Abstract.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 HD 147889. 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 HD 147889 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 HD 147889 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 HD 147889 (and also of course for any other diffuse/translucent cloud) depends strongly on the molecular symmetry and size (number of π 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 HD 147889 is constrained by the observed DIB spectrum to be Gaussian with a mean of 50 carbon atoms.

Detection of Diffuse Interstellar Bands in the Magellanic Clouds Ehrenfreund, P.; Cami, J.; Jimenez-Vicente, J.; Foing, B.H.; Kaper, L.; van der Meer, A.; Cox, N.L.J.; d' Hendecourt, L.; Maier, J.P.; Salama, F.ABSTRACT With the Ultraviolet Visual Echelle Spectrograph mounted at the Very Large Telescope, we have observed at unprecedented spectral resolution the absorption spectrum toward reddened stars in the Magellanic Clouds over the wavelength range of 3500-10500 Å . This range covers the strong transitions associated with neutral and charged large carbon molecules of varying sizes and structures. We report the first detection of diffuse interstellar bands (DIBs) at 5780 and 5797 Å in the Small Magellanic Cloud and the variation of the 6284 Å DIB toward several targets in the Large Magellanic Cloud. The variation of DIBs in the Magellanic Clouds compared with Galactic targets may be governed by a combination of the different chemical processes prevailing in low-metallicity regions and the local environmental conditions.

Recent advances in laboratory spectroscopy lead to the claim of ionized Buckminsterfullerene (C + 60 ) as the carrier of two diffuse interstellar bands (DIBs) in the near-infrared. However, irrefutable identification of interstellar C + 60 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 + 60 9348, 9365, 9428 and 9577 Å absorption bands toward seven heavily-reddened stars. We focus in particular on searching for the weaker laboratory C + 60 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 + 60 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 C + 60 .

In this work we present the results of a systematic search for diffuse bands (DBs, hereafter) in the circumstellar envelopes of a carefully selected sample of post-AGB stars. We concentrated on analyzing 9 of the DBs most commonly found in the interstellar medium. The strength of these features is determined using high-resolution optical spectroscopy, and the results obtained are compared with literature data on field stars affected only by interstellar reddening. Based on the weak features observed in the subsample of post-AGB stars dominated by circumstellar reddening, we conclude that the carrier(s) of these DBs must not be present in the circumstellar environment of these sources, or at least not under the excitation conditions in which DBs are formed. This conclusion is applicable to all the post-AGB stars studied, irrespective of the dominant chemistry or the spectral type of the star considered. A detailed radial velocity analysis of the features observed in individual sources confirms this result, as the Doppler shifts measured are found to be consistent with an interstellar origin.

Aims. Three-dimensional (3D) maps of Galactic interstellar dust are a tool for a wide range of uses. We aim to construct 3D maps of dust extinction in the Local Arm and surrounding regions. Methods. To do this, Gaia EDR3 photometric data were combined with 2MASS measurements to derive extinction towards stars with accurate photometry and relative uncertainties on EDR3 parallaxes of less than 20%. We applied our hierarchical inversion algorithm adapted to inhomogeneous spatial distributions of target stars to this catalogue of individual extinctions. Results. We present the updated 3D dust extinction distribution and provide an estimate of the error on integrated extinctions from the Sun to each area in the 3D map. The full computational area is similar to the one of the previous DR2 map, that is to say with a 6 × 6 × 0.8 kpc3 volume around the Sun. Due to the addition of fainter target stars, the volume in which the clouds can be reconstructed has increased. Due to the improved accuracy of the parallaxes and photometric data in EDR3, extinctions among neighbouring targets are more consistent, allowing one to reach an increased contrast in the dense areas, while cavity contours are more regular. We show several comparisons with recent results on dust and star distributions. The wavy pattern around the Plane of the dust concentrations is better seen and exists over large regions. Its mean vertical peak-to-peak amplitude is of the order of 300 pc; interestingly, it is similar to the vertical period of the spectacular snail-shaped stellar kinematical pattern discovered in Gaia data. Conclusions. The Gaia EDR3 catalogue allows for a significant improvement of the extinction maps to be made, both in extent and quality. The hierarchical technique confirms its efficiency in the inversion of massive datasets. Future comparisons between 3D maps of interstellar matter and stellar distributions may help to understand which mergers or internal perturbations have shaped the Galaxy within the first 3 kpc.

Emission of fullerenes in their infrared vibrational bands has been detected in space near hot stars. The proposed attribution of the diffuse interstellar bands at 9577 and 9632 Å to electronic transitions of the buckminsterfullerene cation (i.e. C + 60 ) was recently supported by new laboratory data, confirming the presence of this species in the diffuse interstellar medium (ISM). In this Letter, we present the detection, also in the diffuse ISM, of the 17.4 and 18.9 µm emission bands commonly attributed to vibrational bands of neutral C 60 . According to classical models that compute the charge state of large molecules in space, C 60 is expected to be mostly neutral in the diffuse ISM. This is in agreement with the abundances of diffuse C 60 we derive here from observations.

Abstract. High-resolution optical spectra of the high-mass X-ray binary 4U 1907+09 covering the wavelength range 4680−10 400 Å indicate that the X-ray pulsar's massive companion is an O8/O9 supergiant with a dense stellar wind. The interstellar atomic lines of Na  and K  of this strongly reddened source (E (B−V) = 3.45 mag) are used to set a lower limit to its distance: d ≈ 5 kpc, excluding the possibility that the massive companion is a Be star. We re-evaluate the system parameters; the new spectral classification of 4U 1907+09's O supergiant companion is discussed in the context of its similarity in X-ray and orbital properties to Wray 15-977/GX 301-2. The Hα line shows variability similar to what is observed in other wind-fed systems. A remarkable feature is the strong He  4686 Å emission line, which possibly originates in the accretion flow towards the X-ray pulsar.

Abstract. 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 Å are identified. We show that the observed 5797 Å DIB strength is in line with the Galactic correlation with reddening, whereas the 5780 Å 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 C 2 rotational transitions. Members of one DIB family (5797, 6379 Å and 6196, 6613 Å) 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  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 ), respectively, using strongly reddened sightlines towards another four distant HMXBs. The 5780 Å DIB, and tentatively also the 5797 and 6613 Å DIBs, are better correlated to the gas tracer H  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  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.