Diffuse Interstellar Bands Research Articles

The optical and NIR spectrum of the Crab pulsar with X-shooter

Context. Pulsars are well studied all over the electromagnetic spectrum, and the Crab pulsar may be the most studied object in the sky. Nevertheless, a high-quality optical to near-infrared (NIR) spectrum of the Crab or any other pulsar has not been published to date. Aims. Obtaining a properly flux-calibrated spectrum enables us to measure the spectral index of the pulsar emission, without many of the caveats from previous studies. This was the main aim of this project, but in addition we could also detect absorption and emission features from the pulsar and nebula over an unprecedentedly wide wavelength range. Methods. A spectrum was obtained with the X-shooter spectrograph on the Very Large Telescope. Special care was given to the flux-calibration of these data. Results. A high signal-to-noise spectrum of the Crab pulsar was obtained from 300 nm to 2400 nm. The spectral index fit to this spectrum is flat with αν = 0.16 ± 0.07. For the emission lines we measured a maximum velocity of ∼1600 km s−1, whereas the absorption lines from the material between us and the pulsar is unresolved at the ∼50 km s−1 resolution. A number of diffuse interstellar bands and a few NIR emission lines that have previously not been reported from the Crab are highlighted.

Stellar population astrophysics (SPA) with the TNG

Aims. The Scutum complex in the inner disk of the Galaxy hosts a number of young clusters and associations of red supergiant stars that are heavily obscured by dust extinction. These stars are important tracers of the recent star formation and chemical enrichment history in the inner Galaxy. Methods. Within the SPA Large Programme at the TNG, we secured GIANO-B high-resolution (R ≃ 50 000) YJHK spectra of 11 red supergiants toward the Alicante 7 and Alicante 10 associations near the RSGC3 cluster. Taking advantage of the full YJHK spectral coverage of GIANO in a single exposure, we were able to measure several hundreds of atomic and molecular lines that are suitable for chemical abundance determinations. We also measured a prominent diffuse interstellar band at λ1317.8 nm (vacuum). This provides an independent reddening estimate. Results. The radial velocities, Gaia proper motions, and extinction of seven red supergiants in Alicante 7 and three in Alicante 10 are consistent with them being members of the associations. One star toward Alicante 10 has kinematics and low extinction that are inconsistent with a membership. By means of spectral synthesis and line equivalent width measurements, we obtained chemical abundances for iron-peak, CNO, alpha, other light, and a few neutron-capture elements. We found average slightly subsolar iron abundances and solar-scaled [X/Fe] abundance patterns for most of the elements, consistent with a thin-disk chemistry. We found depletion of [C/Fe], enhancement of [N/Fe], and relatively low 12C/13C < 15, which is consistent with CN cycled material and possibly some additional mixing in their atmospheres.

Probing the missing link between the diffuse interstellar bands and the total-to-selective extinction ratio $R_V\,\!-\!$ I. Extinction versus reddening

ABSTRACT The carriers of the still (mostly) unidentified diffuse interstellar bands (DIBs) have been a long-standing mystery ever since their first discovery exactly 100 yr ago. In recent years, the ubiquitous detection of a large number of DIBs in a wide range of Galactic and extragalactic environments has led to renewed interest in connecting the occurrence and properties of DIBs to the physical and chemical conditions of the interstellar clouds, with particular attention paid to whether the DIB strength is related to the shape of the interstellar extinction curve. To shed light on the nature and origin of the DIB carriers, we investigate the relation between the DIB strength and RV, the total-to-selective extinction ratio, which characterizes how the extinction varies with wavelength (i.e. the shape of the extinction curve). We find that the DIB strength and RV are not related if we represent the strength of a DIB by its reddening-normalized equivalent width (EW), in contrast to the earlier finding of an anticorrelation in which the DIB strength is measured by the extinction-normalized EW. This raises a fundamental question about the appropriate normalization for the DIB EW. We argue that the hydrogen column density is a more appropriate normalization than extinction and reddening.

Electronic spectroscopy of carbon chains (C2n+1, n = 7–10) of astrophysical importance. II. Quantum dynamics

In continuation with Paper I [S. R. Reddy et al., J. Chem. Phys. 151, 054303 (2019)], the vibronic structure and dynamics of the 1Σu+ electronic state of C15, C17, C19, and C21 chains in the coupled manifold of 1Σu+–1Πg–1Πu– 1Σg+ electronic states have been investigated in this paper. The model vibronic Hamiltonian developed through extensive ab initio quantum chemistry calculations in Paper I is employed, and first principles nuclear dynamics calculations are carried out to obtain the photoabsorption band of the 1Σu+ electronic state. Both time-independent and time-dependent quantum mechanical calculations are carried out to precisely locate the vibrational levels, assign them with the progression of vibrational modes, and elucidate the impact of both Renner-Teller and pseudo-Renner-Teller couplings on them. The nonradiative decay of the 1Σu+ electronic state is studied, and it is found that the decay rate is comparable with the prediction made for them to be qualified as a carrier of diffuse interstellar bands in the literature. The theoretical results are found to be in good accord with the available experimental results.

Electronic spectroscopy of carbon chains (C2n+1, n = 7–10) of astrophysical importance. I. Quantum chemistry

Carbon chains have been predicted to be potential carriers of diffuse interstellar band features in astrophysical observations. Motivated by numerous predictions, we set out to carry out extensive ab initio quantum chemistry calculations to establish the ground and excited electronic potential energy surfaces and their coupling surfaces for carbon chains containing an odd number of carbon atoms (C2n+1, n = 7–10). Vibronic coupling models are established with the aid of the calculated electronic energies to investigate nuclear dynamics from first principles. The latter are reported in Ghosh et al. [J. Chem. Phys. 151, 054304 (2019)]. The mentioned carbon chains possess a linear cumulenic structure at the equilibrium minimum of their electronic ground state, and an electronic excited state of the Σu+1 term appears to be extremely bright optically and absorbs in the visible region of the electromagnetic spectrum. Vertical excitation energy of this state decreases and transition dipole moment increases, and as a result, the oscillator strength of this state linearly increases with an increase of the chain length. There are states belonging to 1Πg, 1Πu, Σg+1, 1Δg, and 1Δu terms, in the immediate vicinity of the Σu+1 state, which are optically dark but can gain intensity through vibronic coupling with the optically bright Σu+1 state. Construction of a coupling scheme considering the Renner-Teller coupling within the degenerate Π states and pseudo-Renner-Teller coupling between the Renner-Teller split component states as well as with the nondegenerate Σ states is another motivation of this work. The coupled-state Hamiltonian is constructed in a diabatic electronic basis in terms of the dimensionless normal coordinates of the vibrational modes of the carbon chains. Both Renner-Teller and pseudo-Renner-Teller types of couplings are included in the Hamiltonian. The theoretical results are discussed in relation to the experimental findings.

Updated extraction of the APOGEE 1.5273 μm diffuse interstellar band: a Planck view on the carrier depletion in dense cores

Context. Constraining the spatial distribution of diffuse interstellar band (DIB) carriers and their links with gas and dust are mandatory steps in understanding their role in interstellar chemistry. Aims. The latest SDSS/APOGEE data release, DR14, has provided an increased number of stellar spectra in the H band and associated stellar models using an innovative algorithm known as the Cannon. We took advantage of these novelties to extract the 15 273 Å near-infrared DIB and to study its link with dust extinction and emission. Methods. We modified our automated fitting methods dedicated to hot stars and used in earlier studies with some adaptations motivated by the change from early- or intermediate-type stars to red giants. A new method has also been developed to quantify the upper limits on DIB strengths. Careful and thorough examinations were carried out of the DIB parameters, the continuum shape, and the quality of the adjustment of the model to the data. We compared our DIB measurements with the stellar extinctions, AV, from the Starhorse database. We then compared the resulting DIB–extinction ratio with the dust optical depth derived from Planck data, both globally and separately for nearby off-plane cloud complexes. Results. Our analysis has led to the production of a catalog containing 124 064 new measurements of the 15 273 Å DIB, allowing us to revisit the correlation between DIB strength and dust reddening. The new data clearly reveal that the sky-averaged 15 273 Å DIB strength is linearly correlated with AV over two orders as reported by earlier studies but leveling off with respect to extinction for highly reddened lines of sight behind dense clouds. The comparison with Planck individual optical depths reveals in a conspicuous way this DIB depletion in the dense cores and shows it applies to all off-plane dense clouds. Using selected targets located beyond the Orion, Taurus, and Cepheus clouds, we derived empirical relationships between the DIB–extinction ratio and the Planck dust optical depth for the three cloud complexes. Their average is similar to the DIB carrier depletion measured in the dark cloud Barnard 68. Conclusions. APOGEE measurements confirm the ubiquity of the 15 273 Å DIB carrier decrease with respect to dust grains in dense cloud cores, in a manner that can be empirically related to the dust optical depth reached in the cloud. They also show that the ratio between the DIB equivalent width and the extinction AV for sightlines with τ(353GHz) ≲ 2 × 10−5 that do not contain dense molecular gas is about four times higher than the constant limit towards which the ratio tends for very long sightlines with many diffuse and dense phases distributed in distance.

A search of diffuse interstellar bands (DIBs) in planetary nebulae environment

Diffuse Interstellar Bands (DIBs) are set of absorption features, mostly at optical and near infrared wavelengths, that are found in the spectra of reddened stars and other objects. DIBs are actively being investigated to understand the nature of their properties and carriers and also their roles in interstellar matter enrichment. Among of the proposed carriers is large carbonaceous molecules (fullerenes). Interestingly, such as C60 and C70 have been detected in Planetary Nebulae (PN) environment. This detection allows us to study DIBs in fullerene-rich space environment. In this work, we study the presence of DIBs in central star of planetary nebulae (CSPN) by using archival data of optical spectra observed with 8.1-m Gemini South Telescope and Gemini Multi-Object Spectrographs (GMOS, spectral range of 3900 Å - 5900 Å and resolution of R ∼ 2300) and also from 2.5-m Isaac Newton Telescope with IDS spectrograph (R ∼ 2300). We measured the equivalent width of 4430, 5870 and 6283 Å DIB by fitting the detected DIB profile with the empirical template derived from higher resolution data. We investigated 4430, 5870 and 6283 Å DIBs in the spectra of CSPN and their correlation with the colour excess (E(B-V)). The correlation between the DIB strength and E(B-V) has positive value. We found that the 4430 and 5870 Å DIBs are generally more abundant on the PN environment.

Tracing Galactic Arm with 6614 Å Diffuse Interstellar Band and Gaia DR2 Distances

Enigmatic problem in astronomical spectroscopy is diffuse interstellar bands (DIBs). DIBs are ∼ 500 weak and broad absorption features seen in the spectra of stars or other astronomical objects located behind interstellar materials. Despite the unknown ions or molecules that caused the absorptions, except for two near infrared DIBs, DIBs can be used as a potential interstellar medium (ISM) tracer to map Galactic structure. Gaia is a mission of the European Space Agency (ESA) which was launched on 2013. It provides precise distances which greatly improve our knowledge of the structure and history of our Galaxy. In this work, we used previously determined DIB equivalent width (EW) measurements by means of DIB fitting that is a combination of a synthetic stellar spectrum, a synthetic telluric transmission, and empirical DIB profile to the Gaia–ESO Large Public Spectroscopic Survey data. From DIB measurements from stellar spectra of distributed target stars and together with Gaia DR2 distances, we investigated the ISM distribution along particular line of sight, i.e., (l,b)∼(213°, -2°) which probes Local and Perseus Arm.

A Search for Extragalactic Diffuse Interstellar Bands: SAMI Data

Diffuse interstellar bands (DIBs) are puzzling absorption features that can be found in the spectra of reddened objects in our Galaxy, as well as in other observed galaxies. Although we still know too little of the carriers of DIBs, the numerous features along the optical and near infrared wavelengths and the consistency of their measured properties make DIBs potentially promising interstellar material tracers. DIBs studies are mostly based on stellar spectra in our Galaxy, but since DIBs can also be found in other galaxies, we search for DIBs in the spectra of nearby galaxies by perusing Sydney-AAO Multi-object Integral-field unit (SAMI) data. We demonstrate DIB measurement by performing an automated fitting of a combination of a smooth continuum and a model of DIB profile to the spectrum. This preliminary result will be an important input to consider in drawing conclusion about DIBs and their environments.

Cracking an interstellar mystery

Abstract Michael McCabe celebrates the centenary of the first observation of diffuse interstellar bands with an overview of the lengthy – and incomplete – search for their origins.

A three-dimensional map of the hot Local Bubble using diffuse interstellar bands

The Solar System is located within a low-density cavity, known as the Local Bubble, which appears to be filled with an X-ray emitting gas at a temperature of 10$^6$ K. Such conditions are too harsh for typical interstellar atoms and molecules to survive. There exists an enigmatic tracer of interstellar gas, known as Diffuse Interstellar Bands (DIB), which often appears as absorption features in stellar spectra. The carriers of these bands remain largely unidentified. Here we report the three-dimensional structure of the Local Bubble using two different DIB tracers ($\lambda$5780 and $\lambda$5797), which reveals that DIB carriers are present within the Bubble. The map shows low ratios of $\lambda$5797/$\lambda$5780 inside the Bubble compared to the outside. This finding proves that the carrier of the $\lambda$5780 DIB can withstand X-ray photo-dissociation and sputtering by fast ions, where the carrier of the $\lambda$5797 DIB succumbs. This would mean that DIB carriers can be more stable than hitherto thought and that the carrier of the $\lambda$5780 DIB must be larger than that of the $\lambda$5797 DIB. Alternatively, small-scale denser (and cooler) structures that shield some of the DIB carriers must be prevalent within the Bubble, implying that such structures may be an intrinsic feature of supernova-driven bubbles.

The Apache Point Observatory Catalog of Optical Diffuse Interstellar Bands

Abstract Aiming for a new and more comprehensive DIB catalog between 4000 and 9000 Å, we revisited the Atlas Catalog based on the observations of HD 183143 and HD 204827. Twenty-five medium to highly reddened sight lines were selected, sampling a variety of spectral types of the background star and the interstellar environments. The median signal-to-noise ratio (S/N) of these spectra is ∼1300 around 6400 Å. Compared to the Atlas Catalog, 22 new DIBs were found, and the boundaries of 27 (sets of) DIBs were adjusted, resulting in an updated catalog containing 559 DIBs that we refer to as the Apache Point Observatory Catalog of Optical Diffuse Interstellar Bands. Measurements were then made based on this catalog. We found our survey most sensitive between 5500 and 7000 Å, due largely to the local S/N of the spectra, the relative absence of interfering stellar lines, and the weakness of telluric residuals. For our data sample, the number of DIBs detected in a given sight line is mostly dependent on E B−V and less on the spectral type of the background star. Some dependence on the molecular fraction f H2 is observed, but it is less well determined owing to the limited size of the data sample. The variations of the wavelengths of each DIB in different sight lines are generally larger than those of the known interstellar lines CH+, CH, and K i. Those variations could be due to the inherent error in the measurement, or to differences in the velocity components among sight lines.

Red Rain Cells of Kerala as a Possible Carrier of the Diffuse Interstellar Bands and the UV Extinction Bump

Red Rain Cells of Kerala as a Possible Carrier of the Diffuse Interstellar Bands and the UV Extinction Bump

On the visible continuum and bands in the interstellar extinction curve

This work purports to help understand the InterStellar Extinction Curve in and near the visible range. In this range, crystalline materials are known to be transparent, so amorphous dust is needed. Molecular modelling experiments are used to compute the electronic spectra of various, relatively large, carbon and silicate structures. Hardly any transition shows up beyond 0.4 μm when the structure is in its ground state (the lowest, most stable state, usually crystalline). This is no longer the case as soon as the structure is distorted in any way. Examples of simulated distortions (or ‘defects') are: angular or linear bond alteration, insertion of free radicals near the main structure, dangling bonds; their cumulative effects lead to the amorphous state. It is shown that, in this state, a structure bears a majority of weak transitions and a minority of strong ones. As the structure grows in size, the former ultimately form a weak continuum already detected experimentally, in the visible, on amorphous carbons and silicates. The stronger transitions will manage to emerge above the continuum, especially when they bunch together by chance near the same wavelength. Parallels are drawn between several properties of the computed continua and transitions and the observed continuum and Diffuse Interstellar Bands. .

Polyacenes and diffuse interstellar bands

The identification of the carriers of the diffuse interstellar bands (DIBs) remains to be established, with the exception of five bands attributed to C60+, although it is generally agreed that DIB carriers should be large carbon-based molecules (with ~10–100 atoms) in the gas phase, such as polycyclic aromatic hydrocarbons (PAHs), long carbon chains or fullerenes. The aim of this paper is to investigate more specific possible carriers among PAHs, namely elongated molecules, which could explain a correlation between the DIB wavelength and the apparent UV resilience of their carriers. More specifically, we address the case of polyacenes, C4N+2H2N+4, with N ~ 10–18 fused rectilinear aligned hexagons. Polyacenes are attractive DIB carrier candidates because their high symmetry and large linear size allow them to form regular series of bands in the visible range with strengths larger than most other PAHs, as confirmed by recent laboratory results up to undecacene (C46H26). Those with very strong bands in the DIB spectral domain are just at the limit of stability against UV photodissociation. They are part of the prominent PAH family of interstellar carbon compounds, meaning that only ~10−5 of the total PAH abundance is enough to account for a medium-strength DIB. After summarizing the limited current knowledge about the complex properties of polyacenes and recent laboratory results, the likelihood that they might meet the criteria for being carriers of some DIBs is addressed by reviewing the following properties: wavelength and strength of their series of visible bands; interstellar stability and abundances, charge state and hydrogenation; and DIB rotation profiles. No definite inconsistency has been identified that precludes polyacenes from being the carriers of some DIBs with medium or weak strength, including the so-called C2 DIBs. But, despite their many interesting properties, additional experimental data about long acenes and their visible bands are needed to make robust conclusions.

Diffuse Interstellar Bands in the High-Resolution GAOES Data

Diffuse interstellar bands (DIBs) are an enigmatic set of interstellar absorption features, observed at optical and near infrared wavelengths in the spectra of reddened stars in our Galaxy also in other galaxies. Identifying the carriers of DIBs has been a longstanding challenge and is still very much a work in progress. In recent years, surveys of DIBs have been conducted, but mostly focus on strong bands. In this work, we extracted and measured DIBs recorded in the high-resolution stellar spectra of ε Auriga (ε Aur) and IQ Per observed by using Gunma Astronomical Observatory Echelle Spectrograph (GAOES) (spectral resolution R∼65000). Observations were aimed at obtaining high resolution spectra of double-lined detached eclipsing binaries. For each selected DIB, we performed spectral fitting of a combination of a smooth stellar continuum (polynomial function), an empirical DIB model, and a synthetic telluric transmission. The high-quality data allow us to measure not only strong and well-known DIBs, but also weaker DIBs.

Diffuse Interstellar Bands (DIBs) as a tracer of warp in the Third Galactic Quadrant: Preliminary Results

We measured 6613.6 Å diffuse interstellar band (DIB) recorded in the 318 stellar spectra of stars located in the Third Galactic Quadrant. In agreement with previous studies, we found that the DIB strength has a positive correlation with the interstellar extinction, even though significant deviations were found. The correlation study between the DIB strength and the extinction is limited by the fact that the 3D extinction survey in the Galaxy is incomplete. The DIB spatial distribution toward the South Galactic Pole indicates that the DIB carrier(s) are distributed similarly as other Galactic warp tracer. More data, especially DIB measurements from target stars above Galactic principal plane, are needed to demonstrate that DIB can be used as a tracer of Galactic warp.

The relation between 5780 and 5797 diffuse interstellar bands, CH/CH+ molecules, and atomic or molecular hydrogen

Correlations between column densities of neutral and molecular hydrogen and strengths of major 5780 and 5797 Å diffuse interstellar bands (DIBs) based on spectra of 66 OB stars were analyzed. We confirm that the 5797 Å DIB is more tightly correlated with column density of molecular hydrogen while the 5780 DIB – with that of atomic hydrogen. This leads to a reasonably tight relation between the molecular fraction of H2 and equivalent width ratio of the 5797 and 5780 major diffuse bands with correlation coefficient equal to 0.77 ± 0.05. Column densities of CH and CH+ molecules were used to analyze correlations between abundances of CH/CH+ molecules and strengths of the major 5780 and 5797 DIBs. The 5780 DIB is better correlated with the column density of methydyline cation than the 5797 DIB. A relation with correlation coefficient equal to 0.95 ± 0.02 based on precise column densities, between column densities of CH and H2 molecules, is also presented; in other words, the column densitiy ratio in the case of H2 and CH molecules in the ISM is equal to (2.01 ± 0.09) × 107.

Pyrene radical cation and the diffuse interstellar band at 4430 Å: A theoretical survey

Multistate and multimode vibronic dynamics on the first six coupled (X˜2B1g - A˜2B2g - B˜2B3u - C˜2Au - D˜2B3u - E˜2B3g ) electronic states of pyrene radical cation are investigated with the aid of ab initio electronic structure calculations of potential energy surfaces (PESs) and first principle quantum dynamical methods. Several low-lying conical intersections among various electronic states are found and their impact on the nuclear dynamics is discussed. The simulation of nuclear motion on the underlying coupled electronic states of pyrene radical cation is examined by both time-independent and time-dependent quantum mechanical methods. The combination of time-independent and time-dependent quantum mechanical simulations render a satisfactory explaination of a large part of the complex vibronic spectrum of Py+ which is in very good agreement with the photoelectron spectroscopic data. Furthermore, this study also establishes that polycyclic aromatic hydrocarbons can be the molecular carrier of diffused interstellar bands and ultrafast non-radiative deactivation of their excited electronic states makes them photostable against strong UV irradiation in the interstellar medium.

Confirming Interstellar C60 + Using the Hubble Space Telescope

Abstract Recent advances in laboratory spectroscopy lead to the claim of ionized Buckminsterfullerene (C ) as the carrier of two diffuse interstellar bands (DIBs) in the near-infrared. However, irrefutable identification of interstellar C requires a match between the wavelengths and the expected strengths of all absorption features detectable in the laboratory and in space. Here we present Hubble Space Telescope (HST) spectra of the region covering the C 9348, 9365, 9428, and 9577 Å absorption bands toward seven heavily reddened stars. We focus in particular on searching for the weaker laboratory C bands, the very presence of which has been a matter for recent debate. Using the novel STIS-scanning technique to obtain ultra-high signal-to-noise spectra without contamination from telluric absorption that afflicted previous ground-based observations, we obtained reliable detections of the (weak) 9365, 9428 Å and (strong) 9577 Å C bands. The band wavelengths and strength ratios are sufficiently similar to those determined in the latest laboratory experiments that we consider this the first robust identification of the 9428 Å band, and a conclusive confirmation of interstellar .