Profiles Of Diffuse Interstellar Bands Research Articles

Organic molecules in translucent interstellar clouds.

Absorption spectra of translucent interstellar clouds contain many known molecular bands of CN, CH+, CH, OH, OH+, NH, C2 and C3. Moreover, one can observe more than 400 unidentified absorption features, known as diffuse interstellar bands (DIBs), commonly believed to be carried by complex, carbon-bearing molecules. DIBs have been observed in extragalactic sources as well. High S/N spectra allow to determine precisely the corresponding column densities of the identified molecules, rotational temperatures which differ significantly from object to object in cases of centrosymmetric molecular species, and even the 12C/13C abundance ratio. Despite many laboratory based studies of possible DIB carriers, it has not been possible to unambiguously link these bands to specific species. An identification of DIBs would substantially contribute to our understanding of chemical processes in the diffuse interstellar medium. The presence of substructures inside DIB profiles supports the idea that DIBs are very likely features of gas phase molecules. So far only three out of more than 400 DIBs have been linked to specific molecules but none of these links was confirmed beyond doubt. A DIB identification clearly requires a close cooperation between observers and experimentalists. The review presents the state-of-the-art of the investigations of the chemistry of interstellar translucent clouds i.e. how far our observations are sufficient to allow some hints concerning the chemistry of, the most common in the Galaxy, translucent interstellar clouds, likely situated quite far from the sources of radiation (stars).

Automated measurements of diffuse interstellar bands in early-type star spectra

Stellar spectroscopic surveys may bring useful statistical information on the links between Diffuse Interstellar Bands (DIBs) and interstellar environment. DIB databases can also be used as a complementary tool for locating interstellar (IS) clouds. Our goal is to develop fully automated methods of DIB measurements to be applied to extensive data from stellar surveys. We present a method appropriate for early-type nearby stars, its application to high-resolution spectra of 130 targets recorded with ESO FEROS spectrograph, and comparisons with other determinations. Using a DIB average profile deduced from the most reddened stars, we performed an automated fitting of a combination of a smooth stellar continuum, the DIB profile, and, when necessary, a synthetic telluric transmission. Measurements are presented for 16 DIBs in the optical domain that could be extracted automatically: 4726.8, 4762.6, 4963.9, 5780.4, 5797.1, 5849.8, 6089.8, 6196.0, 6203.0-6204.5, 6269.8, 6283.8, 6379.3, 6445.3, 6613.6, 6660.7, and 6699.3 {\AA}.

Diffuse Interstellar Band Profiles

AbstractA short review of the profiles of diffuse interstellar bands and their possible interpretation is given, largely from the perspective of absorption by gas-phase molecules. Potentially a crucial source of information on the nature of the diffuse band carriers, profile studies provide a promising avenue of research towards finding the solution to their identification. Detection and modelling of fine structure in particular offers a valuable benchmark against which any proposed carriers(s) can be tested.

Dirty H2 Molecular Clusters as the DIB Sources: Spectroscopic and Physical Properties

AbstractWe 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.

H2MOLECULAR CLUSTERS WITH EMBEDDED MOLECULES AND ATOMS AS THE SOURCE OF THE DIFFUSE INTERSTELLAR BANDS

We suggest 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. 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 contaminated H2 clusters (CHCs). We show that 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 ∼centimeter-sized, dirty H2 ice balls, called contaminated H2 ice macro-particles (CHIMPs), 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 centimeter-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. We also show that the CHCs offer a natural explanation for the anomalous microwave emission feature in the ∼10–100 GHz spectral region.

Linear and circular spectropolarimetry of diffuse interstellar bands

Context. The identification of the carriers of diffuse interstellar bands (DIBs) remains one of the long-standing mysteries in astronomy. The detection of a polarisation signal in a DIB profile can be used to distinguish between a dust or gas-phase carrier. The polarisation profile can give additional information on the grain or molecular properties of the absorber. In order to detect and measure the linear and circular polarisation of the DIBs we observed reddened lines of sight showing continuum polarisation. For this study we selected two stars HD 197770 and HD 194279. We used high-resolution (R~64.000) spectropolarimetry in the wavelength range from 3700 to 10480 Angstrom with the ESPaDOnS echelle spectrograph mounted at the CFHT. Results. High S/N and high resolution Stokes V (circular), Q and U (linear) spectra were obtained. We constrained upper limits by a factor of 10 for previously observed DIBs. Furthermore, we analysed ~30 additional DIBs for which no spectropolarimetry data has been obtained before. This included the 9577 A DIB and the 8621 A DIB. Conclusions. The lack of polarisation in 45 DIB profiles suggests that none of the absorption lines is induced by a grain-type carrier. The strict upper limits, less than ~0.01%, derived for the observed lines-of-sight imply that if DIBs are due to gas-phase molecules these carriers have polarisation efficiencies which are at least 6 times, and up to 300 times, smaller than those predicted for grain-related carriers.

Doppler splitting in diffuse interstellar bands

We demonstrate evident Doppler splitting in the profiles of many narrow diffuse interstellar bands (DIBs). Their components are compared with those of atomic and molecular lines of interstellar origin. It is shown that it is likely that the species that shares both wavelength shifts and strength ratios of the Doppler components seen in the DIB profiles is the CH radical. Because of this profile resemblance, it is recommended that CH lines can be used to shift spectra to the rest velocity frame for interstellar features, especially to determine the DIB rest wavelengths on the basis of the CH line at 4300.3132 A in the spectrum of HD 147889. Our results strongly support a molecular origin of diffuse bands.

High‐Resolution Profiles of Diffuse Interstellar Bands

High-resolution (R = 115,000) echelle spectra (acquired with the ESO/HARPS spectrograph) of a few OB reddened stars, where no Doppler splitting is observed in interstellar atomic lines, are analyzed. The observed profile shapes allow us to divide the diffuse interstellar bands (DIBs) into several sets, characterized by very similar shapes and widths and thus likely carried by similar molecular species. However, not all DIBs behave in the same way; profiles of broader features remain unchanged (are difficult to divide into sets of similar shapes), while evident differences are seen in the narrow ones. Profiles of DIBs may change slightly from object to object, apparently due to different physical parameters in the intervening clouds; this supports the idea of their molecular origin. We also introduce a new parameter, which is a result of dividing the equivalent width of a diffuse band (in mÅ) by the depth of the feature (in percent). This parameter facilitates grouping of similar DIBs, as well as providing some quantitative information that allows us to estimate the broadness of a feature even for very complex profiles, vastly differing from Gaussian.

Fine structure in the profiles of strong diffuse interstellar bands

High-resolution echelle spectra of five reddened OB stars are used to analyse details of the profiles of diffuse interstellar bands (DIBs) at 4726, 4735, 5418, 5850, 6196, 6376, 6379 A together with the well studied 5797, 5780 and 6614 ones. All targets do not show Doppler splitting in the interstellar CH 4300 band with the applied resolving power R = 115 000. The profiles of DIBs differ from object to object except 5418 which looks invariant in our sample: the DIB is a good candidate for further study of possible isotopic effect. We also found that the width of bands at 5797 and 6614 A probably is more sensitive to the temperature (and density) effects than that of other DIBs in the sample, though almost all diffuse bands are narrowest in most ζ -type object (HD 179406) of the sample. On the other hand, it can be just peculiar effect as in a case of unusual line of sight HD 147889 which demonstrates very broad 5780 and 6614 A diffuse bands what hardly can be explained by temperature effects.

A search for fine structure inside high resolution profiles of weak diffuse interstellar bands

This paper presents a survey of the high-resolution profiles of selected, moderately weak diffuse interstellar bands (DIBs) between 4725 and 6730 A. In very high signal-to-noise spectra, obtained as a result of averaging several individual exposures of reddened, early-type stars that show Doppler splitting of < 2k m s −1 in interstellar gas lines, the profiles seem to have a substructure. This supports the molecular origin hypothesis for DIBs. We studied the profiles of the diffuse interstellar bands at wavelengths of 4726.33, 4963.85, 5418.89, 5541.74, 5544.95, 5546.46, 5762.73, 5766.05, 5769.09, 6439.41, 6445.53, 6449.16, 6729.28 A.

Profiles of Very Weak Diffuse Interstellar Bands around 6440 A

Profiles of very weak diffuse interstellar bands (DIBs) between 6400 and 6470 A observed with high resolution and very high S/N are demonstrated. We show that with the increasing quality of reddened stellar spectra, the whole spectral range is covered with weak or very weak DIBs—at least one every 2-3 A. We also present the details of the profiles of a few stronger features; the presence of substructures resembles the profiles of strong DIBs observed in high resolution during the last decade and supports the hypothesis of a molecular origin of at least a majority of DIBs.

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.

Testing the “strong” PAHs hypothesis

The so-called strong Polycyclic Aromatic Hydrocarbons (PAHs) hypothesis postulates that isolated PAHs, which are thought to be the carriers of the Unidentified Infrared Bands, ought to be also responsible for a large number of Diffuse Interstellar Bands (DIBs). In this framework, the spectral profile of such DIBs should be due to unresolved rotational structure of vibronic absorption bands, the rotation of the molecule being by and large governed by the interaction with the interstellar radiation field. In this paper we quantitatively test the above hypothesis against the observational constraint of DIBs profile invariance, by using Monte-Carlo methods to model the photophysics of a prototypical interstellar PAH, namely the ovalene cation (C32H +). Our results show that the predicted rotational band profiles are remarkably insensitive to both the ambient conditions and the assumed values of some poorly known parameters. The present model therefore offers a quantitative link between any given PAH and the observed DIB profiles, providing a valuable tool for molecular identification.

Fine structure of profiles of weak diffuse interstellar bands

We present a very high resolution (R = 120 000-220 000) analysis of profiles of selected diffuse interstellar bands (DIBs): 4964, 4980, 4985, 5850 A. The 4980 A feature is a very weak one, yet some substructures are proved to be present, as well as within the profiles of another stronger ones where this phenomenon was found several years ago. The presence of substructures inside DIB profiles supports their molecular origin.

Very high resolution profiles of 6196 Å and 6614 Å diffuse interstellar bands

We present a careful analysis of very high resolution (R = 220000) profiles of two well correlated diffuse interstellar bands (DIBs): 6196 and 6614 A observed along 7 lines of sight free of the Doppler splitting in interstellar atomic lines. The high signal-to-noise ratio (S/N) of the spectra (∼600-1000) allows us to draw definite conclusion that the ratio of equivalent widths of the two DIBs is not always exactly the same, i.e. they do not seem to originate at the same carrier. The DIB profiles vary from object to object but the variations are different for every DIB. The width of 6196 DIB may change by up to 1.5 times lowest value while that of 6614 remains almost constant. The broadening of 6196 is not caused by the Doppler splitting, absent in atomic interstellar lines. Possibly this fact reflects a strong dependence of the carrier excitation pattern on small variations of physical parameters. The variations of substructure strength ratios inside the 6614 DIB profile, evident in our spectra, are apparently caused by a mechanism different from that which broadens DIB 6196.

Further Evidence for Carbon Isotopic Structure in the λλ5797 and 6614 Diffuse Interstellar Bands

Profiles of the λλ5797 and 6614 diffuse interstellar bands (DIBs) in our high-resolution spectra (R = 120,000-60,000) of five stars provide strong support for the model proposed by Webster, in which 13C atoms are randomly substituted for 12C in highly symmetric molecules. These stars have narrow interstellar Na D lines, and the DIB profiles can be reproduced, as predicted, by a band of equally spaced, equally wide Gaussians with a Poisson intensity distribution. Other good fits are possible with components of unequal FWHM, but the widths, positions, and relative amplitudes of the components then differ from star to star, which is less credible for a common absorber. The average numbers of 13C atoms per molecule derived from the model are about 0.95 and 1.41 for the λλ5797 and 6614 DIBs, respectively, which correspond to between 40 and 100 carbon atoms per molecule. Sharp features in the cores of the λ5797 DIB are not exactly matched by the Gaussians, which implies a more complex intrinsic structure. For HD 37061 in Orion, both DIBs are weak and have a component on the short-wavelength side of the normal profile, possibly indicating higher rotational temperatures.

Molecular rotational contour fitting of ultra-high-resolution profiles of diffuse interstellar bands

The first application of molecular rotational contour fitting to the analysis of high-resolution profiles of the lambda lambda 6614 and 5797 diffuse interstellar absorption bands is described. A range of selected molecular, electronic and transition dipole symmetries is considered. It is found that the profile of lambda 6614 is well reproduced by a Coriolis-induced perpendicular electronic transition of a planar oblate symmetric top molecule. Based on this result and other considerations, large carbon ring molecules emerge as good candidates for the carriers of the lambda 6614 and (possibly) lambda 5797 bands. Molecular constants, molecular rotational temperatures and linewidth parameters consistent with the observational data for lambda 6614 are reported and compared with those anticipated for candidate diffuse band carriers and with astronomical data obtained from studies of molecules in the same lines of sight. Some discrepancies between observed and fitted profiles are noted.

Diffuse interstellar bands — an observational review (Review)

AbstractThe paper presents a review of observational data concerning the unidentified diffuse interstellar bands (DIBs). The history of the investigation of these fascinating spectral features is presented together with very recent results obtained with the aid of solid state detectors. The high signal‐to‐noise spectra allowed to divide diffuse bands into at least 3 families defined as sets of these features characterized by constant intensity ratios in every observed spectrum. “Intrinsic” profiles of DIBs, i.e. profiles that originated in single interstellar clouds free of any Doppler splitting were derived from high resolution spectra. The same observations revealed convincingly the Doppler structures inside DIB profiles observed in distant objects. It is shown that DIBs may originate in clouds of very low opacity — their agent(s) being thus resistant against diffuse UV radiation. The agent(s) seems not to be the dust causing the visual extinction as the bands may be absent in some of reddened spectra. Relations between DIBs and other interstellar absorptions are also briefly reviewed.

DIFFUSE INTERSTELLAR BANDS

ABSTRACT The paper presents a review of the observational investigations of the diffuse interstellar bands (DIBs) which, despite being the object of considerable research, remain unidentified since 1921. The review is focused on the most recent investigations done with the aid of solid-state detectors which make it possible to obtain high S/N spectra. It is shown that DIBs may be divided into three (or more) families probably caused by different agents. The observed profiles are usually altered by Doppler splitting when reddened stars are observed through several clouds. Also, single clouds may differ considerably in their optical properties. The paper presents 'intrinsic' (i.e., originating in single clouds) high-resolution profiles of several DIBs. These profiles are not dependent on the structure of the interstellar medium toward the observed objects.