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M103 and Struve 131

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#1 Pete-LH



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Posted 10 November 2013 - 11:25 AM

I had actually observed M103 a couple weeks ago with my 102mm refractor. After making a sketch I checked on line for other information to see if I correctly identified Struve 131 which is listed in the CDSA as a double at 7.3 and 9.9 magnitude and 14 arc sec separation. It turned out Mike on the Star Gazers Lounge had posted some old photos of M131 and ID'd Struve 131 as a triple.

I was puzzled so waited for the next good night and tried again. Again visually I could not see the third star as in Mike's photo. So I switched to my C8 and could then faintly make out the third star. A few nights later I tried with my C9.25 and it was more obvious. With averted vision it started to pop out like that seen in the photo. I guess this indicates the visibility limitations in my sky here south of Philadelphia.

Is that third star an actual companion? Maybe those making the CDSA had the same magnitude limitations ... hard to believe. My Skytools 3 software lists it as a triple with the C component separation as 28.2 arc sec and magnitude 11.59. The software only indicates easy detectablilty for my C8 and greater aperture. I should make better use of that software.

Here is my second sketch (reverse image since I was using a diagonal). I am just starting out sketching so please forgive some mis-alignments and lack of directional information.

Any similar experience?

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#2 GlennLeDrew


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Posted 11 November 2013 - 08:35 AM

Given the distance to M103 (at least 5,000 l-y?), there is every chance this is merely an optical system, involving either cluster members or cluster/field stars. Visual pairs in distant clusters are not so likely.

#3 fred1871


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Posted 11 November 2013 - 08:20 PM

Various estimates of the distance to M103 seem to start at around 8,000 ly and go outwards (larger) from there.

The STF 131 double is generally regarded as a foreground object - a B5 supergiant with a companion B3 subgiant (WDS data, which lists MK class Iab for the primary, IV for the secondary, in the notes).

Hipparcos parallax for the primary of STF 131 is only 2.44mas, with a relatively large error bar of 1.3mas - so it's ~1300 ly +/- a lot :grin:
If the parallax is as small as 1.14 mas, then ~2800 ly is the result. Still much closer than M 103.
Working out a spectroscopic parallax of the stars is difficult without having a good figure for interstellar absorption.

It could be a genuine binary, very wide, extremely long period; and the measures over the period 1827-2010 show no real change (PA 143-144; sep 13.8"-14.0" - within the errors of measuring). Proper motions are similar and tiny. It's consistent with a binary of huge orbital size, period in millenia, so relative motion over a mere interval of nearly 2 centuries is effectively indiscernible (it may have changed slightly, but the old measures are not accurate enough to show if that's the case). It could be an optical pair, but the difference in apparent magnitudes of the stars is consistent with them being at about the same distance from us, given their spectral types, which allows estimating their Absolute Magnitudes.

The brightest star in M103 that's a genuine member seems to be a 10.6-mag red giant. If the more recent age estimates of the cluster are reasonably right, then a blue supergiant (primary of STF 131) won't be a cluster member due to the short life expectancy of that type of star at that stage of its life cycle - too young.

#4 Pete-LH



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Posted 12 November 2013 - 08:21 AM

Fred, Glenn,
Thanks for those responses and information. I had read that STF 131 was closer and not part of M103. I wonder if that includes all two or three stars (were they all measure and found to be a similar distance +/- that large error ... thanks for that description Fred)

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