Searching for Interstellar ${{\rm{C}}}_{60}^{+}$ Using a New Method for High Signal-to-noise HST/STIS Spectroscopy

The diffuse interstellar band (DIB) problem is the longest-standing puzzle in interstellar chemistry, seemingly impenetrable despite the dedicated efforts of astronomers and laboratory chemists since the early 20th century (Herbig 1995; Sarre 2006; Snow 2014). The DIBs manifest as broad spectroscopic absorption features in the optical to near-infrared (NIR) spectra of stars as their light passes through the diffuse interstellar medium (ISM), indicating the presence of a large quantity of (mostly carbonaceous) unidentified molecular material (e.g., Cordiner 2011). Foing & Ehrenfreund (1994) discovered two DIBs at 9577 Å and 9632 Å that they assigned to ${{\rm{C}}}_{60}^{+}$ (ionized Buckminsterfullerene) based on a similarity with the absorption wavelengths that are seen in neon matrix spectroscopy (d'Hendecourt et al. 1992; Fulara et al. 1993). Recently obtained gas-phase spectra of ${{\rm{C}}}_{60}^{+}$−He_n complexes (for n ≤ 4; Campbell et al. 2015, 2016a; Kuhn et al. 2016) show that the match between the interstellar and laboratory wavelengths is accurate within the observational uncertainties (a fraction of an angström). The bare ${{\rm{C}}}_{60}^{+}$ absorption wavelengths were confirmed in an independent laboratory study by Spieler et al. (2017) using ${{\rm{C}}}_{60}^{+}$ cations embedded in He-droplets.

If confirmed, the detection of interstellar ${{\rm{C}}}_{60}^{+}$ will constitute a major breakthrough in interstellar chemistry and may provide, for the first time, an insight into the true scale of the chemical complexity in the diffuse ISM. Measurements of the NIR electronic transitions of ${{\rm{C}}}_{60}^{+}$ (in absorption) would also provide a unique complement to the discovery of mid-infrared C₆₀ and ${{\rm{C}}}_{60}^{+}$ emission bands in circumstellar and interstellar environments (Cami et al. 2010; Sellgren et al. 2010; Berné et al. 2013).

Despite dedicated observational studies (Walker et al. 2015, 2016; Galazutdinov et al. 2017), the case for interstellar ${{\rm{C}}}_{60}^{+}$ has not yet been proven beyond reasonable doubt. Based on the laboratory measurements, five absorption features are expected (at 9348.4, 9365.2, 9427.8, 9577.0, and 9632.1 Å, with strength ratios 0.07:0.2:0.3:1.0:0.8; Campbell et al. 2016b). Galazutdinov et al. (2017) were unable to confirm the presence of the weakest 3 features in a sample of 19 heavily reddened Galactic sightlines observed from the ground. Instead of a constant ratio for the two strongest ${{\rm{C}}}_{60}^{+}$ DIBs (λ9577 and λ9632), as expected for electronic transitions arising from a ${}^{2}{A}_{1u}$ ground vibronic state, Galazutdinov et al. (2017) found that the interstellar band ratio was highly variable among different lines of sight. We note, however, that if the observed transitions involve lower levels above the ground state (such as the split levels arising from Jahn–Teller distortion), a variable ratio could potentially occur. Although Walker et al. (2016) claimed interstellar detections of all five ${{\rm{C}}}_{60}^{+}$ bands, the three weaker ${{\rm{C}}}_{60}^{+}$ bands are problematic as they fall in a wavelength region heavily obscured by absorption due to water vapor in the Earth's atmosphere (see Galazutdinov et al. 2000, 2017). Telluric-correction methods for such weak interstellar absorption features are error-prone as a result of incomplete telluric line cancelation, and from the possible presence of weak (unseen) stellar lines in the telluric standard spectrum. To rigorously confirm the identification of interstellar ${{\rm{C}}}_{60}^{+}$, high signal-to-noise observations of all five absorption bands are required, preferably from outside Earth's atmosphere.

In this Letter, we present the first ever ultra-high signal-to-noise ratio (S/N), high resolution stellar/interstellar spectra obtained using the Hubble Space Telescope (HST) imaging spectrograph, targeting four of the five ${{\rm{C}}}_{60}^{+}$ features (λ9349, λ9365, λ9428, and λ9577) along a heavily reddened line of sight. Very high continuum signal-to-noise ratios (S/N > 500 per spectral channel) are commonly achieved using ground-based optical telescopes, but such sensitivity has not previously been obtained using (direct) HST spectroscopy at red and near-IR wavelengths due to severe CCD fringing. After successfully employing a previously untested "STIS scan" observing mode to trail the target star along the spectrograph slit (crossing hundreds of CCD rows to facilitate fringe cancelation), unprecedented S/Ns ∼ 600–800 per dispersion pixel have been obtained. These spectra have permitted the first search for weak interstellar absorption features in the NIR, unhindered by telluric absorption.

To reliably detect the weaker ${{\rm{C}}}_{60}^{+}$ DIBs, a high-S/N interstellar spectrum is required with minimal contamination from interloping stellar photospheric lines. The heavily reddened (E_B–V = 1.01) B0 I star BD+63 1964 was selected due to its extremely strong DIBs—the majority of DIBs are among the strongest for stars of comparable extinction (Tuairisg et al. 2000)—as well as its early spectral type and relatively clean predicted spectrum in the vicinity of the λ9365 and λ9428 ${{\rm{C}}}_{60}^{+}$ bands. The lightly reddened B0 I star 69 Cygni (E_B–V = 0.14) was selected for comparison, to assist in the discrimination of stellar and interstellar features.

2.1. HST/STIS

Data were acquired over a single orbit for 69 Cyg and two orbits for BD+63 1964 using the G750M grating with a central wavelength of 9336 Å (covering the range 9050–9610 Å). This covers four of the five known ${{\rm{C}}}_{60}^{+}$ bands; a change of grating tilt would be required to obtain the fifth band. We used the 52'' × 01 slit, and the plate scale was 005 per pixel. Following peakup and focusing maneuvers, a series of STIS-scan exposures was performed for each star, trailing the star along the slit at a constant rate with the shutter held open. For 69 Cyg, six exposures were obtained: two long scans spanning up to 1000 detector rows and four shorter scans spanning ∼300 rows. The scan direction was alternated for successive exposures. For BD+63 1964 (observed in a separate visit after the optimal observing sequence had been established), eight identical forward scans were obtained spanning 700 detector rows each, toward the upper part of the CCD. A pair of flat-field exposures (contemporaneous fringe flats) was obtained in sequence following each pair of target star exposures, and 5–6 additional pairs of flats were obtained to fill in the remaining orbital time during occultation. Exposures of a Pt/Cr–Ne lamp were obtained during each orbit for wavelength calibration. Subsequent conversion from vacuum to air wavelengths was performed using the Morton (2000) formula.

The STIS CCD suffers from charge transfer inefficiency (CTI) during readout, as a result of long-term radiation damage. The raw frames were corrected for CTI using the procedure outlined by Anderson & Bedin (2010)¹⁴ , which significantly reduces the severity of bad CCD columns. Standard dark current, bias, geometric distortion, and wavelength corrections were performed using the Iraf calstis package (Hodge et al. 1998). For BD+63 1964, the four trailed science exposures in each orbit were sufficiently well matched to allow statistical cosmic-ray rejection using ocrreject. This was not possible for 69 Cyg due to imperfect registration of the exposed rows between each frame, which may have occurred due to errors in the guide star tracking, as well as possible exposure timing errors.

Spectral extraction was performed by summing the counts along CCD columns. Bad pixels and residual cosmic rays were rejected based on their standard deviation (using a 3σ threshold), after normalizing the individual CCD rows by their median levels. Such row normalization was required due to variations in the illumination function along the slit, resulting from slight variations in the slit width and telescope scan rate. Scattered-light subtraction was performed with a low-order fit to the light under the two occulting bars (positioned 1/3 and 2/3 of the way along the slit). Extracted Pt/Cr–Ne lamp line widths were consistent with the nominal instrumental resolving power of 10,000 (30 km s⁻¹).

The STIS CCD suffers from severe fringing due to internal reflections at wavelengths greater than 700 nm, where the chip starts to become transparent to incoming light. Fringe amplitudes are variable with wavelength, with a maximum of ±20% near 900 nm (see, e.g., Figure 1). The aim of our fringe cancelation strategy was to match as closely as possible the illumination pattern of the science exposure to that of the flat field by STIS scanning. This approach has the major advantages of (1) reducing the impact of individual bad pixels and cosmic-ray hits, (2) allowing for orders of magnitude increases in the number of counts per science exposure, and (3) eliminating the need to change the slit mechanism during observations, resulting in a consistent optical setup for the science and flat-field exposures.

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As shown in Figure 1, scanning the exposed star across hundreds of CCD rows results in significantly reduced fringe amplitudes (a five-fold reduction in the spectral rms in this case). Following correction using a contemporaneous fringe flat, we obtained apparently complete fringe cancelation down to the level of the statistical noise.

2.2. CFHT ESPaDOnS

2.3. Comparison between STIS and ESPaDOnS Spectra

The main purpose of these ESPaDOnS spectra was to verify the reliability of our nonstandard STIS observation and reduction procedures. A comparison between the HST/STIS and the telluric-corrected CFHT/ESPaDOnS spectra is shown in Figure 2. Telluric-correction artifacts in panels (a) and (b) are severe, preventing a reliable comparison in the vicinity of the weaker ${{\rm{C}}}_{60}^{+}$ bands. However, in the less problematic range 9500–9600 Å (panel (c)), the agreement between the two instruments is excellent, which demonstrates the success of our STIS observation and data reduction strategy. Slight discrepancies may be due to differences in the instrumental resolving power, telluric-correction residuals, or uncertainties in the continuum level of the high-dispersion CFHT spectrum. Some uncertainty in the STIS scattered-light correction also remains, leading to uncertainty in the zero level. A more detailed characterization of the (2D) scattered-light properties for STIS scan exposures awaits further study.

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The combined scattered-light-subtracted exposures of BD+63 1964 and 69 Cyg reached total counts of 8 × 10⁵ and 5.5 × 10⁵, respectively (per dispersion pixel). The presence of weak stellar and interstellar features precludes an accurate measurement of the continuum rms, but in regions free of any obvious lines, S/N = 600–800 was found for BD+63 1964 and ∼700 for 69 Cyg. To our knowledge, this is the highest S/N ever demonstrated with STIS for direct near-infrared spectroscopy at full grating resolution. The continuum-normalized spectra are shown in Figure 3.

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Both stars are B0 supergiants and have nearly identical stellar spectra. We used the non-LTE model atmosphere code CMFGEN (Hillier & Miller 1998, 1999; Hillier 2011), which solves the radiative-transfer equation for a spherically symmetric wind in the comoving frame under the constraints of radiative and statistical equilibrium. An effective temperature of 27,000 K, macro-turbulent velocity of 120 km s⁻¹, and projected rotational velocity of 85 km s⁻¹ were derived for both of the stars. The modeled lines are identified in Figure 3. Four features stand out in the BD+63 1964 spectrum that are not present in 69 Cyg or in the stellar model, which are likely due to interstellar absorption. The first three of these (at 9088, 9302, and 9412 Å) are identified as new DIBs; Galazutdinov et al. (2000) also found a possible DIB near 9412 Å. The absorption feature at 9577 Å coincides with the laboratory ${{\rm{C}}}_{60}^{+}$ wavelength. 69 Cyg also shows evidence for weak absorptions at 9577 and 9412 Å.

The equivalent width (EW) of the λ9577 band is 74 ± 3 mÅ. A Gaussian fit to its profile gives EW = 76 ± 3 mÅ, with a central wavelength of 9576.60 ± 0.03 Å, a central depth of 2.3%, and FWHM = 3.1 ± 0.1 Å. These line strength measurements may be considered accurate to within about ±10% due to uncertainties in the STIS scattered-light subtraction.

A close-up view of the four observed ${{\rm{C}}}_{60}^{+}$ band regions is shown in Figure 4. A spectral model for ${{\rm{C}}}_{60}^{+}$ is overlaid using the laboratory wavelengths and band strengths of Campbell et al. (2016a). We adopt Gaussian band shapes because a Lorentzian profile provides a poor match for the observed λ9577 band wings (Figure 4). The model has been Doppler shifted to match the interstellar K i velocity. As with most heavily reddened diffuse sightlines, the K i line toward BD+63 1964 (recorded in our ESPaDOnS data) shows a complex velocity structure, with three main components at −16, −27, and −33 km s⁻¹. We take the average of these (−25 km s⁻¹) as the interstellar ${{\rm{C}}}_{60}^{+}$ radial velocity. The λ9577 band carrier abundance could be variable across the three K i clouds (due to variations in chemical abundances and ionization levels), so we assign an error margin of ±10 km s⁻¹, which corresponds to ±0.3 Å. Within the uncertainties, our inferred (Doppler corrected) λ9577 rest wavelength of 9577.4 Å matches the laboratory value of 9577.0 ± 0.2 Å; the slight redshift could indicate that ${{\rm{C}}}_{60}^{+}$ is associated more with the higher-velocity K i gas. The 1.0 Å instrumental resolution contributes 0.2 Å to the observed DIB FWHM. The additional (0.4 Å) of broadening in the observed band compared with the laboratory measurement of 2.5 Å is consistent with the spread of interstellar K i velocities in this sightline.

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We find weak absorption features in our BD+63 1964 spectrum close to the wavelengths of the λ9348, λ9365, and λ9428 ${{\rm{C}}}_{60}^{+}$ bands. These features are not present toward our standard star 69 Cyg, and may therefore be due to interstellar ${{\rm{C}}}_{60}^{+}$. However, the appearance of the λ9348 band is far from clear due to contamination by another possible weak DIB at 9350 Å (previously identified at a rest wavelength of 9351 Å by Walker et al. 2016), as well as overlapping stellar He ii absorption. The λ9365 feature is partially contaminated by a weak stellar O ii line, and is broader and deeper than expected. The λ9428 feature is too weak for a reliable measurement.

The strengths of different DIBs are known to vary among sightlines with differing physical and chemical properties, independent of the total amount of interstellar material (e.g., Krełowski & Walker 1987; Ehrenfreund & Jenniskens 1995; Cami et al. 1997; Elyajouri et al. 2017; Ensor et al. 2017). The EW per unit reddening (EW/E_B–V) provides a measure of the relative strength of a given DIB. For BD+63 1964 we find EW(9577)/E_B–V = 73 mÅ, which is among the smallest values known for this band. Unfortunately for our present study, this means that BD+63 1964 is one of the least favorable heavily reddened sightlines in which to search for ${{\rm{C}}}_{60}^{+}$, which explains to some degree our difficulties in establishing (or disproving) the presence of the weaker ${{\rm{C}}}_{60}^{+}$ absorption bands.

The extreme weakness of the λ9577 band per unit E_B−V may be due to a lower than normal degree of ionization in this sightline. The well-known DIB at 6283 Å has a large EW/E_B–V value in diffuse clouds that are strongly irradiated by UV (see above references), while the 5797 Å DIB tends to be favored in more neutral, less irradiated clouds. The λ5797/λ6283 EW ratio is thus considered to be a tracer of the radiation field strength in the line of sight. Data have been taken from compilations of optical DIB measurements by Xiang et al. (2017) and the λ9577 measurements of Galazutdinov et al. (2000) to study the relationship between EW(9577)/E_B–V and the λ5797/λ6283 DIB strength ratio. Additional λ6283 and λ5797 measurements were obtained from archival data from the CFHT and Telescope Bernard Lyot. The results for nine sightlines are shown in Figure 5, including the present results for BD+63 1964. Even without HD 37022 (a highly irradiated sightline through the Orion Nebula with unusually high EW(9577)/E_B–V), the trend is for increasing λ9577 strength in gas pervaded by stronger radiation fields. In a survey of DIBs and atomic lines toward BD+63 1964 by Ehrenfreund et al. (1997), a relatively high gas density and low level of ionization was inferred, and the extremely low λ5797/λ6283 ratio is suggestive of weakly irradiated gas. The trend for increasing EW(9577)/E_B–V with radiation field strength is consistent with the assignment to ${{\rm{C}}}_{60}^{+}$ or another species whose ionization stage becomes dominant in more irradiated diffuse clouds (see also Foing & Ehrenfreund 1997).

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We performed the first successful test of the HST "STIS scan" spectroscopic observational mode. The resulting spectroscopic S/N (up to ∼800) is, to our knowledge, by far the highest demonstrated with STIS for direct stellar measurements at full grating resolution. This technical demonstration opens up a new wavelength range for interstellar, stellar, and exoplanetary spectroscopy, with unprecedented sensitivity.

We obtained the first "clean" measurements of the spectral region covering the λ9348, λ9365, λ9428, and λ9577 ${{\rm{C}}}_{60}^{+}$ bands without the confounding presence of telluric contamination or fringing artifacts. The λ9577 band exhibits a closely Gaussian shape with FWHM and peak wavelength consistent with laboratory measurements. However, due to the surprising weakness of the ${{\rm{C}}}_{60}^{+}$ features in our chosen sightline, the strengths and profiles of the (intrinsically weaker) λ9348, λ9365, or λ9428 bands could not be reliably measured. We identify a correlation between the λ9577 band strength (per unit reddening) and the λ5797/λ6283 DIB strength ratio (an indicator for radiation field strength), which suggests that the weakness of λ9577 toward BD+63 1964 is due to a low-radiation environment, consistent with the assignment of this DIB to an ion of a species whose first ionization energy is much less than that of neutral hydrogen (13.6 eV), and whose second ionization energy is ≳13.6 eV. Using the STIS scanning technique, high-S/N HST spectroscopy of more strongly UV-irradiated heavily reddened sightlines may provide the best opportunity for a conclusive identification of all five ${{\rm{C}}}_{60}^{+}$ bands, which will be required to place the detection of this molecule beyond reasonable doubt.

Possible new DIBs are reported at 9088, 9302, and 9412 Å. Their interstellar origin may be confirmed by follow-up studies of stars with differing spectral types and degrees of extinction.

Based on observations made with the NASA/ESA Hubble Space Telescope (program #14705). Support was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. F.N. acknowledges Spanish grants FIS2012-39162-C06-01, ESP2013-47809-C3-1-R, and ESP2015-65597-C4-1-R. ESPaDOnS observations were obtained under program 16BD86 at the Canada–France–Hawaii Telescope, which is operated by the National Research Council of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii.

Facilities: HST (STIS), CFHT(ESPaDOnS).