Is this star a variable?

[iamjulian]

I took this image of the M103 cluster last night. The orange star at the centre, has two stars of similar brightness right next to it, at 10 o'clock and 2 o'clock. According to the star catalogue, the star at 2 o'clock should be mag 13.16 (and that does match the relative brightness of other stars in the image), but the one on the left which appears about as bright, should only be mag 16.24! Is the catalogue incorrect, or is this a variable? What is the best way to find out? The star's number in the catalogue is 3UC 302-039395.

I know it looks like my focus is off, but this is a 300% increase of a 75% crop. Or something like that. Don't ask.

Thanks.

[lunator]

I would make contact with the BAA and see if there is any info on this.

Cheers

Ian

[FraserClarke]

No, I don't think so. It seems to be missing from the USNO catalogue. GSC-2 gives it at 12.6 (V band I think). USNO (and DSS image) do have a fainter star which would be at ~11:45 on your image; but is too faint for you to see in that image. I suspect that is the one you're picking up from the catalogue. Catalogues can go a bit wonky near bright stars, but it's surprising this one is missing.

Always worth having a look at the DSS images; ESO Online Digitized Sky Survey ; They go down to ~19th, so you can see if there are obvious discrepancies...

[Ben Ritchie]

The attached is an image from Sanner et al. 1999, A&A, 349, 448

Photometric and kinematic studies of open star clusters. I. NGC 581 (M 103)

which shows the two stars in question near their 'star C'. They look to be broadly equivalent in terms of brightness, although i've not gone to their catalogue data to extract the magnitudes

[iamjulian]

Many thanks! Both those images show it the same as mine.

The UCAC3 catalogue must be mistaken.

[Mr Spock]

The identification of the stars is a bit suspect. I have:-

The central red star is BD+59 274 at mag 8.49

At 10 o'clock is BD+59 271 at mag 7.23

At 2 o'clock is TYC 4031-00558-1 at 10.46

I've added a few more to the image below

[iamjulian]

Thanks for that. My description of 10 o'clock and 2 o'clock wasn't very good. I mean the stars that are almost touching the central red star in my image. Your map is interesting however, because it shows one star much brighter than the other, just as the UCAC3 catalogue suggests. Yet the three images above do not appear to agree.

[Ben Ritchie]

If you look at SIMBAD you should be able to track down the star in question and see what research papers reference it - if it's a variable then it'll be in something

SIMBAD Astronomical Database

edit: doing this, the stars appear to be:

10 o'clock -> #123 (B 13.28, V 13.12)

12 o'clock -> #125 (B 13.51, V 13.40)

so almost the same brightness. Neither is listed as variable. There is another fainter star close to #123, which is likely what's being reported in the catalogue

[higginsdj]

In answer to your question, yes the catalogue could be wrong. It just depends on what catalogue you are using. Close stars can appear merged in older images and are thus very difficult to accurately measure. The other point to consider is what is the colour response of your chip. ie if your chip is Red sensitive and the fainter star was red but the brighter star was blue then a colour image may show them to appear of similar brightness!

Cheers

David