Gum 48d HII region is a well evolved and well studied HII region [1-3] located at a distance of about 3.5 kpc [2, 3] in the Southern part of Centaurus constellation, in the vicinity of Beta Cen (Hadar). At this distance the edge of the frame corresponds to about 60 ly, while the apparent diameter of the region is about 0.25 deg, so a lot of structural details can be captured in it. The region has a distinct crescent shape resembling the shape of one iconic SciFi artifact [4], and for that it can be called the “Bat'leth Nebula”. 

The central part of the Gum 48d HII region is dominated by a yellow supergiant star HD 119796 [5] (aka V766 Cen, HD 5171A, HIP 67261, typically staying around +6.4m magnitude but dropping as low as +7.3m once during the history of observation [6, 7]), that is found to be a contact eclipsing binary system by interferometry observations with AMBER/VLTI [6-9]. The primary component of HR 5171A binary turns out to be one of the largest known stars with the physical radius estimated at 1575 ± 400 solar radii [10, 11], while it’s secondary component is estimated to have a physical radius of “only” 650 ± 150 solar radii [10, 11]. The HR 5171A binary in turn is a component of triple star system together with another supergiant star HR 5171B (+9.7m according to UCAC3), where the HR 5171B component is identified as the current primary source of ionization for the region [2, 3]. The view to the central part of the image based on the 8x OIII 600 sec subset of exposures separately processed for preserving full details for the stellar component of the image and fully suppressing the diffuse component is given in Fig. 1. 


Fig. 1. Cropped version of the frame centered on HD 119796 (aka HR 5171A) at 400% of original resolution. The dashed circle of 9.7 arc. sec radius centered on HD 119796 is matching the angular separation between HR 5171A and HR 5171B components [8, 9]. This angular separation corresponds to 0.54 ly at the distance of 3.5kpc.


The history of formation of Gum 48d HII region and the most likely explanation for its unusual present day crescent shape is outlined in detail in Section 4.2 and Fig. 9 of [2, 3]. According to that outline the process started “only” about 10 Myr ago with a group of OB stars creating the initial ionized bubble within a giant molecular cloud that continued expanding until it encompassed most of the material of the cloud at about 4 Myr ago. At that time the HR 5171A and HR 5171B stars formed on the periphery of the original HII shell creating their own ionized bubble that continued expanding until about 1 Myr ago [2, 3]. At a later time the ionization balance of the secondary bubble has shifted, as it transited to the current configuration in which the bubble is getting ionized only by one star HR 5171B [2, 3].

Based on this model, the schematic view of the structure of Gum 48d HII region can be devised as shown in Fig. 2, with following spherical bubble sub-regions identified in it:

• (a) the primary HII region that is now extinct but still representing a relatively low density volume of space in the top left corner of the image (most of it is out of the FOV of he image),
• (b) the molecular cloud ridge remaining from the original giant molecular cloud from which the region was formed (most of which is also out of the FOV),
• (c) inner shell centered on V766 Cen (HD 5171A) of the estimated radius of 7.9 ly (at a distance of 3.5 kpc),
• (d) secondary shell centered on V766 Cen of the estimated radius of 15.6 ly (assuming same distance),
• (e) outer shell centered on V766 Cen of the estimated radius of 24.8 ly (assuming same distance).

Fig. 2. The original image with spherical bubble structure overlays described above.


The geometry of the shells (c) and (d) identified in Fig. 2 appears to be significantly distorted, likely due to the presence of the remnant of the original HII bubble (a). The attempt to trace the boundaries of these shells by the peak intensity for the boundary of each is shown in Fig. 3, with the full set of features identified in it listed below:

• (a) and (b) are the same as in Fig. 2,
• (c) and (d) are the results of tracing the two most prominent density fronts,
• (e) a hint of another spherical bubble adjacent to the front (d),
• (f) teardrop shaped compact dark nebula (DN) likely located in front of the region with estimated length of about 2.8 ly (at a distance of 3.5 kpc),
• (g) a peculiar diffuse component of the inner part of the region revealing itself in Ha and SII channels.

Fig. 3. The original image with density front overlays and selected compact structures described above.


Fig 4. shows the cropped version of the image showing the areas around the central star and the features identified earlier in Fig. 3 as diffuse component (f) and compact DN (g) at 150% of the original resolution. One more region of interest identified in Fig. 4 is the area (h) that shows a peculiar periodic structure with a period of about 8 arc. sec which corresponds to 0.44 ly = 161 light days at a distance of 3.5 kpc.

The intensity profile for the line (L), a radial line aligned with the central star of the region, is added in the top right corner in Fig. 4 and shows the estimated linear size of the gaps between local minima of that profile. This periodic structure is unlikely to be a result of a light echo of some kind as comparing the way how this area looks in Ha channel based on exposure integrals collected in Apr 2023 and and Apr-May 2024 doesn’t reveal any significant change in those structures. Hence it is likely caused by medium density variations / neutral gas tendrils embedded into the bubble. The two characteristic periods associated with V766 Cen are 263 days (radial velocity / spectral variation) [12, 13] and 1304 days (orbital period of the secondary component around the primary one in this tight binary) [6, 7], which would correspond to 13 arc. sec and 65 arc. sec visible distance between the intensity peaks in the medium in a hypothetical light echo scenario at the estimated distance to Gum 48d. Future observations might be able to detect such effects in the nebula due to variability of its central star.


Fig. 4. The cropped version of the image showing the areas around the central star and the features identified earlier in Fig. 3 as diffuse component (f) and compact DN (g) at 150% of the original resolution. Region of interest (h) is provided with intensity profile along the line (L) shown in the right top corner of the image.


Data and initial calibration/integration: Alexandr Zaytsev https://www.astrobin.com/users/m57ring/

ASA Ritchey-Chretien RC-1000: D=1m, f/6.8 on alt-azimuthal direct drive fork mount, FLI ProLine 16803 with secondary mirror based motorized focusing and automatic de-rotation (Telescope #1 system of ChileScope observatory, Río Hurtado Valley, Chile).

23h10min of combined integral LRGBHSO exposure collected over 14 imaging sessions using Chilescope Telescope #1 system spanning the period of 13 months, and structured as follows:

• 10x Ha + 8x OIII + 8x SII guided 600 sec exposures (4h20min of combined integral) collected over 3 imaging sessions carried out on Apr 17, 2023 and Apr 16, 2024.
• 7x L + 7x R + 7x G + 8x B guided 600 sec exposures (4h50min of combined integral) collected over 7 imaging sessions carried out on Apr 17, 2024 and May 3, 5, 9, 25, 26, 29, 2024.
• 10x Ha + 9x OIII + 9x SII guided 1800 sec exposures (14h of combined integral) collected over 9 imaging sessions carried out on Apr 18, 19, 2024 and May 1, 2, 3, 5, 9, 28, 29, 2024.

Image Processing: Mark Hanson https://www.hansonastronomy.com


Enjoy, Mark and Alex 


[1] http://galaxymap.org/cat/view/gum/48d

[2] https://iopscience.iop.org/article/10.1088/0004-637X/697/1/133

[3] https://iopscience.iop.org/article/10.1088/0004-637X/697/1/133/pdf

[4] https://en.wikipedia.org/wiki/Bat%27leth

[5] https://simbad.cds.unistra.fr/simbad/sim-id?Ident=HD119796&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id

[6] https://arxiv.org/abs/1401.2628

[7] https://arxiv.org/pdf/1401.2628

[8] https://www.eso.org/sci/facilities/paranal/decommissioned/amber.html

[9] https://www.aanda.org/articles/aa/pdf/2007/10/aa6496-06.pdf

[10] https://arxiv.org/abs/1709.09430

[11] https://arxiv.org/pdf/1709.09430

[12] https://ui.adsabs.harvard.edu/abs/1982MNRAS.201..105B/abstract

[13] https://articles.adsabs.harvard.edu/pdf/1982MNRAS.201..105B