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

TL;DR

This paper reports a peculiar pair of unresolved HI clouds at velocities $+12$ km s$^{-1}$ and $-6$ km s$^{-1}$ that align with a faint white dwarf within a large HI cavity around the Upper Sco-Cen OB2 Association. Using high-resolution HI4PI and ancillary surveys, the authors argue these features, plus surrounding filamentary HI, are the relics of a very old planetary nebula that expanded into a low-density cavity possibly carved by an ancient supernova, with an age of about $3 \times 10^{5}$ years and a total HI mass near $0.7$ M$_{\odot}$. The study combines kinematic decomposition, geometric considerations, and environmental context (including potential SN remnants and the cavity’s depth) to support this relic PN interpretation, while acknowledging uncertainties such as the lack of CO data and complex ISM structure. The findings highlight how PN remnants can be detectable in 21-cm HI only when they expand into pre-cleared, low-density environments, providing insight into late stellar evolution and the interaction between remnants and large-scale ISM cavities.

Abstract

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.

Figures (8)

• Figure 1: The HI4PI single channel l-b map at +12 km s$^{-1}$ showing the ring of HI surrounding the USco Association in the center of the area outlined by the red dashed circle that extends from about l = 333 to 0 and b = 15 to 35. The area inside the ring is relatively devoid of HI at this velocity as discussed in the text. The boundaries of the Ophiuchus area discussed in the text are indicated by a red dashed-line rectangle. The small HI peak associated with a white dwarf star at (l, b) = (346.13, +27.38) is located just above the upper right corner of the rectangle. Legend for the HI brightness is in K km s$^{-1}$.
• Figure 2: a) A close-up view of the mystery features (Table 1) in a l-b map at (l, b) = (346.08, 27.33) in a HI4PI one channel (1.28 km s$^{-1}$) wide map at a velocity of +12 km s$^{-1}$. The apparent point source has the appearance of a beam shape with a small asymmetry to the north. b) The same area at -6 km s$^{-1}$ that shows an HI peak at (l, b) = (346.25, 27.33). The location of the white dwarf star discussed in the text is indicated by the star symbol. The nature of other structure in both maps is discussed in §3. Legends are in unit of K. km s$^{-1}$.
• Figure 3: Neutral hydrogen emission profiles from two surveys. a) HI4PI data at the position of the mystery source at (l, b) = (346.08, 27.33). b) The closest LAB profile at (l, b) = (346.0, 27.5) shows the same component at +12 km s$^{-1}$.
• Figure 4: LABl-b data in a 2 km s$^{-1}$ wide band showing the HI structure at +12 km s$^{-1}$ toward the USco Association. The locations of three spectroscopic binaries discussed in the text are shown as red filled diamonds. Legend in K. km s$^{-1}$.
• Figure 5: LABl-b data in 5 km s$^{-1}$ wide bands at the velocities indicated showing the outlines of the shell of HI surrounding the USco Association. At +8 km s$^{-1}$ the shell is as clearly visible as it is in Fig. 4. Its outlines are also distinguishable at +28 km s$^{-1}$. At -4 km s$^{-1}$ its outlines can still be seen and in addition there is evidence for a smaller ring of gas inside the larger one. The excess structure in the center of the area may include gas at the near-side face of the shell surrounding the USco Association. At -12 km s$^{-1}$ a hint of structure related to the main shell is visible. Legends are in K. km s$^{-1}$.