DOUBLE STAR NOTES : DISCREPANCIES IN
DOUBLE STAR CATALOGUES
INTRODUCTION
One of the first things in science, or any other
matters disseminating information, is the open assumption that all
measures and statements have been produced in such a manner to avoid
any bias or dishonest practices. Here we live in the true hope that
observers, both present and past, are not giving any deliberate
‘errors’.
We do normally assume, and have
‘faith’
that these kind of mistakes are just unfortunate transcription errors
or widespread slip-ups. If such dishonest methodologies were indeed
found to be true, this makes the task of sorting out real changes in
true double star positions or other related information an absolute
nightmare! (Let us here assume that it is not!)
The wholesale problem of finding and understanding
various discrepancies within any double star data is certainly a
difficult and challenging one. As a rule-of-thumb, observers should
always check and question their data using AT LEAST three
different reliable sources, and if anything else, as many sources
as practicably possible. I.e. Use original sources, say from the;
IDS 1961, WDS 1984, WDS 2001, WDS 2003
WDS 2006 and the latest, WDS 2011.
Alternatively use the original published measures, or
sensible or logical options, taken the expertise in from the many
catalogues, journals or even other common independent references.
There is much work that can also be done by detailed
investigations through all the available catalogues. Fortunately for
the observer, I so think the most serious problems existing say
twenty or thirty years ago have already started to
fade. Yet in regards many of the lesser issues there is much an
visual observer can do on cloudy nights, which can usefully
contribute to the science of double, multiple and binary stars. Most
do not require vigour mathematical or astrophysical expertise and
know-how; but often only requires clearing the head, applying some
common sense and the acquired ability to see the unusual or incorrect
information. Once found, these can be notified to the appropriate
astronomical authority so the same pitfalls can be avoided into the
future or assessed in some distant future time or date.
To explain this concept better, perhaps it is best to
speak from some useful experiences — using my own perspective
as a southern observer over many decades. In my humble view,
experience has regularly revealed many shortcomings in common data or
of the chronicled available measures. I also think this same
judgement also probably applies equally to the northern pairs as
well, even though the historical basis and longer history might not
pose exactly the same troubles, problems or issues. So it is likely
best here to talk from my own experiences and give some examples,
labels, and discuss my own standpoint.
Some Southern Experiences
We find much of the earlier visual pair data was then based much
on the data within the Index of Double Stars (IDS) (1963) and all the
data quoted, in say the three volume
Burnham’s Celestial Handbook or
E.J. Hartung’s Astronomical Objects
for Southern Telescopes (AOST1) (1968), etc. For example, the
majority of Hartung’s AOST1 pairs are
measures made in 1960, though some of these quote the earlier
measures. Today (2012), this is now fifty years or more years
ago!
Even today, many common texts on visual double stars still gives
this now almost antique data. In the southern pairs, much of the
information of these works usually come from one single source
— the R.T.A. Innes great southern work; Reference Catalogue
of Southern Double Stars, published in 1899 in the Annals of the
Royal Observatory at the Cape of Good Hope. Here Innes sets out the
general framework of the behaviours and available observations made
prior to 1899. This 316-page document has most of the
‘common’
southern pairs with useful and critical evaluations of positional or
scalar errors, gravitational association, colours, magnitudes, with
many general relevant historical statements of interest. It contains
2,140 pairs listed in right ascension order by hours, all roughly
south of −25° declination. Several important updates
appear in the first two decades of the 20th Century. These texts then
set the scene for the next sixty or seventy years. When you read
either Burnham’s or
Hartung’s, you become quite acutely
aware of the origin of data and comments they are stating> Hence, the
distinct limitations when observing the pairs for yourself. As time
progresses, these older discrepancies become even more obvious.
Although both these amateur references became very popular,
especially due to the glorious writing style and overall
authoritativeness, and moreover, the sheer number and variety of
objects, they also do have their drawbacks. As you may be already
aware, this was at the time about these were the only useful common
references available. For double stars, the main detraction was often
just using two isolated data points to access variations or changes.*
This often meant that when, say, trying to predicting any future (or
past) separations and/or position angles, become or was nearly always
fraught with some kind of intrinsic or intractable problem. In some
cases they are just plainly wrong.
* Historically, for the southern observers, much of the ‘modern’ double
star data was produced during frantic three periods. These were
between the 1890s, around about 1928 and then again in 1955. In
roughly the late-1970s these problems were decidedly quite notable
when looking through the telescope. This was because many of the
discussed pairs were significantly out in their position, brightness
or against the current published data. Furthermore few data were
available to find those pairs who needed closer inspection or had
significantly moved.
In the example of the mainly wide Dunlop pairs, which
I’ve been slowly studying and
reassessing, we find more often than not, that the data available on
these were and remain very poor. It was not usual, for arguments
sake, that the last observation being made on some Dunlop pair was
something like 1910 or 1928! This was during a period for observers
that was more than fifty years later! Needless to say the information
was an observational minefield!
Southern Double Star Examples
Perhaps the classic example of all were the standard pairs
presented in the annual BAA Handbook and many standards of
systems with presumably known widths — specifically advising
for calibrating simple types of double star micrometers. This was
with the notable southern double placed at the apex of the
Southern Cross, γ Cru / Gamma Crucis /
DUN 124 AB (1.83V & 6.45V magnitude) with the quoted width of 111
arcsec. PA 31° as measured in 1919. After the WDS 2001 the
separation was listed as 125.4 arcsec PA 26°, using the
Hipparcos data of 1991. Because of Dunlop’s reputation for having poor separation and
PA measures the 1826 observation of (93.4″, 46°) was
declared as plainly wrong. To show this dramatic effect of this
conclusion, some useful references like Megastar 4.0 (for example,
published in 2000) still used the 111 arcsec value of 1919! (Fixed in
Megastar 5.0) A micrometer calibration would have had its own ‘R-value’
consistently about 12% too low! Hence, every future measure using
this as a calibration star would be out by this constant amount or,
if the observer ended-up using several different pairs, would be left
with just confusing estimations of this R-value.
Since about 1984, as stated in the introduction of the Washington
Double Star Catalogue (WDS), the general detail available about many
individual pairs —both north and south — has increased
enormously. In Gamma Crucis case, perhaps the most significant
changes of all were the 1991 positions from Hipparcos astrometric
satellite — giving the additional useful stellar parameters
(other than just Separation and P.A.) including accurate positions of
the individual stars, the sometime individual parallax(es) and the
proper motion(s).
I.e. 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. Even worst, in the main 2000
epoch FK5 frame reference, the PPM (South) (Positions and
Proper Motions) star catalogue, the immediate successor to the
SAO (Smithsonian Astrophysical Observatory) star catalogue,
had assumed the measured proper motions for A and B were exactly the
same!
Comments on the Changes in Current Versions of the WDS
One of the biggest difference is between the different versions of
the WDS. This catalogue is now being updated continuously — all
without formal notification — and it is important to state the
Version being used. (I have used things like ‘WDS03’ or
‘WDS03 Sept’, but the USNO never gives such changes in
these terms. Perhaps we should request or just use a version number
base on the download date? I.e. 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 two of the latest WDS (WDS12 June) AND the
WDS03. The reason is that the USNO has changed the FIRST and LAST
DATE of OBSERVATION.
I.e. HJ 4573 (See Example 1 Below.) The WDS 01
(and WDS 84) gives the positions for 1913 and 1924, while the WDS 03
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 !
SOME GENERAL EXAMPLES
Below are several different examples I have been
working on. They are just mainstream investigations from the commonly
available information or data. (Each have been added to an observing
list for further study.)
Example 1
HJ 4573 13137-5616 (Cen)
10.0 /11.0 mag.
HJ 4573 (7.6″, 57°) (1924) WDS03
HJ 4573 (7.7″, 57°) (1913) WDS84
HJ 4573 (4.0″, 57°) (1834) WDS03
This pair looks like it should by in the order of 10 arcsec along
PA 57°. HJ 4573 is unresolved in GSC and single in both the
Tycho and Hipparcos catalogue.
Conclusion: This pair requires new observations and
should be added to a neglected list.
Example 2
HJ 4590 13332-7734
Cha (6.5 and 9.3 mag) F6V
1837 137° 25.0″ WDS 03
1880 134° 22.4″ WDS 84**
1991 133° 22.4″ WDS 03
2000 133° 22.1″ WDS 12
NOTES: HJ 4590 A is variable S Cha, B CPD-76°769.
** Russell 1880.413 134.9° 22.41
HJ 4590 A: pmRA = -367 pmDec=-153 mas per year
HJ 4590 B: pmRA = -351 pmDec=-118 mas per year
HJ 4590 A: Parallax = 28.45± 1.13 mas
HJ 4590 B: Parallax = 52.09±39.91 mas
* mas (milli-arcseconds)
Here the WDS84 only gives the 1880 position and nothing else but
n=3 (Number of observations), while the WDS03 has fixed up the
problem by quoting the first and last observations 1837 (J.Herschel)
and 1991 (Hipparcos) n=11. In WDS12, n=13. 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 mas. gives the distance as 34.14pc. or
114.6±4.56 ly. At this distance, the stars true separation of
25″ corresponds to 878 AU (1.3×1011 km) and an
estimate maximum period is about 1.8×104 years. Using
the 6.5v and 9.3v component magnitudes, the absolute magnitudes are
about 3.8 and 5.6. Using the Mass-Luminosity Relationship (MLR) the
solar masses are about 1.2M⊙ and 0.7⊙, respectively.
(Agreeing well within the given Spectral Type)
Theoretical Catalogue Limit (TCL) Heinze (1979) for attachment is
173″ at 34.14pc. Where;
TCL= 10^(2.8 − (0.2 ×
Δm)) or
TCL= 10(2.8 − (0.2 ×
Δm))
Assuming linear motion, the prediction for this pair in the near
future is;
YEAR Sep ″ PA (°)
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 (190×)
estimate 21 arcsec and 134°.
Conclusion : Confirms the pair is acting as expected.
Update Comment: October 2008
Since writing this text in 2006 the U.S.N.O. has
published on its site the “preliminary”
Catalogue of Rectilinear Elements of many such optical pairs
or systems. This includes notes and a useful ephemeris prediction for
the next twenty-five years. Using the resources as the repository of
double star measures, these linear elements are determined by
weighting the various available measures based on the accuracy of
each known measure.
This is far more detailed than the example here, but
it does emulates the way for assessing which pairs to measure or
study.
Example 3
If I find significant change that cannot be explained,
I get all the positional information from the USNO and plot all these
values against the one in question. For a broad example, at the
moment I have found some problems with the H.C. Russell pair R71
AB;
R71 AB / SLR22 BC (07144-4440) is a white triple star some
8.6′E (PA 99°) of L2 Puppis. R 71
AB is the brighter of the two that contains the 9.6v and 10.2v (9.61V
& 9.77V) magnitude stars separated by 15.9 arcsec along PA
256°. R71 suggests it has shown some change in the positions,
reducing in PA from 288° to 256° and 18.4″ to
15.9″ in the last 125 years. (2004) Russell, incidentally, gave
the magnitudes as “10,10.” R 71 was discovered at Sydney
Observatory by H.C. Russell just before midnight on the 1st April
1879 (1879.248), measuring 15.39″ along PA 77.465°. It
seems that he probably just evaluated the brighter component
incorrectly so the position angle had 180° added to make the new
PA as 257.465 or 257.5°. However, the WDS03 gives this 1879
separation here as 288° and 18.4 arcsec which does not match at
all 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 figure or that another observation was
made in 1879 by another observer that preceded Russell’s own discovery. In the earlier versions of
the WDS, the 1879 position is given as 15.2 arcsec along PA 256°
— matching Russell’s
observations. His position angle has likely been precessed from the
time of the 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
257°. Since discovery by R.P. Sellors at Sydney Observatory in
1896, the position angle has reduced from 266° to 257°
while the separation remains fixed.
Common proper motions here suggests that SLR 22 is likely a true
binary star but the brighter ‘A’ component is almost certainly an optical
companion. All three stars form a straight line that is proportioned
by the ratio of about 9:1. Given spectral classes are A5, A5 and
A7III, respectively. The whole system contrasts nicely with the
orange-red colour of L2 Puppis in the field, being easily
visible in 7.5cm.
Example 4
As for the 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
STF2822 BD 6.1 6.9 206.1 204.8 54 53 1902 1909 A5
STF2822 AB 4.5 5.9 5.6 2.0 109 307 1823 1997 F6
STF2822 AC 4.7 11.53 5.3 68.2 263 289 1878 1987
STF2822 AD 4.5 6.9 217.4 198.3 61 46 1823 1991 F7
WDS03 Oct
Desig Cmpnts PMag SMag Sep1 Sep2 PA1 PA2 Yr1 Yr2 Sp pmRA/Dec pmRA/Dec
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 3 5.3 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
Abbreviations
Desig.=Designation;
Cmpnts =Components;
PMag= Primary Magnitude
SMag = Secondary magnitude;
Sep1 & Sep2 = First & Last Separation;
PA1 & PA2 = First & Last Position Angle;
Yr1 & Yr2 = Year First and Last;
Sp = Spectral Class
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 brightnesses can differ by 0.4 or 0.5
magnitudes depending on the particular colour of the star in
question.
STF 2822AB (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 : 312° ; +2°)
NOTE: Statistically it has been known for
sometime that for moderate to wide pairs the repeatability of
observations 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, that are needed to watch out for. Us
amateurs should mainly concentrate on pairs between 2 arcsec and 4
arcsec for any changes. This is because the Hipparcos satellite had
restrictions 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.
Some Comments on Binary Star Orbits
As for binary star orbits, it is the grading of precision that is
useful here. In the general 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.
Differences between the good (seeing a number of orbits) and the
bad — and usually only a part of the orbit in partial arcs.
Yet it does depending on when the orbit was written, where it is not
unusual to find expressed some range of dates in which the data is
useful. I.e. 1975 to 2040 etc. This is used in multiple systems where
orbits are constantly changing I.e. By apsidal motion, etc., causing
small variations changing elements 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 arcsec. A mean eccentricity for the orbit
‘e’
is 0.523.
Of these;
**********************
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
**********************
Whilst 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 is with
refining the orbits for these differences are essentially minor.
To elaborate more on Tom’s
statement;
“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.”
As W. Heintz says in “Double
Stars” (1970) p.53;
“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 are 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 the 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 eccentricities
exceeding 0.6
— Most orbits are obtained from stars within 100 parsecs 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 Hipparcos
parallaxes. I.e.
“a”
semi-major axis in AU is calculated by a =
‘a’
in arcsec / parallax in arcsec.
More Comments on Discrepancies
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 cannot 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
ORIGINAL POSTS (That inspired the text above from the S33
Group.
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 277°. The observing list of doubles at
the Astronomical Society has it at 50°! 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, moreover, one often gets
disagreement as to which component should be regarded as the primary,
leading to 180° “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-inch 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
Last Update : 4th October 2012
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