M31 - OB Association A102

[Steve Gottlieb]

A few months ago, there was a thread on the "twist" in M31 and Ivan mentioned A102, a star cloud or OB-association at the extreme northeast end of the galaxy. I added it to my observing list and was able to take a close look last week with my 24-inch f/3.7 at my "local" observing site, about 85 miles north of San Francisco. Here's what I found --

M31-A102 is situated at the northeast end of M31, 1.2° from the center and is often not shown on images of M31. At 200x, three separate HII complexes in a NNW-SSE string could be detected. C372/373 at the southeast end is the brightest and appeared faint, small, round, ~12" diameter, contains a slightly brighter stellar point at center. On the SDSS, C372/373 is a group of several HII complexes and I was probably picking up the combined glow of the two northern knots. Just 1' NNW and 2' NNW are fainter C378 and C381. C378 was extremely faint and small, round, 8" diameter, only occasionally popped. C381 was initially seen fairly easily at 200x, forming the southwest vertex of an equilateral triangle with two mag 11/12 star. But then it took more effort to reacquire a very faint 8" glow.

Has anyone else observed this object? The labeled image below is from the SDSS.

M31-A 102.jpg

[Ivan Maly]

Nice observation, Steve! Here are my notes from September (16" f/4.5, SQM-L 21.66):

...The actual NE end is marked by a large bright star cloud, much more distinct than the SW tip. This star cloud forms a right triangle with 2 bright stars that are oriented parallel to the SE edge of the NE tip. The star cloud is closer to the more distal of the stars. This star cloud is A102, unmarked as such on chart 33.


With the 8 mm Ethos, a very close, touching pair of the group formed by C377, G324, and C373 and a fainter, solitary C372 is visible, as is a fainter C378, widely separated to the N. This group of clusters is inside A102. Наблюдение подтверждено на 330х, но вид хуже.

[Steve Gottlieb]

Thanks for posting your excellent, detailed notes. I didn't include G324 in the C372/373/377 complex (on the south side of A102) since its a misclassified globular cluster (see the Revised Bologna Catalogue. Next time out I'll take a look at the southwest end.

[Ivan Maly]

Thanks, Steve. Good to know. It was suspicious for a true globular to be aligned so well with this young association in the outer disc, and without knowing the modern classification I discounted the "G" designation. In the Magellanic Clouds, at least, the "young blue globulars" from the visual observer's perspective are not so much a misclassification as a useful morphological designation (they tend to look globular). In M31, the rare nonstellar globular looks the same to my eyes as a nonstellar open cluster.

[Preston Pendergraft]

This is probably beyond the scope of this thread, but if G324 isn't a true globular cluster, what is it?

[Steve Gottlieb]

G324 is apparently not a globular, but a compact HII region within the OB-association A102. I haven't checked the literature, but this is according to the Revised Bologna Catalogue of M31 globular clusters and candidates. It's included in Table 1, but with a type 5, which stands for "confirmed H II Region".

I just checked SIMBAD and its also listed as an HII region with a primary designation of Bol 488, but also with an alias of G324 (called SKHB 324). The listed V mag is 16.8.

By the way, this situation isn't unusual. A number of M31 globular cluster candidates have turned out to be foreground stars, background galaxies and HII regions, once they were analyzed.