Abstract:
We present the first comprehensive study of the most massive globular cluster, Omega Centauri, in the far ultraviolet (FUV), extending from the center to ∼28% of the tidal radius using the Ultraviolet Imaging Telescope
on board AstroSat. A comparison of the FUV-optical color–magnitude diagrams with available canonical models
reveals that horizontal branch (HB) stars bluer than the knee (hHBs) and the white dwarfs (WDs) are fainter in the
FUV by ∼0.5 mag than model predictions. They are also fainter than their counterparts in M13, another massive
cluster. We simulated HB with at least five subpopulations, including three He-rich populations with a substantial
He enrichment of Y up to 0.43 dex, to reproduce the observed FUV distribution. We find the He-rich younger
subpopulations to be radially more segregated than the He-normal older ones, suggesting an in situ enrichment
from older generations. The ω Cen hHBs span the same Teff range as their M13 counterparts, but some have smaller
radii and lower luminosities. This may suggest that a fraction of ω Cen hHBs are less massive than those of M13,
similar to the result derived from earlier spectroscopic studies of outer extreme HB stars. The WDs in ω Cen and
M13 have similar luminosity–radius–Teff parameters, and 0.44–0.46 Me He-core WD model tracks evolving from
progenitors with Y = 0.4 dex are found to fit the majority of these. This study provides constraints on the formation
models of ω Cen based on the estimated range in age, [Fe/H], and Y (in particular) for the HB stars.
