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 T eff 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-T eff parameters, and 0.44-0.46 M ⊙ 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.
|Journal||Astrophysical Journal Letters|
|Publication status||Published - 2022 Nov 1|
Bibliographical noteFunding Information:
We thank the anonymous referee for the helpful comments and suggestions. We thank Andrea Bellini for making the HST photometric data available. We are grateful to Pierre Bergeron and Leandro Althaus for making the WD models available. D.S.P. thanks Ranjan Kumar for sharing the results of M13, and Simone Zaggia and Yazan Momany for the useful discussions. This publication utilizes the data from the AstroSat mission’s UVIT, which is archived at the Indian Space Science Data Centre (ISSDC). The UVIT project is a result of collaboration between IIA, Bengaluru, IUCAA, Pune, TIFR, Mumbai, several centers of ISRO, and CSA. This research made use of VOSA, developed under the Spanish Virtual Observatory project supported by the Spanish MINECO through grant AyA2017-84089. A.S. acknowledges support from a SERB Power fellowship. C.C. acknowledges support from the National Research Foundation of Korea (2022R1A2C3002992, 2022R1A6A1A03053472). N.W.C.L. gratefully acknowledges the generous support of Fondecyt Iniciación grant 11180005, as well as support from Millennium Nucleus NCN19-058 (TITANs) and funding via the BASAL Centro de Excelencia en Astrofisica y Tecnologias Afines (CATA) grant PFB-06/2007. N.W.C.L. is also thankful for support from ANID BASAL project ACE210002 and ANID BASAL projects ACE210002 and FB210003. A.A.R.V. acknowledges the funding from ANID, Millennium Science Initiative, ICN12_009.
© 2022. The Author(s). Published by the American Astronomical Society.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science