Searching for Interstellar Using a New Method for High Signal-to-noise HST/STIS Spectroscopy

Abstract

A note on versions:The version presented here may differ from the published version or from the version of record. If you wish to cite this item you are advised to consult the publisher's version. Please see the repository url above for details on accessing the published version and note that access may require a subscription. AbstractDue to recent advances in laboratory spectroscopy, the first optical detection of a very large molecule has been claimed in the diffuse interstellar medium (ISM): C 60 + (ionized Buckminsterfullerene). Confirming the presence of this molecule would have significant implications regarding the carbon budget and chemical complexity of the ISM. Here we present results from a new method for ultra-high signal-to-noise ratio (S/N) spectroscopy of background stars in the near-infrared (at wavelengths of 0.9-1 μm), using the Hubble Space Telescope (HST) Imaging Spectrograph (STIS) in a previously untested "STIS scan" mode. The use of HST provides the crucial benefit of eliminating the need for error-prone telluric-correction methods in the part of the spectrum where the C 60 + bands lie and where the terrestrial water vapor contamination is severe. Our STIS spectrum of the heavily reddened B0 supergiant star BD+63 1964 reaches an unprecedented S/N for this instrument (∼600-800), allowing the detection of the diffuse interstellar band (DIB) at 9577 Å attributed to C 60 + , as well as new DIBs in the near-IR.Unfortunately, the presence of overlapping stellar lines, and the unexpected weakness of the C 60 + bands in this sightline, prevents conclusive detection of the weaker C 60 + bands. A probable correlation between the 9577 Å DIB strength and interstellar radiation field is identified, which suggests that more strongly irradiated interstellar sightlines will provide the optimal targets for future C 60 + searches.

The EDIBLES survey II. The detectability of C60+ bands

“…The two DIBs increase in this region dominated by UV radiation, as expected for the C 60 molecule ionization and recombination properties. The low ionization potential of C 60 (7.61 eV) favors its ionization in the diffuse medium outside dark clouds, a trend demonstrated by Cordiner et al (2017). Krełowski et al (2015) and have analyzed the 9577 and 9632 Å DIBs in more recent spectra of this star.…”

“…Secondly, weak stellar lines are present in the spectra of the early-type stars used for the DIB detections. In the infrared, their widths and depths can be very similar to those of the DIBs and they may overlap and mimic the weak DIBs (Foing & Ehrenfreund 1994;Cordiner et al 2017;. Finally, additional weak DIBs are also partially blended with the expected bands (Walker et al 2015(Walker et al , 2016(Walker et al , 2017.…”

“…Finally, additional weak DIBs are also partially blended with the expected bands (Walker et al 2015(Walker et al , 2016(Walker et al , 2017. Very recently, a new technique using the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST) has been developed by Cordiner et al (2017) and brought promising results. Data from this technique that are free of telluric contamination, are expected to close the controversy about the C + 60 identification.…”

and the Diffuse Interstellar Bands: An Independent Laboratory Check

“…Heavy telluric pollution in this wavelength region, particularly by water features, complicates the astronomical data interpretation. As stated earlier, a first attempt to use the Hubble Space Telescope for DIB research in the 1 µm region has been successful (Cordiner et al, 2017), but signals for bands 3, 4, and 5, if any, were found to be at the noise level and a follow up study for different target stars is needed for an unambiguous conclusion. Both C 60 (Cami et al, 2010;Sellgren et al, 2010) and C + 60 (Berné et al, 2013) have been detected in other environments in space through their mid-infrared emission spectra.…”

“…As stated before, spectral pollution, mainly due to atmospheric water, severely complicates the analysis of all these ground-based data around 950 nm. For this reason, in another very recent study, Cordiner et al (2017) presented the first Hubble Space Telescope data (i.e., telluric-free) around 1 µm. Several new near-infrared DIBs were reported, but the chosen target star was found to be suboptimal for the DIBs linked to C + 60 .…”