UBV Photoelectric Sequences in SA 92-115

The stars upon which this system is based are tabulated in Johnson and Morgan (1953), Johnson and Harris (1954) and Johnson (1955,1963). The stars which define the system are nearly all bright stars. The recent large body of southern hemisphere standards (Moreno, Moreno, Stock, Torres and Wroblewski 1966) also consists of bright stars.

The stars now recommended for use as UBV standard stars were redefined at the XIVth General Assembly of the IAU to be those objects brighter than V = 5.0 mag between DE = +- 10 deg; however, most of them are still brighter than desirable for the observing techniques and photometers used by most astronomers. There exist, scattered around the sky, photoelectric sequences of fainter stars which are tied into the UBV system.

Such sequences, more often than not, are located in galactic or globular star clusters. However, they really are not all on the same precise photometric system, as an intercomparison of two or more observers' data on a given cluster many times shows (see, for example, Landolt 1964); slight zero-point differences and color equation problems occur. Many times, sequences are defined only by one, two or three observations per star.

Hence, one ought not use these sequences for extension to fainter objets in extended programs around the sky. There has been much discussion, both privately and in the literature (Blaauw 1955; Walker 1959; Stoy 1958, 1961; Greaves 1955), of the desirability of a faint sequence of standard stars distributed over the sky. Although a variety of useful photometric systems has been established in recent years, the broad-band UBV system still has an important future role in astronomical research.

Particularly is this true for the fainter objects to be observed with the big reflectors, where a well defined, widely adopted standard photometric system can be used to great advantage in the initial reconnaissance of faint celestial objects. An attempt is made in this paper to provide a homogeneous set of UBV standard stars in the celestial equatorial Selected Areas. By doing so, astronomers in both hemispheres will have access to faint standard stars readily accessible to the largest telescopes.

It is not claimed that this system is a priori better than other fine photoelectric sequences scattered in various regions about the celestial sphere. However, the stars observed herein were thoroughly tied together completely around the sky during many observing sessions from the same site, and using the same type of equipment throughout the duration of the observing program. Therefore, the system should be internally quite accurate and consistent.

Many, but not all, of the stars in this catalog are included in later catalogs: 2118 and 2183.