He2-103 in Circinus

Although moderately tough to see, the planetary nebula He2-103 lies on the northern boundary of Circinus with Centaurus. I personally do like this region of the sky as it contains so many interesting objects with numerous bright stars to help locate these tempting objects. Roughly over in the next few Neat Southern Planetaries instalments we will cover the area surrounding the rough square” of sky of Alpha and Beta Centauri, Alpha Circini and the north-eastern edge of the Coal Sack or Black Magellanic Cloud. First here is He2-103 which has mostly been selected because of its close proximity to the Circinus Dwarf Galaxy. As you may have noticed throughout this series, most of the PNe are located near bright objects — reflected by my perhaps more lazier or antiquated methods of finding objects via star hopping. I have sometimes found over the years exclusively use of setting circles can only make you miss out on some of the more interesting objects surrounding your particular target.

(Note: The most challenging deep-sky that I know, and not obeying this rule is the dread and hard southern Pavo PNe, He2-434 (NSP06).) If you do look at any of these planetaries, an O-III or UHC filter will prove a godsend.

He2-103 / Sa2-103 / Wray 16-144 / ESO 97-11 / PK 310-2.1 / PN G310.7-02.9 (14056-6441) [U452] is another faint and challenging planetary in northern Circinus. Although discovered by Karl Henize in 1964, it is often listed as Sa2-103, and is wrongly listed by some sources (like Steven Hynes Planetary Nebulae” (p.134)) as being in Centaurus. It lies 2.2°NE (PA 34°) from NSP 14s NGC 5315, along an imaginary line between NGC 5315 and a point halfway between α and β Centauri. Alternatively, use α Circini, and move the telescope 3.4°W to find the field. I found He2-103 by simply placing β Centauri on the western edge using a low powered eyepiece, and then moving the telescope 4.3°S. Approaching the field of He2-103, three nearly equal 9th magnitude stars appear in a straight line, while another 9th magnitude star lies 12′ S. Continue another 10′S from this last star, and this marks the place of the PNe.

He2-103 Wide Field
He2-103 Close Field

The field teems with a scattering of moderately faint stars. I could just see the small disk of He2-103 using both averted vision and high power in 30cm, but this was not easy. Using moderate to low magnifications it became impossible to see, likely because of the overwhelming brightness of the sky background. He2-103 subtends the diameter of 20.0 arcsec, and is 14.0v or 13.9p magnitude, first obtained from photographic measures made by Cahn/Kaler. (AJ.Sup.Ser, 223, 319-368 (1971)) I found the use of an O-III made only a slight improvement to this faint, but featureless, smudge. Looking at the planetary nebula data from the ESO-Strasbourg PNe Catalogue (1996), I suspect it would appear much better in the UHC filter than the O-III filter. Apertures greater than 35cm should have little trouble seeing it, though I would firmly recommend that observers attempt this from a dark sky site.

The scant observations available on the PNN have not found any magnitude information, though it is suspected to be of spectral class O VI. Overall it is presently moving towards us at -29.9 km.s-1, while the expansion velocity of the gas is estimated to be less than 6 km.s-1. Other than a NIR magnitude determination, little information has been obtained from other studies. Statistical distances average about 2.8 kpc, which likely accounts for the planetaries faintness in a fairly dusty region of space. The most recent distance is 3.419 kpc. (2008)

Overall, He2-103 maybe a decent challenge for larger apertures.

Surrounding Fields of He2-103

NGC 5288 Cr 278 / C* 1345-644 / OCL 910 (13487-6440) is a moderately bright star cluster with only a few stars visible, and is not listed in AOST1 or 2 nor in Sky Atlas 2000.0. The cluster is 1.8°E of Sa2-103 — about halfway between the PNe and the eastern edge of the Coal Sack. It can be found in the NE corner of Circinus, in an odd-looking small piece of square sky once manufactured by Delporté in 1928 to accommodate the two already named eclipsing binaries of T and V Circini — thus keeping them within Circinus boundaries.

Circinus Dwarf Galaxy Classed as 2 1 p −, compact NGC 5288 contains some twenty-five stars in an area about 4.0′ across. Some debate earlier existed on the reality of the cluster, mainly because of its apparent faintness compared to the background star field. When John Herschel observed it in 1835, he claimed to see a …very compact knot of Milky Way”, and estimated the mean stellar magnitude as 14th, though the RNGC gives the mean magnitude as closer to 12th.

In 20cm, NGC 5288 looks nothing at all like Herschels description. Just 2.6῎W and slightly south of the heart of the cluster, is the 7.8 orangish star HIP 67356 / HD 119941 (13483-6441), as Herschel describes; …a star 8th magnitude precedes.”. Another 8th magnitude star 14′ preceding the cluster. This is reddish-orange 7.8 magnitude star HIP 67226/ HD 119661 (13465-6441). Herschel fails to mention this rather fairly prominent field star in his description of the cluster. Is Herschel indicting that this is the preceding star in his description? (HIP 67226 is the same field as the cluster.) How John Herschel twice found this cluster remains a bit of a mystery, as I expect 30cm. might have some trouble seeing more than a dozen of its stars.

At the given position of NGC 5288, there are only two 16th mag. stars indicated in Megastar 5.0, and I could just see in 20cm. perhaps a handful more. Attached is the STScI image, that shows the cluster and the bright star in the field is HIP 67356.

An important new examination of this cluster appeared in a paper by Piatti, et al. (2006). Using CCD photometry, they find the distance as 2.1±0.3 kpc. with an age of 130+40-30 Myr, and conclude; that NGC 5288 is moderately young and probably more metal-rich than the Sun.”.

References : NGC 5288

  1. Piatti, A.E., Claria, J.J., Ahumada, A.V., First estimates of the fundamental parameters of the relatively bright Galactic open cluster NGC 5288”, MNRAS., 367, 599 (2006)) [Link]

The Circinus Dwarf Galaxy / ESO 97-13 / ESO 97-G13 / PGC 50779 (14131-6530) is a field step away from He2-103, being the only galaxy within cooee in this part of the sky. Located 1°SE of He2-103 along PA 129°, it appears on Map 452 in Uranometria 2000.0 and is listed as object #631 in AOST2, with the AAT Schmidt image on p.209.

Discovered in 1977 using the Uppsala Schmidt telescope at Mt. Stromlo, this galaxy shows a band of obscuring dust swathing across the very southern parts of the disk however, this is mostly invisible in all amateur telescopes.

Circinus Dwarf GalaxyIn 20cm, and possibly even in 15cm., this ovoid 2′×1.5′ glowing smudge is aligned along PA 40°, appearing as a mostly featureless disk in a whole heap of field stars. This 9.84V, 10.89V (older sources say 12.1b) magnitude galaxy has the high surface brightness of 13.0 magnitude. Photographical the size of this small spiral galaxy extends 6.9′×3.0′. ESO 97-13 own spectra reveals the proximity of the galaxy, as the radial velocity is 426 k.ms-1 away from us.

The Circinus Dwarf Galaxy is surprisingly bright for an galaxy appearing within the light of the Milky Way, especially where you would expect obscuration by intervening gas to exterminate its considerable brightness. The literature quotes that this dwarf galaxy appears in a convenient less dense corridor of Milky Way, but there is no significant evidence to support this except for the highish B-V magnitude of 1.46. It is likely if this galaxy appeared away from the galactic plane, it would rate on amateur lists of as one of the better of the deep-sky galaxies in the southern skies. For observers to see this, you may have to wait a long time. I estimate some thirty million years would have to pass for the Sun to orbit the Milky Way sufficiently to bring it into view. AOST2 quotes the distance as 4.0 Mpc. to 4.2 Mpc (about 13 million ly), suggesting that it is well away from our Milky Way conglomeration of companion galaxies.


Last Update : 04th November 2011

Southern Astronomical Delights © (2011)

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