                The Photometric Magnitude Difference Catalog

            Charles E. Worley, Brian D. Mason, & Gary L. Wycoff

                Astrometry Department, U.S. Naval Observatory
              3450 Massachusetts Ave, NW, Washington, DC 20392
              bdm@draco.usno.navy.mil, glw@draco.usno.navy.mil

The Photometric Magnitude Difference Catalog, maintained at the United
States Naval Observatory, is a collection of magnitude difference measures
for double stars and serves as a repository for double star observations
where no astrometry is given. At the present time, it consists of 19,589
measures of 10,473 systems with a mean Delta-m of 1.90. Due to the
non-uniformity of filters and the possible variability of one or more
components in a system no ``WDS-like'' summary line is provided. Rather, all
the data are presented, and their interpretation is left as an ``exercise
for the reader.'' This catalog is one of four USNO double star catalogs to
be included on a CDROM now in preparation. A brief summary and statistical
analysis of the contents of the catalog are presented.

Introduction to the Differential Magnitude Problem

The problem of obtaining accurate magnitude differences in binary star
astronomy is a long-standing one. The crux of the problem, as in accurate
binary star astrometry, is the lack of standards against which systems can
be measured. Binary star astronomers have, since the time of John Herschel,
maintained a consistency in the way in which fundamental astrometric data
are published (usually, Besselian year, position angle, and separation in
seconds of arc). However, the publication of photometric information has
been uneven - while sometimes individual magnitudes or magnitude differences
were estimated or measured, on other occasions catalog magnitudes were
published with no indication of the source of the measurement, yielding no
clear way to determine when a measure of photometry was made and when it was
not.

Yet the importance of accurate differential photometry of binary stars
cannot be overstated. While there has been more overlap of late between
spectroscopic and visual binaries, historically the characteristics of
visual binaries were known through orbital analysis and parallax, leading to
a mass sum. A measure of Delta-m can place the individual components of a
binary star system on an empirical mass/luminosity plot, which (although
model dependent) can aid in the refinement and convergence of models of
stellar characteristics. In addition, measurement of Delta-m at two or more
wavelengths ties the masses and luminosities to an effective temperature.

When looking at publications providing binary star measurements which are
primarily listings of relative astrometry, a measure citing a magnitude
difference can typically be believed as a true measure. However, unless it
is made explicitly clear in the publication, cases where magnitudes are
published for both companions typically cannot be trusted. In considering
measures contained in the Washington Double Star (WDS) Catalog, the only
ones which can be trusted as bona fide Delta-m measures are those where only
a magnitude difference is cited or where it is known that individual
magnitudes of the components were measured (as was the case, for example,
for Hipparcos and Tycho). Additional complications to these matters are the
different (and often non-standard) central wavelengths and passbands of
measurements, as well as the photometric variability of some of these
companions.

Sources of Photometric Magnitude Differences

In addition to this small subset of relatively reliable magnitude difference
measurements, there are a few cases where the primary purpose of the work
has been the measurement of photometry of the components of binary stars.
Double-image photometers and micrometers were used from the late 19th
Century (Pickering 1879) until the late-20th (Worley 1969); these permitted
moderate Delta-m measurements over a wide range of separations, sometimes as
close as half a second of arc, although both of these (and any other)
techniques have a number of smaller Delta-m systems which serve to calibrate
the technique against other standards. Kuiper (see Baize 1950) pioneered
work in objective gratings which, although limited in the separations of
systems for which they could effectively observe, were capable of measuring
larger magnitude differences. This work culminated in measures by Lindenblad
(1970) of the Sirius AB system. Wider systems have had a variety of
techniques applied, including the area scanner observations by Rakos and
Franz (see, e.g., Franz 1970), as well as more conventional photographic
measures. This was standardized by Strand (1969). Recent efforts have used
CCDs (e.g., Tokovinin & Shatskii 1995). A listing of the different
techniques, their contribution and mean Delta-m is provided here.

Systems which are separated by an arcsecond or less are typically not
observable by the aforementioned techniques. Unfortunately, it is often
these systems which are the most astrophysically interesting. The
occultation of stars by the Moon can be used to determine Delta-m
measurements for quite close systems. However, these events are restricted
to zodiacal band stars and are infrequent and unrepeatable occurrences.
Also, the Delta-m determinations from lunar occultation have suffered from
large errors and were often taken with non-standard filters (Mason 1996,
1997). In high angular resolution work, the ``fork'' algorithm (Bagnuolo &
Sowell 1988) is only applicable to a photon-noise-limited detector with more
than 5 photons per speckle, while the ``directed vector-autocorrelation''
has errors which are too large without laborious calibrations (which in
turn, decreases observational efficiency by as much as a factor of 40; Mason
et al. 1993). Full image reconstruction from non-redundant aperture masking
or bispectrum analysis is possible; however, the intensification used with
CCDs in most speckle observations seems to prevent accurate determination in
all but photon-limited data (Roberts 1999). Some of these data from selected
papers are included below. While the technique of adaptive optics appears
quite promising (e.g., ten Brummelaar et al. 1996, 2000), it is quite labor
intensive and has yet to see effective throughput. The most effective
technique of late for measurement of the delta magnitude of close binaries
has been the determination of V and B magnitudes of Hipparcos stars measured
by the Tycho instrument (Fabricius & Makarov 2000).

The Catalog

Measures in the Photometric Magnitude Difference Catalog have been
collected, collated, and maintained as an internal USNO publication for over
30 years. Presented in order are the WDS 2000 coordinate, the discovery (and
component) designation, the measured magnitude difference, the number of
measures contributing in the mean, a reference code, a method code and a
column of notes. These notes include but are not limited to wavelength
characteristics (in nanometers) and error of the measurements (sigma). Links
to the complete Delta-m Catalog are below, as are links to the reference
list and a description of observing methods. All these materials are also
included on a CDROM of USNO double star catalogs. Copies of this CDROM can
be obtained from the authors.

     Available Files:

   * The Delta-M Catalog
   * Method Key to the above file
   * Number and mean Delta-m for various methods
   * Reference file

     External Links:

   * Current Delta-M Catalog

     Internal (CD) Links:

   * WDS, 2001
   * Fifth Orbit Catalog
   * Calibration Systems
   * Third Interferometric Catalog

References

   * Bagnuolo, Jr., W.G. & Sowell, J.R. 1988, AJ 96, 1056
   * Baize, P. 1950, J. Obs. 33, 1
   * ten Brummelaar, T.A., Mason, B.D., Bagnuolo, Jr., W.G., Hartkopf, W.I.,
     McAlister, H.A., & Turner, N.H. 1996, AJ, 112, 1180
   * ten Brummelaar, T.A., Mason, B.D., McAlister, H.A., Roberts, Jr., L.C.,
     Turner, N.H., Hartkopf, W.I., & Bagnuolo, Jr., W.G. 2000, AJ, 119, 2403
   * ESA 1997, The Hipparcos and Tycho Catalogues (ESA SP-1200) (Noordwijk:
     ESA)
   * Fabricius, C. & Makarov, V.V. 2000, A&A 356, 141
   * Franz, O.G. 1970, Lowell Obs. Bull., 17, 191
   * Lindenblad, I.W. 1970, AJ 75, 841
   * Mason, B.D. 1996, AJ 112, 2260
   * Mason, B.D. 1997, AJ 114, 808
   * Mason, B.D., McAlister, H.A., Hartkopf, W.I., & Bagnuolo, Jr., W.G.
     1993, AJ 105, 220
   * Pickering, E. 1879, Harvard Annals 11, 105
   * Roberts, Jr., L.C. 1999, PASP, 111, 650
   * Strand, K.A. 1969, Pub. USNO 18, Pt. 5
   * Tokovinin, A.A. & Shatskii, N.I. 1995, Pisma Astron. Zhur. 21, 523
   * Worley, C.E. 1969, AJ 74, 764

Concluding Remarks

If the WDS and associated databases were helpful for your research work, the
following acknowledgement would be appreciated:

``This research has made use of the Washington Double Star Catalogs
maintained at the U.S. Naval Observatory.''

A notification of references to relevant papers is appreciated.

This page is maintained by Brian Mason

Approved by Theodore Rafferty, Astrometry Department, USNO

Updated: 14 August 2001

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