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Old 12-04-2014, 04:00 AM
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Weltevreden SA (Dana)
Dana in SA

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Location: Nieu Bethesda, Karoo, South Africa
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Wd 2—the Feathers of Grandfather Fire

Wd 2—The Feathers of Grandfather Fire

Last month IIS’er Simmo logged Wd 2 with the observation, ‘It is an elongated object much like the shape of an ice cream cone . . . This cluster contains some of the hottest, brightest and largest stars known.’ Paddy added even more observing data from his 16”. Those posts piqued my interest. Overlooked or underreported objects do to me what the sound of the food dish being filled does to my cat.

Image 01: Source: Torres TriAtlas B, chart 201.

Bengt Westerlund first studied the Wd 2 – NGC 3199 region in a 1959 paper (see his Figure 2) but he did not recognize Wd 2 as an intense star-forming region. He simply noted, ‘a group of faint spectra were found to cover a small fraction of the core region [of the associated nebulosity RCW49] in a way that is typical for small, dense clusters, e.g., in the Magellanic Clouds.’ Today we know the Wd 2 region’s spectra were so faint because Wd 2 lies only 0.3° below the centreline of the Galactic Plane where Galactic dust bunnies come in light-year size.

Wd 2 is rewarding to we eyepiece-warmers only if we know exactly what we are looking at and why it appears as it does. Wd 2 is a bit hard to identify at first because it lies a very small patch of faint mag 13-14 stars in a region crowded with much brighter stars. At 150x and above in a 150mm or larger scope Wd 2 yields up five tightly packed stellar components. My 100mm achro at 96x does show it, but as a near-the-limit fuzzy star. Even in a 200mm scope at 300x, all I see is a tiny kite of five stars which can be glimpsed in direct and held steadily in averted. Reduce to 150x and add a UHC filter and the cluster blossoms with associated nebulosity. Paddy’s Wd 2 post says he saw 20 stars in this tight ball. Ahh, aperture, sweet aperture, the intriguing smile blossoms into a comely maid.

The surrounding environs are worth an evening all unto itself. The two-degree finder field around Wd 2 embraces the older and more spread-out cluster NGC 3247, scads of field stars of all brightnesses, and a very fine nebular cluster RCW48/NGC 3199 1.5 degrees WSW. But once spotted, Wd 2 is like finding the tastiest bonbon in a candy store, you can’t eat just one. It is listed as WESTR2 on Chart 201 of my Torres B set and on Chart 86 of Michael Vlasov’s set.

Image 02 below is a more detailed view from Jose Torres’ TriAtlas C. This cutaway also nicely frames the spangled young cluster NGC 3293 just above Eta Carinae—worth a special detour in its own right because it rivals the nearby Jewel Box (Kappa Crucis) for sheer prettiness.

Image 02 source: Torres TriAtlas C chart 533.

Also be sure to take a little side-tour to nearby nebular arc RCW48, a third of a degree in diameter and 1.5° SSW. This is part of a gas ring expelled from the high-emission mag 10.8 point source which excites RCW48’s HII emission, the nitrogen-dominant Wolfe-Rayet HD 89358. HD 89358 lies within the sparse cluster NGC 3199. RCW48’s pretty arc is quite a nice show all by itself., and arc-shaped because parts of it are extinguished by MW dust.

Configure your go-to to RA 10:24:00 Dec −57:45:26 and you’ll plop right into the middle of Wd 2. A hi-rez WikiSky printout of the Wd 2 area helps distinguish the cluster from the cluster’s hot HII region RCW49, which is part of Wd 2’s early-formation ejecta. RCW49 fascinates the professionals as much as the cluster itself because of RCW49’s unusually high radio emissions. (Beware: the ‘RCW49’ on the Torres C chart is mislabeled; it is actually RCW48, an optical, radio, and x-ray emission patch that was also studied by Westerlund in 1963. RCW 48 is suspected to be a rapidly rotating neutron star associated with nebulosity NGC 3199.)

The RCW49 described in Simmo and Paddy’s 27 Feb 2014 IIS posts is 17 arcmin E of Wd 2’s tiny 3 arcmin ball of stars. While discernible unfiltered, it becomes a standout in UHC. Once centered in the eyepiece, Wd 2 doesn’t exactly knock one’s socks off. It looks like a faint, tight mag 12.5 asterism of the type we see by the cartload every time we scan the Milky Way. Across six observing sessions under excellent skies and seeing from 0.8 to 1.4 arcsecs, at 150x to 300x in 15 to 20 cm scopes, the five tiny specks I logged in Wd 2’s 2 arcmin lozenge-shaped core are most likely the Wd 2’s five brightest stars, photometrically measured in descending order as:

MSP 018, vis.mag 12.88 at RA 10:24:02, Dec -57:44:35, an O7 giant radiating at 40,100 K
MSP 203, vis.mag 13.31 at RA 10:24:02, Dec -57:45:35, an O7:V giant at 38,600 K
MSP 188, vis.mag 13.41 at RA 10:24:01, Dec -57:45:31, an O7:V giant at 42,600 K
MSP 183, vis.mag 13.57 at RA 10:24:02, Dec -57:45:31, an O7:V giant at 43,900 K
MSP 175, vis.mag 13.98 at RA 10:24:01, Dec -57:45:30, an O6V-III giant at 39,000 K

Paddy’s 20 specks are about all anyone with a 16” scope can see. The next 40-odd stars clumped from vis.mag 14 down to 18 are so tightly packed they require very high mags and sub-0.5 arcsec air quality to bring out. Observatory scopes use equipment with sub 0.1 arcsec per pixel resolution.

Image 03 source: ESO

Twenty-nine of these are O giants, one of the densest concentrations in the Galaxy. Wd 2 lies between 13,700 and 26,000 light years away. There is a vigourous difference of opinion on the subject of Wd 2’s distance (and therefore age). It’s amazing how two different groups can draw dramatically different conclusions about one single object. Given the vigour with which both sides enlist equations into their respective estimates, we amateurs are better off just looking at the thing and enjoying the show.

I think of Wd 2 and its neighbourhood the way an 11th century Chinese poet described the ‘Guest Star’ we now know to be the 1054 supernova behind the Crab Nebula, M1:

‘Upon seeing this, would you have known
you were seeing the feathers of Grandfather Fire?’
Attached Thumbnails
Click for full-size image (02 Wd 2 fm Torres TriAtlas C 533.jpg)
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Click for full-size image (03 Wd 2 & RCW49.jpg)
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Attached Files
File Type: pdf 01 Wd 2 field fm Torres TriAtla B chart 201.pdf (444.5 KB, 15 views)
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