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STF 2146 - Mistake or Mix-up?

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#1 VanJan

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Posted 09 August 2021 - 06:15 AM

Follows is the current data of STF 2146 in Draco at 17h 13.1m +54° 08'

 

AB = 6.92, 8.80   2.8"   225°   (2018)   A9III     AC = 6.95, 8.87   89.3"   235°   (2016)

 

I first observed this double with my 20cm reflector at 40X and noted the AC pairing. I then observed the A star at 200X. No B star visible. Went to 250X. Still no B star. But I did notice that the C component had a 10th magnitude star at approximately the separation and position angle listed for the AB pair. Hmm . . . confused1.gif

 

Back inside at my computer, I pulled up Memoirs: Volume 56 by Royal Astronomical Society (1906), and it lists the data for STF 2146 as  [ 8.0, 10.0   2.84"   225.2°   (1879) ]  with no mention of a third star. It further states the proper motion of BD +54°1868 and concludes "as there is no perceptible change [in the double star measurements] we must regard this as a physical pair." Pulling up BD+54°1868 on the Simbad database equates it to HD 156161, and the accompanying photo clearly shows that this star is the C component of the current WDS data.

 

So . . . from what I observed and the old data, I must conclude that the current data has an A star that was not originally part of the Struve pair and lists the true A component as the C component. Or I'm seeing things. Or I'm NOT seeing things. Or the current data is mistaken or mixed up.

 

help.gif   

 

 


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#2 c2m2t

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Posted 12 August 2021 - 03:32 PM

Hi VanJan!

You have uncovered an interesting anomaly. I have attached a screen shot from Aladin containing the UCAC4 data. The entries for the component data are in alphabetic order...A, B, C. Given what the UCAC4 data shows, one is likely not to see anything at the "B" position...the data shows an fmag of 14.244 and nothing for the J and K band filters meaning  that this supposed companion is very dim. Note also, from the star portion of the image, the  "C" companion appears to have no companions at 2.8" distance...if there is, the surveys did not detect it.

 

Therefore, I am not sure what to tell you. Something that appears pretty conclusive is that there is no mag. 8.80 "B" companion at the stated location. As far as your certainty of there being a close optical companion star to "C", this wants to be check by a few sets of eyes. 

 

Bottom line, there are issues with the WDS data that need investigation. Given the 32 observations since discovery, something is definitely amiss. 

We need a Sherlock for this one. 

 

Cheers, Chris.

Attached Thumbnails

  • stf 2146.JPG

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#3 fred1871

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Posted 12 August 2021 - 10:44 PM

A couple of issues here. To begin with the second note above: a survey photo of a very bright star has so much image spread that a 2 magnitudes less bright star less than 3 arcseconds away will disappear within the image spread. So there's no reason to think the usual survey photos will show the two stars.

 

I checked the 2MASS image via Aladin-Lite - that also doesn't show more than image spread around the primary. 2MASS can with fainter stars show resolution to about 1.5 arcseconds as an elongation, but only if the stars are dim, getting towards the 2MASS faintness limit - so that can work for 11th and 12th magnitude stars (if both are faint). When there's a bright star involved, it can be as unable to show close secondaries as the red and blue plates of other sky surveys.

 

Gaia does not help because it does not list the 'B' star. However I've found quite a few cases where Gaia does not list the secondary star of fairly close doubles, especially when one or both are bright.

 

Regarding the observation by VanJan - it may simply be that it was a failed resolution of the AB pair, fairly close and ~2 mags brightness difference. That level of brightness difference can be surprising in how much difference it makes. Especially on a night of less than very good seeing.

 

Simbad does bring up two stars for the C component - no magnitude listed for the secondary, but a position about 2.5 arcseconds from the 'C' primary. And the 2MASS image shows a very slight hint of elongation in the image of 'C' in PA about 225. When I checked Gaia for 'C', only one star came up there as well, with the expected Gmag (similar to Vmag). Regarding the absence in WDS of a listed companion for 'C', this happens with some "companions" that for a number of reasons don't get listed in WDS - it can be because it's outside the parameters such as those Aitken suggested, or non-cpm, or...?

 

So - as it seems unlikely with 32 measures over time that there could be no companion to 'A', we would have to expect a mix-up of the AB data which should be given for CD instead.  However, as VanJan points out, the 1906 catalogue by Lewis (a double star specialist at Greenwich) in the RAS Memoirs gives the early measures and they're consistent with the later information. The WDS for 1996 has the same identifications as WDS 2021 (current); the 1976 Lick-IDS agrees (the data is reproduced for brighter pairs in the Sky Catalog 2000 vol 2).

 

My suggestion at the moment is that some observers should check out STF 2146, on a steady night, and preferably with apertures of 20cm and above. The AB pairing (which according to Lewis goes back to Struve) was first measured with the Dorpat refractor, about 9 1/2-inch aperture (it's described as 9 Paris inches, not quite the same as British/Imperial). AB appears not to have changed very much, if at all. So that gives a (refractor) aperture for seeing and measuring. One of the old measures Lewis gives is by Dembowski, presumably with his 7-inch refractor. That points to a slightly more accessible pairing. 

 

If no one under good conditions with telescopes of adequate size can see the 'B' star, we have a likely confusion in the data. This would seem unlikely given the long and consistent listing of AB. However if all 4 stars exist, we have another 'double-double'. At least for medium (in amateur terms) apertures.


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#4 VanJan

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Posted 13 August 2021 - 06:33 AM

A ginormous THANK YOU! to Chris and fred1871 for your investigations. Much appreciated. Now I am really looking forward to what other observers uncover here. popcorn.gif  Not excluding another peek, myself. wink.gif  


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#5 VanJan

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Posted 14 August 2021 - 11:31 PM

Ok. I observed this pair again, and I saw exactly the same thing as I did the first time I looked at it. I did carefully examine what the current data calls the A star at 313X and saw no comes whatsoever. What the current data calls the C star does have a comes of approximately 10th magnitude at what appears to be the current data for the non-existent AB pair (2.8" 225°). What I see of the C plus comes at 200X, 250X, and 313X matches the magnitudes (8.0, 10.0) and separation and PA (2.84" 225°) by Burnham in 1879. Aside from having no visible comes, the currently called A star is too bright (6.92) to match the historical data.

 

Pending further observations or evidence to the contrary, my conclusion is that the current data is in error as far as labeling. What is currently termed the C star, plus observed comes, should be listed as the AB pair, and what is currently called the A star should either be dropped entirely or termed a later measured C component.

 

That's my story, and I'm sticking to it. sumo.gif 

 

At least until someone convincingly shows me the error of my ways. brick.gif  


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#6 fred1871

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Posted 15 August 2021 - 01:34 AM

Mix-up - transfer of data from what would be Cx on current lettering to a supposed AB has always been possible, but unlikely - unless of course it was done very early, somehow, and observers kept measuring Cx while reporting the results as an AB measure. If that happened we would get a new lettering - perhaps Cx could become BC, with the unaccompanied current A retaining that letter. And the data revised, to AB as a wide pairing, and BC as a close one.

 

The factor of plausibility might be helped by your ability, VanJan, to see the companion to current C, but not for A. That makes a companion to the current C real (with backups from others), and the current B being either non-existent or out of reach. For now I'll ignore the slight differences in the separation measures in recent years, examples I found range from 2.4" up to 2.8". Think I'd better get hold of the full data, to see who and how the measures have been done.

 

I have also wondered about the very similar magnitudes for B and C. Coincidence? or an erroneous data transfer?

 

I'll still suggest observations with larger scopes to confirm the absence of the B companion close to A, before sending off a note to the WDS "keepers of the double stars" discussing the whole matter. Best to cover all possibilities before that. I can't help with observations as from my latitude, as STF 2146 barely comes above my northern horizon. Which is not where you want a close unequal pair to be. frown.gif


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#7 VanJan

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Posted 18 August 2021 - 01:27 PM

Through no agency of my own instigation, the WDS has revised its listing for STF 2146. The WDS data now reads:

 

AC = 6.95, 8.87   89.3"   235°   (2016)   A9III     CD = 8.87, 10.5   2.8"   225°   (2018)

 

which matches the telescopic view. I'm pleased the disconnect between labels and stellar presentation has been resolved. imawake.gif

 

 


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#8 flt158

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Posted 18 August 2021 - 04:30 PM

Hello Van Jan. 

 

This is all very good to know!

And a big thank you for studying this triple or quadruple system. 

 

The sooner Stelle Doppie follows suit the better it will be for the rest of us. whistling.gif fingertap.gif

I'm glad some of their team regularly visit the Double Star forum in recent times.   

 

Clear skies from Aubrey. 


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#9 tdfwds

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Posted 19 August 2021 - 06:31 PM

What in fact happened was that checking of VanJan's object in the WDS confirmed what he was saying.

 

However, contrary to what has been stated, the situation was quite evident in GAIA EDR3 data.  It was plainly obvious from the high proper motions that the brightest star, 6th magnitude "A" was optical and had no "B", and in fact that a "BC" pair was 89" distant, said "BC" pair being the 2.8"/2.9" 8th and 10th magnitude pair, and therefore likely originally the true STF2146AB.

 

The Washington Double Star Supplement (WDSS), which consists primarily of results from professional data mining practices (well, nowadays it does) independently confirmed this, and no doubt due the the historical AB-C situation then currently in the WDSS the WDSS pair had been logged as CD.  All this came from the proper motions and parallaxes of the stars involved.

 

Having a fair amount of data available that seemed to show what the case was, even if having no historical information to go by, this info was sent off to WDS (I don't tend to send anything to WDS as a problem unless I am confident of having found the fix for presenting to them) including at the beginning of the email the public forum archive link of VanJan's post to show where the issue was first noted.  In other words, VanJan saw the error and that's why it was looked into, so his post was linked foremost in the notification email to WDS.

 

As some have noted this led to a correction being made to the WDS main file at the GSU site around last Monday (it still hadn't been updated the day before on checking).  Stelle Doppie is about 8 months behind at present (unless updated today, I haven't checked), and the Vizier holding for WDS is about ten days old and not likely to be updated again before early next week.

 

The decision made by the WDS has been to have AC and CD as the solution, although CD are the true STF 2146 I can see the point in this because renaming it to STF2146AB would lead to confusion with old records and literature and people would still end up confused by it all.  And as a consequence B has disappeared, or at least appears to have done so.

 

The response email received from WDS on the matter revealed that when originally imported into the IDS the only coordinates where those of the one used in the identifier and those were the same for the three stars, and this had carried over into the WDS.  The number of measures reveal that the object is rarely measured and it is not clear without the source data how many of these measures predate the IDS, which has a formal publish date of 1963 I think, but was prepared over several years before being put on punch card.

 

If there are newer measures I can envision why they did not lead to earlier noticing of the issue if they were simply reported as for STF2146.

 

Or maybe something else happened.

 

The wide A-CD pair is such that the proper motion in declination for CD (roughly 100+ mas/y) is around twice that of the A star so relative separation must have changed over time for the wide optical alignment.  There are significant differences in the CD pair proper motions especially for declination, but not enough to stop them being common proper motion, but sufficient to suggest relative orbital motion.  Possibly the object is approaching periastron at present, as the separation seems to have been stable for some time?  On the other hand it could be poor data or just a red herring.  It's a time will tell thing, future measures needed.

 

And that's that, basically.  But for VanJan and GAIA data it may have gone unnoticed for years to come.

 

Except as a final note there is a star E, noted in the WDSS.  It has the same large proper motions and parallax as C and D although it lies ten to eleven arcminutes away.  Certainly a common motion object.  Feel free to recheck this point, I haven't when writing this.  The GAIA colours and derived Absolute G magnitude suggest this object may be a low mass red subdwarf, certainly it isn't anywhere small enough to be a brown dwarf.  The data this is based on is from EDR3.  I wouldn't bother trying to look for it, it's about 21st magnitude visually.


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#10 Adam Long

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Posted 29 August 2021 - 04:10 AM

Had a look at this group last night, after first locating nearby Arrakis/ mu Dra.

 

At 180x in the 14" f/4 A was clearly brighter, with the C component widely separated. D was not immediately obvious in the middling seeing - just a suggestion of a messy snowman - so I swung back to mu Dra to tweak focus. Returning to CD gave a neat split at this mag, although D is fairly faint. All exactly as reported by VanJan.

 

Worth noing that the error is duplicates in CDSAII - the separation matches CD but the mags match AC. Stelledoppie now seems to show the updated AC, CD arrangement.

 

Also noted some other faint stars within a couple of arcminutes matching the brighter ones in Chris' aladdin screenshot. I have a sketch but probably not worth uploading.


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#11 blakesphere

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Posted 30 August 2021 - 05:32 PM

I've been looking at this system for a long time...

 

JUN 16, 2014, backyard, city limits, poor seeing, loaner 8" f/6 Dob. Did not see B.

 

JUL 20, 2015, dark sky observatory, poor seeing, Celestron 14-inch SCT, Tele Vue 101 refractor.  Did not see B.

 

OCT 07, 2016, backyard, suburban, average seeing, C8". Did not see B.

 

AUG 11, 2018, dark sky observatory, poor seeing, GSO 16-inch RC, Tele Vue 101 refractor. Might have seen B star. Distracted. Some trouble with the software field of view...

 

SEP 25, 2018, robotic observatory, RC 24-inch, ½ second exposure, poor seeing.

http://blog.lumpydar...t-stf-2146.html

 

This imaging system generally does not resolve below 5 arc-seconds. But I'm getting down to magnitude 15 and 16.

 

 


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#12 Ihtegla Sar

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Posted 24 June 2023 - 10:49 AM

Through no agency of my own instigation, the WDS has revised its listing for STF 2146. The WDS data now reads:

AC = 6.95, 8.87 89.3" 235° (2016) A9III CD = 8.87, 10.5 2.8" 225° (2018)

which matches the telescopic view. I'm pleased the disconnect between labels and stellar presentation has been resolved. imawake.gif


I am glad Stelle Doppie has this correctly listed. My Cambridge Double Star Atlas says it's a mag 6.9/8.8 2.6" double with a 224 PA.

I was confused last night when I saw a mag 7ish star widely seperated from a mag 9ish star that had a tight (about 2.5") mag 11ish companion at a 224 degree PA. But after checking Stelle Doppie and this thread, all is right with the world.

#13 rugby

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Posted 25 June 2023 - 10:35 AM

Observed it with a 10-inch f5.6 under very good skies using 470x. I saw what Van Jan reports.

My uneducated guess is this was a mixup.



#14 Ihtegla Sar

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Posted 25 June 2023 - 11:34 AM

It was a mixup but it has been fixed in WDS/Stelle Doppie, but not in Cambridge's printed Atlas.


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