Addressing confusion in double star nomenclature:
The Washington Multiplicity Catalog

Introduction: A Welcome ``Problem”''

The new observing and reduction techniques available to astronomers have led to remarkable changes in the field of double and multiple stars. New classes of companions, such as brown dwarfs and exoplanets, have been discovered. Binaries which previously constituted distinct classes are now observable by multiple techniques (witness, for example, the increasing overlap between the visual and spectroscopic regimes). With many long-baseline optical interferometers operational or planned, with improvements in other techniques (e.g., absorption-cell RV work), and with astrometric space-based missions in various states of planning and funding, the situation is likely to become more complicated. The result is greater understanding for the scientist, but greater challenges for the cataloger!

The ``problem'' is that purveyors of different techniques use different nomenclature. Visual binaries are given discoverer designations, based on observer's name (e.g., STF 13, BU 96), while spectroscopic binaries are usually identified by their HD number, eclipsing binaries by their variable star designation, occultation binaries by SAO or ZC number, and so on. Binaries analyzed by multiple methods may wind up with multiple designations.

While multiple designations are confusing, a large cross-reference list (such as SIMBAD) can usually handle these problems. Component confusion is even worse, however, as one person's AB pair may be another's ab or BA or BC or primary/secondary! It is this problem that we wish to address.

Addressing the Problem

An electronic discussion among an informal working group was begun in 1999. Over time, these discussions resulted in four suggested schemes:

• KoMa: a hierarchical scheme developed by D. Kovaleva and O. Malkov. Using a variety of upper/lower case alphabetic, numeric and Roman numerals this scheme indicated both hierarchy and type of companion (e.g., stellar, planetary, etc.).
• UC: developed by S. Urban and T. Corbin. This is a numeric only, backside expandable scheme similar to that used for library call numbers.
• Sequential: a non-hierarchical scheme developed by L. Dickel and P. Dubois. In this numeric scheme all components are assigned numbers in the order of their discovery with no heed given to their relationship with other components.
• WMC: the Washington Multiplicity Catalog. This method, while based on the venerable series of upper and lower case letters used in the WDS, extends to multiple levels through use of additional numbers and letters.

At IAU Symposium 200 the attendees seemed to favor the WMC, with UC a close second. The sequential scheme, while not favored, was sufficiently different from others so that it continued to be discussed.

At IAU-GA XXIV interested parties met to discuss various methods for clearing up the nomenclature ambiguities. As a result of those discussions the WMC was endorsed and the following resolution was ratified by Commissions 5 (Documentation & Astronomical Data), 8+24 (Astrometry), 26 (Double & Multiple Stars), 42 (Close Binary Stars), and later 45 (Stellar Classification). The resolution read as follows:

         On Designating Components of Binary/Multiple Star Systems

Recognizing

 - the increasing synergy of techniques for the investigation of stellar 
   companions blurring the traditional distinction between astrometric, 
   spectroscopic, and photometric binary and multiple stars;
 - the detection of substellar (including planets) as well as stellar components
   by these techniques and,
 - the need for a simple, unambiguous, flexible, and computer friendly 
   designation scheme for components of binary and multiple star systems,

Noting that future ground and space-based telescope projects have the potential
to detect both substellar as well as stellar components in increasingly large 
numbers, 

Recommends that

a uniform designation scheme, based on expansion of the new WDS system, be 
developed during the next 3 years to include all types of components and that 
this be reviewed in time for its adoption to be considered at General Assembly 
XXV.

Implementation of the scheme was to be as follows:

1. Present a sample of the resulting scheme to Commission 26 at Colloquium 191.
2. Make any needed modifications based on suggestions from participants, then
3. present this modified scheme to the SOC of IAU-GA XXV SPS 3 (Special Session 3: A New Classification Scheme for Double Stars).
4. Make additional modifications if necessary, then present the further modified scheme at SPS3, July 18, 2003.
5. If approved, present the all-sky WMC at IAU-GA XXVI in 2006, and continue to update and maintain.

Roots of the WMC

The root of the WMC is the Washington Double Star Catalog (WDS). The WDS, maintained at the United States Naval Observatory (USNO), is the principal database of astrometric double and multiple star data for the astronomical community. It contains (as of February 2003) nearly 600,000 mean positions for nearly 100,000 pairs, and is updated nightly. The USNO double-star program also maintains catalogs of differential magnitudes, visual orbital elements, and interferometric and other high-resolution observations.

While the WDS is a complete listing of all resolved systems (i.e., visual and interferometric doubles), many components are detected but not resolved. These include:

• spectroscopic doubles (single- or double- lined),
• photometric or eclipsing binaries,
• astrometric doubles,
• lunar occultation doubles,
• other doubles, and
• planets and other substellar objects.

However, the WDS nomenclature rules (with slight modification) can accommodate all types of double stars.

The WDS and Hierarchy

The WDS is system-based rather than object-based. In an object-based scheme, N objects yields N entries, but in a system-based scheme, N objects can yield up to N(N - 1)/2 entries. The WDS may also contain measures between photocenters. Multiple systems may become quite complex as N increases. However, the WDS lists only pairings actually measured, and the observer's common sense usually implies the system hierarchy.

Generally, orbital period and/or separation are used to assign the hierarchical structure. The 3:1 ratio of semi-major axes determined by R.S. Harrington is generally followed, although separations > 1 arcsecond are usually given upper-case letters. This 3:1 ratio assumes physicality. However, most visual doubles do not have enough measures to determine whether motion is Keplerian or rectilinear. In general, then, all hierarchies in the WMC are apparent rather than absolute. It is assumed that all double stars within some small separation are of interest (if only as a warning of possible image blending), so are retained in the WDS and WMC even if shown to be optical.

Rules of Component Designation

The WDS at present extends nomenclature to second level hierarchies. The WMC will extend this nomenclature to cover more complex systems, however, as follows:

• Level 1: capital letters (e.g., STF1523 AB)
• Level 2: lower case letters (e.g., FIN 347 Aa,Ab)
• Level 3: numbers (e.g., BNK 1 Ab1,Ab2)
• Higher levels: alternate lower case letters and numbers (no examples yet found)

A comma will be used as the delimiter between components in a system, with the full component identifier before and after the comma (e.g., Aa,Ab). The only exceptions: if only two characters are provided the delimiter is assumed (e.g., WAK 8CD = WAK 8C,D)

While the WMC will strive to maintain hierarchies in the assignment of letter and/or number, this is not always possible, given our often very limited knowledge. Also, if a subsystem is found that cannot be assigned unequivocally to a higher order component, a tentative “best guess” assignment will be given and a note added to the catalog.

Figure 1 illustrates a (ficticious) system, growing increasingly complex as new components are discovered.

Illustration of nomenclature assignment as a (ficticious) system grows more complex:
• 1850: a visual pair is discovered
• 1900: a wide common proper motion companion is found
• 1975: the B component is found to be a spectroscopic binary
• 1985: the C component is split by speckle interferometry
• 1990: an additional speckle C component is resolved
• 1995: a planet is found orbitting the A component
• 1998: a second planet is found
• 2005: the primary of B is resolved by long-baseline interferometry

The Sample WMC

The 11h -- 11h30m band of RA was selected for the sample WMC. In addition to being historically compelling (it's the ``home'' of Xi UMa), it includes a variety of component types:

• astrometric binaries,
• X-ray binaries, cataclysmic variables, and related objects,
• eclipsing binaries,
• occultation binaries,
• spectroscopic (SB1 and SB2) and spectrum binaries,
• interferometric binaries,
• visual binaries, and
• planetary companions (by extending to 10h59.5m).

• Downes et al., 2001, A Catalog and Atlas of Cataclysmic Variables: On-line Version, PASP 113, 764,
• Ritter & Kolb 1998, Catalogue of cataclysmic binaries, low-mass X-ray binaries, and related objects (Sixth edition), A&AS 129, 83,
• Batten et al., 1989, Eighth Catalog of the Orbital Elements of Spectroscopic Binary Systems, Pub. DAO, 17,
• Pourbaix et al., 2003, Ninth Catalogue of Spectroscopic Binary Orbits,
• Svechnikov & Bessonova 1984, Catalog of Orbital elements, Masses and Luminosities of close double stars, Bull. Inf. CDS 26, 99,
• van Paradijs 1995, A Catalogue of X-Ray Binaries, in X-ray Binaries, Lewin et al, eds., Cambridge Univ. Press, ch. 14, pp 536-577,
• California & Carnegie Planet Search website and links therein.

         95.8%  visual binaries and optical pairs
         50.6%  interferometric binaries and optical pairs
          1.7%  spectroscopic binaries
          1.4%  cataclysmic variables or related objects
          1.0%  occultation binaries
          0.3%  astrometric binaries
          0.2%  eclipsing binaries
          0.2%  X-ray binaries
          0.1%  spectrum binaries
          0.1%  planets

Coordinate Matching

System matches are based on the stars' arcsecond precise coordinates. The most time-consuming aspect of the WMC construction (by far!) was the improvement of the WDS arcminute coordinates: Some 80% of the 1,645 stars were matched to Hipparcos or Tycho-2 objects. An additional 19% matched to GSC2, USNO A2, 2MASS, etc. via individual inspection using ALADIN. There remain 1.3% (21 pairs) which still have only arcminute accuracy coordinates. Nearly all of these are older, unconfirmed, visual doubles (including some very wide common proper motion pairs and some with suspect coordinates).

Following coordinate matching we found a total of 1,465 systems in this slice of the sky. These may be broken down as follows:

       1,336   (91  %)   simple binaries 
          80   ( 5.5%)   non-hierarchical triples 
          16   ( 1.1%)   non-hierarchical systems, > 3 components 
          25   ( 1.7%)   hierarchical triples 
           8   ( 0.5%)   hierarchical systems, > 3 components 

The Catalog

A sample page from the catalog is shown below, and an explanation of the columns follows. (Note: In the explanation for columns 3 and 4, the names in parentheses indicate the usual order of preference for that column.).

Sample Washington Multiplicity Catalog                                                                                     

WMC........... Comp.....  ?? NAME.............  ?? NAME..............  Sep....* Period.y   Mag1.f  Mag2.f  Spec1...... Spec2...  Parallax  Mass1.....* Mass2.*  Btype.  References...... RA,Dec (2000).
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
1059280+402549 AB         *  47 UMa             HD  95128                0.14     2.982y    5.05           G0V         planet      71.05h                2.54j  S1      CCP              1059280+402549
1059280+402549 AC         *  47 UMa             HD  95128                0.26     7.1  y    5.05           G0V         planet      71.05h                0.76j  S1      CCP              1059280+402549

1100004+413608 AB         BD +42 2173           T2 3012 01054 1          0.005a   0.899y   10.08     .     R...        .            5.29h     .           .     A       O6               1100004+413608

1100006-285044 AB         ** TDS7598            T2 6647 01291 1          0.46      .       11.13   11.52   .           .             .        .           .     V       WDS,I4           1100006-285044

1100175-295159 AB         *  EC 10578-2935      G2 S12221102974           .        .       15.9      .     ?           .             .        .           .     CV      CCV              1100175-295159

1101493+295217 Aa,Ab      HD  95515             T2 1980 00145 1          0.262     .        7.8      .     K0          .            9.00h     .           .     V       WDS,I4           1101493+295217
1101493+295217 Aa,B       HD  95515             T2 1980 00145 1          1.13      .        7.00    9.36   K0          .            9.00h     .           .     V       WDS,I4           1101493+295217

1108028-774227 Aa,Ab      ** GHE  35            G2 S1101231136           0.72      .        7.03k   8.38k  ?e          .            7.1 r     .           .     V       WDS,I4           1108028-774227
1108028-774227 Ab1,Ab2    ** BNK   1            G2 S1101231136           0.100     .        8.92k   9.32k  .           .            7.1 r     .           .     V       WDS,I4           1108028-774227
1108028-774227 AB         ** GHE  35            G2 S1101231136           2.7       .        7.03k   9.5 k  ?e          .            7.1 r     .           .     V       WDS,I4           1108028-774227
1108028-774227 AC         ** SZR   1            G2 S1101231136          17.        .        7.03k  10.1 k  ?e          .            7.1 r     .           .     V       WDS,I4           1108028-774227

1108059-615608 AB         V* RS Car             HD  96830                 .       0.083d    7.0 b  18.0    nova        .             .        .           .     CV      CCV              1108059-615608

1113063-421643 AB         CD -41 6409           T2 7734 00204 1          2.503     .       10.55   11.69   .           .             .        .           .     V       WDS,I4           1113063-421643
1113063-421643 AC         CD -41 6409           T2 7734 00204 1          5.46      .       10.55   13.6    .           .             .        .           .     V       WDS              1113063-421643

1113125-262754 AB         V* TT Hya             HD  97528                0.65 m   6.953d    7.60    8.90   A2Ve        K1.5IV       6.50h    2.08        0.65   E,S1    SvB,SB8,SB9      1113125-262754

1115073-611539 Aa,Ab      HD  97950             T2 8959 01959 1          0.339     .       10.5    11.1    .           .             .        .           .     V       WDS,I4           1115073-611539
1115073-611539 Aa,Ac      HD  97950             T2 8959 01959 1          0.371     .       10.5    11.3    .           .             .        .           .     V       WDS,I4           1115073-611539
1115073-611539 Aa,Ad      HD  97950             T2 8959 01959 1          0.784     .       10.5    10.3    WN...       WN...         .        .           .     V       WDS,I4           1115073-611539
1115073-611539 AB         HD  97950             T2 8959 01959 1          1.81      .        9.03   10.8    WN...       WN...         .        .           .     V       WDS              1115073-611539
1115073-611539 AC         HD  97950             T2 8959 01959 1          2.69      .        9.03   11.3    WN...       .             .        .           .     V       WDS              1115073-611539
1115073-611539 AD         HD  97950             T2 8959 01959 1          3.54      .        9.03   12.3    WN...       .             .        .           .     V       WDS              1115073-611539
1115073-611539 AE         HD  97950             T2 8959 01959 1          4.59      .        9.03   11.8    WN...       .             .        .           .     V       WDS              1115073-611539

1118047+361614 AB         ** KZA  16            2M 1118046+361614        8.08      .       11.0    11.5    .           .             .        .           .     V       WDS              1118047+361614

1118109+313145 Aa,Ab      *  xi UMa             T2 2520 02634 1          0.053  669.18 d    4.41     .     G0V         M3V?       127.       0.022   f   0.37:  A,S1    O6,SB8           1118109+313145
1118109+313145 AB         HD  98231             T2 2520 02634 1          2.536   59.878y    4.41    4.87   G0V         F8.5V      127.       2.21    t    .     V       WDS,O6,I4        1118109+313145
1118109+313145 AC         HD  98231             T2 2520 02634 1         54.31      .        4.41   15.0    G0V         .          127.        .           .     V       WDS              1118109+313145
1118109+313145 Ba,Bb      *  xi UMa             T2 2520 02634 2          0.33 m   3.981d    4.87     .     F8.5V       .          127.       0.000052f    .     S1      SB8              1118108+313145
1118109+313145 Ba,Bc      HD  98230             T2 2520 02634 2          0.026     .        4.87     .     F8.5V       K2-3V?     127.        .           .     V       WDS,I4           1118108+313145

1124349+021238 AB         ** SLE 597            A2 0900-06920936        40.68      .       11.9    12.5    .           .             .        .           .     V       WDS              1124349+021238

1129041+392013 AB         HD  99787             T2 3013 02482 1          5.45      .        5.35   10.67   A2V         .           15.59h     .           .     V       WDS,I4           1129041+392013
1129041+392013 AC         HD  99787             T2 3013 02482 1        216.55      .        5.35   11.6    A2V         .           15.59h     .           .     V       WDS              1129041+392013
1129041+392013 AD         HD  99787             T2 3013 02482 1        344.36      .        5.35    7.73   A2V         K0          15.59h     .           .     V       WDS,I4           1129041+392013
1129041+392013 AE         HD  99787             T2 3013 02482 1        100.33      .        5.35   10.6    A2V         .           15.59h     .           .     V       WDS              1129041+392013
1129041+392013 AF         HD  99787             T2 3013 02482 1        131.03      .        5.35   10.3    A2V         .           15.59h     .           .     V       WDS              1129041+392013
1129041+392013 AG         HD  99787             T2 3013 02482 1        330.27      .        5.35   10.3    A2V         .           15.59h     .           .     V       WDS              1129041+392013
1129041+392013 DE         HD  99720             T2 3013 01407 1         99.06      .        7.73   10.6    K0          .            4.23h     .           .     V       WDS,I4           1128359+391829
1129041+392013 DF         HD  99720             T2 3013 01407 1        128.54      .        7.73   10.3    K0          .            4.23h     .           .     V       WDS              1128359+391829
1129041+392013 EF         ** STF1543            T2 3013 01935 1         43.88      .       10.6    10.3    .           .             .        .           .     V       WDS,I4           1128353+391650

Column Description
1. WMC designation (WDS or J2000 arcsecond coordinates of system primary)*
2. component designation (AB; Aa,Ab; etc.)
3. catalog and name (Bayer/Flamsteed/variable star designation, HD, DM, discoverer designation, etc.)
4. catalog and name (HD, Tycho-2, GSC2, USNO A2, 2MASS, etc.)
5. angular separation (including vector separations, separations predicted from spectroscopic orbit and parallax, etc.)
6. orbital period
7. primary magnitude (filter codes if not V, flags for variability)
8. secondary magnitude
9. primary spectral type
10. secondary spectral type
11. parallax (from Hipparcos, orbital parallax, etc)
12. primary mass (or mass function, mass ratio. etc.)
13. secondary mass
14. binary type (visual, spectroscopic, X-ray, etc.)
15. references (with web links to on-line catalogs)
16. J2000 arcsecond coordinates of principal star of pair

Examples from the sample WMC

A few systems from the portion of the sample WMC shown above are noted below. These illustrate some of the features of the WMC, as well as some of the difficulties to be encountered in creating the full catalog.

• 1100004+413608, 1100006-285044, etc.: These simple doubles, whether of visual, astrometric, spectroscopic, or other discovery origin, are by far the most common entries in the WMC.
• 1059280+402549 and 1113063-421643: These two triples are treated in the same manner, even though one consists of three stars and the other a star and its two planets. In both cases, the apparent separations of the AB and AC pairs are comparable, so all components are considered to be at the same hierarchy.
• 1101493+295217: This is an example of a simple hierarchical triple.
• 1108028-774227: This is the only system in the sample WMC showing all three levels of hierarchy.
• 1129041+392013: All components of this system (apparently a small cluster) are wide, so no hierarchical structure was assigned, despite the wide range in separation.
• 1118109+313145: The Xi UMa system is another complicated multiple. The A component is a spectroscopic binary; the B component has also been split spectroscopically (with separation estimated as 0.33 milliarcseconds). There is also a component of B resolved by speckle interferometry. Based on the separations, one would ordinarily assign the speckle pair the second level of hierarchy (e.g., Ba,Bb) and assume one of those components was split spectroscopically (e.g., Ba1,Ba2). However, we are familiar with the speckle pair and suspect the resolution may be spurious, as it has never been confirmed. We're thus reluctant to assign a different hierarchy to the very close pair.
• 1115073-611539: This is another apparent small cluster. Since the component we call A was later resolved into four stars, we assigned those stars to a second apparent hierarchical level.

Help!

Your suggestions and opinions are needed, be they on format, information to add or delete, etc.). See the WMC website (linked from the WDS website: http://ad.usno.navy.mil/wds/wds.html).

One question to ponder: Should old component designations be changed to the new scheme or should they remain as they are? On the one hand, some of these older designations may have been in the literature for many years, and changing them could lead to confusion. On the other hand, some designations (such as those for systems given different designations by different techniques) will have to change. Others have argued that consistency in new designations will lead to less confusion in the future. (One comment heard: ``The future is longer than the past!'')

Your data are needed (for the WDS, visual and spectroscopic orbit catalogs, etc., as well as the WMC). Send comments, etc. to wih@usno.navy.mil or bdm@usno.navy.mil. Comments and questions regarding the WMC will be posted as deemed appropriate.