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:
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!