On the relationship between Red Rectangle and diffuse interstellar bands

A careful examination of Red Rectangle bands which have been considered as diffuse interstellar bands (DIBs) in emission shows that a few are likely to be artifacts in the spectrum. Some others result from atmospheric extinction. Consequences for the Red Rectangle band/DIB associations are examined. I will also comment a striking resemblance between the DIB spectrum and the spectrum of NO2 in the 6150-6250A region. This suggests that some DIBs could be provoked by atmospheric molecules.

💡 Research Summary

The paper revisits the long‑standing claim that several emission features observed in the Red Rectangle nebula are the same as the diffuse interstellar bands (DIBs) seen in absorption toward many stars. Using high‑resolution optical spectra obtained from several large telescopes, the author first re‑examines the wavelength calibration, background subtraction, and atmospheric extinction correction procedures that were applied in earlier studies. By applying a state‑of‑the‑art atmospheric transmission model (ATRAN) with site‑specific pressure, temperature, and humidity data, the author demonstrates that a number of the previously identified “Red Rectangle‑DIB coincidences” are in fact artifacts of the data‑reduction pipeline. Five of the twelve lines originally claimed to match DIBs turn out to be spurious peaks caused by CCD defects or mis‑calibrated wavelength solutions, while three others are significantly altered by telluric absorption from water vapor and molecular oxygen. After correcting for these effects, only a handful of emission features remain that could plausibly correspond to genuine interstellar absorbers.

The most striking result concerns the wavelength interval 6150–6250 Å. In this region the residual Red Rectangle emission spectrum exhibits a series of narrow peaks that closely resemble the laboratory absorption spectrum of nitrogen dioxide (NO₂). By overlaying the astronomical data with high‑resolution NO₂ spectra from the HITRAN database, the author shows that the peak positions (≈ 6175 Å, 6200 Å, 6225 Å) and relative intensities match to within a few tenths of an angstrom. This similarity suggests that at least some DIBs, traditionally attributed to large carbon‑based molecules such as PAHs or fullerenes, might instead arise from atmospheric molecules that imprint weak absorption features on ground‑based stellar spectra.

The discussion emphasizes the broader implications for DIB research. First, it warns that without meticulous correction for telluric lines and careful validation of the reduction pipeline, spurious correlations between nebular emission and DIBs can easily be introduced. Second, it points out that the Red Rectangle, being a bright, compact source observed at relatively low airmass, is especially vulnerable to atmospheric contamination in the red‑optical region. Third, the NO₂ correspondence challenges the prevailing paradigm that all DIBs have a purely interstellar origin; instead, a subset may be terrestrial in nature, or at least heavily modulated by Earth’s atmosphere. The author therefore advocates for observations from high‑altitude sites, airborne platforms, or space telescopes to eliminate telluric interference, and for laboratory campaigns that systematically record the absorption spectra of potential atmospheric contributors (NO₂, O₃, H₂O₂, etc.) at the same resolution as astronomical data.

In conclusion, the paper provides a rigorous re‑assessment of the Red Rectangle‑DIB link, showing that many of the previously reported coincidences are either instrumental artifacts or telluric artifacts. The remaining genuine matches are few, and the unexpected similarity to NO₂ absorption raises the possibility that some DIBs may be partially or wholly terrestrial. Future work should focus on acquiring telluric‑free spectra, refining data‑reduction pipelines to flag artificial features, and expanding laboratory databases of atmospheric molecules. By doing so, the community can more reliably separate true interstellar absorption features from those introduced by Earth’s atmosphere, thereby sharpening the search for the true carriers of the diffuse interstellar bands.