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Questions about STF 88, the WDS, and binary systems

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#1 Organic Astrochemist

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Posted 26 November 2019 - 10:45 AM

I recently was observing in Pisces and recorded some spectra of 74 Psc (also known as Psi1 Psc or STF 88). This is a nearby bright pair (V mag 5.273 and 5.455) widely separated (29.2", position angle 160°). 
 

I posted the gory details here on the Observational Astrophysics forum, but I’d like some opinions from the double star experts here.

 

Stelle Doppie (WDS) says the primary is B9.5V and the secondary is A0V and that the parallax and common proper motion confirm this pair as physical. But Simbad says that the brighter primary is actually a cooler, dimmer A1V and the secondary is actually a hotter, brighter A0V.  Sky Safari is emphatic that the two are an optical pair.

 

I don’t think everyone can be exactly right here but there should be some reconciliation possible.

 

Do people often check the WDS vs Simbad for spectral classification? How often is there a discrepancy? 

 

With physical pairs, how often is the brighter primary the cooler star? There are scenarios where this could occur such as unresolved companions and evolved stars. How often are these seen? In closely matched pairs, how common is the subgiant luminosity class IV?

 

How reliable are parallax and common proper motion to confirming the physical nature? What if the spectroscopy argues against it?

 

STF 88 is an easily observed pair. Any comments or images are welcome.


Edited by Organic Astrochemist, 26 November 2019 - 10:47 AM.

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#2 dmdouglass

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Posted 26 November 2019 - 01:26 PM

Well, you said: "STF 88 is an easily observed pair. Any comments or images are welcome."

So.... STF-88 is also H(erschel) IV 9  [ H-IV-9 ] and i observed it on Sep 7, 2018... Here is my image and measurement report...https://www.az-dahut...DS-20180907.JPG

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H-4-009-DS-20180907.jpg


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#3 fred1871

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Posted 27 November 2019 - 01:02 AM

The spectral types given in WDS are a better ft for the listed magnitudes than those via Simbad: with star A having a brighter apparent magnitude, if the A and B stars are a binary they'll be at the same distance from us, so the (slightly) brighter star fits for a slightly earlier spectral type. The source for spectral types isn't given in WDS nor is it upfront in Simbad.

 

The Proper Motion (PM) numbers listed in WDS and those from Simbad are sufficiently similar (atar to star) to suggest there's no problem for this being a gravitational pair. Likewise, the parallaxes from Simbad, Hipparcos and Gaia, are very similar.

 

I would treat this combination as most likely a gravitational pair. Even Bruce McEvoy, who's very keen to exclude possible as well as definite optical pairs, has kept this one in the gravitational fold in CDSA2. 

 

I would ignore Sky Safari on these matters because its reliability is not high on binaries or astrophysical criteria. Perhaps someone didn't know that various confirmed binaries (such as 61 Cygni) can have noticeable differences in PM without that suggesting merely optical alignment. Of course, the ideal is to see arced motion, confirming gravity, but this one has barely changed in over 250 years.

 

Differences in spectral classification between Simbad and WDS occur more than rarely; however one would need to trace the origin of the classifications to evaluate which is better when they differ. Here, as I noted above, the WDS spectral types are a better fit than Simbad's, if we assume binarity.



#4 WRAK

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Posted 27 November 2019 - 03:37 AM

GAIA DR2 parallaxes and proper motion values are similar enough to assume at first look a physical.

 

Closer look shows that parallax errors are too large to take this as a given. Smallest possible spatial distance between the components (with the given measurement errors used as standard deviation for normal distributions) is ~2,500 AU but the spread goes up to > 5 mio AU so the likelihood for gravitational relationship is with the given data quite small.

Even when we take the smalles possible distance as given then a potential orbit would still have a period of nearly 60,000 years (using the StarHorse median masses of 2.35 and 2.15 Sun mass).

 

For this reason no change in angular separation and position angle is to be observed in a human time span and also proper motion speed is too small to notice a significant position change against the background stars - looks like this object  is of little interest for visual observation



#5 fred1871

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Posted 27 November 2019 - 05:18 AM

Certainly not a double that matters for orbit calculation if it is gravitational - far too wide and huge period.

 

However - Gaia parallaxes are not always of the accuracy we might hope to be achieved when the final numbers are released. Some Gaia DR2 parallaxes are more than the formal error bars changed from DR1. And I've found the occasional one where the DR2 parallax is clearly wrong, based on the spectral type, and suggested also by the PM numbers not matching - for example, large PM numbers with a parallax suggesting very large distance, where the spectroscopic parallax suggests it's much closer.

 

I've encountered a few cases where the parallaxes are available from DR2 for both stars of a definite binary that shows orbital motion; but the parallaxes differ by more than three times the formal errors. If I can locate some of these examples I'll offer them here - they're usually a by-product of work I'm doing for USNO on neglected doubles, and not my primary interest, so I'm not assiduously recording them.  Though I do record them in adequate detail in my working notes.



#6 ssmith

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Posted 27 November 2019 - 10:05 AM

The spectral data from SIMBAD appear to come from the following reference and the tabular data can be found here (lines 52 & 53). 

 

Correction: 

After a more complete reading of the above referenced paper it appears that the spectral types in the table above were not generated as a result of the research but were pulled from SIMBAD.

 

I took a quick look at some of the referenced papers for these stars and the spectral types have been rather unsettled over the years.  Here is a summary from some of the sources.

 

1959  B9.5V    A3V

1963  B9V       A0V

1981  B9.5Vn  B9.5Vn

1987     ---      A0IV-Vnn

SIM    A1Vn     A0Vn

WDS  B9.5V    A0V

 

A full data request for this system from the WDS may reveal the source for their data.

 

Looking at the parallax data I dont think there is much likelyhood of the pair being orbital in any meaningful sense.

 

Psi1 Psc 120mm 11-23-17 4fr.jpg


Edited by ssmith, 27 November 2019 - 02:00 PM.

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#7 fred1871

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Posted 28 November 2019 - 12:02 AM

The stars may well turn out to be co-moving through space, rather than orbital. My main point on orbital vs optical is that we don't really know. The data isn't certain enough to decide one way or the other. Better data may well be available after Gaia's work has finished. At present, some of the data is still in a condition of some uncertainty. 

 

Interesting variations on spectral type, Steve. Not a great range, but enough to make aspects of the story more uncertain. There is a preference by head-count of spectral types, say B9.5V for star A, A0V for star B. The biggest variation is the 1959 spectral classification for star B as A3V. Without detailed reasons for the variants the best we can say is the WDS classes look probable.... and will be close, judging by the modest range of variation.



#8 John Fitzgerald

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Posted 28 November 2019 - 09:36 AM

Co moving through space and not orbital means that over many thousands of years they will drift apart due to differential attraction from external sources.



#9 Organic Astrochemist

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Posted 28 November 2019 - 09:33 PM

Thanks to all for the interesting and insightful responses. I’m definitely going to include a trip to Gaia data for my double star observing.

Where are people looking for their Gaia distance data? I’m using this catalog, but perhaps there are better sources:
http://vizier.u-stra...rce=I/347&-to=3

Thanks for the spectral types Steve. Interesting that only Simbad has the primary as cooler than the secondary. I would think that if the two spectra are taken at the same time (and position on the sky) it should be easy to tell which is hotter. The Balmer and Ca II lines are very sensitive to temperature in these spectral types. Interesting that the 1981 reference had them the same at B9.5V. Maybe I’ll observe them again some day and see if I get the same result, in agreement with Simbad.

Edited by Organic Astrochemist, 28 November 2019 - 09:34 PM.


#10 fred1871

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Posted 02 December 2019 - 01:45 AM

I've added some material regarding Gaia parallax issues in the thread about 10 Ari, which is another example of problems with the current data release (DR2).



#11 Organic Astrochemist

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Posted 02 December 2019 - 02:51 PM

Thanks Fred. While there no doubt are errors with Gaia, they won't all be of the same kind and degree. In the case you posted about 10 Ari, within the uncertainties of Gaia DR2 the pair are separated more than five parsecs which is opposed to 200 years of observation that suggest they are separated by about 90 AU. In the case of STF 88, as Wilfried points out, the Gaia data are consistent with all other data that suggest a widely separated pair. Further, I think the distances are consistent with the spectroscopic data in that the hotter, more luminous secondary is further away and the cooler, less luminous primary is closer.

 

I did note that the pair is listed by Bruce McEvoy in CDSA2. I think it's a shame he doesn't seem to contribute here any more. But I think that part of his philosophy was to enable and encourage visual observation of the full gamut of double stars, from as closely separated as possible to very wide systems including common proper motion pairs. I understood what Wilfried meant when he said "looks like this object  is of little interest for visual observation", but I think Bruce was advocating a different double star visual observing philosophy in contrast to precisely this kind of sentiment.

 

How are we to observe these very wide systems? Perhaps not only by the way that we observe closer systems because a) they might not fit in the same field of view b) the separation and angle may never change on human time scales and c) without recourse to some additional information the observation may be no different than observing random pairs of stars in the sky.

 

I think spectroscopic stellar classification and parallax distances are useful to observing these wide pairs, but it is important to consider their limitations. I'm starting to see the value of embracing aspects of astronomy that are uncertain.



#12 fred1871

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Posted 02 December 2019 - 09:15 PM

Gaia not being reliable on parallaxes (yet), we're left with decisions regarding the status of the pair essentially depending on the spectral types of the two stars. And the spectroscopy is somewhat uncertain, with some differences in spectral classification for each of the stars. That leaves us with an assessment based on preferences - which data points seem (to each of us) more likely to be accurate.

 

I'd not put my money on either horse - the very wide gravitational horse or the merely optical horse. On further looking at data sources I'm still uncertain about status, with perhaps a slight leaning towards co-moving.

 

Regarding Bruce McEvoy, and encouraging "visual observation of the full gamut of double stars", it's rather the case that

he didn't want to include double stars that were likely or definitely optical, hence his decision to, shall we say, "de-platform" those that were not, or possibly not, gravitational or wide co-moving in CDSA2. It puzzled me because in one of his submissions here on CN he agreed to the idea of aesthetic observing of doubles being legitimate, and some definitely optical pairs such as HJ 3945 in Canis Major (the 'Winter Albireo') are definitely high on the aesthetic list.



#13 WRAK

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Posted Yesterday, 09:14 AM

As a matter of fact GAIA DR2 does not even claim to provide accurate data for physical star systems because it is restricted to a single star model assuming that there is no such thing as a specific additional movement due to gravitational forces between close stars. Despite this weakness DR2 delivers parallax and proper motion data usually precise enough to identify physical systems as such if DR2 delivers data for all of the components - which is the case only for a fraction of physicals going down to zero for angular separations <0.4". This said I consider DR2 as extremely valuable source.

 

The property if physical or not is in my opinion completely irrelevant for visual observation with one important exception: Proper motion is high enough to be noticed by changes in position relative to background stars and in perfect cases orbit speed is high enough to be noticed by changes in position angle and angular separation from year to year.


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#14 fred1871

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Posted Today, 04:50 AM

The absence of both primary and secondary stars in some definite binaries is a surprise, with the primary star examples I've come across being in the range from 7th to 11th magnitude. I don't expect all the brighter stars to be included. But the non-appearance of 7th to 11th mag stars is unexpected.

 

Some reasonably wide pairs have the primary included but not the secondary. And of course with so much data to process there are cases where a star is included but has no parallax or PM numbers - that last is unsurprising.

 

In my work on binaries one thing I do from the start is try to scale the orbit, at least in terms of the projected separation. For that, a parallax is the best starting point if it's reasonably accurate. Where no parallax can be found I estimate based on spectral type. Where it becomes interesting is when there is a Gaia parallax and the spectroscopic parallax disagrees substantially with it.

 

Even so, the large majority of Gaia parallaxes I've looked at thus far are consistent with other data. The most problematic are the cases such as 10 Arietis, very clearly a binary, so the stars are effectively at the same distance from us, but with significantly differing Gaia parallaxes that don't reconcile with that reality.




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