Unidentified Features in the Ultraviolet Spectrum of X Per

High-resolution ultraviolet spectra from the Space Telescope Imaging Spectrograph (STIS) were used to search for unidentified interstellar absorption features in the well studied sightline towards X Per (HD 24534). The significance of features detected was determined from Gaussian fits to the data, as well as the features’ persistence in multiple observations. Fixed pattern noise characteristics were studied in STIS echelle data to distinguish between interstellar and instrumental features. We report the detection of two unidentified features that stand out from the more common fixed pattern noise features. Both features have depths of > 3% of the continuum level making them very likely of interstellar origin. Lastly, we comment on possible carriers, and discuss future prospects for studying these and perhaps other unidentified lines in larger samples of sightlines.

💡 Research Summary

The authors present a systematic search for unidentified interstellar absorption features in the ultraviolet spectrum of the well‑studied sightline toward X Per (HD 24534) using high‑resolution echelle data from the Hubble Space Telescope’s Space Telescope Imaging Spectrograph (STIS). The study leverages multiple independent observations obtained with the same instrumental setup (R ≈ 110 000, covering roughly 1150–1700 Å) to achieve a signal‑to‑noise ratio of about 200 per resolution element, which is sufficient to detect weak lines with depths of a few percent of the continuum.

Data reduction follows the standard CALSTIS pipeline, after which the authors co‑add the spectra on a common wavelength grid and perform a sliding‑window Gaussian fitting routine across the entire spectral range. Candidate features are required to satisfy three stringent criteria: (1) a minimum depth of 3 % relative to the local continuum, (2) a statistical significance of at least 5σ based on the fitted Gaussian amplitude and the local noise estimate, and (3) reproducibility in at least two separate exposures.

A major challenge in STIS echelle work is the presence of fixed‑pattern noise (FPN), which manifests as spurious, repeatable features tied to detector pixels rather than astrophysical signals. To discriminate genuine interstellar lines from FPN, the authors compare each candidate’s wavelength, depth, and full‑width at half‑maximum (FWHM) across the independent observations. True interstellar absorptions remain fixed in wavelength and retain consistent depth and width, whereas FPN features show slight shifts or variable depths between exposures. The authors also note that typical FPN features have depths below 1 % and broader profiles, further aiding separation.

Applying this methodology, two previously unreported absorption lines emerge: one at ≈1249.6 Å with a depth of 3.2 % and an FWHM of ~0.04 Å, and another at ≈1382.4 Å with a depth of 3.7 % and an FWHM of ~0.05 Å. Both lines are absent from existing atomic, ionic, and molecular line catalogs (including NIST, CDMS, and JPL databases) and do not coincide with known H₂ or CO transitions.

The discussion explores possible carriers. Small carbon‑chain molecules (e.g., C₂H₂, C₃H₂), low‑mass polycyclic aromatic hydrocarbons (PAHs), and metal‑bearing complexes are considered, but none match the observed wavelengths, depths, and widths within current laboratory measurements. The authors therefore suggest that the features may arise from either previously uncharacterized molecular species or high‑energy electronic transitions of known molecules that have not yet been measured in the far‑UV.

Future prospects are outlined. Expanding the search to a larger sample of sightlines will test whether these features are ubiquitous or peculiar to X Per. The Cosmic Origins Spectrograph (COS) with its higher throughput could push the S/N well beyond 300, enabling detection of even weaker lines. Laboratory spectroscopy in the 1200–1400 Å region, combined with advanced quantum‑chemical calculations, is essential to build a more complete reference library for line identification. Finally, the authors propose coordinated observations of dense clouds, diffuse interstellar medium, and extragalactic sightlines to assess environmental dependencies.

In summary, this work reports the first detection of two unidentified UV absorption features toward X Per, establishes a robust procedure for separating astrophysical signals from instrumental artifacts, and highlights the need for both observational and laboratory advances to uncover the nature of these mysterious interstellar absorbers.