intense bands calculated from He doubly excited states embedded in Rydberg Matter

The interpretation of the almost 300 diffuse interstellar bands (DIBs) is one of the most long-standing problems in interstellar spectra. The only model showing agreement with a large number of DIBs (>60 bands) is based on transitions in doubly excited atoms embedded in the condensed phase named Rydberg Matter (RM). A similar model is now used to precisely calculate all intense bands from the high-resolution survey by Galazutdinov et al., 63 bands in total. Thus, the RM model interprets at present 120 DIBs accurately in a consistent manner, almost half of the number of DIBs. The origin of almost all intense DIBs is shown to be absorption during electron transitions between co-planar doubly excited He atoms inside RM, and from singly excited He atoms to doubly excited He atoms inside RM. The average error in the assignments is 4 cm−1 (1.4 Å), comparable to the band width. The intense but broad DIB 578.0 nm is part of a series of 10 bands interpreted as RM transitions from n″= 5 to the conduction band limit. The shape of DIB 579.7 nm is consistent with a rotational band from symmetric top RM clusters at approximately 100 K. Good agreement with observational results like band shapes and correlations among various bands is found.

Interstellar gas, dust and diffuse bands in the SMC

Aims. In order to gain new insight into the unidentified identity of the diffuse interstellar band (DIB) carriers, this paper describes research into possible links between the shape of the interstellar extinction curve (including the 2175 A bump and far-UV rise), the presence or absence of DIBs, and physical and chemical conditions of the diffuse interstellar medium (gas and dust) in the Small Magellanic Cloud (SMC). Methods. We searched for DIB absorption features in VLT/UVES spectra of early-type stars in the SMC whose reddened lines-ofsight probe the diffuse interstellar medium of the SMC. Apparent column density profiles of interstellar atomic species (Na i ,K i ,C aii and Ti ii) are constructed to provide information on the distribution and conditions of the interstellar gas. Results. The characteristics of eight DIBs detected toward the SMC wing target AzV 456 are studied and upper limits are derived for the DIB equivalent widths toward the SMC stars AzV 398, AzV 214, AzV 18, AzV 65 and Sk 191. The amount of reddening is derived for these SMC sightlines, and, using RV and the H i column density, converted into a gas-to-dust ratio. From the atomic column density ratios we infer an indication of the strength of the interstellar radiation field, the titanium depletion level and a relative measure of turbulence/quiescence. The presence or absence of DIBs appears to be related to the shape of the extinction curve, in particular with respect to the presence or absence of the 2175 A feature. Our measurements indicate that the DIB characteristics depend on the local physical conditions and chemical composition of the interstellar medium of the SMC, which apparently determine the rate of formation (and/or) destruction of the DIB carriers. The UV radiation field (via photoionisation and photo-destruction) and the metallicity (i.e. carbon abundance) are important factors in determining diffuse band strengths which can differ greatly both between and within galaxies.

Linear and circular polarisation of diffuse interstellar bands

Context: The detection or absence of a polarisation signal in the diffuse interstellar band (DIB) profile can possibly give important clues on the identity of its carrier. For molecular gas-phase carriers the effect of polarisation on its electronic transitions depends, although uncertain how exactly, on its physical properties (e.g. molecular structure). On the other hand, if the carriers are grains, impurities or defects in grain mantles are also expected to show up in the polarisation of DIBs. Aims: Our aim is to detect a polarisation signal in a DIB profile or to derive stricter upper limits on the polarisation efficiency of DIB carriers. Methods: In order to detect and measure the linear and circular polarisation of the DIBs we observe reddened lines of sight showing continuum polarisation; for this study we select the stars HD 21219 , HD 198478 , HD 197770 , HD 183143 and HD 163472 . We use spectropolarimetry in the wavelength range 4480 to 6620 A with the MuSiCoS echelle spectrograph mounted at the Telescope Bernard Lyot. Results: Linear polarisation spectra are constructed in order to search for a polarisation signal in the DIB profiles. No significant change in the linear polarisation degree is found. We obtain a 2sigma polarisation detection limit (per DIB FWHM) of 0.01-0.04% for HD 21219 , 0.04-0.14% for HD 197770 , 0.01-0.14% for HD 183143 and 0.01-0.14% for HD 198478 , for the six investigated DIBs. We derive upper limits for the polarisation efficiency factor f for six strong narrow DIBs; f_max of 0.31, 0.44, 0.45, 0.18, 0.47 and 0.68 for the lambdalambda 5780, 5797, 6196, 6284, 6379 and 6613 DIBs, respectively. The derived detection limits are similar to those derived by Adamson & Whittet (1995), although for more lines of sight and more DIBs. Circular polarisation (Stokes V) spectra of high signal-to-noise have been obtained for the first time for the 5780, 5797, 6196, 6203, 6284, 6376, 6379 and 6613 A DIB profiles. No circular polarisation signal is detected. The 2sigmaV (per 0.1 A) noise level limits are 1.0-2.5% for the DIBs in the line of sight towards HD 197770 and 0.06-0.10% for the DIBs towards HD 163472 . Conclusions: . The lack of polarisation of the DIB profiles is consistent with DIB carriers that are not directly related to solid features, like impurities in grain mantles. However, large (carbonaceous) gas phase molecules remain viable DIB carrier candidates. Based on observation at the Observatoir du Pic du Midi, France with the MuSiCoS spectropolarimeter.

Gas-Phase Study of Coronene Cation Reactivity of Interstellar Relevance

Recent research suggests that cations of polycyclic aromatic hydrocarbons (PAHs) may be responsible for the observed diffuse interstellar bands. Determining the reaction rate constants and reactivity trends of these ions with species relevant to the interstellar medium will help lead the search for specific diffuse interstellar band carriers and develop pathways for their transformation. This Letter presents the gas-phase ion-neutral reaction rate constants of a specific PAH cation, the parent molecular cation of coronene (C24H), with H atom, N atom, O atom, and H2, and discusses the reactivity trends of PAH cations of increasing size with these small interstellar species. This study examines the kinetics of the largest PAH cation to date. These experiments were carried out using a flowing afterglow-selected ion flow tube coupled to a quadrupole mass spectrometer. The reaction rate constants of the coronene cation with H atom, O atom, N atom, and H2 have been measured to be 1.4 × 10-10, 1.3 × 10-10, <1 × 10-12, and <5 × 10-13 cm3 s-1, respectively.

The effects of metallicity, radiation field and dust extinction on the charge state of PAHs in diffuse clouds: implications for the DIB carrier

Context.The unidentified diffuse interstellar bands (DIB) are observed throughout the Galaxy, the Local Group and beyond. Their carriers are possibly related to complex carbonaceous gas-phase molecules, such as (cationic) polycyclic aromatic hydrocarbons and fullerenes.

The Large Magellanic Cloud: diffuse interstellar bands, atomic lines and the local environmental conditions

The Large Magellanic Cloud (LMC) offers a unique laboratory to study the diffuse interstellar bands (DIBs) under conditions that are profoundly different from those in the Galaxy. DIB carrier abundances depend on several environmental factors, in particular the local UV radiation field. In this paper we present measurements of twelve DIBs in five lines of sight to early-type stars in the LMC, including the 30 Doradus region. From the high resolution spectra obtained with VLT/UVES we also derive environmental parameters that characterise the local interstellar medium (ISM) in the probed LMC clouds. These include the column density components (including total column density) for the atomic resonance lines of Na I, Ca II, Ti II, K I. In addition, we derive the H I column density from 21 cm line profiles, the total-to-selective visual extinction RV and the gas-to-dust ratio N(H I)/A_V. Furthermore, from atomic line ratios we derive the ionisation balance and relative UV field strength in these environments. We discuss the properties of the LMC ISM in the context of DIB carrier formation. The behaviour of DIBs in the LMC is compared to that of DIBs in different local environmental conditions in the Milky Way. A key result is that in most cases the diffuse band strengths are weak (up to factor 5) with respect to Galactic lines of sight of comparable reddening, EB-V. In the line of sight towards Sk -69 223 the 5780 and 5797 �DIBs are very similar in strength and profile to those observed towards HD 144217 , which is typical of an environment exposed to a strong

Graphite, Tubular PAHs, and the Diffuse Interstellar Bands

Planar polycyclic aromatic hydrocarbons (PAHs) are candidates for the diffuse interstellar band (DIB) carriers. Nanometer-size Fe metallic particles, expected to be present during astrophysical graphite growth, are catalytic for formation of a related species—large tubular PAH molecules. We propose that tubular PAH molecules are a component of the interstellar medium. Electronic structure calculations, on a specific family of tubular PAH molecules derived from elongated C60, reveal intense electronic transitions in the visible and near-IR, which vary systematically with length. We analyze these molecules as DIB carriers within the known constraints. Subject headings: astrochemistry — dust, extinction — ISM: molecules — stars: carbon

High resolution laser induced fluorescence excitation spectroscopy of jet-cooled terrylene

High resolution laser induced fluorescence excitation spectra upon absorption in the A 1 B 3 u ← X 1 A g band of jet-cooled terrylene have been recorded. Precise energies of three vibronic transitions are deduced. Low lying vibrations are found in both electronic states. Rotational constants in ground and excited state are determined by band contour analysis. Terrylene is a medium-size polycyclic aromatic hydrocarbon (PAH) and a possible carrier of diffuse interstellar bands (DIB). The results of the jet experiments are discussed regarding the PAH–DIB hypothesis.

Diffuse interstellar bands of unprecedented strength in the line of sight towards high-mass X-ray binary 4U 1907+09

High-resolution VLT/UVES spectra of the strongly reddened O supergiant companion to the X-ray pulsar 4U 1907+09 provide a unique opportunity to study the nature of the diffuse interstellar bands (DIBs) at unprecedented strength. We detect about 180 known DIBs, of which about 25 were listed as tentative and are now confirmed. A dozen new DIB candidates longwards of 6900 A are identified. We show that the observed 5797 A DIB strength is in line with the Galactic correlation with reddening, whereas the 5780 A DIB strength is relatively weak. This indicates the contribution of denser regions, where the UV penetration is reduced. The presence of dense cloud cores is supported by the detection of C2 rotational transitions. Members of one DIB family (5797, 6379 A and 6196, 6613 A) behave coherently, although one can make a distinction between the two correlated pairs. The broadened profiles of narrow DIBs are shown to be consistent with the premise that each of the main clouds in the line of sight discerned in the interstellar K I profile is contributing proportionally to the DIB profile. We complement and extend the relation of DIB strength with reddening {E} (B-V) , as well as with neutral hydrogen column density N(H I), respectively, using strongly reddened sightlines towards another four distant HMXBs. The 5780 A DIB, and tentatively also the 5797 and 6613 A DIBs, are better correlated to the gas tracer H I than to the dust tracer {E} (B-V) . The resulting relationship can be applied to any line of sight to obtain an estimate of the H I column density. In the search for the nature of the DIB carrier, this strongly reddened line of sight is a complementary addition to single cloud line of sight studies.

The Optical Spectroscopy of Extraterrestrial Molecules

AbstractFor Abstract see ChemInform Abstract in Full Text.

Correlation patterns between 11 diffuse interstellar bands and ultraviolet extinction

We relate the equivalent widths of 11 diffuse interstellar bands, measured in the spectra of 49 stars, to different colour excesses in the ultraviolet. We find that most of the observed bands correlate positively with the extinction in the neighbourhood of the 2175-A bump. Correlation with colour excesses in other parts of the extinction curve is more variable from one diffuse interstellar band to another; we find that some diffuse bands (5797, 5850 and 6376 A) correlate positively with the overall slope of the extinction curve, while others (5780 and 6284 A) exhibit negative correlation. We discuss the implications of these results on the links between the diffuse interstellar band carriers and the properties of the interstellar grains.

PAH charge state distribution and DIB carriers: Implications from the line of sight toward HD 147889

We have computed physical parameters such as density, degree of ionization and temperature, constrained by a large observational data set on atomic and molecular species, for the line of sight toward the single cloud HD147889. Diffuse interstellar bands ( DIBs) produced along this line of sight are well documented and can be used to test the PAH hypothesis. To this effect, the charge state fractions of different polycyclic aromatic hydrocarbons (PAHs) are calculated in HD147889 as a function of depth for the derived density, electron abundance and temperature profile. As input for the construction of these charge state distributions, the microscopic properties of the PAHs, e. g., ionization potential and electron affinity, are determined for a series of symmetry groups. The combination of a physical model for the chemical and thermal balance of the gas toward HD147889 with a detailed treatment of the PAH charge state distribution, and laboratory and theoretical data on specific PAHs, allow us to compute electronic spectra of gas phase PAH molecules and to draw conclusions about the required properties of PAHs as DIB carriers. We find the following. 1) The variation of the total charge state distribution of each specific class ( series) of PAH in the translucent cloud toward HD147889 ( and also of course for any other diffuse/ translucent cloud) depends strongly on the molecular symmetry and size ( number of p electrons). This is due to the strong effects of these parameters on the ionization potential of a PAH. 2) Different wavelength regions in the DIB spectrum are populated preferentially by different PAH charge states depending on the underlying PAH size distribution. 3) The PAH size distribution for HD147889 is constrained by the observed DIB spectrum to be Gaussian with a mean of 50 carbon atoms. 4) For the given PAH size distribution it is possible to constrain the total small catacondensed PAH column density along the line of sight to HD147889 to 2.4 x 10(14) cm(-2) by comparing the total observed UV extinction to the strong UV absorptions of neutral PAHs in the 2000-3000 Angstrom region. 5) Catacondensed PAHs with sizes above some 40 C-atoms are expected to show strong DIBS longward of 10 000 Angstrom. Large condensed PAHs in the series, pyrene, coronene, ovalene,...., on the other hand, mainly absorb between 4000 and 10 000 Angstrom but extrapolation to even larger pericondensed PAHs in this series also shows strong absorptions longward of 10 000 Angstrom. 6) Only the weak DIBs in HD147889 could be reproduced by a mix of small catacondensed PAHs (<50 C atoms) while for large pericondensed PAHs ( 50 <C atoms <100) the intermediate DIBs are well reproduced. Small catacondensed PAHs cannot contribute more than 50% of the total observed equivalent width toward HD147889. Strong DIBs can only be reproduced by addition of very specific PAH molecules or homologue series to the sample set (i.e., a small number of PAHs with high oscillator strength or a large number of PAHs with a low oscillator strength). An outline is provided for a more general application of this method to other lines of sight, which can be used as a pipeline to compute the spectroscopic response of a PAH or group of PAHs in a physical environment constrained by independent (non-DIB) observations.

The Optical Spectroscopy of Extraterrestrial Molecules

The ongoing quest to identify molecules in the interstellar medium by their electronic spectra in the visible region is reviewed. Identification of molecular absorption is described in the context of the elucidation of the carriers of the unidentified Diffuse Interstellar Bands, and molecular emission is discussed with reference to the unidentified Red Rectangle bands. The experimental techniques employed in undertaking studies on the optical spectroscopy of extraterrestrial molecules are described and critiqued in the context of their application.

Correlations between diffuse interstellar bands and atomic lines

We present and discuss correlations between strengths of the well-known, strong interstellar atomic lines of K I and Ca II, and four selected, strong unidentified diffuse interstellar bands (DIBs): 5780, 5797, 5850 and 6614. In order to analyse a homogeneous sample of echelle high-resolution spectra it has been chosen to use measurements from Terskol Observatory in Northern Caucasus plus a selected number of higher resolution observations performed using other instruments. We demonstrate that the strength of certain DIBs correlate well with neutral potassium lines and to a much lower degree with ionized calcium lines. This fact suggests that the degree of irradiation of a cloud with UV photons, capable to ionize interstellar atoms, plays a crucial role in the formation/maintenance of certain molecular species: possible carriers of DIBs.

Cavity ring-down laser absorption spectroscopy of jet-cooled anthracene

Polycyclic aromatic hydrocarbons (PAHs) have been suggested as possible carriers of diffuse interstellar bands. To verify this, absorption spectra of PAHs under conditions similar to those encountered in the interstellar medium should be obtained. We report here the application of cavity ring-down laser absorption spectroscopy to the study of neutral anthracene expanded in supersonic jets. Absorption spectra of the S1←S0 transition of anthracene near 361 nm have been obtained with significantly higher resolution than reported previously. Assignments of the vibrational modes involved in the sequence bands observed on the red side of the origin band are proposed.

The Rotational Excitation Temperature of the λ6614 Diffuse Interstellar Band Carrier

Analysis of high spectral resolution observations of the λ6614 diffuse interstellar band (DIB) line profile shows systematic variations in the positions of the peaks in the substructure of the profile. These variations—shown here for the first time—can be understood most naturally in the framework of rotational contours of large molecules, where the variations are caused by changes in the rotational excitation temperature. We show that the rotational excitation temperature for the DIB carrier is likely significantly lower than the gas kinetic temperature—indicating that for this particular DIB carrier angular momentum buildup is not very efficient.

On the Possible Role of Carbon Chains as Carriers of Diffuse Interstellar Bands

Because the laboratory gas-phase electronic spectra of only the three polyatomic bare carbon chains C3, C4 and C5 are available, we have made a further attempt to detect the origin bands of C4 (3789 Å) and C5 (5109 Å) in the diffuse cloud toward ζ Oph. The measurements provide an improved 3 σ limit to their column densities: N(C4) ≤ 5 × 1011 cm-2 and N(C5) ≤ 1 × 1011 cm-2, with a signal-to-noise ratio (S/N) of 42,000 and 31,000 Å-1, respectively, at a resolution of 110,000. The limits to N(C4) and N(C5) are lower than predictions from the chemical model used by Roueff and coworkers for such diffuse clouds. In conjunction with observations of related, hydrogen-containing polar chains in the millimeter region and laboratory studies of the electronic transitions of a number of homologous series, these results lead to interesting conclusions about the role of carbon chains as potential carriers of the diffuse interstellar bands (DIBs). First, the abundance and oscillator strength of smaller chains, say up to 10 atoms, are too small to account for the stronger DIBs. Second, because of the electronic configurations of these open-shell species, the lowest energy π-π transition does not have a large oscillator strength but the higher energy one in the UV does, and the chains would have to be prohibitively long for these absorptions to shift into the DIB 4000-9000 Å wavelength region. The exceptions are closed-shell systems such as the odd-numbered bare carbon chains, and the ones in the C15-C31 size range that have their very strong transitions in this region. These species should be a major goal for laboratory and subsequent astronomical studies.

Rydberg Matter as the diffuse interstellar band (DIB) carrier in interstellar space: the model and accurate calculations of band centres

A model for the interpretation of a large number of the 280 diffuse interstellar bands (DIBs) is presented. The bands are proposed to be due to electronic excitation processes in Rydberg Matter leading to doubly excited coplanar Rydberg molecules. Rydberg Matter is composed of atoms and small molecules like H2 in condensed coherent circular Rydberg states. It has been studied in the laboratory especially with laser time-of-flight experiments and in laser Raman spectroscopy studies. Rydberg Matter is used as the lasing medium in an ultra wide-band tunable IR laser (Chem. Phys. Lett. 376 (2003) 812). Rydberg Matter has recently been proposed to be part of the dark matter in the Universe, to be the source of the so called UIR emission bands from interstellar space and to give rise to the Faraday rotation in intergalactic space. The potential energy of the doubly excited coplanar Rydberg states, which are the final states in the DIB transitions, is now calculated accurately in the classical limit. More than 60 of the DIB bands are calculated with good accuracy. The intensity distributions of the transitions are relatively smooth without unexplained missing bands. The model used does not predict too many bands. The calculations are very accurate, in this optimal wavelength region, relative to other calculations in systems of a complexity comparable to Rydberg Matter.

Very high resolution profiles of four diffuse interstellar bands

Ultra-high-resolution (R ∼ 300 000) profiles of four diffuse interstellar bands (DIBs) are presented. The λλ 5797-, 5850-, 6196- and 6379- ˚ A DIBs were observed towards the reddened supergiant HD 24398, a line of sight free of Doppler splitting; thus the observed profiles can be considered as intrinsic to the DIB carriers. Three of the profiles show substructure which supports the hypothesis of a molecular origin for these DIBs.

Time‐dependent Density Functional Theory Calculations of Large Compact Polycyclic Aromatic Hydrocarbon Cations: Implications for the Diffuse Interstellar Bands

We investigate the electronic absorption spectra of several maximally pericondensed polycyclic aromatic hydrocarbon radical cations with time-dependent density functional theory calculations. We find interesting trends in the vertical excitation energies and oscillator strengths for this series containing pyrene through circumcircumcoronene, the largest species containing close to 100 carbon atoms. We discuss the implications of these new results for the size and structure distribution of the diffuse interstellar band carriers.