Diffuse bands 9577 and 9633 -- relations to other interstellar features
aa r X i v : . [ a s t r o - ph . GA ] F e b Diffuse bands 9577 and 9633 – relations to other interstellar features
G.A. Galazutdinov
Instituto de Astronomia, Universidad Catolica del Norte Av. Angamos 0610, Antofagasta, ChilePulkovo Observatory, Pulkovskoe Shosse 65, Saint-Petersburg 196140, RussiaSpecial Astrophysical Observatory of the Russian AS, Nizhnij Arkhyz 369167, Russia [email protected]
G. Valyavin
Special Astrophysical Observatory of the Russian AS, Nizhnij Arkhyz 369167, Russia
N.R. Ikhsanov
Pulkovo Observatory, Pulkovskoe Shosse 65, Saint-Petersburg 196140, Russia
J. Krełowski
Materials Spectroscopy Laboratory, University of Rzeszów, Pigonia 1 Street, 35-310, Rzeszów,Poland
ABSTRACT
We study, for the first time, the relations of two strong diffuse bands (DIBs) at 9633and 9577 Å, commonly attributed to C +60 , to other interstellar features seen in opticaland UV spectra including H i , Ca i , Fe ii , Na i , Ti ii , CN, CH, CH + , and C and DIBs5780, 5797, 6196, 6269, 6284, and 6614. We analyzed 62 lines of sight where the stellarcontamination by Mg ii was corrected or found negligible for DIB 9633. Equivalentwidths of DIB 9577 were measured in 62 lines of sight. Poor mutual correlation betweenthe strengths of the above features and the major diffuse bands (5780 and 5797) aswell as with other DIBs (with some exceptions) were revealed. The considered DIBsare also poorly correlated with the features of neutral hydrogen, molecular carbon, andthose of simple interstellar radicals. Perhaps this phenomenon can be explained if thediffuse band 9577 is an unresolved blend of two or more interstellar features. There areindications that 9633 and 9577 diffuse bands are stronger in σ -type clouds, i.e. thesefeatures resemble the behavior of reasonably broad DIBs, which are strong in the linesof sight where the UV flux from the very hot nearby stars plays an important role. Subject headings:
ISM: atoms, molecules – lines and bands 2 –
1. Introduction
Two relatively strong and broad interstellar features near the diffuse bands (DIBs) 9577 and9633 were first proposed to be possibly carried by the C +60 molecule by Foing & Ehrenfreund (1994).The features are situated in the near-infrared (near-IR) region of the spectrum, heavily contaminatedby telluric lines. A determination of their exact profiles and strengths requires thus a telluric linedivisor. Moreover, the DIB 9633 feature is inextricably intertwined with the stellar Mg ii line(Galazutdinov et al. (2000); Galazutdinov et al. (2017)); the latter is negligibly weak only in thespectra of the hottest (O-type) stars.Cami et al. (2010) reported the first detection of infrared emissions carried by neutral C ,seemingly present on the surfaces of solid material (dust particles). It was observed only in thevicinity of the peculiar planetary nebula Tc1. Independently, Sellgren et al. (2010) reported thepresence of neutral C in the NGC 7023 reflection nebula illuminated by the B star HD 200775.The molecule was detected only in the regions closest to the star. All these reports support the ideaof a presence of the buckminsterfullerene cation in translucent interstellar clouds.The reasonably recent publications by Campbell et al. (2015, 2016a, 2016b) restarted thediscussion on whether the “soccer ball” molecule may carry the abovementioned near-IR spectralfeatures. According to Campbell et al. (2015), C +60 exhibits four relatively strong spectral lines,centered at 9365.9 ± ± ± ± ii stellar line from the DIB 9633 profile. Thelatter was done by means of calculating non-LTE synthetic spectra of the atmospheres of hot starsused to estimate the strength of the contaminating line with respect to another non–blended one,near 4481 Å (see details in Galazutdinov et al. 2017). The result was discouraging: the 9577/9633strength ratio seems to be severely variable.Another problem is the presence of weaker bands seen in laboratory spectra. Their relativeequivalent width ratios should mimic the observed ones. Here the problem of telluric contaminationis even more severe. The expected features are weak, while the telluric lines are strong; thuscomplete elimination of telluric contamination is not always possible. The identification of the C +60 molecule in translucent interstellar clouds was disputed by Galazutdinov & Krełowski (2017); theobserved strength ratios of the postulated C +60 features do not follow the laboratory predictions ofCampbell et al. (2016a). The observed strength ratios were demonstrated to be variable, i.e. notthe same from object to object. In their following publication, Campbell & Maier (2018) changedthe laboratory spectrum: the C +60 lines are at 9365.2 ± ± ± ± +60 feature is weaker than that at 9365 Å in contrast to the results reported in 2015. This newspectrum matches the observed interstellar one better with one caution: the very weak 9428 Å DIB 3 –is severely contaminated by telluric lines.The latter problem should be solved if the observations are done outside the atmosphere. Thisis the subject of a recent publication by Cordiner et al. (2019). The authors used the spectrographonboard the Hubble Space Telescope (HST) to record the near-IR spectra of seven reddened andfour unreddened stars. Despite their claim that C +60 is eventually discovered beyond a doubt, theformerly missing feature (9428) is clearly seen only in BD +40 4220, albeit without detailed analysisof possible stellar contamination. The presence of the 9428 Å feature in other objects is doubtful.Moreover, the HST spectrograph does not cover the second strongest C +60 DIB – the 9633 one. Thusthe authors of the latter paper were unable to analyze the relation between the two strong bands.We mentioned some problems with this in Galazutdinov & Krełowski (2017). The lack of the majorDIB 9633 in the observed wavelength range reduces the significance of the reported conclusions.Apparently the statistics of the existing measurements are a serious problem. One needs moretargets to convince everybody that the claimed (especially weak) features really exist and that theirstrength ratios mimic the laboratory predictions. In this paper we present a sample of reddenedtargets collected to check the possible relations of the proposed C +60 features to other interstel-lar ones. We emphasize that almost all interstellar features are correlated, in many cases quitetightly (Moutou et al. 1999). Diffuse bands usually demonstrate good mutual correlations with the6196 – 6614 relation being the “champion” (Krełowski et al. 2016). However, the latter publicationwarns that even a very tight correlation does not guarantee that the two features share a carrier(Krełowski et al. 2020).Walker (2015) state that the weak C +60 bands near 9366 and 9428 Å can be traced in the spectraof HD183143 (only 9366) and HD169454 (both). Walker (2016) reported the detection of diffusebands 9632, 9577, 9428, 9365 and 9348 towards the stars HD 46711, HD 169454, HD 183143. Inany case the conclusions were based on very small samples. It is the natural consequence of the factthat IR DIBs (especially weak ones) can be traced only in the spectra of heavily reddened targets.In this paper we present an analysis of the largest sample so far of targets with measured near-IRinterstellar features attributed to C +60 . We carefully corrected not only the telluric line contaminationbut also that caused by stellar lines (Galazutdinov et al. 2017). The latter contamination posesspecial difficulties while dealing with broad interstellar features with a width comparable to stellarlines. Here we use the results from Galazutdinov et al. (2017) supplemented with measurements ofhot objects lacking a strong Mg ii effect.We also analyze possible relations between the two strong infrared bands (C +60 ) and the otherwell-known DIBs, as well as abundances of simple interstellar molecules and atoms. 4 –
2. Spectral data
Our observations have been collected using several high resolving power echelle spectrographs.They are as follows.• The Ultraviolet and Visual Echelle Spectrograph (UVES) is fed with the 8m Kueyen VLTmirror (ESO, Paranal, Chile; (Dekker et al. 2000)). The spectral resolving power reachesR=80,000 in the blue range and R=110,000 in the red. The telescope size allows one to gethigh signal-to-noise ratio (S/N) spectra even for rather faint stars.• The Fiber-fed Extended Range Optical Spectrograph (FEROS) is fed with the 2.2m MPG/ESOtelescope (ESO, LaSilla) (Kaufer et al. 1999). It allows one to record in a single exposure aspectral range of 3600 to 9200 Å divided into 39 echelle orders. The resolving power ofFeros spectra is R=48,000. Feros spectra cover a broad wavelength range per each order,which makes the spectrograph a very useful tool for checking the spectral types and lumi-nosity classes of the observed targets and for measuring DIBs and atomic/molecular features.However, the C +60 features are out of its range.• The Echelle SpectroPolarimetric Device for the Observation of Stars (ESPaDOnS) spectro-graph is a bench-mounted high resolving power echelle spectrograph/spectropolarimeter, at-tached to the 3.58m Canada-France-Hawaii telescope at Maunakea (Hawaii, USA). It is de-signed to obtain a complete optical spectrum in the range of 3700 to 10050 Å. The wholespectrum is divided into 40 echelle orders. The resolving power is about 68,000. Spectra fromESPaDOnS were obtained during the runs 05Ao5 (in 2010, PI B. Foing) and 15AD83 (in 2015,PI G. Walker). The high altitude (more than 4 km) and exceptionally low humidity on the sitemake Mauna Kea an ideal place for the study of IR DIBs. The quality of telluric line removalin ESPaDOnS spectra is the best among all the spectral data we have used. Unfortunately,the major DIB 9633 is out of the observed spectral range in the spectra from ESPaDOnS. Thisexplains why the number of the DIB 9633 measurements is lower than that for 9577 (Tab. 1).• The Bohyunsan Echelle Spectrograph (BOES) of the Korean National Observatory (Kim et al.2007) is installed at the 1.8m telescope of the Bohyunsan Observatory in Korea. The spectro-graph has three observational modes allowing resolving powers of 30,000, 45,000, and 90,000.In any mode, the spectrograph covers the whole spectral range of ∼ ∼ ∼ m with a sufficient ( ∼ codes. The spectral resolutions provided by the abovementioned instruments arenot identical but all are high enough to precisely measure the strengths of atomic and molecularinterstellar lines and, especially, of the broad DIBs.Measuring the DIBs 9633 and 9577 is not a simple procedure because of strong telluric lineremnants and, sometimes, a high level of noise. In most cases, to measure the equivalent width, weused the manual profile fit method: a cubic spline fit over manually set profile points (see Fig. 1).The method provides the possibility to measure lines of any complex, irregular shape. The resultingdata for the DIBs 9577 and 9633 are presented in Tab.1. The equivalent width errors were estimatedusing Eq. 7 from Vollmann & Eversberg (2006) where both spectral noise and uncertainty of thecontinuum normalization are taken into account. The near-IR wavelength range suffers strong contamination by telluric lines. To eliminate thecontamination we applied the classical method based on the use of a divisor — a spectrum of ahot, unreddened, and preferably rapidly rotating star. The method permits one to adjust both thepositional and intensity differences of the telluric lines in the studied object and divisor spectra.We used several stars as telluric line divisors, always trying to observe the one closest to the chosentarget. As a divisor we used spectra of Spica (HD 116658, B1V), HD120315 (B3V), HD218045(B9III) and some other targets with characteristics satisfying the requirements given above. Ourmethod is described in detail by Galazutdinov et al. (2017). Here we used the same procedures,only the number of analyzed targets is more than twice as large as in Galazutdinov et al. (2017).Complex and variable spectra of the chosen divisors, when used for the removal of telluriclines, may introduce unwanted distortions into the resulting profiles of relatively broad bands, e.g.9633, since real spectra contain stellar lines. On the other hand the telluric lines (mostly of theatmospheric H O) are in many cases saturated, which makes their removal practically impossible.Thus some remnants of the telluric spectrum are still seen in our resultant spectra. With suchsmall contamination we can still reliably measure the chosen DIBs (manually fitting the profile tothe undisturbed points). However, in many cases the telluric contamination is the main source ofuncertainty for the equivalent width measurements given in Tab. 1.It is worth mentioning that the stellar contamination of the DIB 9633 depends on the effective ii line from the profileof the DIB 9633 was discussed broadly in Galazutdinov et al. (2017). The applied method wascriticized by Lallement et al. (2018) who claimed that the ratio of the two strong features, 9577/9633correlates with the surface gravity of the star (see their Fig. 1), i.e. the stellar contaminationeffect is not properly addressed. However, the linear fit, shown in the abovementioned figure, wascalculated neglecting the weighting procedure by the individual error bars. If the straight line iscalculated using proper weights the trend disappears, i.e. the determinations of stellar parametersby Galazutdinov et al. (2017) are correct and we use these values also in the current paper (see Fig.2). However, to improve the statistics, we have added additional targets. Some of them are very hotstars (similar to HD76341) where the Mg ii contamination to the DIB 9633 profile is negligibly small,although the equivalent width of the DIB 9633 may be slightly overestimated. For cooler targetswe have measured the DIB 9577 only. It is of basic importance to check whether the strength ratioof the two strong DIBs 9577 and 9633 is close or not to the laboratory predictions. If not, the ideathat C +60 is their carrier must be postponed until we have a lab spectra of C +60 with a variable ratioof these major features due to the variation of some physical/chemical parameter, e.g. rotationaltemperature.
3. Results
Using our sample of reddened stars we measured the DIB 9577 in 62 objects and the DIB 9633in 43 objects. In some of our targets the latter DIB is not available or is contaminated by the Mg ii stellar line (Galazutdinov et al. 2017). In our sample of DIB 9633 measurements we included onlythe hottest stars where the stellar contamination is negligible. If both the 9633 and 9577 bands,attributed to C +60 , share the same carrier, their ratio should be similar to that obtained in thelaboratory measurements and thus their strengths should correlate tightly. The correlation betweenthe two features, shown in Fig. 3, is surprisingly poor in contrast to the expectations. Moreover, thecorrelation coefficient between the DIBs 9633 and 9577 is among the lowest for any pair of diffusebands. Perhaps this can be partially explained by the relatively high uncertainty of the measuredequivalent width due to the imperfection of the telluric line removal procedure. However, as is shownin the bottom part of Fig.3, the variability of the 9577/9633 ratio cannot be completely explainedby the influence of telluric lines. It is worth mentioning that all of the correlation coefficients werecalculated with the measurement error σ taken as a weight in the form of 1/ σ .The poor correlation between the DIBs 9577 and 9633 casts doubts on whether they are of thesame origin, i.e. whether they do represent the spectrum of the C +60 molecule. We now check to seehow both DIB carriers are possibly related to other interstellar species.It is certainly of interest to investigate whether C +60 is mixed well with the interstellar atomichydrogen. In Fig. 4 we try to relate both DIBs to the hydrogen column density taken fromthe compilation of Gudennavar et al.(2012), based on measurements of absorptions observed by 7 – N o r m a li z ed I n t en s i t y Wavelength (¯)HD 78344, 9577 DIB
Fig. 1.— An example of the equivalent width measurement procedure with a manually set diffuseband profile (red).Table 1: Observed targets and equivalent widths of the DIBs 9633 and 9577 (mÅ). Spectral type,luminosity class and v sin i are taken from the SIMBAD database. T eff , log g are from Galazutdinovet al. (2017). Object SpL or Teff/lgg v sin i v sin i ±
15 107 ±
10 HD148379 17000/1.7 51 80 ±
11 137 ± ±
16 168 ±
11 HD148605 20500/4.2 145 119 ±
35 80 ± >
200 263 ±
62 350 ±
78 HD148937 O6fp 143 ±
23 130 ± ±
14 368 ±
150 HD149038 O9.7Iab 52 100 ±
27 86 ± ±
14 235 ±
55 HD149757 O9.2IVnn 303 96 ±
30 72 ± ±
45 HD150136 O4III+O8 108 ±
27 120 ± ±
70 HD151804 O8Iaf 72 126 ±
19 75 ± ±
54 HD152408 O8Iape 110 ±
20 95 ± ±
22 HD152424 OC9.2Ia 59 161 ±
50 136 ± ±
46 77 ±
34 HD153919 O6Iafcp 114 ±
24 110 ± ±
17 50 ±
30 HD155806 O7.5V 52 86 ±
40 80 ± ±
22 70 ±
40 HD167264 29000/3.2 82 82 ±
20 60 ± ±
15 71 ±
20 HD167971 O8Iaf(n)+O4/5 65 178 ±
13 190 ± ±
14 94 ±
40 HD168607 B9Iaep 335 ± ±
23 67 ±
35 HD168625 B6Iap 194 ±
25 320 ± ±
20 60 ±
20 HD169454 21000/2.1 39 130 ±
20 82 ± ±
20 HD170740 21000/3.9 40 150 ±
20 93 ± ±
17 94 ±
47 HD183143 11500/1.4 37 105 ±
20 300 ± ±
30 110 ±
20 HD184915 27000/3.4 220 70 ±
25 70 ± ±
13 294 ±
15 HD190603 B4 Ia 45 155 ±
111 150 ± ±
11 160 ±
10 HD194279 B5Ia 48 161 ± ±
33 84 ±
28 HD204827 O9.5IV 10 ± ±
40 100 ±
40 HD208501 B8Iab 41 97 ± ±
38 113 ±
64 HD219287 B0Ia+ 144 ± ±
20 195 ±
15 HD226868 O9.7Ib 106 129 ± ±
26 HD228712 B0.5Ia 222 ± ±
21 58 ±
25 HD228779 O9Ia 104 ± ±
20 120 ±
30 HD229059 B2Iab 205 ± ±
33 190 ±
36 HD235825 O9IV 110 ± ±
12 70 ±
20 HD254577 B0.5Ib 185 ± ±
18 HD281159 binary 584 227 ± Equation y = a + b*xWeight No WeightingResidual Sum of Squares 2.08299Pearson’s r 0.67524Adj. R-Square 0.42195 Value Standard ErrorrCorr Intercept 0.1125 0.27427Slope 0.32863 0.08974 E W / E W Equation y = a + b*xWeight InstrumentalResidual Sum of Squares 52.70883Pearson’s r 0.34546Adj. R-Square 0.0643 Value Standard ErrorrCorr Intercept 0.35347 0.16685Slope 0.10047 0.06823 lgg
Fig. 2.— Fit over the lg(g) versus EW(9633)/EW(9577) relation with no error bars taken intoaccount (top) and the weighted fit (bottom). Note the low correlation magnitude in the secondplot. 9 – E W ( ) , m ¯ EW(9633),m¯ R=0.37 N o r m a li z ed I n t en s i t y Wavelength (¯) hd 76341 hd 78344
Fig. 3.— Top: correlation between the equivalent widths (mÅ) of the DIBs 9577 and 9633. Notethe low correlation coefficient (R). The broken line represents the laboratory strength ratio of thesebands. Bottom: an example of a different strength ratio for the DIBs 9633 and 9577 in two objectsof similar spectral and luminosity class. 10 –Table 2: Correlation coefficient R estimated for n pairspair of DIBs R n pair of DIBs R n9633/9577 0.37 37 9633/NaI 3303 0.23 369633/E(B-V) 0.50 45 9577/NaI 3303 0.01 339577/E(B-V) 0.47 60 9633/TiII 3242 -0.25 329633/6196 0.54 45 9577/TiII 3242 0.02 309577/6196 0.70 62 9633/FeI 3860 0.02 419633/6284 0.25 45 9577/FeI 3860 0.30 389577/6284 0.65 62 9633/CN 3874.6 0.40 419633/5780 0.44 44 9577/CN 3874.6 0.11 399577/5780 0.71 61 9633/CaI 4227 0.09 439633/5797 0.36 43 9577/CaI 4227 0.27 439577/5797 0.52 60 9633/CH + + C o l u m n den s i t y , c m - N(HI)
Fig. 4.— The lack of correlation between the equivalent widths of DIBs 9633, 9577, and the columndensity of neutral hydrogen.(The complete figure set (5 images) is available in the online Journal.) 11 –space-born instruments. As seen in Fig. 4 both of the considered DIBs hardly correlate with thehydrogen column density. The correlation coefficients are 0.35 and 0.46 for the DIBs 9633 and 9577,respectively. Apparently the DIB carriers are not evenly distributed in the interstellar H i cloudsand the H i column density does not allow one to predict the intensities of both DIBs. The samesituation is observed in the cases of all atomic gases: Ti ii , Na i , Ca i or Fe i ; for all these relationsthe correlation coefficients are very small (see the online figure set associated with Fig. 4).Another interesting question is whether our DIBs are related in some way to interstellar dust.The optical depth of the latter is usually measured using the color excess E(B-V). Traditionally afeature that correlates with E(B-V) is considered as interstellar. Fig. 6 relates both strong DIBsto E(B-V) and proves a rather poor correlation between the DIB carriers and the dust grains.The color intensity of the symbols in the plots is associated with the equivalent width ratio of themajor DIBs at 5797 and 5780 Å, i.e. darker colors correspond to a stronger ζ effect, i.e. a higherequivalent width (5797/5780)) ratio. It is evident that ζ -type objects have a tendency to presentweak C +60 associated DIBs. A rather speculative issue is the possible presence of two sequences inthe relation between the equivalent width of the DIB 9577 and the reddening, roughly marked bythe red dashed lines in the right panel. This possible split cannot be explained by ζ / σ progressions.Indeed, a heavily reddened ζ -type object HD204827 and the progenitor of ζ -type clouds HD149757both exhibit a very weak DIB 9633. If any, the split can be explained, for example, by a presence oftwo independent diffuse bands at λ +60 DIBs. 9577 is also quite wellcorrelated with the DIB 5780 (R=0.71). However, the same correlation coefficient for 9633 is only0.44. All correlation coefficients are listed in Tab. 2. Generally they are smaller for the DIB 9633than for DIB 9577, although the DIB 9633 sample size is systematically smaller. This is anotherargument for the binary origin of the DIB 9577: like the cumulative effect of the average of manyclouds increases the magnitude of mutual correlation between diffuse bands, the blending of diffusebands smears the peculiarities and, might provoke an increasing correlation coefficient.It is of interest whether the considered DIBs show any correlation with interstellar featuresof simple radicals. Fig. 8 illustrates the relation between the two DIBs and the strongest line ofa simple CH molecule. The correlation coefficient is 0.25 and 0.22 for the DIBs 9633 and 9577,respectively, which is in fact negligible. Apparently the DIB carriers are not well mixed with the 12 – E W ( m ¯ ) R= -0.25
EW(m¯)
R=0.02
Fig. 5.— The same as Fig. 4 but for Ti ii . R is the correlation coefficient. R=0.50 -59 2735 EW /EW E W ( m ¯ ) R=0.47
Fig. 6.— The correlation between the equivalent widths (in mÅ) of the DIBs 9577, 9633 and E(B-V). Note the low correlation coefficients (R). The relation for 9577 may suggest two sequences, eachthus correlating more tightly (see the text). 13 –
R=0.54 E W ( m ¯ ) EW(m¯)
R=0.70
Fig. 7.— The correlation between the equivalent widths (in mÅ) of the DIBs 9577, 9633 and theequivalent width of the narrow DIB 6196 (in mÅ). The correlation coefficient for 9577 (R=0.70) isamong the highest in our sample.
The complete figure set (6 images) is available in theonline Journal.
R=0.25 E W ( m ¯ ) EW(m¯)
R=0.22
Fig. 8.— The correlation between the equivalent widths (in mÅ) of the DIBs 9577, 9633 and theequivalent width of the CH 4300 Å line. R is the correlation coefficient.
The complete figure set(3 images) is available in the online Journal.
14 –simple radicals. The correlations for CH + and CN are not much better. On other hand, thecorrelation of DIBs 9577,9633 with CH + is almost two times higher than that with CH. Perhapsthis is not just a statistical fluctuation but an observational fact supporting our idea that theseDIBs are stronger in σ -type clouds. The C radical seems to be the most interesting, as such small,carbon molecules may be considered as building blocks for more complex species such as C +60 . Fig.9 shows that apparently the abundance of C is not related in any way to C +60 . This casts additionaldoubt on the identification of the DIBs 9577 and 9633 as carried by C +60 .Since 1988 (Krełowski & Westerlund 1988) interstellar clouds are being divided into σ and ζ types. This division is based on the strength ratio of the major DIBs: 5780 or 6284 and 5797 or6379. It is thus interesting how the intensities of 9577 and 9633 react to the 5797/6284 ratio. Thisis illustrated in Fig. 10. The points in this figure are distributed in the form of the letter "L".Narrow diffuse bands may be very strong when our so-called C +60 DIBs are very weak. The oppositesituation is possible as well. However, both sequences intercept each other. This may suggest thatDIBs 9577 and 9633 are stronger in σ -type clouds, i.e. the carriers of these features follow theintensity of reasonably broad DIBs which are strong in the lines of sight where the UV flux fromthe very hot nearby stars plays an important role.
4. Conclusions
Our considerations allow us to infer the following conclusions.• The two strong, so-called C +60
DIBs (9577 and 9633), exhibit a poor mutual correlation, whichcasts doubt on their common origin. However the lack of correlation can be explained, e.g. ifthe DIB 9577 is a blend of two features (see Fig. 6 and the text).• The relation of the two DIBs to E(B-V) is also quite poor. Again, this might be due to apossible blending effect in DIB 9577.• The intensities of the two strong DIBs are poorly related to the H i column density and,generally, to all molecular/atomic features analyzed in this article, although the correlationcoefficients vary from feature to feature (see Tab. 2). In particular, the two DIB carriers donot seem to be related to simple interstellar radicals such as CH but also CH + and CN. Aparticular difficulty concerns C as the latter may be considered as a building block for C .• Also, the DIBs 9633 and 9577 are stronger in σ -type clouds, i.e. the carriers of these featuresfollow the intensity of reasonably broad DIBs, which are strong in the lines of sight where theUV flux from very hot nearby stars plays an important role. An additional argument in favorof this suggestion is the correlation of DIBs 9577,9633 with CH + , quite weak though but twiceas high as that with CH.Poor mutual correlation of the 9577 and 9633 diffuse bands as well poor correlations of these 15 – I n t en s i t y Wavelength (¯) C EspadonsBD +404220HD 183143
Fig. 9.— An example of poor correlation of the DIB 9577 with molecular carbon. The profile of9577 is identical in the spectra of two heavily reddened stars while the intensity of the C bandsdiffers drastically. Also, this figure is a good example of a comparison of the DIB 9577 profilesobserved in spectra of fast (BD+40 4220) and slow (HD 183143) rotating stars. E W ( m ¯ ) Fig. 10.— The plots divide our targets into two groups following either the strong narrow DIBs(5797) or the broad ones (6284). 16 –DIBs with other interstellar features may be partially explained by the influence of: (i) imperfectionof the telluric line removal procedure, (ii) contamination by stellar lines, and finally, by (iii) anunresolved blend with other DIBs. The remedy for the first two issues is evident though not easyto perform: extensive observation of both reddened and unreddened targets with the aid of spacetelescopes, with subsequent correction of the intrinsic parameters of the stellar lines in order toprecisely model the stellar spectra. The third issue might be quite difficult to resolve withoutessentially increasing measurement quality and the quantity of the studied lines of sight. Indeed, itis difficult to detect and measure precisely the moderately reddened targets in the near-IR becauseof the weakness of near-IR DIBs in objects lacking an essential amount of dust.There are widely spread reports in favor of the identification of interstellar C +60 (see, e.g. Lin-nartz et al. 2020, Woods 2020). However, in our opinion the identification cannot be decisivelyrecognized without explaining the issues raised from the observational facts presented in this article.This paper includes data gathered with the VLT and UVES spectrograph, programs 067.C-0281(A), 082.C-0566(A), 092.C-0019(A). The authors are grateful to Dr. Jacco Th. van Loon for thecareful reading of the paper and the valuable suggestions and comments. G.A.G., G.V. and N.R.I.acknowledge the support of Ministry of Science and Higher Education of the Russian Federationunder the grant 075-15-2020-780 (N13.1902.21.0039). J.K. acknowledges the financial support ofthe Polish National Science Center —the grant UMO-2017/25/B/ST9/01524 for the period 2018 –2021. G.A.G. and J.K. acknowledge the Chilean fund CONICYT grant REDES180136 for financialsupport of their international collaboration.
REFERENCES
Bailey, M., van Loon, J. T., Farhang, A., et al. 2016, A&A, 585, A12Cami J., Bernard-Salas J., Peeters E., Malek S.E., 2010, Science, 329, 1180Campbell E. K., Holz M., Gerlich D., Maier J. P., 2015, Nature, 523, 322Campbell E. K., Holz M., Maier J.P., Gerlich D., Walker G.A.H., Bohlender D., 2016, ApJ, 822, 17Campbell E. K., Holz M., Maier J. P., 2016, ApJ, 826, L4Campbell, E. K. & Maier, J. P., 2018, ApJ, 858, 36Cordiner, M. A., Linnartz, H., Cox, N. L. J., Cami, J. et al. 2019, ApJ, 875, 28Dekker, H., D’Odorico, S, Kaufer, A., Delabre, B. & Kotzlowski, H., 2000, Proc. SPIE 4008, p. 534Foing B. H., Ehrenfreund P., 1994, Nature, 369, 296 17 –Galazutdinov G.A., Krełowski J., Musaev F. A., Ehrenfreund P., Foing B.H., 2000, MNRAS, 317,750Galazutdinov, G. A. & Krełowski, J. 2017, Acta Astronomica 67, 159Galazutdinov G. A., Shimansky V. V., Bondar A., Valyavin G. & Krełowski, J. 2017, MNRAS, 465,3956Gudennavar S.B., Bubbly S.G., Preethi K., Murthy J., 2012, ApJ Suppl. Ser., 199, 8Kaufer, A. et al. 1999, The Messenger 95, 8Kim K.-M., Han I., Valyavin G.G., Plachinda S. et al., 2007, PASP, 119, 1052Krełowski, J. & Westerlund, B. E., 1988, A&A, 190, 339Krełowski, J., Galazutdinov, G. A., Bondar, A. & Beletsky, Y., 2016, MNRAS, 460, 2706Krełowski, J., Galazutdinov, G., & Bondar, A. 2019, MNRAS, 486, 3537.Krełowski, J., Galazutdinov, G. A., & Siebenmorgen, R. 2020, ApJ, 899, L2Kroto H.W., Heath J.R., Obrien S.C., Curl R.F., Smalley R.E., 1985, Nature, 318, 162Lallement, R., Cox, N. L. J., Cami, J., Smoker, J. et al. 2018, A&A, 614, 28Linnartz, H., Cami, J., Cordiner, M., et al. 2020, Journal of Molecular Spectroscopy, 367, 111243Moutou, C.; Krełowski, J., D’Hendecourt, L. & Jamroszczak, J. 1999, A&A, 351, 680Musaev, F. A., Galazutdinov, G. A., Sergeev, A. V., et al. 1999, Kinematics and Physics of CelestialBodies, 15, 216Sellgren K., Werner, M.W., Ingalls J.G., Smith J.D.T., Carleton T. M., Joblin C., 2010, ApJ, 722,L54Tody D., 1986, "The IRAF Data Reduction and Analysis System" in Proc. SPIE, Instrumentationin Astronomy VI, ed. D.L. Crawford, 627, 733Vollmann, K., Eversberg, T. 2006, Astron. Nachr., 327, 862Walker G. A. H., Bohlender D. A., Maier J. P., Campbell E. K., 2015, ApJL, 812, 8Walker G. A. H., Campbell E. K., Maier J. P. et al. 2016, ApJ, 831, 130Woods, P. 2020, Nature Astronomy, 4, 299
This preprint was prepared with the AAS L A TEX macros v5.2.
18 –
R=0.23 E W ( m ¯ ) EW(m¯)
R=0.01
Fig. 11.— The same as Fig. 4 but for Na i . R=0.09 E W ( m ¯ ) EW(m¯)
R=0.27
Fig. 12.— The same as Fig. 4 but for Ca i . 19 – R=0.02
FeI 3860 E W ( m ¯ ) EW(m¯)
R=0.30
Fig. 13.— The same as Fig. 4 but for Fe i . R=0.36 E W ( m ¯ ) EW(m¯)
R=0.52
Fig. 14.— The same as Fig. 7 but for DIB 5797. 20 – E W ( m ¯ ) EW(m¯)
R=0.44 R=0.71
Fig. 15.— The same as Fig. 7 but for DIB 5780. E W ( m ¯ ) EW(m¯)
R=0.25 R=0.65
Fig. 16.— The same as Fig. 7 but for DIB 6284. 21 – E W ( m ¯ ) EW(m¯)
R=0.32 R=0.66
Fig. 17.— The same as Fig. 7 but for DIB 6269.
R=0.37 E W ( m ¯ ) EW(m¯)
R=0.77
Fig. 18.— The same as Fig. 7 but for DIB 6614. 22 – E W ( m ¯ ) EW(m¯)
R=0.44
R=0.36
Fig. 19.— The same as Fig. 8 but for CH + R=0.40 E W ( m ¯ ) EW(m¯)