interstellar features

Transcription

1 Mon. Not. R. Astron. Soc. 317, 750±758 (2000) On the identification of the C 1 60 interstellar features G. A. Galazutdinov, 1w J. Kreøowski, 2w F. A. Musaev, 1w P. Ehrenfreund 3w and B. H. Foing 4w 1 Special Astrophysical Observatory, Nizhnij Arkhyz , Russia 2 Center for Astronomy, Nicholas Copernicus University, Gagarina 11, Pl TorunÂ, Poland 3 Leiden Observatory, PO Box 9513, 2300 RA Leiden, the Netherlands 4 ESA Space Science Department, ESTEC/SCI-SO, 2200 AG Noordwijk, the Netherlands Accepted 2000 March 20. Received 1999 December 17; in original form 1998 August 24 ABSTRACT The identity of the carriers of the diffuse interstellar bands (DIBs) is one of the most fascinating puzzles of modern spectroscopy. Over the last few years the number of known DIBs has grown substantially. In this paper we discuss the two recently discovered nearinfrared weak interstellar features which have already been proposed as fingerprints of the buckminsterfullerene C 1 60 : We present and discuss measurements of the two related DIBs within a larger sample of reddened targets, observed with different spectrometers, telescopes and site conditions. We provide additional arguments in favour of the interstellar origin of the two bands. We find evidence around the 9577-AÊ DIB of far-wing structures, which may affect broad-band measurements. We estimate corrections and errors for telluric and stellar blends, and show that the cores of the two DIBs are well correlated with a ratio near unity within 20 per cent. Finally, we discuss their relation to the laboratory spectra of C 1 60 ; and the search for two expected weaker C 1 60 transitions. Key words: dust, extinction ± ISM: molecules ± infrared: ISM: lines and bands. 1 INTRODUCTION The identification of the carriers of the diffuse interstellar bands (DIBs) is still one of the most fascinating puzzles 75 years after the discovery of the first two spectral features: the yellow bands centred near 5780 and 5797 AÊ (Heger 1922). The last surveys of the interstellar spectra (Jenniskens & DeÂsert 1994; Kreøowski, Sneden & Hiltgen 1995; Herbig 1995) have shown hundreds of weak unidentified features. Most of the newly discovered features are, however, very weak ones, observed only in spectra of very high signal-to-noise ratio or towards heavily reddened stars. This very rich spectrum of the interstellar medium contains, without a doubt, a lot of information concerning the physical conditions inside H i clouds and the complicated chemistry of these objects. C 60 and its polyhedral geometry were discussed extensively by Kroto et al. (1985). The synthesis of C 60 in macroscopic quantities was achieved by Kraetschmer et al. (1990). The presence of sooty material in C-rich stars, and the spontaneous formation and remarkable stability of the fullerene cage suggest the presence of fullerene compounds in interstellar space. It is assumed that complex molecular ions are very abundant in the interstellar medium, and that some of them may be the origin of the w gala@sao.ru (GAG); jacek@astri.uni.torun.pl (JK); faig@sao.ru (FAM); pascale@strw.leidenuniv.nl (PE); bfoing@estec.esa.nl (BHF) unidentified interstellar features. This has led to the proposal that the ion C 1 60 might be more abundant than the neutral species, and thus that it is an attractive proposition for a DIB carrier (Kroto 1987; LeÂger et al. 1988). The recent papers of Foing & Ehrenfreund (1994, 1997) report the discovery of two spectral features in the near-infrared which may originate in the cation of the buckminsterfullerene C 1 60 : They have observed these two DIBs in a few targets, and thus any additional observations in the region of the two (9577 and 9632 AÊ ) features would be of importance. Recently Jenniskens et al. (1997) discussed these near-infrared bands and their intensity ratios, which appear different from those of the laboratory measurements of Fulara, Jakobi & Maier (1993). However, both of the considered DIBs are weak and were observed only in spectra of relatively late B supergiants in which the 9632-AÊ band is blended with the stellar Mg ii line which made the measurements uncertain. The region of the two features is strongly contaminated by telluric lines, and their proper subtraction plays a crucial role. The difficulties of separation of the above-mentioned DIBs from stellar and residual features after telluric correction are illustrated in Fig. 1. The and 9632-AÊ features are shown in Fig. 2 and appear clearly in heavily reddened stellar spectra. They are the strongest interstellar absorption features in a surveyed range from 940 to 1000 nm (Foing & Ehrenfreund 1995). Fig. 1 demonstrates also q 2000 RAS

2 The C 60 interstellar features 751 Figure 1. Spectra of the 954±967 nm range of HD and b Ori, showing the proposed DIBs at 9577 and 9632 AÊ in the reddened target. The spectra are corrected using the telluric divisor HD (B3 V, v sin i ˆ 205 km s 21 : Note the blending of the 9632-AÊ interstellar feature with the stellar Mg ii line, which is relatively strong in late B-type supergiants as shown by the synthetic stellar spectrum (above). This blend is partly corrected by the B3 V divisor, with a residual of 80 maê measured in b Ori compared with a total absorption (stellar residual interstellar of 282 maê in HD Both interstellar features are below the level of detection in the spectrum of the hot, rapidly rotating O6 star HD Figure 2. The near-infrared interstellar absorption features as seen in the Terskol and SAO spectra. For the parameters of the stars, see Table 1. Note the strong telluric contamination which cannot be divided out completely owing to the saturation of the atmospheric water vapour lines.

3 752 G. A. Galazutdinov et al. that the 9632-AÊ DIB is blended with the stellar Mg ii line, especially in spectra of relatively late B-type stars like the wellknown star HD However, the Mg ii blend does not exceed 80 maê for b Ori (of the same spectral type as HD ). For spectral types O7±B3 I, the Mg ii blend should not exceed 50 maê. Therefore this blend does not dominate the 9632-AÊ DIB equivalent width measurement, but it should be corrected for better accuracy. In this case, this makes the measurements of the strengths more imprecise (to the 30 per cent level), owing to the task of separating stellar and interstellar components. Therefore the most reliable 9632-AÊ DIB measurements come from lines of sight towards the O- and early B-type stars. However, the main limitation resides in the telluric correction in this wavelength range, unless one has exceptionally dry conditions and adequate divisors at the same airmass. 2 THE OBSERVATIONAL DATA The main set of data considered here comprises spectra acquired at the Russian Special Astrophysical Observatory (SAO), with the aid of the new coudeâ echelle spectrometer fed by the SAO 1-m telescope (Musaev 1993), and also spectra acquired with the aid of the similar coudeâ echelle spectrometer fed by the 2-m telescope of the observatory on top of Terskol peak (Northern Caucasus). With the Wright Instruments CCD matrix (pixel size 22:5 22:5 mm 2 the spectrometer covers in a single exposure the range,3500± AÊ with resolutions R ˆ (SAO) and (Terskol). Our reduction of the echelle spectra was made using the dech code (Galazutdinov 1992). This program allows flat-field division, bias/background subtraction, one-dimensional spectrum extraction from the two-dimensional images, correction for the diffuse light, spectrum addition and excision of cosmic ray features among the standard operations. The dech code also allows location of a fiducial continuum, measurements of the line equivalent widths, line positions and shifts, and other measurements. The spectral range, covered in every exposure, contains strong and well-identified atomic interstellar lines: Ca ii, Cai, Nai and K i. This allows us to determine precisely the radial velocities of the intervening interstellar clouds at any moment of any observation with high precision. Seven reddened targets were observed at the Observatoire de Haute Provence (OHP) using the AureÂlie Spectrograph on the 1.52-m telescope. The resolving power was R ˆ over the range 950±970 nm using a 1200 groove mm 21 grating. The spectra were reduced at OHP and at ESA Space Science Department (SSD). For the telluric corrections we observed immediately, at the same airmass, unreddened reference stars of similar spectral type. This procedure had been tested on earlier runs, and was optimized on this occasion, also taking advantage of the relatively dry conditions at the OHP site in 1993 November. Additional observations were obtained at the 3.6-m Canada±France±Hawaii Telescope (CFHT) on Mauna Kea, Hawaii (coudeâ f =8 spectrometer, 1995 March; coudeâ Gecko spectrometer, 1996 November), and at the ESO 1.5-m Caide Auxiliary Telescope (CAT) (Coude Echelle Spectrometer CES, 1994 November and 1996 November). The spectral resolving power was and the signal-to-noise ratio better than 300 in areas free of telluric lines. The conditions were dry during most of these periods, and exceptionally dry at CFHT in 1995 March. We used also echelle spectra obtained with the ESA-MUSICOS spectrometer newly commissioned on the 2.5-m Isaac Newton Telescope (INT) at La Palma observatory in 1996 November. The ESA-MUSICOS echelle spectra were reduced using the midas software. The segments of our SAO and Terskol spectra, covering the range of possible DIBs, are shown in Fig. 2. The telluric lines have not been removed completely from these spectra to avoid raising the continuum above the real level in places where the telluric features are especially dense and deep. Fortunately the suspected DIBs are not blended with strong atmospheric features. It is evident that the suspected C 1 60 features can be observed towards heavily reddened stars. Similar segments of OHP spectra are depicted by Foing & Ehrenfreund (1994). In this case the removal of the telluric features was improved by the immediate observation of standards at same airmass, but the general signal-to-noise ratio arising from photon noise was lower. However, it is evident that the and 9632-AÊ features are clearly seen in displayed spectra from all sources. The increase of the strength of bands with reddening E B 2 V is consistent with an interstellar origin. The difficult problem is the strong telluric contamination of the investigated spectral range. The telluric lines are in many cases almost saturated, which makes it impossible to remove them by division with the telluric standard. Any attempt to do this by means of re-scaling the depths of this features can affect the continuum level, especially close to `empty' regions i.e. the close vicinities of both the and 9577-AÊ features. It should be emphasized that the two DIBs are not measurable in any spectrum of moderately reddened stars that could be obscured by only single clouds, owing to the photon and telluric noise in the data. It is thus not yet possible to describe their behaviour in very different media, as has been done for other DIBs in the visible (see Kreøowski et al. 1995; Cami et al. 1997; Sonnentrucker et al. 1997). However, if the features are indeed of the same origin, their strength ratio should remain similar toward all observed objects. To search for possible C 1 60 features we have chosen a set of heavily reddened stars. This facilitates the search for weak interstellar features. However, even in these cases, measurements of the intensities of our bands are difficult, both because of the direct contamination of their profiles and because of the uncertain continuum level after attempts to remove the atmospheric (strong) lines. 3 RESULTS 3.1 Interstellar origin of the 9577/9632-A Ê features A first test with the new data (acquired at SAO and Terskol observatories) is to confirm whether the two features described by Foing & Ehrenfreund (1994, 1997) are really interstellar, and to estimate possible stellar and telluric correction residuals in order to study whether their intensity ratio remains constant along every sightline. A problem for the 9632-AÊ DIB may come from blending with the stellar Mg ii line which has equivalent width,50±80 maê in late B supergiants (Fig. 1). Foing & Ehrenfreund reported that this feature was divided out in most of their spectra together with the telluric lines, with the choice of adequate standards. This was shown by a good correction of the stellar blend for low-reddening stars. However, it is very difficult to find a standard with the same stellar parameters (with the same Mg ii line contribution) at the same airmass (observed immediately to minimize the residuals of the telluric correction owing to short-term atmospheric water variations). For our SAO and Terskol targets we have used two telluric line divisors: i.e. the unreddened, early-type rapid rotators listed in Table 1; the OHP divisors (different for each target) are also listed

4 The C 60 interstellar features 753 Table 1. Basic data on `stellar targets'. The divisors are presented as the last eight rows of the table. Note that HD is used for HD only. HD/BD Sp/L V B 2 V E B 2 V Observatory B6 Ia OHP A0 Iab TER, OHP O9.5 Iae SAO, OHP B8 Iab SAO O6pe ESO, INT, CFH O9 III INT O9.5 Ibe ESO B0.5 II OHP B2/3 Iae CFH O8/ TER O TER O CFH B7 Ia TER, OHP, CFH B8 Ia TER B1.5 Iae SAO, OHP, CFH B2pe CFH B1.5 Ia TER O9.5 Ia SAO B3 Iae CFH B2 Ib CFH O9 IIe TER O6 Iab SAO B3 Ia TER O7e TER, OHP B3 V SAO, TER A0 V CFH B9 III SAO 5394 B0 IVe ESO B5 III OHP, TER B3 V ESO B0 III ESO B1 IV OHP in the table. We have also observed the star b Ori (HD 34085) ± the typical unreddened comparison star for the heavily reddened HD (see e.g. Herbig 1975). Fig. 3 presents the spectra of two of our heavily reddened targets together with the synthetic spectra of stellar lines calculated for the assumed stellar effective temperature, gravity and rotational velocity. It is clear that only some weak Mg ii contaminations can be expected inside the DIB profiles, namely less than 50 maê for types O7±B3 I. They must be very weak as the stronger neighbouring He i line is not visible ± most probably due to the fast rotation of our targets. The lack of any features similar to 9632 and 9577 AÊ in the synthetic spectra confirms once again that both features are interstellar. The wavelength scale is adjusted to the rest wavelength for the interstellar medium, using the wavelength shifts measured in the interstellar atomic lines seen in the same spectra. Our targets are of very different apparent rotational speed. This allows one additional check of the interstellar origin of both suspected DIBs. The profiles of the stellar lines should vary substantially from object to object owing to the rotation. In spectra of very fast rotators only the strongest stellar lines can be seen. We have thus selected the spectral region containing the strong He i 5876-AÊ line, and Fig. 4 shows this region in spectra of all our SAO and Terskol targets. It is evident that the B7 I HD rotates very slowly whereas BD is a very fast rotator, possibly with a circumstellar disc. On the other hand, as seen in Fig. 2, the suspected DIB profiles are nearly identical in the spectra of these two stars. This is another strong argument in favour of the dominant interstellar origin of both features. 3.2 Equivalent widths and shifts of the 9577/9632-A Ê DIBs The equivalent widths of both spectral features under consideration have been measured and listed as EW1 and EW2 in Tables 2 and 3. They have been measured using a single-gaussian fit Figure 3. The spectra of two heavily reddened hot stars with the corresponding synthetic ones. The spectra have been corrected using the telluric divisor HD The spectra are shifted to match the laboratory wavelength scale of the stellar lines.

5 754 G. A. Galazutdinov et al. Figure 4. The profiles of the stellar He i line in the spectra of the same stars as in Fig. 2. The profile seen in the last target BD shows a very high rotational velocity in contrast to that of HD The profile of the neighbouring 5850-AÊ DIB is nearly identical in both spectra, as is that of the Na i D 2 line. Table 2. SAO/Terskol DIB core measurements of equivalent widths EW1 and EW2 in maê using the single-gaussian fit method. Measurements of wavelength differences dl1 and dl2 from the average wavelengths and AÊ are given. Typical equivalent width measurement errors are 30 maê. EW2 0 is the estimate of the 9632-AÊ DIB after correction for the Mg ii stellar blend in the targets. The DIB core ratio EW2 0 /EW1 is also given with the estimated error (in per cent). HD/BD dl1 EW1 dl2 EW2 EW2 0 EW2 0 Error AÊ maê AÊ maê maê /EW1 in % invis. ± ± method on spectra over the intervals 9573±9581 and 9628± 9636 AÊ. We have also estimated from observed spectral standards and synthetic spectra the effect in the 9632-AÊ DIB measuring window of stellar blends of Mg ii 9632 AÊ and the He i 9625-AÊ wing, present both in the reddened star and in the fast-rotating telluric divisor. We have assumed typical variations of the stellar blend EWs of (Mg ii, He i) in (maê, maê ) from stars of spectral type O7 (30, 30), O9.5 (35, 60), B1.5 (45, 100), B3 I (55, 180) and B7 I (80, 100) to A0 I (100, 30). We have assumed blend EWs of (30, 50) for the B3 V telluric divisor h UMa, but calculated the bias in the measuring window taking into account its fast-rotation Doppler broadening. While testing the interstellar origin of any spectral feature, it is important to check whether its laboratory wavelength (i.e. that Table 3. CFHT, INT and ESO DIB equivalent width (in maê ) Gaussian fit DIB core measurements over the selected wavelength intervals 9573±9581 and 9628±9636 AÊ, for reddened targets corrected with telluric/spectral standards (STD). HD/BD STD EW1 EW2 EW2 0 EW2 0 Error maê maê maê /EW1 in % CFHT ±125 INT ,15 ESO calculated using the Doppler shift of identified interstellar atomic or molecular features) remains constant in different targets. We have used the well-known spectral features of interstellar Ca ii, Na i and K i to determine the Doppler shifts of the unidentified interstellar features. We found the latter the most reliable, as the K i lines are never saturated in our spectra, but are strong enough to be precisely measured. The Ca ii and Na i lines are frequently saturated, and thus their widths prevent precise measurements of their wavelengths, and of bulk velocities. Our near-infrared DIBs can be observed only in heavily reddened objects (see Fig. 2), and thus we can always expect several interstellar clouds along any

6 The C 60 interstellar features 755 such sightline. The broad sodium line profiles, seen e.g. in Fig. 4, provide evidence in favour of this assumption. This makes any attempt to determine their intrinsic profiles very difficult. In cases of saturation the strong components of interstellar lines are so broad that one cannot separate them. The inferred rest wavelengths of the and 9577-AÊ DIBs, determined using the K i line reference velocity, are not exactly the same in the different lines of sight (Table 2), but their profiles can differ because the stars are observed through several clouds. However, they have a dispersion of the order of 0.3 AÊ rms. Part of this dispersion comes from measurement noise, but it also could be due to some velocity gradient in the interstellar clouds between the DIB carrier C 1 60 and K i material. The line EW ratios for the measurements of the SAO/Terskol range around unity (1.0), with a dispersion of 20 per cent consistent with the measurement uncertainties. The equivalent width measurements may differ slightly between various data sets, owing to the selected wavelength intervals. In the different data sets, EW rms errors range from 15 to 30 maê for DIB core measurements, and are more than 50 maê for broad wavelength intervals. This quantifies the uncertainties of line ratios, and explains some of the non-detections in the lowreddening targets. In Tables 2 and 3, we have not listed any measurements for the three stars HD 24912, and HD In these cases, we found both features below the confident level of detection. In HD (corrected with the divisor HD B9 III), we found weak features in SAO spectra, difficult to measure compared with those measured at ESO under better telluric conditions and better spectral standard matching by Foing & Ehrenfreund (1997), and confirmed by new INT and CFHT observations (see Table 3). 3.3 Evidence for DIB far-wing structures In the spectra of the most heavily reddened stars, one can see a broad feature centred on the 9577-AÊ DIB. This may be either an additional DIB or a very broad wing related to the 9577-AÊ DIB (Fig. 5). The possible broad DIB is especially well seen in the heavily reddened spectrum of BD in which the telluric features are especially well removed, or in composite average spectra of several very reddened targets. The total DIB equivalent widths towards this star over a 15-AÊ broad measurement interval are 500 maê at Terskol and 490 maê for the data taken in 1993 at The Observatoire Haute Provence (OHP93). Here, the broad DIB wing contributes 200 maê compared with about 300 maê for the DIB core. This illustrates that, for the same wavelength intervals, the equivalent width measurements are consistent from site to site. Apparently part of the discrepancy between the published measurements comes from this notable broad wing contribution. We note that large wings for the two DIBs extending out to 10 AÊ from line centre were reported on an average composite spectrum by Foing & Ehrenfreund (1994). That is the reason why they measured the DIB strength in OHP93 data over large intervals, specifically 9571±9580 and 9625±9643 AÊ. However, OHP93 discovery data were affected by telluric residuals and limited signal-to-noise ratio. There was also a compromise to limit problems with strong telluric residuals at 9581 and 9645 AÊ. OHP93 data were obtained with a normal grating at first order and do not suffer from continuum ripples. The same broad features were found on the CFHT 1995 data also obtained with a normal coudeâ spectrograph. This is not the case with data obtained with echelle spectrographs (SAO/Terskol, CES and ESA-MUSICOS) where the blaze function is not perfectly corrected by the flat-fielding. For these data, we selected several pseudo-continuum points in the Figure 5. The possible broad feature and far wings around the 9577-AÊ DIB core. These additional features may explain the apparent discrepancies between DIB equivalent widths obtained with different measuring intervals and methods, as well as differences between DIB ratios and C 60 laboratory ratios obtained from Ne-matrix low-resolution measurements.

7 756 G. A. Galazutdinov et al. spectrum, and fitted a spline residual blaze correction function which was later divided out. As a result the spectra are rectified, but this can remove broad wing features if the pseudo-continuum points are selected in these areas. Fortunately, there are points free of telluric absorption sufficiently far out from the 9577-AÊ DIB line centre allowing the signature of the line wings in the SAO data to be retained. Therefore we believe that the 9577-AÊ DIB is characterized by large wings extending to 10 AÊ from the line centre, or is superposed upon a broader DIB. The case for a broad wing around the 9632-AÊ DIB deserves further study, as part of the possible wings below 100 maê could be masked by the telluric division with broadened Mg ii and He i stellar lines from the fastrotating telluric standards. The differences in the quality of the telluric conditions and residual corrections, and the width of the measurement intervals, largely explain the differences in the reported equivalent widths. For late B-type stars, it is also important to achieve a proper correction for spectral line residuals. 4 DISCUSSION 4.1 Correlation between the and 9632-A Ê DIBs The new spectra confirm the interstellar nature of the two features at 9577 and 9632 AÊ as bona fide DIBs. When proper care is taken to correct for telluric contamination and stellar line blending, one can measure the widths of the profiles and their equivalent widths in different lines of sight. Foing & Ehrenfreund (1994, 1997) found that the two features are correlated, with similar depletion in dense clouds such as HD or enhancement in the strongly UV-irradiated region in front of HD This supports a link between the carriers of the two DIBs. The same measured width of the DIBs is consistent with the same or a very similar carrier. The actual width of 3 cm 21 is in fact compatible with the rotational contour of a molecule with a similar moment of inertia to a fullerene. Based on the more accurate DIB core equivalent width measurements from these new data sets, including also some improvement for corrections for stellar blends in the 9632-AÊ DIB measuring window, we show in Fig. 6 the relation between the cores of the two DIBs. Figure 6. The correlation between the core equivalent widths of the and 9632-AÊ DIBs. The correlation coefficient is larger than 90 per cent and the intercept does not exceed the typical measurement error. The diagram shows one of the highest correlations between DIB features (with a correlation coefficient of more than 90 per cent). This supports the indication that the cores of the two DIBs originate from very similar molecules, or indeed from the same molecule. 4.2 Comparison with laboratory measurements Foing & Ehrenfreund (1994, 1997) have proposed that the DIBs correspond to the C 1 60 molecule. Criteria for this identification, and proposed verifications, were discussed in the 1994 original paper and following papers (Ehrenfreund & Foing 1995, 1997). They found a shift between the DIBs and the measured positions of C 1 60 in a frozen Ne matrix of respectively 3 and 10 cm 21 for the and 9632-AÊ bands (note that the quoted shifts of 7 and 14 cm 21 by Maier (1994) seem to be in error because of confusion between air and vacuum wavelengths for the DIBs). There is a general problem with trying to identify laboratory spectral features of some molecules with certain weak DIBs in the visible range. The weak DIBs are so densely packed in every heavily reddened spectrum that only when very precise wavelengths of the molecular features are known is it possible to ensure a more reliable identification. However, most of the existing laboratory spectra are obtained in noble gas matrices and thus a wavelength shift is expected. In such cases we can usually see several interstellar features inside a typical laboratory `error box', which makes the task of identification very difficult if not impossible. Depending on the molecules and the wavelength range considered, this shift can range from 5±10 cm 21 in the nearinfrared at 1000 nm to 20±50 cm 21 in the visible at 500 nm, for a molecule such as C 1 60 : The facts that the and 9632-A Ê features are the strongest DIBs observed in the range 900± 1000 nm, and that they are consistent with the strongest fundamental transitions of C 1 60 give weight to the identification. 4.3 Search for additional C 1 60 transitions The paper of Fulara et al (1993) reports two other weaker features of C 1 60 in a frozen Ne matrix at and A Ê (vacuum wavelengths). Assuming a similar matrix shift of 10 cm 21 for these transitions, we calculate expected positions for the predicted additional C 1 60 DIBs of 9420 and 9363 ^ 10 A (air wavelength). Neither feature could be detected by Jenniskens et al. (1997). Our most heavily reddened spectrum (of BD ) contains a probably interstellar feature around 9410 AÊ, i.e. inside the range of uncertainty (Fig. 7). The feature has a similar width to the 9632-AÊ DIB, but is only about a factor of 2, not a factor of 6, weaker than either of the two considered above. There is a stellar line blend arising from Si ii that has to be corrected to estimate the real strength of the DIB feature. It is thus difficult to say whether that feature is related to the buckminsterfullerene C 1 60 or whether it is some other DIB. It must be emphasized that the range is even more severely contaminated with telluric features than that of the and 9632-AÊ DIBs, limiting the search for some weak spectral features. Only observations in exceptionally dry conditions, and optimal telluric correction at sufficient spectral resolution and signal-to-noise ratio (such as at the CFHT in 1995), may contribute further to this issue. We have not identified any possible interstellar feature in the wavelength range 9363^10 AÊ that is affected by strong telluric water absorption.

8 The C 60 interstellar features 757 Figure 7. The possible weak DIB interstellar feature at 9410 AÊ. 5 CONCLUSION AND PERSPECTIVES We can confirm that both the features at 9577 and 9632 AÊ are predominantly of interstellar origin. We could improve the DIB equivalent width estimates by correcting for weak stellar blends in the 9632-AÊ DIB window. The core measurements of the two DIBs are among the best correlated data on DIBs in regions of different environmental conditions; this finding supports a common origin and is consistent with the C 1 60 assignment. The DIB core band ratio is around unity and can be measured with an accuracy of 20 per cent in most sources, as it is affected by residuals from the telluric correction. However, the possibly present broad, overlapping wings (which could contribute up to 50 per cent of the overall features) make the determination of the 9577 AÊ /9632 AÊ overall intensity ratio difficult. This explains some discrepancies with previous measurements integrated over large wavelength intervals. It makes also more difficult the direct intensity comparison with laboratory matrix measurements (which do not resolve the core from the wings), and gas-phase measurements (which are not yet very reliable in intensity). A third C 1 60 feature can be described as possibly present. A more extensive sample of high signal-tonoise ratio, heavily reddened spectra is necessary to determine the abundance variations of the C 1 60 buckminsterfullerene in different interstellar clouds. It is necessary to include measurements of these absorption lines in single-cloud environments (Kreøowski, Schmidt & Snow 1997; Kreøowski & Schmidt 1997), for comparison with the behaviour of other DIBs (Cami et al. 1997; Sonnentrucker et al. 1997). Laboratory measurements should be encouraged, to contribute further to knowledge in this area. ACKNOWLEDGMENTS We thank the staff of the SAO, OHP, CFHT, ESO and INT for support during the observations. This paper has been supported by the II US±Poland Maria Skøodowska±Curie Joint Fund under grant MEN/NSF±94±196, and by The Polish National Committee for Scientific Research under grant 2 P03D GAG expresses his thanks to the Nicolaus Copernicus University in Torun for supporting his stay at the Center for Astronomy during which the project was finished and the paper prepared. This paper is based on observations obtained at the Russian Special Astrophysical Observatory (SAO), Terskol Observatory (TER), the Canada±France±Hawaii Telescope (CFHT), the European Southern Observatory (ESO), the Observatoire de Haute-Provence (OHP) and the Isaac Newton Telescope (INT). REFERENCES Cami J., Sonnentrucker P., Ehrenfreund P., Foing B. H., 1997, A&A, 326, 822 Ehrenfreund P., Foing B. H., 1995, Planet. Space Sci., 43, 10/11, 1183 Ehrenfreund P., Foing B. H., 1997, Adv. Space Res., 7, 19, 1033 Foing B. H., Ehrenfreund P., 1994, Nat, 369, 296 Foing B. H., Ehrenfreund P., 1995, in Tielens A., Snow T., eds, Diffuse Interstellar Bands. Kluwer, Dordrecht, p. 65 Foing B. H., Ehrenfreund P., 1997, A&A, 317, L59 Fulara J., Jakobi M., Maier J. P., 1993, Chem. Phys. Lett., 211, 227 Galazutdinov G. A., 1992, Spec. Astrofis. Obs., Preprint No. 92 Heger M. L., 1922, Lick Obs. Bull., 10, 146 Herbig G. H., 1975, ApJ, 196, 129 Herbig G. H., 1995, ARA&A, 33, 19 Jenniskens P., DeÂsert X., 1994, A&AS, 160, 39 Jenniskens P., Mulas G., Porceddu I., Benvenuti P., 1997, A&A, 327, 337 Kraetschmer W., Lamb L., Fostiropoulos K., Huffman D. R., 1990, Nat, 347, 354 Kreøowski J., Schmidt M., 1997, ApJ, 477, 209 Kreøowski J., Sneden C., Hiltgen D., 1995, Planet. Space Sci., 43, 1195 Kreøowski J., Schmidt M., Snow T. P., 1997, PASP, 109, 1135

9 758 G. A. Galazutdinov et al. Kroto H. W., 1987, in LeÂger A., d'hendecourt L., eds, Polycyclic Aromatic Hydrocarbons in the Galaxy. Reidel, Dordrecht, p. 197 Kroto H. W., Heath J. R., O'Brien S. C., Curl R. F., Smalley R. E., 1985, Nat, 318, 162 LeÂger A., d'hendecourt L., Verstraete L., Schmidt W., 1988, A&A, 203, 145 Maier J. P., 1994, Nat. Corres., 370, 423 Musaev F. A., 1993, Sov. Astron. Lett., 19, 776 Sonnentrucker P., Cami J., Ehrenfreund P., Foing B. H., 1997, A&A, 327, 1215 This paper has been typeset from a TEX/LATEX file prepared by the author.