A Photometric Study of Ara OB1

Extract

Photoelectric and photographic UBVRI photometery have been obtained for over 700 stars in the field of Ara OB1, including many in the central cluster NGC 6193. Association members were identified mainly on the basis of their reddenings. The association sequence is well populated blueward of (B-V)_O = −0^m.10, (U-B)_O = −0^m.35. Beyond this “turn-up” point stars lie only above the ZAMS. There are clear gaps in the color magnitude diagrams just redward of the turn-up points. The width of these gaps is 0.07 mag. in (U-B). They cannot be easily explained by pre-main sequence models involving only gravitational contraction or by those involving ³He burning. The difficulty of identifying the faint pre-main sequence stars (if they exist) amidst the field stars is discussed and a limiting luminosity function for the association is derived.