Alejandro Feinstein

Photometric study of the young open cluster NGC 3293

Deep and extensive CCD photometric observations at UB V(RI)CH were carried out in the area of the open cluster NGC 3293. The new data set allows to see the entire cluster sequence down to MV +4:5, revealing that stars with MV + 2a re placed above it. According to our analysis, the cluster distance is d= 2750 250 pc (V0 MV = 12:2 0:2) and its nuclear age is 8 1 Myr. NGC 3293 contains an important fraction of pre-main sequence (PMS) stars distributed along a parallel band to the ZAMS with masses from 1 to 2:5M and a mean contraction age of 10 Myr. This last value does not dier too much from the nuclear age estimate. If we take into account the many factors that may aect the PMS star positions on the colour- magnitude diagram, both ages can be perfectly reconciled. The star formation rate, on the other hand, suggests that NGC 3293 stars formed surely in one single event, therefore favouring a coeval process of star formation. Using the H data, we detected nineteen stars with signs of H emission in the region of NGC 3293, another indication that the star formation process is still active in the region. The computed initial mass function for the cluster has a slope of x= 1:2 0:2, a bit flatter than the typical slope for field stars and similar to the values found for other young open clusters.

THE OPEN CLUSTER NGC 3293 AND THE OB COMPLEX IN CARINA.

Photoelectric measures in UBVRI of 46 stars in and around the open cluster NGC 3293 are presented. A distance modulus of V0 - Mv equals 12.1 plus or minus 0.15 (s.d.), corresponding to 2.6 kpc, was derived from the photometric data for 34 member stars. This is similar to distances found for other young groups in the Carina region. One M supergiant and one Be star are members of the cluster. The brightest stars on the main sequence are of type B1. Several stars that have evolved away from the upper main sequence have been classified as giants and supergiants. The age of the cluster is about 7-million years. The star HD 91824 (no. 1), spectral type O7 V((f)), is not a member of the group. The insterstellar reddening is normal in front of this cluster.

NEAR INFRARED EXCESSES IN BE STARS.

Sixty-eight bright Be stars were observed in the R and I bands of the Johnson system. Nearly all of them lie above the main sequence in a (B - V,V- I) diagram, which is interpreted as a near-infrared excess (ΔI) in their energy distributions. From the computed difference with the standard main-sequence relation, a correlation of this ΔI value with the emission strength of the Hα line, previously defined as eα is found. On the other hand, the ultraviolet excesses derived from the Q values shows no evidence of correlation with the infrared excesses. But a correlation with polarimetric data appears reliable.

FAINT OB STARS BEHIND THE COALSACK

An investigation of OB stars in the Coalsack region is done by means of UBV photographic photometry. The study includes previously known OB stars within a 1 radius circle centered at = 302? 4, b = -0? 2, and newly discovered faint OB stars in five small areas (80 square arc minutes each) within the same circle. Total visual absorption values and minimum distance moduli are derived for all the stars, and the absorption vs. distance modulus diagram is presented. A possible distant OB association is noted in the region sur rounding the compact cluster Hogg 15. Key words: interstellar matter - reddening - OB stars - spiral structure