An unusual pair of interstellar HI features and a related white dwarf star inside the HI cavity surrounding the Upper Sco-Cen OB2 Association

Two mysterious unresolved HI structures at velocities of +12 and -6 km/s were discovered in high resolution 21-cm survey data in the direction of a faint white dwarf star. Examination of the HI morphology in this area of sky shows that the star and HI features exists in a large cavity in interstellar HI surrounding the Upper Sco-Cen OB2 Association. The cavity may have been created by an ancient supernova. It is hypothesized that the pair of HI features and filamentary HI structure found in its immediate vicinity may be the remnants of a planetary nebula some 3 x 10^5 years old that have cooled to the point that the gas is neutral and emitting the 21-cm spectral line. This remnant has maintained the morphological characteristics of the original planetary nebula because it expanded into a volume of space relatively devoid of interstellar gas that would otherwise have absorbed any traces of the original nebula.

💡 Research Summary

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The authors report the discovery of two compact, unresolved neutral hydrogen (HI) features at radial velocities of +12 km s⁻¹ and –6 km s⁻¹ in high‑resolution 21‑cm survey data (HI4PI and LAB). Both features are located at nearly the same Galactic coordinates (ℓ≈346°, b≈27.3°) and lie in the direction of a faint white dwarf (WD) identified in the GAIA DR3 catalogue (distance ≈110 pc). A SIMBAD query revealed two white‑dwarf candidates; the nearer one (WD #1) is within 0.1° of the HI peaks and is therefore considered the most plausible counterpart.

The authors measured the observed angular widths of the two HI peaks, deconvolved the telescope beam (16′ FWHM) assuming a Gaussian source profile, and derived intrinsic source sizes of roughly 0.5–0.7 pc for the –6 km s⁻¹ component and 0.3 pc for the +12 km s⁻¹ component. Using Gaussian fits to the spectra they estimated a column density of N_HI≈3 × 10¹⁹ cm⁻², which translates into a total neutral hydrogen mass of order 0.01–0.02 M⊙ for each feature. These values are comparable to the masses of known planetary nebulae (PNe), although on the high side for very old, dispersed nebulae.

The broader context is a large HI cavity (or “bubble”) surrounding the Upper Scorpius (USco) subgroup of the Scorpius‑Centaurus OB2 association. In the HI4PI l‑b maps a ring of HI emission at +8 km s⁻¹ encircles a region of markedly reduced HI brightness that extends roughly from ℓ≈333° to 0° and b≈15°–35°. The authors argue that this cavity was likely carved by an ancient supernova explosion (or the combined winds of massive OB stars) and that its depth may be comparable to its angular width, implying a line‑of‑sight extent of ~80 pc. With the USco distance estimates ranging from 140 to 160 pc, the white dwarf at 110 pc would lie inside or near the near side of this cavity.

Based on these spatial coincidences, the authors propose that the two HI features represent the relic of a planetary nebula ejected by the white dwarf roughly 3 × 10⁵ years ago. In this scenario the nebula expanded into the low‑density cavity, preserving its original bipolar morphology (the two opposite‑velocity components) while cooling enough for the hydrogen to become neutral and emit the 21‑cm line. The filamentary HI structures seen around the star in several velocity channels (±4–8 km s⁻¹) are interpreted as the remnants of the nebular shell, which would be expected to appear near the systemic velocity (~+3 km s⁻¹, the average of the two components).

The paper discusses alternative explanations only briefly. The possibility that the features are unrelated high‑velocity clouds, small dense HI clumps, or chance alignments with the white dwarf is not ruled out. Moreover, the authors acknowledge that no optical or infrared emission (e.g., Hα,