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View Full Version here: : A tale of two clusters: ESO 92-18


Weltevreden SA
16-03-2016, 09:46 PM
It was the best of times, it was the worst of times. On my week-long March dark-site visit, one night of stars, two of mist, four of rain. Skies crackling electrical spiders and roar-bruised air. The brontophobe cat hid under the bed. The dog on his blanket feigned sleep but his eyes darted all over the room at every jolt. Nobody got a wink. Mates, count our blessings we’re not cows.

And then there was the best. Seeing a new object for the first time sweeps the entire history of astronomy into one exciting glimpse. For the duration of a glimpse we are Aristotle, Brahe, Galileo, Kepler, Herschel. And not just one object this visit: two. Normally I don’t submit observing reports of deep southern objects to northern observers because it’s frustrating to read about some fascinating cluster or galaxy they will likely never see. But the remote and ancient open cluster clusters ESO 92-18 (http://www.univie.ac.at/webda/cgi-bin/ocl_page.cgi?cluster=ESO+92-SC05) and likewise remote but young globular Lyngå 7 really justify the air fare and auto rental, so I cross-posted this report to our pals up north on CN. Besides, you Ozzies can reaffirm what positive geniuses you are with what can be done on an outdoor grill while watching the sky get dark.

See image 01 below, José Torrès' ESO 92-18 finder chart from TriAtlas C chart 496.

I never heard of ESO 92-18 until Timo Karhula reported it from Geraldton, NSW. Using a 10-inch scope he relates, “With 150x magnification, I saw a faint but pretty large patch of light. With averted vision, I could glimpse some stars over it. These are foreground stars overlaying the cluster it would seem — the brightest true cluster members are around the mag 15.5 mark.” An online search shows that ESO 92-18 has been reported only twice in the amateur forums. Both started with Timo, the latest on Feb 8 2016 during his recent starathon at Geraldton (http://www.cloudynights.com/topic/526628-reports-from-geraldton-western-australia/), These revisited his sightings reported on IIS 6 years ago (http://www.iceinspace.com.au/forum/archive/index.php/t-59242.html) (near bottom of thread).

Timo alluded to it’s being an almost-a-Pal. It probably would be a Pal if the POSS plates extended to 60° south. ESO 92-18 (http://simbad.u-strasbg.fr/simbad/sim-coo?Coord=10+14+59.85+-64+36+22.7&CooFrame=FK5&CooEpoch=2000&CooEqui=2000&CooDefinedFrames=none&Radius=2&Radius.unit=arcmin&submit=submit+query&CoordList=), for all its shy look, is a contrarian cluster. It looks like what it isn’t and it lives where it shouldn’t. It presents like a globular but is actually an ancient open cluster about 4–5 gyr old.

It is an easy star hop with the naked eye because it lies almost exactly halfway between the grandly named Southern Pleiades (Collinder 102) and mag 2.9 Upsilon Carinae (http://simbad.u-strasbg.fr/simbad/sim-id?Ident=upsilon+Carinae&NbIdent=1&Radius=2&Radius.unit=arcmin&submit=submit+id) 3° westward. (υ Carinae is a pretty, unequal mag double worth a pause on its own.) I hate to be curmudgeonly but the Southern Pleiades looks about as Pleiadean as a toad, I’m sticking with Collinder.

At 9.5 kpc or 30,100 ly from us ESO 92-18 may well be the most distant OC we hobbyists can see. It presents as faint, flat, and fragile. For go-to users ESO 92-18 may not be in your handpad list, so key in RA 10 14 59.8, Dec –64 36 22.7. Using José Torrès TriAtlas C charts (stars to Mv 13, DSOs to Mv 15) and switching from eye on the sky to eye in the eyepiece at 40x I found it easily in a 6-inch Astro-Tele refractor. Find the pretty little line of four mag 7 & 8 stars in the shape of a shallow bow. They are clearly seen in the above chart. The easternmost star is just a bit further apart than the other four. Proceed SW from this star at a 45° angle for half the length of the four-star asterism, and there will be a mag 8.2 star and a second star SW below it at mag 9.3. ESO 92-18 forms a 3rd vertex of an imaginary shallow isosceles triangle between these two stars. Conveniently, there is a 12th mag star occupying the 3rd vertex point just opposite of ESO 92-18.

It opened up considerably in a 6-inch Intes MN61 at 191x. Two stars held in direct and four more hovered in the wings of averted. Checking their proper motions on Simbad, these are unrelated field stars overlying the wan white luminance of unresolved fainter stars. Raising the stakes to an 8-inch Wirth-Intes MN86 unveiled a mottled patchy surface of Mv 13–14.5 stars approx. 1.5 arcmins dia. The overall effect was like a remote halo globular seen laterally through the Galactic thin disc. ESO 92-18 is about the same visual appearance as the globulars Pal 2 in Auriga, E3 in Chamaeleon, or NGC 6419 in Aquila, though it doesn’t exhibit the murkiness of these three reddened globulars. At a Landoldt value of 0.79 (2.5 visual magnitudes) ESO 92-18 has been somewhat less reddened by intervening dust than the above three globulars. It is nestled in a sector of the Galactic disc that looks like the stellar equivalent of your living room floor after the kids have had a popcorn party.

ESO 92-18 emits about the same tepid surface brightness as the Fornax Dwarf’s four globulars (mag 12.5–13.6) but is twice times the visual diameter. In the 8-inch MN86 at 212x I counted 3 more stars below mag 14 amid the ashen haze of unseeable stars. From the look of the cluster and its listed location in the MW Galactic inter-arm region between our Sagg Arm and the 5,000 ly distant Perseus Arm, I wondered if it might be a multi-gigayear-old open cluster.

But here the cluster gets contrarian. Old compact open clusters aren’t supposed to reside in the outer inter-arm regions. Inter-arms aren’t star-forming regions, they are the rest home for molecular cloud remnants and the dissipated remains of unbound open clusters which have been exhausted by their passage through a spiral arm. Inter-arms are where matter density and energy density return to viriality, the equilibrium between total kinetic energy of all the massive objects in a system and their total potential kinetic energy that keeps them bound in that system. Stasis gives inter-arm regions their flat, boring appearance compared with the fireworks of spiral arms, but inter-arm regions serve a vital function in the life of a galaxy: they mix together all the residual products of the previous spiral arm crossing so that the next crossing will be a summation of the galaxy’s historical chemical evolution.

ESO 92-18’s locale is true to form in a second way: compact bound open clusters gradually migrate vertically away from the Galactic thin disc into the thick disc as they age. Many if not most star clusters are born with a vertical energy moment that they never wholly lose. When their original molecular clouds merged with a spiral arm, many of them entered into the thin disc from above or below in addition to their lateral sideways motion along the disc plane. Some progenitor clouds are called HVC or high-velocity clouds because they descend from deep space into the Galactic disc. Their entry angle and velocity is partly carried through the disc, so some clusters leave the disc instead of remaining in it. Some HVCs crash through at such a high velocity that they form an open cluster crossing the disc and then promptly fly out the other side without losing much of their momentum. The parent cloud of the cluster Blanco I in Sculptor came in almost perpendicular to the disc at 270 km/sec. Its stars formed rapidly some 100 million years ago and kept right on going without so much as a fare-the-well. Its current Galactic declination is –79° at 253 pc (825 ly) below the Galactic plane. The cluster is irretrievably on its way out into whatever plans deep space has for it.

The most disruptive place in a galaxy is the disc. Lucky are those clusters whose formation gas came in at a substantial angle. We see this naked-eye in the very ancient open clusters NGC 188, NGC 6791, M67, and Collinder 261 in Musca. These reside many parsecs above or below the Milky Way band and are still moving gradually away from the disc. The further away they move from the disc plane, the safer they are.

If ESO 92-18 is roughly 5 billion years old it is the wrong kind of cluster in the wrong kind of place, is it unique? Are there other old open clusters in the same category that give evidence of their being formed via a different mechanism which deposits them already formed in their present locales?

Actually, quite a lot. Friel 1995 (http://cdsads.u-strasbg.fr/abs/1995ARA%26A..33..381F) lists 74 older than the Hyades (>600 myr), many of them discovered only recently. One of ESO 92-18’s nearby neighbours is ESO 92-SC05 (http://www.univie.ac.at/webda/cgi-bin/ocl_page.cgi?cluster=ESO+92-SC05). It is 35,500 ly away from us at –7.5° below the disc, and may be part of the same disrupted dwarf galaxy that appears to have made ES) 92-18 some 3-plus billion years ago. Some astronomers believe that at least some other old open clusters outside the outer solar circle also may have been absorbed from accreted dwarf galaxies. We’d be a bit threadbare too if we lived the kind of life old open clusters have to put up with.

If you want to learn more about the lives and lifestyles of old open clusters, Friel 1995 (http://cdsads.u-strasbg.fr/abs/1995ARA%26A..33..381F) is an easy-to-read primer. If you hanker after meatier stuff, check out Portegeis Zwart 2010 (http://arxiv.org/abs/1002.196).

As I took what turned out to be the final look of this dark visit, my mind flashed through all the pictures we see of elderly men and woman in the remote tribal villages of Kazakhstan, Bolivia, or Hokkaido, faces creased like pond mud after years of no rain, eyes that have seen through a century the way gas clouds see through a spiral. Why does such beauty have to be made of hard times?

Hey little ESO 92-18 far far away there, not farewell. Fare forward.

simmo
16-03-2016, 10:41 PM
Very cool reports Dana.

Bombardon
19-03-2016, 01:20 PM
Great report, Dana. Yet another swashbuckling adventure from the intrepid night stalker of the Transvaal; this time with domesticated pets in tow and Dickens in his back pocket, an enviable array of optics at large and not a growling leopard in sight! Loved not only the hunt for your esoteric ESO – can I get this one in my trusty 10? – but a well-deserved swipe at our Southern Pleiades (will even the Jewel Box be safe from this man?) but the things you learn about the vertical energy in our ocs are fascinating. I have tracked and found your maps – thanks, Dana- and I agree, they leave no stone unturned. The good news from BBQ land is that the humid monsoonal weather has packed its clammy skirts and taken its dripping humidity north to the tropics and soon the whales will be on the move and those tingling days of Autumn with all of Virgo’s treasures will arrive. Keep these Dark Adventures coming, Eugene

ngcles
22-03-2016, 06:38 PM
Hi Dana & All,

Thanks very much for your report that I greatly enjoyed reading. I echo the comments of Euene above that I could not have expressed better. In a thread you link to I added ESO 92-18 to the observing challenge objects back in 2010. I have no independent recollection of having pursued it after that note and I can't find it in my log, so it goes to the top of my list after this full moon gets out of the way.

Best,

L.

Bombardon
01-04-2016, 10:05 AM
Hi Dana, Les and keen viewers. After much cloud we had a reasonable sky on the south coast last night but with a fair bit of moisture and sky glow. Having settled my eyes on the Leo Triplet and abandoned Jupiter as a bit dull and unclear, I headed for Dana’s marker for ESO 092-18. Trouble at the start as a triplet of stars similar to the Altair visual three (about 1 degree NW) did not show well in Dana’s map. I discovered after a bit (when compared to my Interstellarum) that one had disappeared under my light as it was obviously a variable and shown as a circle! I soon got on to the curious double h4306, which is in a wonderful field and two fine blues of similar magnitude. The ‘little bow’ of stars that Dana described were obvious and I reached what I thought were the two markers on each side of the ESO. Nothing in low power (10” F5, 21 Denk 21mm). Serious nature call left me light contaminated and had to start with a fogging finder 20 minutes later. After settling in and hunting Sombrero for a bit, I returned to the scene of the crime. Again, nothing! I changed to a 13mm Baader Hype and suddenly a few stars appeared and the hint of a haze. Down to a Meade 9mm but a bit too much. Back to the wonderful Hyperion and gradually some stardust started to emerge set in a Corvus-like quad of stars. I noticed again the fine blue double just on the edge of the field. I read Dana’s report again later and he describes the cluster well as if a veil were drawn over the haze, which to me gave it an almost globular look. I was well pleased with the hunt and the fact that I succeeded with my simple Dobsonian. The Insterstellarium Desk Edition took a hammering but well worth it. Remembering Dana’s comments about this eerie object added greatly to the experience. Regards, Eugene