PRE-MAIN-SEQUENCE STARS IN THE YOUNG GALACTIC CLUSTER IC-4996 - A CCDPHOTOMETRIC-STUDY

The results of a UBV CCD photometric study of the cluster IC 4996 are presented. We obtain new values for the cluster parameters: E(B-V) = 0 .71 +/- 0.08, V-O, - M-V = 11.9 +/- 0.1, and age = (7.5 +/- 3) x 10(6) yr. The combined evidence from the photometric diagrams suggests the presence of a number of pre-main-sequence (PMS) cluster members with s pectral types similar to A0 to F5. The interpretation of these objects as PMS candidates is further supported by independent results from CC D uvby H beta observations. If confirmed, these stars would bridge the existing gap in the sample of PMS stars, between the coolest Herbig A eBe stars (HAeBe), and the hottest T Tauri stars. These PMS candidates are located some 0.5 and I mag above the main sequence in the V-(B-V) diagram, around the location of spectral types AF. This feature, toge ther with their locations in the (U-B)-(B-V) diagram and the interpret ation that they are PMS members, suggest an additional reddening law w ith a slope alpha = E(U-B)IE(B-V) similar or equal to 0.55, probably c aused by circumstellar material.