Weak Diffuse Interstellar Bands Research Articles

Extended correlations between diffuse interstellar bands

The systematic analysis of the correlations between diffuse interstellar bands (DIBs) is extended to weak DIBs through the comprehensive catalogue of the Apache Peak Observatory (APO) of 559 DIBs in 25 lines of sight with diverse interstellar properties. The main results are the following: 1) An extension of the number of DIBs identified to be related to C2, that is, those that need very shielded interstellar regions for their carriers to survive UV photo-dissociation. Based on the correlations with the reference C2 and ζ DIBs, anticorrelations with UV-favoured (σ) DIBs, and the strength ratios in shielded and unshielded sight lines, we propose 12 new C2 candidates and 34 possible “C2-related” DIBs (mostly at λ <5950 Å) in addition to the ~20 known confirmed C2 DIBs. With these additions, the census of C2 DIBs might approach completion. 2) We discovered that the intensities of a large set of poorly studied DIBs are strongly enhanced in one or two of the sight lines of HD 175156 and HD 148579. This tentative class, denoted χ for the time being, extends over the whole wavelength range of visible DIBs. It might include up to 50–100 members, half at λ > 6000 Å, and a number of C2 DIBs. These possible enhancements might reflect specific formation processes of their carriers that are yet to be identified in the interstellar medium of these two sight lines. The possible matches of the wavelength of five very broad DIBs, including three χ DIBs, with the strong bands that were recently measured by action spectroscopy might favour some long carbon chains and rings as carriers of some DIBs. These correlations and findings justify further theoretical and laboratory efforts for improving our understanding of the complex physics, spectroscopy, and chemistry of the various carbon chains and rings, and their possible formation and destruction in the diffuse interstellar medium.

Electronic Transitions Responsible for C60+ Diffuse Interstellar Bands.

Diffuse interstellar bands (DIBs) are puzzling absorption features believed to contain critical information about molecular evolution in space. Despite the fact that C60+ recently became the first confirmed carrier of several DIBs, the nature of the corresponding transitions is not understood. Using electronic structure methods, we show that the two strong C60+ DIBs cannot be explained by electronic transitions to the two different excited 2 E1 g states or the two spin-orbit components of the lowest 2 E1 g state, as suggested before. We argue that the strong DIBs at 9632 and 9577 Å correspond to the cold excitations from the non-Franck-Condon region of the ground electronic state to the two components of the lowest 2 E1 g state split by Jahn-Teller distortion. The weak DIBs at 9428 and 9365 Å are assigned to the first vibronic transitions involving the low-energy vibrational modes and components of the lowest 2 E1 g electronic state.

The EDIBLES survey II. The detectability of C60+ bands

Gas phase spectroscopic laboratory experiments for the buckminsterfullerene cation C60+ have resulted in accurate rest wavelengths for five C60+ transitions that have been compared with diffuse interstellar bands (DIBs) in the near infra-red. Detecting these in astronomical spectra is difficult because of the strong contamination of ground-based spectra by atmospheric water vapor, to the presence of weak and shallow stellar lines and/or blending with other weak DIBs. The detection of the two strong bands has been claimed by several teams, and the three additional and weaker bands have been detected in a few sources. Certain recent papers have argued against the identification of C60+ based on spectral analyses claiming (i) a large variation in the ratio of the equivalent widths of the 9632 and 9577 Å bands, (ii) a large redshift of the 9632 Å band for the Orion star HD 37022, and (iii) the non-detection of the weaker 9428 Å DIB. Here we address these three points: (i) We show that the model stellar line correction for the 9632 Å DIB overestimates the difference between the strengths of the lines in giant and dwarf star spectra, casting doubts on the conclusions about the ratio variability. (ii) Using high quality stellar spectra from the ESO Diffuse Interstellar Bands Large Exploration Survey (EDIBLES), recorded with the ESO/Paranal Ultraviolet Echelle Spectrograph (UVES) in about the same atmospheric conditions, we find no wavelength shift in the 9632 Å band toward HD 37022. (iii) Using EDIBLES spectra and data from the Echelle SpectroPolarimetric Device for the Observation of Stars (ESPaDOnS) at CFHT we show that the presence of a weak 9428 Å band cannot be ruled out, even in the same observations that a previous study claimed it was not present.

Near-infrared diffuse interstellar bands in APOGEE telluric standard star spectra

Aims. Information on the existence and properties of diffuse interstellar bands (DIBs) outside the optical domain is still limited. Additional infra-red (IR) measurements and IR-optical correlative studies are needed to constrain DIB carriers and locate various absorbers in 3D maps of the interstellar matter. Methods. We extended our study of H-band DIBs in Apache Point Observatory Galactic Evolution Experiment (APOGEE) Telluric Standard Star (TSS) spectra. We used the strong λ15273 band to select the most and least absorbed targets. We used individual spectra of the former subsample to extract weaker DIBs, and we searched the two stacked series for differences that could indicate additional bands. High-resolution NARVAL and SOPHIE optical spectra for a subsample of 55 TSS targets were additionally recorded for NIR/optical correlative studies. Results. From the TSS spectra we extract a catalog of measurements of the poorly studied λλ15617, 15653, and 15673 DIBs in ≃300 sightlines, we obtain a first accurate determination of their rest wavelength and constrained their intrinsic width and shape. In addition, we studied the relationship between these weak bands and the strong λ15273 DIB. We provide a first or second confirmation of several other weak DIBs that have been proposed based on different instruments, and we add new constraints on their widths and locations. We finally propose two new DIB candidates. Conclusions. We compared the strength of the λ15273 absorptions with their optical counterparts λλ5780, 5797, 6196, 6283, and 6614. Using the 5797–5780 ratio as a tracer of shielding against the radiation field, we showed that the λ15273 DIB carrier is significantly more abundant in unshielded (σ-type) clouds, and it responds even more strongly than the λ5780 band carrier to the local ionizing field.

Spatial structure of several diffuse interstellar band carriers

Diffuse interstellar bands (DIBs) hold a lot of information about the state and the structure of the ISM. Structure can most directly be observed by extensive spectroscopic surveys, including surveys of stars where DIBs are especially important, as they are conveniently found in all observed bands. Large surveys lack the quality of spectra to detect weak DIBs, so many spectra from small regions on the sky have to be combined before a sufficient signal-to-noise ratio (SNR) is achieved. However, the clumpiness of the DIB clouds is unknown, which poses a problem, as the measured properties can end up being averaged over a too large area. We use a technique called Gaussian processes to accurately measure profiles of interstellar absorption lines in 145 high SNR and high resolution spectra of hot stars. Together with Bayesian MCMC approach we also get reliable estimates of the uncertainties. We derive scales at which column densities of 18 DIBs, CH, CH$^+$, Ca I, and Ca II show some spatial correlation. This correlation scale is associated with the size of the ISM clouds. Scales expressed as the angle on the sky vary significantly from DIB to DIB between $\sim0.23^\circ$ for the DIB at 5512 {\AA} and 3.5$^\circ$ for the DIB at 6196 {\AA}, suggesting that different DIB carriers have different clumpiness but occupy the same general space. Our study includes lines-of-sight all over the northern Milky Way, as well as out of the Galactic plane, covering regions with different physical conditions. The derived correlation scales therefore represent a general image of the Galactic ISM on the scales of $\sim5$ pc to $100$ pc.

VISIBLE ABSORPTIONS OF POTENTIAL DIFFUSE ISM HYDROCARBONS: C9H9 AND C9H5 RADICALS

ABSTRACT The laboratory detection of previously unobserved resonance-stabilized C9H5 and C9H9 radicals in the supersonic expansion of a hydrocarbon discharge source is reported. The radicals are tentatively assigned as acetylenic-substituted cyclopentadienyl C9H5 and vinyl-substituted benzyl C9H9 species. They are found to feature visible absorption bands that coincide with a few very weak diffuse interstellar bands toward HD183143 and HD204827.

Using Machine Learning to classify the diffuse interstellar bands

Using over a million and a half extragalactic spectra we study the correlations of the Diffuse Interstellar Bands (DIBs) in the Milky Way. We measure the correlation between DIB strength and dust extinction for 142 DIBs using 24 stacked spectra in the reddening range E(B-V) < 0.2, many more lines than ever studied before. Most of the DIBs do not correlate with dust extinction. However, we find 10 weak and barely studied DIBs with correlations that are higher than 0.7 with dust extinction and confirm the high correlation of additional 5 strong DIBs. Furthermore, we find a pair of DIBs, 5925.9A and 5927.5A which exhibits significant negative correlation with dust extinction, indicating that their carrier may be depleted on dust. We use Machine Learning algorithms to divide the DIBs to spectroscopic families based on 250 stacked spectra. By removing the dust dependency we study how DIBs follow their local environment. We thus obtain 6 groups of weak DIBs, 4 of which are tightly associated with C2 or CN absorption lines.

Electronic Spectroscopy of Carbon Chains and Rings of Astrophysical Interest

This perspective is concerned with laboratory measurements of the electronic spectra of carbon chains, rings, and their ions, including derivatives terminated by hydrogen and nitrogen atoms. The selected-species have relevance to astronomical observations through diffuse clouds, absorption features known as diffuse interstellar bands (DIBs). Two indications to decide which molecules should be studied are the observations of polar carbon chains in dense clouds by rotational spectroscopy and the knowledge that a certain number of these have electronic transitions in the DIB region. This information has been obtained initially by measurements of the electronic absorptions in 6 K neon matrixes using mass-selection. This was followed by the gas-phase observations using cavity ringdown and resonance enhanced techniques in combination with pulsed-supersonic discharge sources or via laser vaporization. The gas-phase spectra were then compared with DIB data, all with negative results, except for the detection of C3, but leading to upper limits of their column densities <10(12) cm–2. By reference to mm-wave absorption measurements in the diffuse medium, it is shown that, although species such as H2C3 are present there, the product of the expected column densities and oscillator strength of the transitions will lead to only very weak DIBs. The significant conclusion is that carbon chains and their derivatives containing hydrogen or nitrogen comprising up to a dozen atoms cannot be responsible for stronger DIBs. However, chains with an odd-number of carbon atoms, C17, C19, ···, have very intense transitions in the region above 4400 Å and remain attractive candidates. An uncertainty is the excited electronic state lifetime; if this is less than 70 fs, then the resulting absorptions would be too broad to be astronomically relevant. The electronic absorptions of some of the species studied bear a striking resemblance to DIB data. The two peaked rotational contour of the origin band in the electronic transition of dicyanoacetylene cation is superimposable on a DIB absorption when shifted by 1 Å. The band profiles of cyclic C18 at 100 or 20 K are similar to DIBs but differ in wavelength. This suggests that another set of potential candidates are the carbon rings of sizes up to a hundred of atoms, including ions and heavy atoms, with the requirement of a large oscillator strength. Observations on the absorptions of propadienylidene C3H2 and C60+ are discussed.

IDENTIFICATION OF H2CCC AS A DIFFUSE INTERSTELLAR BAND CARRIER

We present strong evidence that the broad, diffuse interstellar bands (DIBs) at 4881 and 5450\,\AA are caused by the $B\,^1$B$_1$\,$\leftarrow$\,$X\,^1$A$_1$ transition of H$_2$CCC (l-C$_3$H$_2$). The large widths of the bands are due to the short lifetime of the $B\,^1$B$_1$ electronic state. The bands are predicted from absorption measurements in a neon matrix and observed by cavity ring-down in the gas phase and show exact matches to the profiles and wavelengths of the two broad DIBs. The strength of the 5450\,\AA DIB leads to a l-C$_3$H$_2$ column density of $\sim5\times10^{14}$ cm$^{-2}$ towards HD\,183143 and $\sim2\times10^{14}$\,cm$^{-2}$ to HD\,206267. Despite similar values of $E$($B-V$), the 4881 and 5450\,\AA DIBs in HD\,204827 are less than one third their strength in HD\,183143, while the column density of interstellar C$_3$ is unusually high for HD\,204827 but undetectable for HD\,183143. This can be understood if C$_3$ has been depleted by hydrogenation to species such as l-C$_3$H$_2$ towards HD\,183143. There are also three rotationally resolved sets of triplets of l-C$_3$H$_2$ in the 6150$-$6330\,\AA region. Simulations, based on the derived spectroscopic constants and convolved with the expected instrumental and interstellar line broadening, show credible coincidences with sharp, weak DIBs for the two observable sets of triplets. The region of the third set is too obscured by the $\alpha$-band of telluric O$_2$.

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.

On the diffuse bands related to the C2interstellar molecule

The recently published idea that intensities of some weak diffuse interstellar bands (DIBs) are related to the C2 molecule column density have been examined. We use a set of high quality echelle spectra of heavily reddened stars, acquired at the Bohyunsan Optical Astronomical Observatory (South Korea), with a resolution . The high quality (high S/N ratio) of our spectra is proved by the fact that despite using the most widely used Phillips (2, 0) band of the C2 molecule (near 8760 Å), we can trace the (3, 0) Phillips band (near 7725 Å) as well. Equivalent widths of four (5176, 5542, 5546 and 5769 Å) out of 16 examined DIBs demonstrate relatively good correlation with C2 column density. However, a majority of the studied DIBs, already reported as “C2” ones, most likely are not related to this simplest carbon molecule. A removal of peculiar objects like HD 34078 from the analyzed sample does not substantially change the level of correlations.

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 Toward HD 62542

Diffuse interstellar bands (DIBs) have been detected for the first time along the peculiar translucent line of sight toward HD 62542, which passes through a diffuse cloud core. Although only a small fraction (18 out of more than 300) of generally weak DIB features have been shown to correlate with and C_3 (the C_2 DIBs), it is predominantly these DIBs that are observed toward HD 62542. The typically strong DIBs λλ5780 and 5797 are detected but are significantly weaker than toward other lines of sight with similar reddening. Other commonly observed DIBs (such as λλ4430, 6270, and 6284) remain noticeably absent. These observations further support the suggestion that the line of sight toward HD 62542 crosses only the core of a diffuse cloud and show that the correlation between the DIBs and small carbon chains is maintained in environments with very large fractions of molecular hydrogen, f_(H_2) > 0.8. A comparison of CH, CN, C_2, and C_3 column densities and DIB strengths toward HD 62542, HD 204827, and HD 172028 suggests that the line of sight toward HD 204827 passes through a diffuse cloud core similar to that seen toward HD 62542, as well as what might be referred to as a diffuse cloud envelope. This indicates that the bare core toward HD 62542 may not have significantly different relative chemical abundances from other diffuse cloud cores and that the DIBs may serve as a diagnostic of such cores.

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.

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.

Profiles of the λ6196 and λ6379 Diffuse Interstellar Bands

We have looked for rotational structure in the sharp λ6196 and λ6379 diffuse interstellar bands (DIBs) at a resolution ~120,000 in seven stars where the interstellar λ7699 K I line is unresolved. The λ6196 DIB is bell-shaped with a flat core and differs slightly in width from star to star. It is accompanied by a weak DIB at λ6194.7, with which it does not maintain a constant depth ratio. The λ6379 DIB is asymmetric with a sharp double core, but the profile hardly varies between the stars apart from being undetectable for HD 37061. Weak features connect it to the weaker λ6376 DIB, with which it varies in unison. Simple rotational models do not fit the observed profiles of λ6196 and λ6376 at all well because of more prominent branch structure in the models. We achieve an acceptable fit by arbitrarily convolving the modeled profiles with Gaussians (0.2 to 0.3 cm-1). The Gaussians correspond to an unexpected or anomalous broadening process that cannot be explained by the interstellar velocity distribution or the instrumental point-spread function. The fits give upper limits to the ratio of the rotational temperature to the moment of inertia of the molecular carriers. If the former lies in the range 10-100 K, then the molecules must be large, with moments of inertia comparable with that of the fullerene C60. We present evidence that the anomalous broadening of the rotational profiles is intramolecular in origin, but it is not easily explained by broadening processes previously invoked in connection with the DIBs. We suggest that some other process such as a zero-point vibrational isotope shift may be involved that could be characteristic of many of the narrow bands.

The Search for Interstellar C60

The optical region of a number of reddened O-type stars has been examined on Keck I HIRES spectrograms (R = 45,000) for evidence of interstellar C60. No absorption features were detected near the laboratory C60 wavelengths 3857 and 3980 A. An interstellar feature is present at 6220.8 A, but it is unacceptably far from the laboratory gas-phase wavelength of 6217.5 A. It is probably just another of the weak diffuse interstellar bands (DIBs), which are numerous in that spectral region. The most astronomically promising C60 feature was measured in the laboratory at 3284 A in liquid or solid matrices. Its gas-phase wavelength can be inferred either from matrix shifts of C60 features at longer wavelengths or from high-temperature gas-phase measurements. On that evidence, the interstellar feature could fall anywhere between about 3244 and 3306 A. Its width is uncertain but here is taken to be about 1 A . No interstellar absorption fitting these specifications and as strong as 16 mA has been detected in the stars observed, including Cyg OB2/8A of E(B-V) = 1.60. It follows that in that particular line of sight and for the assumed FWHM of 1 A , N(C60) < 4.5 × 1011 cm-2. However, some recent laboratory spectroscopy suggests that its width may be very much larger, in which case this limit would be invalid. At this upper limit, the corresponding number of carbon atoms contained in neutral C60 indicates that that molecule would be only a minor contributor to the total amount of C in that direction, and would be less than 1% of the amount that may be tied up in the DIBs. Stronger C60 bands are known in the laboratory at approximately 2110 and 2566 A, but the upper limit on 3284 A suggests that they will not be easy to detect without high resolution, high signal-to-noise ratio (S/N) satellite spectroscopy and better laboratory gas-phase wavelengths. An estimate of the column density of C, under the assumption that the 9577, 9632 A bands are indeed due to C and that the laboratory f-value is correct, indicates that the C/C60 abundance in that line of sight is greater than 100.

Narrow Diffuse Interstellar Bands: A Survey with Precise Wavelengths

ABSTRACTWe present an atlas of 271 diffuse interstellar bands (DIBs) between 4460 and 8800 Å based on echelle spectra (R = 45,000), of which more than 100 are new DIBs discovered in this survey. The atlas is restricted mostly to narrow features, and we describe the tests for an interstellar origin. The rest wavelength of each DIB was determined in a high‐quality composite spectrum of the star HD 23180 using the interstellar Na i (D1 and D2) lines to establish the radial velocity of the single intervening cloud. DIB wavelengths are quoted to 0.01 Å and are probably accurate to ∼0.1 Å. Other, weak DIBs found in the spectra of heavily reddened stars are included with a lower wavelength precision.

GROUPINGS OF STRONG AND WEAK DIFFUSE INTERSTELLAR BANDS FORMED BY COMMON CARRIERS

ABSTRACT The paper presents a discussion of the relations between strong and weak unidentified interstellar features in the spectral range 5760 and 5860 Å, i.e., between the two strong stellar lines of Si III (5740 Å) and He 1(5876 Å). The analysis is made by means of making average spectra (to raise the signal-to-noise ratio) of rapidly rotating stars which diffuses their spectral lines, leaving only interstellar features in the considered range. We divide our sample of stars, shining through apparently single interstellar clouds, into two subsamples characterized by very different strength ratios of the two prominent diffuse bands at 5780 and 5797 Å. By normalizing both spectra to the central depths of different strong diffuse bands, we are able to show that the weak features are apparently closely related to them.

A survey of weak diffuse interstellar bands in selected ranges between 5600 and 7000 Å

The paper describes briefly the history of the discovery of diffuse interstellar bands (D] Bs) and their “younger relatives”, weak unidentified interstellar features, recently, discovered and described in scientific literature. The features of both kinds form a verv rich spectrum of interstellar clouds, the spectrum varying, however, from object to object. The high quality echetle spectra acquired at McDonald Observatory (R = 60,000 ; S/N = 500) indicate that both strong and weak features form several subsets (“families”) which can be considered as spectra of single species populating the interstellar clouds. This fact is important as any two of the well-known. strong DlBs hardly belong to one such “family” which makes the identification of the DIB carriers so difficult. The paper also discusses the difficulties of the detection of the weak features, especially when one deals with very shallow features. Hundreds of prospectively observed weak unidentified features can give us an important insight into the nature of interstellar clouds, especially their apparently very complicated chemistry.