The problem of finding and understanding discrepancies with double star data is certainly a difficult one. Firstly, I hope people are not giving deliberate ‘errors’ because this makes sorting out real changes in true double star positions an absolute nightmare. I have assume that these are just transcription errors. If this is not, then observers should always check their data using AT LEAST three different sources. Ie. Original sources, IDS 1961, WDS 1984, WDS 2001 and WDS 2003-05.
There is a lot of work that can also be done by a though detailed investigation through the available catalogues. I think the serious problems of say 20 to 30 years ago have started to fade. Much of the earlier visual pair data was then based much on the 1963 Index of Double Stars (IDS) and all the data quoted, in say Burnham's or E.J. Hartung, etc. still gives this data. As you are aware this was about the only reference available, and using two data points meant that predicting any future (or past) Sep. and P.A. was fraught with problems.
Ie. For southern observers, much of the “modern” double star data was produced in frantic two periods c.1928 and c.1955. In the late 1970’s the problems when looking through the telescope were notable only because many pairs were significantly out. Furthermore few data were available to find those pairs who needed closer inspection.
In the example of the mainly wide Dunlop pairs the data available on these were very poor. It was not usual for the last observation being made in 1910 or 1928 when the period of observer was fifty years later. The classic example were the “standard pairs” in the BAA Handbook and many ‘standard’ texts of known width - advising for calibrating simple micrometers. This was with Gamma Crucis / DUN 124 AB (1.83 & 6.45 mag) with the quoted width of 111"arcsec. PA 31o. as measured in 1919. In the WDS 2001 the separation was listed as 125.4"arcsec PA 26o. Because of Dunlop’s reputation for having poor separation and PA measures the 1826 observation of (93.4", 46o) was declared as wrong. To show the effect of this “conclusion”, references like Megastar 4 (for example, published in 2000) still uses the 111"arcsec value of 1919! (Fixed in Megastar 5.0) A calibration of a micrometer would have had its ‘R-value’ c. 12% too low.
Since the introduction of the WDS in 1984 the detail available about pairs has increased enormously. Perhaps the most significant changes were the 1991 positions from Hiparchos - giving the additional parameters (other than just Sep and P.A.) but parallax and proper motion.
Ie. Had the proper motions been known for DUN 124 AB of A= +028 -264 and B=+005 -014 - it became clearly obvious that this optical pair was far from fixed. (The PPM star catalogue assumed the proper motions for A and B were the same!
One of the biggest difference is between the versions of the WDS. This catalogue is being updated continuously - without notification - and it is important to state the Version. (I have used things like ‘WDS03’ or ‘WDS03 Sept’, but the USNO never gives such changes. Perhaps we should request or just use a version number base on the download date ie WDS031510 (year, day, month )
Users of the WDS will find significant differences with the data in say the still standard WDS84 - and still information from here is often quoted in many software and written texts. I have found that I have had to use both the latest WDS (WDS03 Oct) AND the WDS02 Sep. The reason is that the USNO has changed the FIRST and LAST DATE of OBSERVATION.
Ie. HJ 4573 (See Example 1 Below.)
The WDS 01 (and WDS 84) gives the positions for 1913 and 1924, while the WDS 03 now gives the positions of 1834 and 1924.
This can be even more confusing when the FIRST DATE is earlier and the LAST DATE is later than the previous version of the WDS !
HJ 4573 13137-5616 (Cen) 10.0 /11.0 mag. HJ 4573 (7.6", 57o) (1924) WDS03 HJ 4573 (7.7", 57o) (1913) WDS84 HJ 4573 (4.0", 57o) (1834) WDS03
This pair looks like it should by in the order of 10 arcsec along PA 57o. HJ 4573 is unresolved in GSC and single in both the Tycho and Hiparchos catalogue.
HJ 4590 13332-7734 Cha (6.5 and 9.3 mag) F6V 1837 137o 25.0" WDS 03 1880 134o 22.4" WDS 84** 1991 133o 22.4" WDS 03 NOTES: HJ 4590 A is a variable S Cha. B is CPD-76@769. ** Russell 1880.413 134.9o 22.41 HJ 4590 A: pmRA=-367 pmDec=-153 HJ 4590 B: pmRA=-351 pmDec=-118 Parallax= 52.09±39.91 mas HJ 4590 A: Parallax=28.45±1.13 mas (milli arc seconds) HJ 4590 B: Parallax=52.09±39.91 mas
Here the WDS84 only gives the 1880 position and nothing else but n=3 (no. observations), while the WDS03 has fixed up the problem by quoting the first and last observations (1837 (J.Herschel) and 1991 (Hiparchos)) n=11.
This is likely an optical pair based on the differing pmDec. The problems with the error in ‘B’ parallax makes this conclusion uncertain. Using the primary's parallax of 28.45±1.13 gives the distance of 34.14pc. or 114.6±4.56 ly. At this distance, the stars true separation of 25" corresponds to 878 AU (1.3x1011 km) and an estimate maximum period is about 1.8x104 years. Using the 6.5 and 9.3 component magnitudes, the absolute magnitudes are about 3.8 and 5.6. Using the Mass-Luminosity Relationship (MLR) the solar masses are about 1.2 and 0.7, respectively. (Agreeing with the Spectral Type)
Theoretical Catalogue Limit (TCL) Heinze (1979) for attachment is 173" at 34.14pc. Where;
********************** YEAR Sep " PA (o) 2000 21.93 133.83 2005 21.84 133.73 2010 21.74 133.63 2015 21.65 133.53 2020 21.55 133.44 2050 20.99 132.85 2100 20.05 131.88 **********************
Visual observation on 23 June 2003 using 10.5cm f/11 (190x) estimating 21"arcsec and 130o.
If I find significant change that cannot be explained, I get all the positional information from the USNO and plot all these value and the one in question. At the moment I have found some problems with the Russell pair R71 AB;
R71 AB / SLR22BC (07144-4440) is a white triple
star some 8.6'E (PA 99o) of L2
Puppis. R 71 AB is the brighter of the two that
contains the 9.6 and 10.2 (9.61V and 9.77V) magnitude
stars separated by 15.9"arcsec along PA
256o. R71 suggests it has shown some change
in the positions, reducing in PA from 288o
to 256o and 18.4" to 15.9" in the last 125
years. (2004) Russell, incidentally, gave the
magnitudes as “10,10.”
R71 was discovered by H.C. Russell just before
midnight on the 1st April 1879 (1879.248) and then
measured as 15.39" along PA 77.465o. It
seems that he evaluated the brighter component
incorrectly so the position angle had 180o
added to make the new PA as 257.465 or
257.5o. However, the WDS03 gives this 1879
separation here as 288o and 18.4" which
does not match the Russell’s "Sydney
Observatory List of New Double Stars" that appears
written in the 1881 publication. At the time of
writing the original measures are being obtained from
by the USNO, but it does seem that something is quite
wrong with the WDS03 figures or that another
observation was made in 1879 by another observer
preceding Russell’s own discovery. In the
earlier versions of the WDS, the 1879 position is
given as 15.2"arcsec along PA 256o -
matching Russell’s observations. His position
angle has likely been precessed from the time of
Russell’s original measure. Another possibility
is that this is the 1935 measure and not the 1879 one,
as the older versions of the WDS state the same scalar
measures.
SLR 22 finds that the R71 B is again double.
‘C’ is 11.2v (10.90V) magnitude being
separated by 1.7"arcsec along PA 257o.
Since discovery by R.P. Sellors at Sydney Observatory
in 1896, the position angle has reduced from
266o to 257o while the
separation remains fixed.
Common proper motions suggests that SLR 22 is likely a
true binary star but the brighter ‘A’
component is a optical companion. All three stars form
a straight line proportioned by a ratio of about 9:1.
Spectral classes are A5, A5 and A7III, respectively.
The whole system is visible in 7.5cm and contrasts
nicely with the orange-red colour of L2 Puppis in the
field.
As for your example of Mu Cygni, the differences between the two WDS’s are obvious;
21441+2845 Mu Cyg
WDS01
Desig Cmpnts PMag SMag Sep1 Sep2 PA1 PA2 Yr1 Yr2 Sp Component pm pmRA/Dec pmRA/Dec cSTF2822 BD 6.1 6.9 206.1 204.8 54 53 1902 1909 A5 2STF2822 AB 4.5 5.9 5.6 2.0 109 307 1823 1997 F6 2STF2822 AC 4.7 11.5 35.3 68.2 263 289 1878 1987 2STF2822 AD 4.5 6.9 217.4 198.3 61 46 1823 1991 F7 WDS03 Oct STF2822 BD 6.1 6.9 206.1 204.8 54 53 1902 1909 A5 +230-218 -001-055 STF2822 AB 4.75 6.18 5.6 2.0 109 312 1823 2002 F6V G2V +277-251 +277-251 STF2822 AC 4.7 11.5 263 289 35.3 68.2 1878 1987 +277-251 STF2822 AD 4.75 6.94 217.4 197.5 61 45 1823 2001 F7V F2V +277-251 -001-055
Note : Here the ‘D’ star is optical while the others are almost certainly associated. Also the differences are in the magnitudes. WDS01 is visual ‘v’ magnitude while some of the WDS03 Oct is now visual photometric (V) magnitude. Some of these brightness can differ by 0.4 or 0.5 magnitudes depending on the star’s particular colour.
STF2822AB (21441+2845) YEAR PA Sep. 2002 309.9 1.811 2003 310.7 1.793 ** 2004 311.4 1.774 2005 312.2 1.754 2006 313.0 1.735 ** NOTE : This is a Grade 4 Orbit (Hei1995) (Measure in 2002 : 312o ; +2o)
NOTE : Statistically it has been known for some time that for moderate to wide pairs the repeatability of observation need only be once every twenty to thirty years. As most of these will have periods between hundreds if not several thousands of years. It is only the close ones, or those possible few with highly eccentric orbits, to watch out for. Us amateurs should mainly concentrate on pairs between 2" and 4"arc seconds for changes. This is because the Hiparchos satellite had a flaw within this range and could not measure these ones accurately if at all. More importantly these are the ones that can show significant change in motion depending on the orientation and eccentricity of the orbit to our line of sight.
As for binary star orbits, it is the grading of precision that is useful here. In the genaral old grading system (its simpler to describe) Grade 1 (like ALpha Centauri) can be calculated to high precision. Where as a Grade 3 is intermediate, and a Grade 5 is considered poor.
The differences between the good (seeing a number of orbits) and the bad (usually only a part of the orbit in an arc. However, depending on when the orbit was written, it is not unusual to express a range of dates in which the data is useful. Ie. 1975 to 2040 etc. This is used in multiple systems where orbits are constantly changing (apsidal motion) varing slightly from one orbit to the next.
Of the 1789 binary systems with orbital parameters appear in the 6th Orbit Catalogue. The grading is as follows;
*************** Grade No. *************** Grade 1 52 Grade 2 199 Grade 3 307 Grade 4 483 Grade 5 453 Grade 8 8 Grade 9 254 *************** NOTES Grade 8 : Interferometric Orbits (high quality) Grade 9 : Astrometric Orbits (low quality)
The average period of these stars is P=370.84 years and the mean size of the orbit is ‘a’ 1.217 arc seconds. A mean eccentricity for the orbit ‘e’ is 0.523.
********************** Binaries Period (No.) (yr.) ********************** 77 >1000 105 500 to 1000 233 250 to 500 436 100 to 250 245 50 to 100 684 0.01 to 50 **********************
While I do agree that many orbits are incomplete and may not give an exact position in Sep and P.A., those for Grade 1 to 3 accurate enough to quote masses to reasonable accuracy. The problem with refining the orbits for these are essentially minor.
“Few if any orbits are known with complete certainty(even those that one sometimes sees quoted as being reliable). The great majority are pretty conjectural, and are constantly being revised by the few brilliant people who really understand the extremely difficult art (for that is what it is) of orbit computation. ”
“Presently orbits of nearly 700 objects are available (Note; This is now double this), covering a considerable range of periods and other parameters, and - naturally - differing also with regard to reliability. Not too many cases, perhaps 25 (Now 52), meet first class standards. so that elements can be definitive.”“These have; Good quality in observations, One-and-a-half revolutions covered, Semi-major axis large enough to make error small, and All mainly nearby systems.”
Two or three hundred (now 558) are reliable; the elements are substantially correct, and only minor corrections are expected. Many pairs with largely though not completely observed ellipse, and close pair with residual scatter qualify for this category.
[Total 610 or 39.7% (Not counting astrometric binaries) accurate enough]
About 300 (Now 925 or 60.3%) may receive he label ‘preliminary’.
- Substantial changes may be forthcoming, particularly for orbits over 300 years. (515)
- In short period orbits only with limited or inconsistent observations
- Even the basis of mass can be made by a3/P2 even though a and P may not be well known (dynamical parallaxes)
- Orbits are of statistical use.
“About three-quarter (75%) of the orbital periods are in the range of 20 to 25 years (Now 874 or 48.9% - Things are improving!) The very close pairs are too infrequently observable, while most of the long period ones are still too indeterminate.... Finally there are a small group of indeterminate orbits...voided as misinterpretations by subsequent measurements, and some spurious or faulty computed.”
Several points of interest to the discussion regarding radical changes in orbits include;
- One third of binaries have ecentricities exceeding 0.6
- Most orbits are obtained from stars within 100pc. of the Sun (Limit now about 200pc.)
- An improvement to orbit accuracies and a narrowing of the parameter ranges has now occurred from the more accurate Hiparchos parallaxes. Ie. "a" semi-major axis in AU is calculated by a= ‘a’ in arcsec / parallax in arcsec.
As for Tom’s statement;
“Detecting these discrepancies, which doesn’t always require sophisticated equipment, is part of the fun of double-star observing! If published, it can also be scientifically useful. I think you will find that there are several other members of this group who have had similar experiences to my own in this area.”
I agree 110%. You can have more fun researching the discrepancies and errors you find from your own observations. Although the mathematics in working out orbits is likely beyond yours (and my) capabilities, it is the discovery of those system that can break the ‘normal’ view of things.
Bill’s point is very valid here in the sense that a sudden change is PA might be real or not. If anyone does find a true error (such as unexplainable changes in either separation or position angle) that they can’t explain - then don’t be frightened to talk about it. There are enough observers here to point you in the right direction and if you do find something, they have both the experience and likely the necessary reference material. Even if you don't discover something of significance - at least you can correct the error at its source!
Regards and Clear Skies!
Andrew
Message: 15
Date: Mon, 20 Oct 2003 21:27:38 -0000
From: "Bill Green"
Subject: Data Discrepancies
I have noticed variations in data for many doubles.
I can surely understand minor differences but some
are gross discrepancies. For example; the chart in
PJs recent article on Autumn Double lists the PA of
Alpha Piscium as 277o. The observing list
of doubles at the Astronomical Society has it at
50o! There are many others with equal
differences.
Am I reading something wrong or just plain missing
something?
Bill Green
Message: 18
Date: Mon, 20 Oct 2003 23:56:27 +0100
From: "Thomas Teague"
Subject: Re: Data Discrepancies
Bill,
I seem to recall reading recently that there are some deliberate ‘errors’ to be found here and there in some amateur observing lists, designed to ‘catch out’ those who report observations they haven’t actually made! I can’t vouch for this, though.
However, it is very common to find discordant data about double stars. With experience one gets to regard all such data with considerable reserve. As a rule, popular amateur texts don’t bother to cite their sources. One therefore doesn’t know whether they come from predictions based on computed orbits that may have been completely discredited, or from observational data that may well be seriously out-of-date. With some pairs, there can be considerable relative motion in just a few years. Indeed, near periastron, even pairs with quite long orbits may show remarkably rapid relative motion - a good example right now is Gamma Virginis, which will show extraordinary changes in PA over the next few years. With stars of more or less equal brightness, moreove, one often gets disagreement as to which component should be regarded as the primary, leading to 180o ‘discrepancies’. Sometimes, people carelessly give data for AB which actually turns out to be for AC or some other combination of components.
In some handbooks, such as the annual BAA Observing
Handbook, you will find lists of binaries printed
with orbital predictions in which angular
separations are given to 0.01 arc seconds. It is
tempting to assume that these are accurately known
data, which can be taken on trust. Nothing could be
further from the truth. Few if any orbits are known
with complete certainty (even those that one
sometimes sees quoted as being reliable). The great
majority are pretty conjectural, and are constantly
being revised by the few brilliant people who really
understand the extremely difficult art (for that is
what it is) of orbit computation. In practice, one
quite often finds that a casual glance in a small
telescope is enough to reveal that a seemingly
devastatingly accurate orbital prediction is in fact
grossly out. I found this myself with Mu Cygni a few
years ago using a little 2.5" refractor (just
eyeballing - no micrometer), and later found that my
conclusion had been independently confirmed by Bob
Argyle using one of the Cambridge refractors in
conjunction with a filar micrometer. Detecting these
discrepancies, which doesn't always require
sophisticated equipment, is part of the fun of
double-star observing! If published, it can also be
scientifically useful. I think you will find that
there are several other members of this group who
have had similar experiences to my own in this
area.
Tom
(NO ADS!)