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Old 12-04-2014, 05:00 AM
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Weltevreden SA (Dana)
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Wd 2葉he Feathers of Grandfather Fire

Wd 2裕he Feathers of Grandfather Fire

Last month IIS弾r Simmo logged Wd 2 with the observation, 選t 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, 疎 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痴 spectra were so faint because Wd 2 lies only 0.3ー 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痴 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稚 eat just one. It is listed as WESTR2 on Chart 201 of my Torres B set and on Chart 86 of Michael Vlasov痴 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謡orth 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ー SSW. This is part of a gas ring expelled from the high-emission mag 10.8 point source which excites RCW48痴 HII emission, the nitrogen-dominant Wolfe-Rayet HD 89358. HD 89358 lies within the sparse cluster NGC 3199. RCW48痴 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値l 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痴 hot HII region RCW49, which is part of Wd 2痴 early-formation ejecta. RCW49 fascinates the professionals as much as the cluster itself because of RCW49痴 unusually high radio emissions. (Beware: the 然CW49 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痴 27 Feb 2014 IIS posts is 17 arcmin E of Wd 2痴 tiny 3 arcmin ball of stars. While discernible unfiltered, it becomes a standout in UHC. Once centered in the eyepiece, Wd 2 doesn稚 exactly knock one痴 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痴 2 arcmin lozenge-shaped core are most likely the Wd 2痴 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痴 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痴 distance (and therefore age). It痴 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 賎uest Star we now know to be the 1054 supernova behind the Crab Nebula, M1:

繕pon 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, 4 views)
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  #2  
Old 12-04-2014, 12:42 PM
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madbadgalaxyman (Robert)
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G'day there, Dana and Paddy and Simmo and co. ,

Thanks for the extremely good visual observations by Paddy and Simmo.
It is remarkable how much you two blokes are seeing, considering how elusive this object is!

No wonder this cluster is so hard to see.... 6 magnitudes fainter than it would be if there were no foreground interstellar dust!

There have been a number of papers Very Recently on Massive Young star clusters / Super Star Clusters / Young Globular Clusters (take your pick as to what you call them) in M83, M82, etc., and it does seem that young star clusters continue to form (in some galaxies) at a total individual cluster mass (per each cluster) of as much as 100,000 to 1 million solar masses per each cluster. The secret to the formation of massive clusters is that more of the most Massive young Star Clusters form in those galaxies which have a very high star formation rate, in star forming complexes that have very high gas pressures and densities.
The Milky Way has a relatively slovenly total Star Formation Rate of only two solar masses per annum, which is about normal for a massive spiral galaxy, so it does contain at least some young star clusters of 10000 -20000 solar masses, but its population of Massive Rich Young Star Clusters is not very large compared for instance to that of M83
(M83 has a globally elevated star formation activity; hence the numerous giant HII regions in its Spiral Arms which are energized by its super-rich population of OB stars, and M83 has had 6 observed supernovae so far.
See: http://www.spacetelescope.org/images/heic1403a/ for the sharpest ever image of M83, at 0.04 arcseconds resolution!)

Thus it is that massive and rich Star Clusters like Wd2 are undeniably
fascinating and interesting objects.
They are also noted for forming in galaxy mergers, such as NGC 4038/4039 , in the most extreme environments found in the interstellar medium of galaxies.

[[ We have occasionally mentioned Westerlund 1 ( http://www.eso.org/sci/publications/...o142-31-35.pdf )( http://www.ipac.caltech.edu/2mass/ga...lund1atlas.jpg ) ( http://m.iopscience.iop.org/1538-3881/145/2/46 ) in IIS forums as being a potential young Globular Cluster candidate in our own Galaxy ; it weighs in at 50,000 solar masses or possibly higher ]]

With so many O stars, Wd 2 is going to be a massive and rich star cluster. But it is very very much dimmed by several magnitudes (about 6 magnitudes) of visual extinction from foreground interstellar dust.
This paper ( http://adsabs.harvard.edu//abs/2007A%26A...466..137A
) detected some 5000 infrared sources (stars) in imaging, so the total stellar population of this cluster is very large. However, their detection limit is such that thousands of faint stars are undeniably missed in their observations and therefore these numerous stars are not included in this count of the total number of stars. (their observations are only complete down to about 0.8-0.9 solar masses)

Here is the J+H+K (1.2 to 2.2 micrometers) composite image (in the Near-infrared), from the above paper:
Click image for larger version

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Note: dust extinction at 2.2 microns is only 1/10 that at visual wavelengths, so this is the way to easily see a highly obscured cluster like this one.

Total mass of this cluster may be nearly comparable to that of NGC 3603, as the Initial Mass Function (the relative number of stars per each interval of luminosity or interval of magnitude) is relatively invariant in many and varied star-forming environments and clusters. The IMF can be expressed ( 'parameterized' if you want to sound impressive) in various ways, ranging from complex equations...... through to simple verbal statements such as "the relative number of stars varies as the inverse of the stellar mass to the power of 2, implying that if the stellar mass is doubled, then there will only be one quarter as many stars at the larger value of mass."

There are always a lot more low mass stars than high mass stars, so the 29 O stars in thus cluster imply the existence of an enormous number of low mass stars in this cluster. I recall reading that the most numerous stars in galaxies are those of a piddling 0.35 solar masses!
(so the Sun is actually an above average star!)

Looking at my google search results, it seems that there have been several estimates made of the total mass of this cluster; and it seems that I am on solid ground when I say that it should be very comparable in total cluster mass to the remarkable super-compact cluster NGC 3603
(see for example this poster paper http://www.a2omega-conference.net/Po...ionI_7_Hur.pdf
which estimates a total cluster mass of at least 7300 solar masses )
The infrared paper mentioned before estimates about 7000 solar masses in total, but they caution that these estimates of total cluster mass are notoriously rubbery due in part to the need to extrapolate the cluster mass in order to include vast numbers of sub-solar mass stars that are often missed in observations.

The mass distribution of lot of things in nature is quite similar to the relative numbers of stars of different masses that are found in a rich Star Cluster (as is described by the IMF)
For instance, the relative numbers of animals of various sizes, the relative numbers of galaxies of various masses, etc.
So there are always a whole lot more tiny things than big things, and this applies to galaxies and animals as much as does to stars!

Last edited by madbadgalaxyman; 12-04-2014 at 09:13 PM. Reason: Several Corrections. Plus more info.
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Old 12-04-2014, 09:08 PM
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madbadgalaxyman (Robert)
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Oh, and here is one more very pretty picture of both cluster and nebula, from the Aladin deep sky atlas and database (it is a DSS image; don't worry about the strange colours, as they have no accuracy whatsoever)

Click image for larger version

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The general appearance is a little bit reminiscent of the NGC 3603 field.

For those of you crazy about clusters, like Dana, here is a very good and very clear review paper about the Initial Mass Function of stars. It is very detailed, but remarkably accessible for enthusiastic non-astrophysicists like us:

_____IMF_(Kroupa)____[P.Kroupa__2002__Science__ Vol.295__p.82.pdf

Last edited by madbadgalaxyman; 13-04-2014 at 09:17 AM.
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Old 14-04-2014, 09:01 PM
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Paddy (Patrick)
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Thanks for some great information Dana and Robert. Again you have both added so much to the observing experience!
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Old 14-04-2014, 10:12 PM
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Quote:
Originally Posted by Paddy View Post
Thanks for some great information Dana and Robert. Again you have both added so much to the observing experience!
Really massive and luminous Star Clusters are exciting objects;
pity that NGC 3603, Westerlund 1 and Westerlund 2 are greatly dimmed by many magnitudes of dust extinction!

I just came upon this fantastic map that shows where the so-far discovered Very Massive young clusters are in our galaxy!(known clusters of at least 10,000 solar masses)

(note that the distance of Westerlund 2 is not settled as yet)

Click image for larger version

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I suspect that it is quite possible to visually detect some "super star clusters" in other galaxies, though it would be a great challenge. The most massive of these clusters can be three or four magnitudes more luminous than Omega Centauri.
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Old 15-04-2014, 11:24 AM
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Westerlund 2__pretty picture Number 3

And yet another pretty picture of Westerlund 2.

This one was downloaded from the Aladin online sky atlas
{{ http://aladin.u-strasbg.fr/AladinPreview
}}

This is an image from the ESO Sky survey, in the red.
It was digitized with MAMA.

Click image for larger version

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Using the full Aladin applet or the standalone Aladin application, enables coordinates to be easily superposed on this image;
and yes, Dana is absolutely right......... the dense region of multiple very faint stars is only two arcminutes across.
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