Presumed Detection of C₅ Molecule in the Interstellar Medium

Absorption spectra of interstellar clouds contain atomic lines as well as molecular features. The latter are mostly simple two-atom radicals, known since 1937. Some centrosymmetric interstellar molecules were discovered much later as their features are numerous but very shallow. The simple, diatomic, C₂ molecule was detected for the first time by Souza & Lutz (1977) who observed the 1–0 Phillips band ${{\rm{A}}}^{1}{{\rm{\Pi }}}_{u}-{{\rm{X}}}^{1}{{\rm{\Sigma }}}_{g}^{+}$ in the near-IR range. The band was discovered in the spectrum of heavily reddened star Cyg OB2 No. 12 (VI Cyg No. 12, E(B − V) = 3.2). The bands of centrosymmetric species contain many rotational transitions and the energy of the whole band(s) is distributed into many separate, shallow lines. Hobbs (1979) has discovered the 2–0 Phillips band in the spectrum of HD 24398. He also estimated a relatively high (as for the interstellar medium) rotational temperature for this object—78 K. The same band was discovered independently by Chaffee et al. (1980). The shallowness of individual lines of the homonuclear species was the reason why the C₂ molecule was observed much later than the polar species such as CH or CN. The discovery of the C₂ molecule initiated the idea that longer, pure carbon species, may be responsible for the longest standing unsolved puzzle of spectroscopy—the origin of the 100 yr old and still unidentified diffuse interstellar bands (Douglas 1977). The latter idea was based on the argument that chains beyond a certain length are stable against photodissociation and will persist in the low density clouds in which the diffuse interstellar lines are formed.

The next question was: are carbon chains, longer than two atoms, present in interstellar clouds? The first candidate was, of course, C₃. This species was discovered much later than C₂. Its discovery was announced by Maier et al. (2001). They found the C₃, ${{\rm{A}}}^{1}{{\rm{\Pi }}}_{u}-{{\rm{X}}}^{1}{{\rm{\Sigma }}}_{g}^{+}$ band near 4051 Å, i.e., in the blue spectral range, in the spectra of HD 179406, HD149757, and HD24398. The expected lines of C₂ ⁻, C₂ ⁺, and C₃ ⁻ remained below the level of detection. The C₃, ${{\rm{A}}}^{1}{{\rm{\Pi }}}_{u}-{{\rm{X}}}^{1}{{\rm{\Sigma }}}_{g}^{+}$ band was soon after discovered in HD 148184, HD 149757, and HD 152236 by Galazutdinov et al. (2002). It was thus proved that the C₃ molecules are quite popular in the interstellar H i clouds. The most extensive survey of the rotational spectra of the C₃ molecule was made by Ádámkovics et al. (2003). They proved that rotational temperatures of this species may vary in a pretty broad range.

The next chains that could be expected in the interstellar space are, of course, C₄ and C₅. After the positions of their spectral features have been determined in the laboratory (Motylewski et al. 1999; Boguslavskiy & Maier 2006), the observational tests were made as well. Only one feature of C₅, namely 4975, was found in the averaged spectra of several reddened stars (Galazutdinov et al. 2001); the other one (5109), broader but also shallower, remained below the level of detection. The latter was also not discovered in the spectrum of the well known star: ζOph (Maier et al. 2002). The results were thus negative. Neither the C₄ band, near 3789 Å nor the C₅ band near 5109 Å were detected. However, as we mentioned in Galazutdinov et al. (2001), "it should be emphasized that our observations are restricted to interstellar clouds of reasonably low optical depth. The molecular abundances could easily be higher toward heavily reddened stars. However, such objects, being severely obscured and thus attenuated, are too faint to be observed with high resolution and high signal-to-noise ratio (S/N)." Moreover, heavily reddened stars are often shining through many clouds—likely of different physical parameters. Thus one can expect to discover spectral features of long carbon chains in spectra of heavily reddened stars, observed through the medium free of Doppler splitting in atomic and/or molecular lines (of identified species).

In this Paper reasonably high-resolution spectra of two such objects are being analyzed. HD 166734 and HD 80077 were selected from the existing spectral European Southern Observatory (ESO) databases. Their spectral types are very different, which facilitates distinguishing between stellar and interstellar features. These make it possible to detect even very weak (shallow) interstellar spectral features.

Our observational data have been collected using two high-resolution, echelle spectrographs:

• 1.  
  Ultraviolet and visual echelle spectrograph fed by the 8 m Kueyen Very Large Telescope (VLT) mirror (Dekker et al. 2000). The spectral resolution (R = λ/Δλ) is up to 80,000 in the blue range and 110,000 in the red one. The spectral range (3040–10400 Å) is usually divided into six subranges. The telescope size allows to get high S/N spectra of even pretty faint stars. The star HD 80077 was observed using this instrument.
• 2.  
  FEROS spectrograph, which is fed by the 2.2 m ESO LaSilla telescope (Kaufer et al. 1999). It allows to record in a single exposure the spectral range from 3600 to 9200 Å, divided into 39 echelle orders. The resolution of Feros spectra is λ/Δλ = 48,000. The spectrum of HD 166734 were recorded using this instrument.

The raw data were processed and measurements were made in the reduced spectra with the interactive analysis software DECH (Galazutdinov 2019). For the DECH data reduction, we averaged bias images for subsequent correction of all other images. The scattered light was determined as a complex-shaped two-dimensional surface function, which is individually calculated for each stellar and flat-field frame by a two-dimensional cubic-spline approximation over areas of minima between the spectral orders. Then, the pixel-to-pixel variations across the charge-coupled device (CCD) were corrected by dividing all stellar frames by the averaged and normalized flat-field frame. One-dimensional stellar spectra were extracted by simple summation in the cross-dispersion direction along the width of each spectral order. The extracted spectra of the same object, observed in the same night, were averaged to achieve the highest S/N. Fiducial continuum normalization was based on a cubic-spline interpolation over the interactively selected anchor points.

Figure 1 demonstrates the lack of Doppler splitting in the potassium lines, observed in the spectra of our targets. It shows also very high reddening of both stars. The profiles look like different shapes but this follows different resolutions of both spectrographs. The equivalent widths of K i line in both objects are nearly identical. Both spectra have been shifted to the zero radial velocity of the latter line.

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Figure 2 demonstrates the spectra of both targets, covering the ranges of the two expected C₅, ${{\rm{A}}}^{1}{{\rm{\Pi }}}_{u}-{{\rm{X}}}^{1}{{\rm{\Sigma }}}_{g}^{+}$ features: 4975 and 5109 Å. According to Galazutdinov et al. (2001), the former one should be narrower and deeper.

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Figure 2 clearly shows that the two considered targets are of very different spectral types (effective temperatures). It is evident that some spectral lines can be seen only in one of the targets; the stellar lines, seen in both spectra, are not at the same positions and the radial velocities of both stars are not identical. Spectral features, showing the same wavelengths in both spectra are evidently stationary, i.e., interstellar. Note that both spectra were shifted to the zero radial velocity scale of interstellar features using the CH lines depicted in Figure 1.

In the expected position of the 5109 C₅ feature one can see a very specific line (band). The feature demonstrates a very extended blue wing (Figure 3). No such shapes are observed among other diffuse interstellar bands. The deeper part of the feature was marked as 5110.9 Å by Hobbs et al. (2008). Thus, the whole band can be presented as a blend of two independent profiles where their central wavelengths are slightly shifted. The blue component seems to coincide with the laboratory 5109.4 Å C₅ feature. The 5109.4 band is inextricably intertwined with the 5111 diffuse interstellar band (DIB). The strength ratio of the abovementioned features is apparently variable. The C₅ band occupies only the wing of the DIB (Figure 3). Thus it is hardly possible to compare directly the strength ratio of the C₅ bands with those from laboratory where the stellar contaminations do not exist.

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The above figures show that the C₅ species likely exists in the interstellar space. One can observe the two features exactly at the same positions, despite the stellar contaminations and different radial velocities of the two stars. Thus they are evidently interstellar. One can see only a part of the 5109 feature, being apparently much broader than the 4975 one in accordance with the illustration by Galazutdinov et al. (2001). The proposed C₅ bands are very weak (shallow) and, moreover (especially 5109) contaminated with other interstellar features. Thus the observed strength ratios of the bands can hardly mimic that observed in the laboratory. The chosen targets are single cloud ones (i.e., they do shine through one cloud each; Figure 1) and are heavily reddened. Thus the two observed interstellar clouds are likely very dense, which facilitates molecule formation. Most likely the abovementioned spectral features of C₅ can be observed in other objects, providing they shine through sufficiently dense clouds and the spectra are of sufficiently high resolution and S/N. The column densities of the C₅ molecule are, naturally, very low. Using the oscillator strength of the the 5109 band from Maier et al. (2002), i.e., f = 0.001, we got the N(C₅) = 3.1 × 10¹⁰ cm² for HD 80077 and N(C₅) = 5.5 × 10¹⁰ cm² for HD 166734. Motylewski et al. (1999) mention two additional C₅ features: at 4939.1 and 5038.1 Å. They are, however, expected to be even weaker than the above described very weak bands. If the strongest bands are at the edge of detection it seems natural that other ones are below the level of it. Thus there is nothing to be presented.

The applied spectra, being high resolution and high S/N ones, allow to trace weak C₅ features providing the latter are originated in dense, single, interstellar clouds. The attempts to detect C₄ band near 3789 Å also gave a negative result. Very likely longer carbon chains may be more stable or resistant than the shorter ones. High-resolution and high S/N observations seem necessary to make sure that long carbon chains are present everywhere in the interstellar space. Unfortunately such observations are very time consuming and thus the available material is currently very scarce. The existing data suggest that both C₅ features can be detected and that the 5109 C₅ band is blended with the 5111 diffuse band.

The authors acknowledge the financial support of the National Science Centre, Poland (grant No. 2017/25/B/ST9/01524) for the period 2018–2023. This research has made use of the services of the ESO Science Archive Facility and the SIMBAD database, operated at CDS, Strasbourg, France (Wanger et al. 2000).