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# Dim Dawes Double

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### #1 Asbytec

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Posted 30 November 2013 - 09:07 AM

Always curious what a dim Dawes double would look like. I found A 1724 in Auriga, a pair of 10th magnitude stars apparently separated by 0.7" arc. I found out what it looks like. Sketch below. (Can you tell which one it is?)

Hint: the sketch is fairly accurate, magnitudes might be a little off.

### #2 WRAK

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Posted 30 November 2013 - 12:21 PM

Nice riddle - no idea. Not one of the littel fuzzies shows for me a hint for being double.
Wilfried

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Posted 30 November 2013 - 04:47 PM

Well I'm afraid I'm stumped too! I don't detect anything that would suggest one over another

### #4 fred1871

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Posted 30 November 2013 - 06:27 PM

Agree with the others - nothing in the drawing to indicate which one is the double.

Looking at the magnitudes quoted, the combined effect of the two stars would match the 9.8 mag, as marked; and then looking at WDS, I see a wide companion of mag 10.2, which again fits for the 9.8 being A 1724. The high magnification gives the wide separation compared to field size.

So - is your point that Dawes separations don't show as doubles around mag 10.5 instead of mag 6? - Some of us have suspected that might be the case....
the eye doesn't resolve well at low light levels etc etc

Okay - so what secret fiendish plot is brewing here?

As a side note - the change in PA of the close pair, 257 to 111 from 1908 to 1991, suggests - given the near equality of brightness of the stars - that there's been a quadrant reversal between first and last dates of measure. So 1908 might be 257-180=77 as the PA - that still leaves fairly quick change, along with noticeable widening first to last. This suggests a binary changing at a fairly rapid rate for a likely non-nearby pair. Spectrum is vaguely G, no subdivision nor MK class. Assuming a solar-type star, main-sequence (MK 'V'), will give a spectroscopic parallax suggesting a distance of maybe 4-500 ly. If the Absolute Magnitude is about 5.5 (a little less bright than our Sun) the distance is ~300ly. That indicates a large orbit (0.7" in projection is 200+AU), so the listed changes are either rapid for such a wide pair - or the measures are misleading, especially on separation change. Of course, if the vague spectrum proves to be K instead of G, the pair is dimmer intrinsically, therefore nearer to us, the orbit smaller, so rate of change is less surprising.

And, after that brief excursion into conjectural astrophysics, it's back to the drawing, Norme. Tell us what you're thinking.

### #5 Pezdragon

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Posted 30 November 2013 - 07:41 PM

I don't know but the 9.8 star looks slightly asymmetric at 5:00

### #6 Asbytec

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Posted 30 November 2013 - 09:33 PM

Okay, sorry for dragging you through this, but it was the 9.8th magnitude star (magnitudes varied between SS, Stellarium, and it's 10th according to WDS.) And it simply looks like a single star, if I identified it correctly. It was a trick sketch.

Truth is, if the 9.8 magnitude star was it, I could not discern anything resembling a double being that dim and that tight. I put it up maybe to show how difficult it seems to be to observe dim pairs even though magnitude is plenty bright and the separation might be easy enough for some elongation. Any elongation indicated was a purely accidental sketch artifact.

So, what's happening physiologically to human vision (assuming the charts are correct and I actually found it) that makes it so difficult or impossible? It's interestingly difficult.

So - is your point that Dawes separations don't show as doubles around mag 10.5 instead of mag 6?

Yes, give that man a banana, he can have one of mine.

Interesting analysis, Fred, using spectral types to determine orbital distance and speed. Now, that's real double star observing when you can use the data given to work out the possible mechanics of the system. Gotta figure out how to do that, well done.

### #7 WRAK

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Posted 01 December 2013 - 04:35 PM

Dawes himself seemed to have not believed that faint doubles are really different ffrom faint ones - at least Chris Lord reported this and was himself serious about this. One of the greatest weaknesses of Lord's Rule of Thumb algorithm for resolving unequal doubles is the ignorance of the fact that faint doubles are much harder than bright ones.
Wilfried

### #8 Asbytec

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Posted 01 December 2013 - 08:01 PM

Wilfried, definitely more difficult and at some point impossible. It's what I found interesting about NOT splitting this pair. All the specs look easy enough, but the combination of specs certainly is not.

### #9 WRAK

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Posted 02 December 2013 - 05:10 AM

Just forgot: My current RoT algorithm gives a proposed aperture of ~200mm for A1724. This is certainly an extrapolation as my data set consists mostly of limit observations with less than 6" apertures but it seems not this far away from reality. Maybe somebody with an 8" scope with a rather small CO of ~0.2 could check this.
Wilfried

### #10 Asbytec

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Posted 02 December 2013 - 05:55 AM

[quote name="WRAK"]Maybe somebody with an 8" scope with a rather small CO of ~0.2 could check this.[quote]
Pete?

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