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Old 12-05-2016, 09:26 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
One steak: ESO 92-SC05. One sizzle: VDBH 176. II

One steak. One sizzle

Cars run out of gas. So do galaxies. At the Milky Way’s present gas consumption (i.e., star forming) rate of 1 to 2 solar masses every million years, the Milky Way will run out of gas in 2.6 billion years. That will at least get us across town on the long long road to the big city of Virgo Supercluster, but what happens when the gas tank runs low? Where’s the gas station?

Actually, there are two. The far-away one is a quarter of the way to Virgo, and is called Local Filament. Local Filament is a very big place and very complex to navigate. So let’s tank up closer to home. It’s called Galactic Stream System because it has 15 pumps (so far), all named Stream. They dispense fuel blends with names like Pal 5 Stream, Sagittarius Stream, Virgo Stream, Magellanic Stream, Anticenter Stream, Arcturus Stream, Helmi Stream, and a stream whose origins we can’t quite figure out which is called (surprise) Orphan Stream. That one is so far out there that it forms a giant ellipse larger in every dimension than the entire Milky Way disc and takes f-o-r-e-v-e-r to top up our tank. Another pump, called Monoceros Ring, wraps itself around the Milky Way three times and is thought to be the almost digested stars of the Canis Major Dwarf. And we thought putting a tiger in the tank was such a Big Deal.

There’s something a little suspicious about where these pumps are themselves filled. Nine of them list their origin as Defunct Dwarf Galaxy, four others say Defunct Globular Cluster, one says it is Possible Progenitor of Styx Stream, and the rest list Big Name celebrity gas bags like Canis Major Dwarf and Large Magellanic Cloud. It says so right there in Wikipedia and who’s to question Wiki when it comes to reliable source material?

Caption to Fig 1 below: A sample of three stellar streams around the Milky Way. Such streams form when dwarf galaxies or globular clusters fall into the gravitational well of the MW and are tidally disrupted first into slender arcs streaming on either side of the former galaxy core. M54 Sagg, Pal 5, and probably Omega Centauri are surviving cores. Others are lengthy elliptical streams which can completely encircle the disrupting galaxy. The Monocerous Ring encircles the Milky Way three times.

The problem with all these swirly streams eager to get into our gas tank is that none of them actually fill us up where it counts: the stellar disc. Instead, the stars these streams contribute end up in our Galactic halo while the gas ends up in our Galactic corona. Even worse, gas siphoned from dwarf galaxy streams is cool, on average ten thousand Kelvin while the Galactic corona averages two million Kelvin. I’m not so sure we should rush to the Astrophysical Journal with the news of our discovery of a Fifth Law of Thermodynamics, namely “Add cool to warm and you get warmer”. So ipso facto, inflowing gas cools our corona, which robs it of energy, which constrains star formation, which means we’re not going to no Virgo Supercluster anytime soon. Can you imagine the gnashing of teeth and rending of garments if a major oil company dispensed energy-losing fuel at one of their ghastly examples of function-makes-form architecture adorned with even ghastlier studies in signage colours guaranteed to give the colour-blind visions they soon wish they hadn’t seen?

So here we sit, amateur astronomers swelling with pride at our minimarvels of glass and TLC, just itching to somehow prove that we, not just those fellows on mountaintops, can actually spot the solution to the mystery of how our galaxy fills its tank. Can we?

Let’s check out a few possibilities.

ESO 224-8, aka van den Bergh-Hagen 176 (Simbad: BH176), OC Norma

Caption to Fig. 2 below: ESO 224-8 finder chart from Jose Torrčs TriAtlas C chart 497.

“VDBHA176 is a large very low surface brightness globular cluster. It is 6' across and has a slight brightening toward the middle. The sky here is very busy so the cluster is difficult to pick from the background. Numerous 13-14th magnitude stars are scattered about the face of the cluster. High power did not resolve the cluster. The cluster could easily be overlooked as a general sky glow about a grouping of stars. Again a photo taken of the ESO Schmidt allowed me to identify the cluster correctly. Megastar lists the object as an open cluster and is positioned about 20" east of the correct position.” (Andrew Murrell, Ilford NEW, Australia, n.d.)

“Seen at a very low altitude so this globular cluster was difficult, using MegaStar chart and replotted location from William Harris' database of Milky Way Globulars. I saw 5 (13-14th magnitude) stars with a faint background glow about 4' in size. This cluster appears visually more like an open cluster than a globular in the eyepiece.” (Barbara Wilson, Texas USA, 1997)

Caption to Figs 2a and 3 below: BH176 is offset slightly in the circlet of stars.

My observations in Apr 2016: “RA: 15h 39m 5.4s, Dec: −50° 03′ 2″, 6 observing sessions March-Apr 2016, Weltevreden Farm, Karoo 31°8587 S 24°4403 E, Intes 6” and 8” Mak-Newts 48x – 212x, LVM ~7.3–7.5, trans. 8–10 seeing 7–10. Finding details: Fm Mv 9.6, 1.5 arcmin dia. globular NGC 5946 Norma proceed either (a) 40 arcmins NE to a prominent rhombus of four stars Mv 8.6–10.2, then 32 arcmins along longest side of rhombus to Squiggle, a ragged line of seven 10.5–11.2 stars perpendicular to rhombus vector line; or (b) from NGC 5946 proceed 42 arcmins along 350° vector to an isosceles “kite” of Mv 9.2–11.6 stars, then 290° vector 33 arcmins to Squiggle feature. W end of Squiggle has 3 arcmin dogleg of 9.9 & 11.0 stars; ~3 arcmins E of dogleg is Mv 11.1 star w. two Mv 12 stars below left leading to obvious ragged circlet of 6 Mv 12.1–14.2 stars ~4 arcmins dia. MH176 slightly off centre to WSW within Circlet. Observations: Brightest BH176 cluster star is 14.6, well w/in range of 8” scope but spotted only erratically due to Circlet distraction. In 6” Mak-Newt faintest stars in circlet near visibility limit & cluster undetectable across ˝ hour steady gazing on four nights. In 8” Mak-Newt at 212x, four Circlet stars steady in direct & two irregular in averted. BH176 halo glow seen fleetingly four times in six half-hour sessions across four nights. Cluster presents as barely perceptible evanescence for only 1–2 seconds each sighting, approx. 2 arcmins dia. w.no discernible core concentration as seen in deep images. MH176 presents similar visual effect as GC Lyngĺ 7 in Norma, Westerlund 1 Ara, or Pal 2 Auriga. More difficult than any of the Fornax GCs @ Mv 12.6–13.6, NGC 6419 Aquila, or E3 Chamaeleon. Verdict: nice sizzle. No steak.”

ESO* 92-SC05, OC Carina

So far, I have found no previous amateur community observing reports of this interesting, very old (>6 Gyr), elusive open cluster near the Southern Pleiades in Carina. This is not surprising given that the cluster was discovered only in 2000. Even in the professional literature only one paper studies it, Ortolani et al. NMRAS 2008. These make ESO 92-SC05 the least-studied Galactic cluster I have come across. It deserves more attention from both communities. For we amateurs, it is the most difficult cluster I have successfully observed in an 8-inch scope. Professionally, it is interesting because is sidesteps the known Galactic open cluster parameters wrt location, kinematics, chemical content, origin, and evolution. In sum, it’s a rare bird which conceals itself very successfully from our eyes and algorithms.
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* ESO is the acronym for the European Southern Observatory / Uppsala Catalog compiled in 1982 from blue plates taken by the 1 meter Schmidt telescope at La Silla, Chile and the US 48” Schmidt at Siding Spring, Australia. The survey shows 18,438 DSOs in 606 fields centered on 5° sections from Dec -20 to -90 and to MB 21.5. It is sometimes referred to the ESO(B) Catalog because the telescopes recorded images using blue-sensitive Kodak IIa-O plates in combination with a GG 3850 nm filter. For an explanation of the ESO object numbering system see Lauberts 1982.

So let’s go there:

Caption to Fig. 4 below: Wide-field finder for ESO 92-SC05. Source: Jose Torrčs TriAtlas C chart 533.

To get there, return to the shallow arc and proceed SW on 315° vector past a wide pair down to the Asterism marked on Fig. 5 below. Continue 12 arcmins to the Triangle formed by Mv 8.9–10.4 stars. ESO 92-SC05 lies several arcmins due W.

Caption to Fig. 5: ESO 92-SC05 close-in location chart. Source: Processed and annotated from WikiSky field.

What do we find when we arrive?

My observations in Apr 2016: “ESO 92-SC05, 10h 03m 14s, –64° 45’ 12” (Gal: l = 286° b = –7.50°): Summation of six observing sessions March-Apr 2016, Weltevreden Farm, Karoo, S Africa, 31°8587 S 24°4403 E, Intes 6” and 8” Mak-Newts 48x – 212x, LVM ~7.3–7.5, trans. 8–10 seeing 7–10. Finder: ESO 92-SC05 is located 4.2° W of the Southern Pleiades and 2.2° E of μ Carinae. Find the shallow arc of 8.5–10.2 Mv stars midway between the So.Pleiad. and μ Car shown on Fig 4. To the SE of the arc lies the pretty OC ES0 92-18 reported here. ESO 92-18 is visually more luminous than ESO 92-SC05. Seen tremulously in a 6-inch scope, in an 8-inch ESO 92-18’s dull round glow resolves into ±10 Mv 14.5–15.5 stars with the white phorescence of abundant stars below visibility. ESO 92-18 is a tasty amuse-bouche to the dessert of ESO 92-SC05 1.3° to W. The various comparison stars in Fig. 5 help pinpoint ESO 92-SC05. The cluster’s brightest red clump stars are Mv 16.5, which at the cluster’s metallicity of Z=0.004 puts its red giants in the Mv <14.8 region. Most observers with sub-12 inch scopes will simply have to wait for the 1 arcmin white glow of the cluster to flicker feebly in and out during moments of low scintillation. It was unseeable in my 6” Mak-Newt, which can see to Mv 14.5 on the best nights. The 8” revealed a fleeting <30 arcsec Mv 15 phorescence distinguishable from a faint asterism only by the evenness of its patch. The nearby Mv 14.1–15.0 double shown in Fig. 5 is a great practice object because the cluster is somewhat fainter and larger, but round instead of oblong. The core region of the 400 Mlyr spiral PGC 29141* directly beneath the Triangle asterism is somewhat more visible than ESO 92-SC05, hence also a good practice object. Practice, however, doesn’t make the cluster any easier. In the course of five hours observing time total, ESO 92-SC05 was unmistakable only three times out of seven possibles. It’s the only cluster that edges out HP1 in the Tiny Toughie department. Verdict: Nice sizzle, great steak. Seeing it is like seeing an exotic bird on the other side of a leafy grove."
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* PGC 29141 (10 03 18.6 –64 58 02.9) is a Mv 13 spiral at z=0.0073 or ~100 million lyr out and is a member of the galaxy bridge between the Hydra and Antlia galaxy clusters.
Attached Thumbnails
Click for full-size image (Fig 1 Milky Way stellar streams t.m 2007.jpg)
112.1 KB9 views
Click for full-size image (Fig 2 VDBH 176 ESO 224-8 OC Norma TriC.jpg)
194.4 KB9 views
Click for full-size image (Fig 2a BH 176 ESO 224-8 Norma wide view finder.jpg)
222.7 KB9 views
Click for full-size image (Fig 3 BH 176 close-in finder.jpg)
173.7 KB10 views
Click for full-size image (Fig 4 ESO 92-SC05 finder fm Torres Tri-C 533.jpg)
162.3 KB8 views
Click for full-size image (Fig 5 ESO 92-SCo5 close-in field.jpg)
176.6 KB9 views
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