Saturday, October 13, 2018

captured Keid (Halifax)

I was a little surprised to see a couple of emails from the BGO robot this morning. There must have been a break in the clouds. Or better conditions than predicted.

I had had a request for ο (omicron) Eridani aka Keid, the multi-star system. The SMU observatory gathered good data, after aiming at GSC 05313 00997. Looks like a lovely triple!

multi-star Keid in luminance

Luminance only, ½ second subexposures, 20 stacked shots. FITS Liberator, Paint.NET. North is up; east is left.

The part of the reason for imaging this system (Struve 518) was because I wanted to double-check positions. When I last viewed this target on 19 Mar '18 from the backyard, I had noted the B star in a very different position than it was noted in SkyTools 3 Professional. ST3P shows B at a position angle of 65°. At the time I thought it approx. 60° different from that, counterclockwise in my SCT, therefore an increase in the PA, perhaps 125°. It is obvious in the image above that B (and C) are to the east-south-east. That's a PA of roughly 100°. Curiously, ST3P states in the Object Information box that the AB angle is 104° as of 2002. Here's an instance where the visual chart is very different than the OI data and it threw me off back in March.

Also, SkyTools notes another pair: Aa. With a PA of 97 and separation of 77.9. But it seems, early in 2018, I did not see this companion either.

The Washington Double Star database shows the Keid is a 5-star system with the following data:

pair first last PA1 PA2 sep1 sep2
A,BC 1783 2016 108 102 89.2 83.7
AC 1987 2011 117 97 79.4 77.3
AD 1850 1998 197 38 128.3 481.4
AE 1850 1998 279 24 99.4 569.9
BC 1851 2017 160 331 3.0 8.3
BD 1922 1999 196 28 147.0 457.0
BE 1922 1999 356 16 279.5 563.7

SkyTools lists the A, B, and C stars. The D and E stars from the WDS are mag 12-13 stars well away. They are visible to the north-north-east in the image. I'm curious why the bright, closer star beyond BC is not included.

The BC, BD, and BE alternate measures support the main entries.

plot of Keid stars for 2018

A plot, using the current or most recent positions, from my Excel mapper tool. Good correspondence to the image.

It looks like for the A-to-B data, there's been little movement. It seems the C is moving about B. SkyTools and other sources quote the B-C orbital period around 250 years.

But the D and E values are kinda crazy. I'll consider that this is not real motion but errors in the original observations.

The SkyTools Aa entry looks suspiciously like the WDS AC datum. I think therefore I should discard it.

By the way, Wikipedia says 40 Eri is a triple.


Wikipedia link: 40 Eridani.

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