Field 31: Source of photometry

The photometric data given in the Hipparcos Input Catalogue are derived from a wide variety of sources. The correspondence between the letters used in field 19 and the source of the photometry, along with the number of stars whose photometry was taken from this source, is given in Table B2 .

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Stars were proposed for Hipparcos observation on the grounds of their scientific interest, irrespective of the accuracy of available data on position or photometry. The photometric data given by the proposers were taken either from the SIMBAD data base or from various sources of very heterogeneous quality. Of the 214000 proposed stars, photoelectric photometry was available for about 26000 stars, while approximate photographic photometry was available for about 32000 other stars. In addition, it was possible to derive acceptable and magnitudes from the available estimates of blue and visual magnitudes for some 139000 stars. For the remaining 17000 stars, the photometric information was not reliable. These stars belonged primarily to the faint end of the proposed programmes. The observational effort was concentrated on stars having a high probability of being finally retained in the Input Catalogue considering the results of the successive numerical simulations of the mission. The observing lists were updated after each simulation.

As for positional data, the data given in fields 25-31 are the result of extensive compilations (Mermilliod & Mermilliod 1985; Egret 1985; Grenon 1989a), and new observations (Grenon 1985a, 1988a, 1988b; Torra et al. 1988). The photometric contents of the INCA data base, initially identical to those of SIMBAD, were updated in successive steps:

• compilation of photoelectric multi-colour photometry. This work was prepared in Lausanne and Geneva (Mermilliod & Mermilliod 1985; Grenon 1989a) within the framework of the updating of the General Catalogue of Photometric Data (GCPD, Hauck et al. 1990). and magnitudes (Johnson system) and the magnitudes were computed from the data collected in the GCPD for various multi-colour photometric systems after a determination of the transformation formulae from these systems to the Johnson and Hipparcos systems (Grenon 1985b). Identifiers used in the GCPD were incorporated into the INCA data base, facilitating the inclusion of the , and magnitudes;
• new observations in photoelectric multi-colour photometry. This work was organised in Geneva (Grenon 1985a, 1988a, 1988b, 1989a) in order to obtain a Hipparcos magnitude and a colour for all stars for which the available data were not reliable, and in order to perform ground-based observations as far as possible complementary to the outputs expected from Hipparcos and Tycho. The observations were made using multi-colour photometry, with the system selected according to the spectral type in order to be able to derive some basic physical parameters (such as , , [M/H]) in addition to the magnitudes. The selection was made as follows (Grenon 1985a):

  

  All observations were published separately, and only the resulting , and magnitudes are given here. They represent the result of the combined effort of astronomers from Belgium, Denmark, France, Germany, The Netherlands, Spain and Switzerland (Grenon 1989a). The data were introduced into the INCA data base following the same procedure as for the compiled photoelectric data described above. Among these new observations, special mention should be made of the visual magnitudes obtained by the Carlsberg Automatic Meridian Circle at La Palma: it provides magnitudes in a band similar to the Johnson and with an accuracy only slightly lower than that achieved with classical photometric techniques. The magnitudes published in the Carlsberg Catalogues Vols 1-3 (1985, 1986, 1987) were reduced to Johnson using a colour term derived from the spectral type, and were retained when no other photoelectric data were available;

• improvement of non-photoelectric data. For the remaining stars, data from the SIMBAD data base or from the original programmes proposed for Hipparcos observation were used as a basis. The magnitudes available in SIMBAD are the result of a hierarchy of catalogue sources and of a unification of systems performed by Ochsenbein (1974). When the sources available for both and magnitudes were not identical, the more reliable magnitude was retained, and the colour of the stars was computed from the spectral type and a newly-derived extinction model (Arenou et al. 1992);
• for components of double and multiple systems, the photometric data were derived as described above when the components are processed as single stars (i.e. systems with separation smaller than 3 arcsec or systems with secondary components that have a negligible impact on the quality of the Hipparcos observations). In the other cases, new compilations (Oblak & Mermilliod 1988) and new observations (Oblak 1988, Argue & Irwin 1988) were performed. Some magnitudes were taken from the GSC (Lasker et al. 1990). In the absence of additional data, the magnitudes were taken from the preliminary version of the CCDM, which were, in turn, taken from the Index Catalogue of Visual Double Stars (IDS, Jeffers et al. 1963). Most often the CCDM magnitudes are visual estimates but, especially for the faintest systems, they may be photographic. The `magnitude difference' given in field 49 was computed from the CCDM, and may not be consistent with data given in field 27 for the separate components.