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The content of the HyperLeda database results from the compilation of several thousands of articles published in astronomical journals and of larges surveys. This bibliography is stored in the table bref04.
This is the code given to a 'series of measurements', and it usually corresponds to a publication in the astronomical literature. In a small number of cases, in particular for work internally performed by the database team to ascertain the integrity of the database and fix errors, this is an 'internal' code, not associated with a publication.
Measurements grouped with the same iref are normally internally homogeneous. They may be globally effected by common systematic effects, that the homogenization procedures try to characterize and compensate when it computes the mean homogenized parameters.
Some article present different series of measurements that may be internally homogeneous, but possibly affected by different systematic effects. For this reason, such series may be given different iref, despite the fact that they come from the same publication.
The astronomical references have a unique descriptor consisting of 19 characters and coding the year of publication, the journal, the volume and page numbers and the first initial of the first authour's name. It was invented by NED and Simbad in the 80s and is adopted by the NASA/ADS database which is the most complete astronomical bibliographic archive.
The bibcode is given when it exists. And in the rare cases where no code was defined (data acquired privately, internal contribution by the HyperLeda team, ...), the field is left NULL, or an internal code prefixed with the character '*' is used.