Old 28-08-2013, 10:18 AM
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Weltevreden SA (Dana)
Dana in SA

Weltevreden SA is offline
Join Date: Apr 2012
Location: Nieu Bethesda, Karoo, South Africa
Posts: 216
Collinder 261, an overlooked Musca gem

I've come across an interesting and little-mentioned OC in Musca. It's between Crux and Musca’s two glorious globulars, NGC 4372 and NGC 4833, listed on charts as Harvard 6 or Collinder 261.

Forming a shallow isosceles triangle above a line connecting A & B Muscae is a dim patch about the same 5’ visual diameter as Musca’s two GCs, but four magnitudes fainter. Aged 6 to 10 billion years, Cr 261 is exotic for being the oldest open cluster in southern skies, and one of the four oldest in the galaxy. It’s the only old ancient OC that orbits inside the Sun’s orbit; the other old OCs are either primordial halo objects or tidally stripped dwarf galaxy cores. Cr 261 interests professionals because it reflects conditions in the inner part of the galaxy disc about the same time the Sun’s cluster was formed—which is also about the same time the Milky Way's bar was first forming. Yet while the Sun’s ancient brothers & sisters & cousins & aunts long ago merged into the spiral disc, Cr 261 has hung together despite an orbit that sweeps it up and down through the disc roughly very 110 million years. It's one tough little cluster to have survived so long in the wild neighbourhood that comprises a galaxy spiral disc.

Cr 261 can be glimpsed as a faint patch in a good pair of 7x35 or 10x50 binoculars. It is easily spotted at 25x to 40x in a 80mm or 100mm refractor as a soft glow of light that can be mistaken for a small emission nebula. Raise the power to 80x or 100x and the patch looks to be about 5’ x 5’, slightly uneven in light distribution yet noticeably brighter than the nearby field. Tangent to the NW of the cluster is a distinctive line of eight 11th to 12 mag stars about as long as the cluster is in diameter; these make the cluster easier to find.

In 150mm to 200mm scopes at 100x to 225x, Cr 261 presents as a faint soft-edge 5 arcmin rectangle of mag 13 to 14+ stars speckling in and out above a faint haze. The ragged periphery blends into fainter field stars, bordered on the SE by a mag 10.8 star with a pair of 12.3 stars touching the cluster. Longer observing times—half an hour or so—will bring out roughly 20 stars in the mag 13.6 to 14.3 range, but the eye’s hold on them is tenuous at best. They come, they go, but after a time some return regularly enough to call them confirmed.

Cr 261 is 7140 light years away and its brightest star is only mag 13.6. Its overall surface brightness is a tepid 18.0. The nearby Jewel Box Cluster (NGC 4755) is 90% of Cr 261’s distance to the sun, at 6440 light years, yet it stands out like a lighthouse compared to CR 261’s glow-worm appearance. Why the difference? Both have about the same number of stars, ~100.

Age. At roughly 14.1 million years, The Jewel Box has some of the Milky Way’s brightest O – B stars and its most formidable red and yellow supergiants. The Jewel Box's colour-magnitude diagram looks like a steep ladder ascending 8 visual magnitudes. Ca 261 is roughly 425 times older with a minimum estimate of 6 billion years and likely quite a bit more than that. (The uncertainty lies in which stellar evolution model is applied to a cluster’s photometry.) Its CMD has a very squat, truncated look to it, no surprise considering how few large stars it has (thouigh it does have a surprisingly high ratio of 8 blue stragglers). Only 11 of Cr 261's stars are as large as 2 solar masses. All the other 90-odd known cluster members are less than the Sun’s mass, including six red 'giants'. The word ‘giants’ is relative: it refers to the size of a star’s observed photosphere (or radiating surface area) temperature, not its actual mass. That most of Cr 261's stars are less massive than the Sun is a strong hint that it is a very old cluster.

There may be an undocumented small cluster nearby. Ten minutes to the E of Cr 261 is the star HD 109448. Two minutes south of it is a tight asterism of roughly 15 stars mag 12.5 to 14. It shows up well (as does Cr 261) on WikiSky. Disregard the bordering mag 9.9 star; it's an unrelated field star. The suspected cluster position is RA 12 35 38, Dec -68 24 48. I sense that it's a small cluster because it's visual and photometric properties are too condensed and of similar magnitude to be a chance alignment. However, it's never been studied or identified as such. Have a look and tell us what you think.

Give both a try—but do it soon. The position is getting low in the sky and is best observed as soon as astronomical dark arrives. =Dana in SA
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