Some open clusters in the Auriga-Perseus region

[Martin Meredith]

Under a bright moon last night I had a look at some open clusters in the general area of Auriga and Perseus. Auriga is home to Messiers 36, 37 and 38 and I visited them, along with some NGCs, but for me the most interesting objects of the evening were from 'lesser' catalogues. Later I'll post some of the King clusters, but I'll start with Stock 8, which lies about 1.5 degrees due South of M38. This is only the second Stock I've observed. I was forced to observe Stock 24 in Cassiopeia a few weeks ago because it is also Berkeley 3, and I have been 'after' the Berkeleys for a while.... 

Stock 8 lies in the Perseus Arm of our galaxy. Although the catalogue I'm using suggests the cluster is 18' in diameter, more recent studies place it at around 6', which ties up with that I can see here.

The yellow-orange star is naked eye Phi Aur (mag 5.1) at around 450 light years away. The white star is also just about naked eye at mag 5.9,  so this cluster ought to be very easy to locate.

It isn't too clear perhaps in this shot, and is washed out by the moon, but the the cluster is surrounded by nebulosity (from which it presumably formed). This is an HII region known as IC 417 (Sh 2-234). I imagine this would look really interesting in a deeper exposure. Apparently Stock 8 is an extremely young cluster -- only 3 million years. It was fascinating to read a little in [1] of the large scale structure around Stock 8. Apparently, there is evidence for a giant shell of hot gas whose high expansion velocity implies that it was the result of a supernova explosion. This in turn triggered the formation of many stars in the region. Stock 8 is surrounded by 33 early-type massive stars that may have been formed in this way. Stock 8 is moderately massive, with a total of up to 600 solar masses. Fig 1 in [1] shows a striking false colour composite which makes the H-alpha component of the shell very clear.

 

One of the jewels of this field (but not part of the cluster as far as I can tell) is the carbon star OP Aur, shining 'redly' below and to the right of Stock 8. This is a Mira-type variable with a maximum of 12.7 but which gets 3 magnitudes fainter in a cycle of about 500 days. It is so red that the jump when my red subs started coming in was immediate. In fact it doesn't even show up after the blue filter at all. The panel shows individual 15s subs in each filter together with an RGB (not LRGB) composite (width ~ 1').

This cluster doesn't need the long exposure I gave it. I was hoping to pull out more of the nebulosity but will revisit on a moonless night.

cheers

Martin

[1] https://arxiv.org/pdf/1612.00697.pdf

[Mike JW]

Enjoyed the read and OP Aur. Visually I have often hunted out carbon stars - they tend to look orangey red.

Mike

[Mr Spock]

Great looking object, and a nice write up too.

[Martin Meredith]

Thanks Mike and Michael!

Now for the King clusters. I happened upon one, unplanned, and enjoyed it so much that I went for another. Then I felt it would be good to have three Kings (very important here in Spain as the bringers of xmas presents...). I ended up with a rather untidy 4.

I've just been reading about the history of these clusters. They were discovered by Ivan King in 1949 by inspection of plates from the 16" "Metcalf" refractor which I think was at the Harvard Observatory as the paper (which you can read here -- http://articles.adsabs.harvard.edu//full/1949bharo.919...41k/0000042.000.html will only take 1 minute!) was the Harvard College Observatory Bulletin. He listed 21 candidates of which he regarded 1-12 as definites. The list must have been extended slightly as there are 26 now listed, but 3 of them (including the 'definite' King 3) are missing (perhaps it coincides with another prior cluster).

King is famous for his 1966 theory of the dynamic evolution of clusters which I had a quick glance at (a bit too late at night for equations) and see that it has been pretty influential, cited more than 2000 times. There's a brief note and a photo here: https://www.stellar-journeys.org/cluster-king-info.htm

So to the objects themselves.

Unlike Stock 24, King 5 is rather old ~ 1 billion years and highly-reddened. I love the zigzags of stars in this one. It resembles some of the Berkeleys in this sense but has more oomph -- brighter, larger. As for Stock 24, I think the diameter is something of an overestimate.

 

King 7 estimated to be 700 million years, again highly-reddened

 

King 8 is most likely the oldest of these 4 at around 1.1 billion years and rather compact. The visual double in the foreground provides a lovely colour contrast.

 

Finally, King 17, which was first studied in detail as recently as 2008 and found to be of intermediate age.

Unfortunately for the cluster the field is dominated by another carbon star, TX Aur, pretty bright with a range of 8.5 to 9.2. Working backwards thru the various mentions on Simbad, the earliest seems to be in a charming paper from 1899 under the designation BD.+ 38°i035. The paper, "ON THE SPECTRA OF STARS OF CLASS III b." is well worth a look at, if only to read about the observing conditions in Upsala. http://articles.adsabs.harvard.edu/pdf/1899ApJ.....9..119D. The star is now classified as a LB ('slow irregular') variable. 

thanks for looking

Martin

[Mike JW]

Martin.,

A very enjoyable read. Colour definitely helps with the view.  I shall look up the various articles later.

Mike

 

[Bill S]

Very interesting observations. The background reading was fascinating. The 1899 paper is not a good advert for astronomy holidays in Uppsala (or Upsala).
 

Bill