OB Stars on the Outermost Borders of M 31

Abstract

Van den Bergh (1964) has selected and mapped OB associations in M 31 on the basis of plates taken with the Tautenburg 52 inch Schmidt camera. The selection was done by the blinking method. The task of the present investigation is to find if there exist OB stars (single or in association) on the outermost borders of this stellar system. For this purpose we measured the brightness of all stellar objects in UBV down to the magnitude 20.^m0 (B) on Tautenburg plates in a special test field (Figure 1). It includes 0.26 square degrees and its centre has a distance of 104′ = 22 kpc from the centre of M 31. This test field includes too Baade’s field IV with the photoelectric standards in UBV observed by Arp (Baade and Swope, 1963) and the OB associations OB 184, OB 185 and OB 186 of Van den Bergh. The total number of stars brighter than 20.^m0 (B) was 996. The stars within the associations OB 184 and OB 185 are not included in these statistics. From this number, 704 objects could be measured in all 3 colours. We found among them 23 blue objects with U−B ≤ −0.^m25. The mean error of the brightness on the basis of 4 plates in each colour does not exceed ±0.^m7 for stars of 19.^m0 (B).