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                 *             The 90 000 Stars Supplement                 *
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                 *                        to the                           *
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                 *                  PPM Star Catalogue                     *
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                 ***********************************************************
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-                                 S. Roeser and U. Bastian
0                         Astronomisches Rechen-Institut, Heidelberg
-                                        A. Kuzmin
0                          Sternberg Astronomical Institute, Moscow
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0                                Heidelberg, December 1993
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                 *       Introduction to the 90 000 Stars Supplement       *
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0          Preface
           =======
0          The idea of this introduction to the 90 000 Stars Supplement is
           to deliver a short explanation together with the catalogue
           itself.  It briefly describes the catalogue and its format, with
           heavy reference to the introductions to PPM North and PPM South.
           The 90 000 Stars Supplement is also described in a paper by
           Roeser, Bastian and Kuzmin in Astron. Astrophys. Suppl., 1994.
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0          Contents
           ========
-           1. Scope of the 90 000 Stars Supplement
            2. Construction
            3. Properties
            4. Practical usage
            5. Availability
            6. Format of the machine-readable version
0              Table 1
               References
               Acknowledgements
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           1. Scope of the 90 000 Stars Supplement
           =======================================
0          Since its appearance in 1966 the SAO Catalogue (SAO, 1966) has
           been the primary source for stellar positions and proper motions.
           Typical values for the rms errors are 1 arcsec in the positions
           at epoch 1990, and 1.5 arcsec/century in the proper motions.  The
           corresponding figures for the AGK3 (Heckmann et al., 1975) on the
           northern hemisphere are 0.45 arcsec and 0.9 arcsec/century.
           Common to both catalogues is the fact that proper motions are
           derived from two observational epochs only. Both catalogues are
           nominally on the B1950 / FK4 coordinate system.
0          The PPM Star Catalogue (R|ser and Bastian, 1991, Bastian et al.,
           1993; for a short description see R|ser and Bastian, 1993)
           effectively replaced these catalogues by providing more precise
           astrometric data for more stars on the J2000 / FK5 coordinate
           system.  Compared to the SAO Catalogue the improvement in
           precision is about a factor of 3 on the northern and a factor of
           6 to 10 on the southern hemisphere. In addition, the number of
           stars is increased by about 50 percent.  Typical values for the
           rms errors on the northern hemisphere are 0.27 arcsec in the
           positions at epoch 1990, and 0.42 arcsec/century in the proper
           motions. On the southern hemisphere PPM is much better, the
           corresponding figures being 0.11 arcsec and 0.30 arcsec/century.
0          The improvement over the SAO Catalogue was made possible by the
           advent of new big catalogues of position measurements and by the
           inclusion of the century-old Astrographic Catalogue (AC) into the
           derivation of proper motions (for a description of AC see
           Eichhorn, 1974).  But even PPM does not fully exploit the
           treasure of photographic position measurements available in the
           astronomical literature of the last 100 years.
0          The Astrographic Catalogue contains roughly four million stars
           that are not included in PPM.  For most of them no precise
           modern-epoch position measurements exist.  Thus it is not yet
           possible to derive proper motions with PPM quality for all AC
           stars.  But among the 4 million there is a subset of some 100 000
           CPC-2 stars (see below) that are not included in PPM.  These
           stars constitute the 90 000 Stars Supplement to PPM.
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           2. Construction
           ===============
0          The CPC-2 gives 276 000 high-quality photographic star positions
           on the southern hemisphere. The plates were taken around 1966 in
           South Africa and were measured in Great Britain.  The resulting
           positions were published recently (Zacharias et al., 1992).
           172 000 CPC-2 positions were already used in the construction of
           PPM. The remaining 100 000 were not used because no precise
           later-epoch observations were available and because the PPM
           concept aimed at having more than two epochs per star.
0          The 90 000 Stars Supplement provides J2000 positions and proper
           motions for the majority of these remaining 100 000 stars.  It
           was constructed in essentially the same way as PPM, but with only
           CPC-2 and AC as observation catalogues. The former was used as
           published, since it is on the system of FK5.  The latter (AC) was
           used in the fully reduced form that was produced for the PPM
           project.
0          A provisional star list for the Supplement was set up by
           cross-identifying CPC-2 with PPM and retaining only the
           non-identified stars. It contained 102 681 stars.  This list was
           then automatically cross-identified with AC. In cases of doubt
           the GSC, NLTT and the Cape Photographic Durchmusterung were used
           to verify the identity.  If doubts still remained or if the
           individual AC measurements were grossly discordant, the star was
           deleted from the list. Also deleted were all stars having only
           one AC counterpart (the missing of the second one making the star
           dubious), and of course those 4147 objects having no AC
           counterpart at all.
0          Proper motions were thus derived from three or four positions for
           a vast majority of the remaining stars: one from CPC-2 (each
           CPC-2 position in fact is the mean of four independent plate
           measurements) and two or three from the overlapping plates of AC.
           Only a few thousand stars have more than 4 positions. The details
           of the catalogue construction process are largely identical to
           those of PPM South.
0          The resulting star catalogue contains 89 676 stars. Besides the
           astrometric parameters the 90 000 Stars Supplement provides
           magnitudes, spectral types (if available) and
           cross-identifications to other star catalogues (Bonner
           Durchmusterung, Cordoba Durchmusterung, Henry Draper Catalogue).
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           3. Properties of the 90 000 Stars Supplement
           ============================================
-          A rough summary of the astrometric precision is given in Table 1,
           below, along with comparison data for the main parts of PPM. The
           numbers in the table are mean values. The numbers for individual
           objects vary.  The Supplement stars are in fact divided in two
           well-separated groups. The division is caused by two major
           classes of precision of the CPC-2 positions. The division is
           obvious in the precision of the present-day  positions, but less
           so in proper motions, since there is a large variation of epochs
           and mean errors in the AC positions.
0          The positions and proper motions provided by the 90 000 Stars
           Supplement are less precise than those in the southern part of
           PPM, but they are more precise than those of PPM north.  Thus the
           already existing imbalance between the two hemispheres (see
           Table 1) is further increased.
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           4. Practical usage
           ==================
0          The practical usage of the 90 000 Stars Supplement is perfectly
           the same as for the main parts of PPM. Explanations are given in
           the Introductions to PPM North and PPM South.
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           5. Availability
           ===============
-          Magnetic tapes containing PPM and its 90 000 Stars Supplement can
           be obtained from the astronomical data centres, e.g. the Centre
           de Donnees Astronomique de Strasbourg (CDS), Strasbourg, France
           and the Astronomical Data Centre at Goddard Space Flight Centre,
           Greenbelt, USA. There will be no printed version of the 90 000
           Stars Supplement, but inclusion into the various forthcoming
           "astronomical catalogues sampler" CD-ROMs is aimed at.
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           6. Format of the machine-readable version
           =========================================
0          The machine-readable version of the 90 000 Stars Supplement to
           PPM contains 2 physical files. They are perfectly analogous to
           Files 1 and 2 of the machine-readable version of PPM South. Thus
           the description need not be repeated here.




0          Table 1: Error budget of PPM and the 90 000 Stars Supplement
           ============================================================
0          The table gives some information on the accuracy of PPM and the
           90 000 Stars Supplement.  The individual columns of the table
           contain, in turn, the declination zone, the total number of stars
           and average values for a number of characteristic quantities:
           star density (stars per square degree), number of source
           positions used for each star, mean epochs for right ascension and
           declination, mean errors of the proper motions (in
           arcsec/century) and mean errors of the 1990 positions (in
           arcsec).
0          The last two lines give rough statistics of the SAO Catalogue for
           comparison.
-             decl.     no.of   star   no.   mean ep.  mean err.   mean err.
              (deg.)    stars   dens.  obs.   1900+    prop. mot.  pos. 1990
-
            PPM, north
0           +90..-2.5  181731   8.7    6.2  31.5 30.7  0.43 0.42   0.27 0.27
0           PPM, south
0           -2.5..-90  197179  10.3    6.1  61.8 61.6  0.30 0.30   0.11 0.11
0           90 000 Supp.
0           +1.5..-90   89676   4.4    3.4  55.4 54.8  0.46 0.44   0.19 0.18
-           SAOC, north
0           +90..-2.5  133000   6.4    2.0  30   30    1.5  1.5    0.9  0.9
0           SAOC, south
0           -2.5..-90  126000   6.3    2.0  30   30    1.5  1.5    1.2  1.2
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           References.
           ===========
0          Bastian, U., R|ser, S., et al., 1993, PPM Star Catalogue, Vols.
                  III and IV, positions and proper motions of 197179 stars
                  south of -2.5 degrees declination. Astronomisches Rechen-
                  Institut, Heidelberg. Spektrum Akademischer Verlag,
                  Heidelberg, Berlin, New York, 1993.
0          Eichhorn, H., 1974, Astronomy of Star Positions, Frederick
                  Ungar Publ. Co., New York.
0          Heckmann, O., Dieckvoss, W., Kox, H., G}nther, A., Brosterhus, E.,
                  1975, AGK3. Star catalogue of positions and proper
                  motions north of -2.5 declination. Hamburg-Bergedorf 1975.
0          R|ser, S., Bastian, U., 1991, PPM Star Catalogue, Vols. I and II,
                  positions and proper motions of 181731 stars north of
                  -2.5 degrees declination. Astronomisches Rechen-Institut,
                  Heidelberg. Spektrum Akademischer Verlag, Heidelberg,
                  Berlin, New York, 1991.
0          R|ser, S., Bastian, U., 1993, The final PPM Star Catalogue for
                  both hemispheres, Bull. Inform. CDS, 42, 11.
0          R|ser, S., Bastian, U., Kuzmin, A., 1994, PPM Star Catalogue: The
                  90 000 Stars Supplement, Astron. Astrophys. Suppl.,
                  in press.
0          SAO, Staff of, 1966, Star Catalogue. Positions and proper
                  motions of 258997 stars for epoch and equinox of 1950.0.
                  Publ. Smithsonian Inst. of Washington,D.C., No. 4562.
0          Zacharias, N., de Vegt, C., Nicholson, W., Penston, M., 1992,
                  CPC-2 - the Second Cape Photographic Catalogue.
                  A&A 254, 394.
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           Acknowledgements
           ================
0          This work was made possible by the key-punching of the Astrographic
           Catalogue, performed by many patient and careful workers at Moscow,
           supervised by V.V. Nesterov.
           Financial support was provided by Deutsche Forschungsgemeinschaft
           and the Russian Academy of Sciences.
