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Old 16-03-2016, 09:27 PM
Weltevreden SA's Avatar
Weltevreden SA (Dana)
Dana in SA

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[B]A tale of two clusters: Lyngå 7[/B ]

Now here’s one that’s literally off the charts: Lyngå 7 (aka ESO 178-SC011). It does not appear on any of the charts I use, including the highly detailed (stars to mag 13, DSOs to 15) José Torrès TriAtlas C set, chart #496. Its position and distance modulus put it 23,480 ly away into the Galactic disc in the Galactic disc but its original CMD put it in the halo. There’s only one image of it on NED (and WikiSky’s SDSS image is a lot better). It doesn’t show any hits in the GALEX, UV, or IRAS websites, although there are four IR band hits in IRSA WISE. (These are FITS files which might not open in a PC or Mac system image reader.) It’s only been logged a handful of times by amateurs, all using medium-aperture 12-inch to 20-inch scopes, and posting their observations here on IIS. Little turned up in literature searches beyond the discovery and classification papers, 1, 2, 3, 4, 5.

Image #01 below is a clip from the Torrès C chart with Lyngå 7’s position marked with an arrow.

For observers with small aperture scopes like my 6- to 8-inch gear, Lyngå 7 is a tricky bait-and-switch artist. When you arrive at it, the “cluster” shows immediately in a 6-inch at 40x as a tight glittery group of mag 12.5–14 stars that look like a compact open cluster. But a number of the Ice In Space and Deep Sky Observers Companion reports mention difficulty seeing it because of “low surface brightness”. That certainly isn’t what I saw at first glance. Lyngå 7 looked like a fainter version of nearby open clusters Lyngå 6, Ruprecht 115, and Pismis 22 only 3° to 4° N. (A 5° radius circle around this globular is a cluster lover’s paradise, starting with the bright nearby NGC 6067, 6031, Harvard 10 and superimposed Collinder 299.) If your go-to doesn’t have Lyngå 7 in its database, key in the position RA 16 11 03.6, Dec -55 19 04.0.

However, my first obvious-at-a-glance cluster is merely a distracting smoke-and-mirrors illusion. The real Lyngå 7 is faint and small and not attached the glittery asterism I first saw. Here is a DSS2 image that shows what a tricky object it is.

Image 02 below resembles Lyngå 7 in a medium-aperture scope at 200x

Lying only 2.8° below the Galactic plane, Lyngå 7 is highly reddened with a Landolt extinction of 0.54, which translates visually to Aλ 2.9 mags dimmer in the eyepiece than if there was no dust in the way. Closer inspection in my MN86 showed a very faint fuzzy patch about 2 arcmin diameter with no hint of resolution. That’s about right given that Lyngå 7’s half-light diameter is 2.4 arcmins and its brightest RGB stars are well south of mag 16. The cluster’s dimness belies the fact that it is really quite close to us at 26,000 ly (8.4 kpc) from the Sun (closer than M13, NGC 6205). It would look a lot like M56 or M92 without all the dust in the way. Harris doesn’t list Lyngå 7’s central surface brightness, but its faintness matches other globulars with an s.b. of ~24 mags/arcsec. Despite such a low s.b. I was able to hold it steadily in averted at 120x and briefly in direct at 218x.

Once you’ve logged it as certain, there are a few other goodies nearby. The HII hotspot Henize 2-146 (ESO 178-10) is a faint daub 22 x 15 arcsec that has the look of a bipolar planetary tipped on its side.

Lyngå 7 is a bit of an unorthodox globular. For one, it’s the home of the only known carbon star Mira-type variable. As one study sums it, “Since such a variable is expected to have an age of ∼1–2 Gyr and an initial mass of ~1.5M☉, we conclude that this star must be the product of a stellar merger [blue straggler].”

In 1964 when Gosta Lyngå saw this cluster on the emulsion plates he used, he thought it was a remote open cluster like ESO 92-18 mentioned above. In 1993 the Italian globular specialist Sergio Ortolani re-studied Lyngå 7 photometrically and reclassified it as a thick-disc globular. It lies on the far side of the Sagittarius Arm and adjacent to the messy, crowded Norma Cloud at the very end of the southern bar of the Milky Way. It isn’t associated with the bar itself. In my scopes the nearby field was cluttered with field stars. That same field looks very much more than merely cluttered to photometric equipment that registers stars down to mag 22 to 27 (as the HST cameras do). New CMD’s obtained in 2004 by Ata Sarajedini and his team were heavily contaminated by non-cluster stars, but did reveal a prominent core helium-burning red clump and a main-sequence turnoff that suggested an age of ~12 billion years. Unfortunately the CMD was a mess because it included the entire cluster out to its halo. The Milky Way is so star-clotted the CMD required months of mathematical tiptoeing to achieve the rudiments of the cluster’s character. ER doctors have the same problem when parsing from a writhing accident victim’s garbled words what is broken and what is fright.

Image 03 below is Lyngå 7’s initial CMD taken from imagery provided by the HST*, from from the Astronomical Journal, A-J 133:1658–1672. The Sarajedini team’s used the full cluster width. This CMD is a case study in the perils of generating a CMD with a wide-field capture zone in the MW disc. There is so much confusing MW chatter that mathematically untangling the real cluster data was the kind of job astronomers give to their grad students to sort out.

* A word about the Hubble’s F606W and F814W filters used for this image. The HST Advance Camera for Surveys F606W filter corresponds roughly to the Johnson-Cousins (J-C) green V filter at 554 nm. The F814W band lies near the J-C near-infrared 806 nm I band. That means this CMD is equivalent to a V / V– I CMD taken with J–C colorimetry. This particular filter choice minimizes red info from the plot and highlights what we would call the visual band. Removing the IR attenuates the red giant and AGB branch data. If the IR had not been filtered this way, the oversaturated redness of the stars brightness would overwhelm the lower-mass stars in this plot (shown as dots). Even with the filtration, the crosses at the top indicate non-cluster field stars so bright they saturated the HST spectrograph’s pixels, which skewed the brightness of the RGB and AGB enough to make their information not fully reliable. Astronomers go through mathematical contortions to correct their isochrones for this skew, so they mark which stars were affected by it. Here the Sarajedini team used little crosses.


Image 04 is Lyngå 7's CMD after minimizing field crowding by measuring only the central 15 arcsec of the centre instead of the entire cluster.

Pruning out field scatter makes it easier to estimate the cluster’s age, metallicity, stellar population by mass, star-formation history, and from these its evolutionary profile. Note that the main sequence (MS) begins at mag 27 at the bottom, and turns off at mag 20.25. A CMD that measures 7 magnitudes down into the MS reaches the dimmest MS stars in the cluster, at 0.08 solar masses. Stars smaller than this are in the territory of deuterium-burning brown dwarfs too small to compress the core to the 10 million K needed for hydrogen ignition. The short redward shift at the mag 20.25 main sequence turnoff (MSTO) marks the period (1 to 6 million years on average) when the star maintains its temperature/pressure balance by slowly shrinking and releasing gravitational heating. Eventually its central density and pressure become high enough to re-ignite hydrogen fusion in a shell around the core of inert helium ash, and the star ascends the red giant branch.

The above CMD tell us that Lyngå 7 is ~12 billion years old, has a metallicity [He/F] of –0.54 and therefore has had at least two generations of stars, the first of which was already significantly polluted by metals from the universe’s initial Pop III starburst before cosmic reionization. With a mass/luminosity ratio of 1.96 its dark-matter mass is 4% less than its visible matter mass. (By definition star systems without dark matter have a mass/luminosity ratio of 1.00.) Lyngå 7 lost all its remaining gas during the five-billion-year era of high-energy UV generated by first cosmic reionization from z = 6 to z = 2. It lost even more during massive-star UV radiation from z = 2 to z= 0.8 during the peak intensity of galactic star formation. (Watch the whole show here.) It hasn’t witnessed a supernova of its own for 11.996 billion years. It has been bare of gas since ~300 million years after its birth. Lyngå was not born a dense high-mass cluster to begin with, and has lost roughly 60 to 70% of its stars since. It is aging unhurriedly into a future in which it, and everything else, will grow dark. Its one claim to fame is its lone carbon star, which are unusual in globular clusters because carbon stars fall into the 3 – 5 Gyr age bin. It is the only known blue-straggler carbon star.

Yes, this dark time visit was the worst I’ve ever had. But, like the old folks with lined faces whose eyes have seen a century of field crowding finally part to reveal the truth, two simple clusters made the whole thing magnificent. The universe has roughly 300 million galaxies and 7 trillion dwarf galaxies. There aren’t enough numbers to list its Lyngå 7s.

Let's just go have a look at the thing and enjoy what it looks like instead of what it is.
Attached Thumbnails
Click for full-size image (01 Lynga 7 finder fm Torres C.jpg)
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Click for full-size image (02 Lynga 7 fm DSS2.jpg)
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Click for full-size image (03 Lynga Sarajedini CMD before.jpg)
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Click for full-size image (04 Lynga 7 CMD fm Sarajedini 2007 (after).jpg)
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Old 22-03-2016, 06:47 PM
ngcles's Avatar
ngcles
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Hi Dana,

Yes indeed it is a tricky object and here is my observation of it from 2002 with my 31cm f/5.3 Newtonian:

x186 26' TF. Mag ?? Size ??. This took some time to pick upoin the field. It lies within a Tri formed by a mag 8 * a mag 9 * and and inverted "Y" shaped asterism of mag 13-14 *s, which is 5' a side. One or two mag 14-5-15 *s are seem superimposed on the halo. It is hard to believe they are resolved *s. 2' diameter with a weak central brightening and looks like a face on spiral eg of v/lsb. It is probably easier than Vdb-HA 176.

Best,

L.
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