He2-68 in Musca

Our selected challenging planetary nebula this time is the faint He2-68 in the western portion of the constellation of Musca. Musca was first named by Bayer as Apis the Bee, but was renamed by Edmond Halley in the 15th Century as Musca Apis. Until the mid-1870s, these names were used interchangeably but, since this time, the name Musca Australis vel Indica was used, commonly referred to Musca Australis to make it distinct from the defunct constellation of Musca Borealis, then sensibly abbreviated to just Musca. Benjamin Gould assigned some seventy-five stars to the constellation, which is immersed in the most southern of the part of the Milky Way. Included here are some field objects within roughly 5° of He2-68.

He2-68 / SaSt 2-6 / Wray 16-94 / ESO 94-3 / PK 294-4.1 / PN G294.9-04.3 (11318-6558) is a faint PNe that can be found in the north-western corner of Musca, and lies 1.6°NW from 3.6v magnitude λ Mus / Lambda Muscae / HIP 57363 (11456-6644). Discovered by Henize in 1964, He2-68 is extremely faint at 14.9p magnitude and displays the smallish diameter of 9 to 10 arcsec. Located in a moderately starry field, it is certainly a fair challenge for visual observers, especially in glimpsing its small outer disk, but this is likely the case even with the largest of Dobsonians.

I have selected this PNe mainly because the immediate field (Figure 1) contains the pair NZO 23. Its general field also has the eclipsing binary TX Mus, being almost 14 arcmin due north of He2-68. Karl Henize used this star to find object He2-68, stating: 8.5′S of CPD-53° 1683; 9.5′ north, and slightly west, of CPD-52° 3642.”. Together, all three form a compact 60° — 30° right-angle triangle, with NZO pair being on the right-most angle.

He2-68 Small

Fig.,1. He2-68 Aladin 12½′×12½′ (left) ;
Fig. 2. He2-68 Close Image 40″×40″ (right)

Observations of He2-68

The only visual observation I have seen (and likely the first known amateur sighting), was given to me by Kent Wallace in a personal communication. He describes this stellar PNe using his 20cm. C8 SCT as:

At 100×, stellar, very faint, requiring the UHC filter and averted vision. Fair response to the O-III and UHC filters. No response to the Hβ filter. At 200× can see the PNe with averted vision alone as a very faint star, requiring the UHC filter to identify it as the planetary. Identified the field in the [SEC] finder chart.

I have tried to see this object twice myself in 20cm. back in 1984, but failed to see the PNe at all. Although I was very certain of the position, though I would admit would have loved to look for this object in 30cm or greater. The radial velocity suggests He2-68 is now receding from us at about +40.5km.s-1.

Looking at the attached magnified image (Figure 2, roughly 40×40 arcsec), shows the obvious peanut shape or in three dimensions the ring-like shape aligned diagonally across the field. Careful inspection reveals four protuberances facing at the points of the compass, almost as a bipolar nebular type of known as quadrupoles. This image was created as a composite of the AAO/R/DSS2 Images from Aladin: Image (red), SERC/I/ DSS2 (blue) and the average of the two (green). Red here most probably signifies the Hα emissions, which usually predominate PNe spectra. The front of the nebulosity is at its brightest, giving most of the observed light, which some 40cm Dobsonian might reveal beyond the stellar-like appearance as stated by Kent Wallace. He2-68 here reminds me very much of Musca‖s other interesting but particularly difficult and faint planetary, NGC 4071 / He2-75 / PK 298-4.1 / PN G298.3-4.8 (12043-6718). [See NSP 19.]


Last Update : 04th November 2011

Southern Astronomical Delights © (2011)

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