Discovery of a hot post-AGB star in Galactic globular cluster E3

• Published in: arXiv.org 2024-04
• Main Authors: Kumar, R, Moharana, A, Piridi, S, Pradhan, A C, Hełminiak, K G, Ikonnikova, N, Dodin, A, Szczerba, R, Giersz, M, Ojha, D K, Samal, M R
• Format: Article
• Language: English
• Subjects: Abundance; Asymptotic giant branch stars; Binary stars; Galactic clusters; Globular clusters; High resolution; Iron; Kinematics; Metallicity; Parallax; Parameter identification; Photometry; Physical properties; Spectral energy distribution; Stellar atmospheres
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2403.12907

We report a new hot post-asymptotic giant branch (PAGB) star in the Galactic globular cluster (GC) E3, which is one of the first of the identified PAGB stars in a GC to show a binary signature. The star stands out as the brightest source in E3 in the \mbox{{\em Astrosat}}/UVIT images. We confirmed its membership with the cluster E3 using Gaia DR3 kinematics and parallax measurements. We supplemented the photometric observations with radial velocities (RVs) from high-resolution spectroscopic observations at two epochs and with ground- and space-based photometric observations from 0.13 \(\mu\)m to 22 \(\mu\)m. We find that the RVs vary over \(\sim\)6 \kms\ between the two epochs. This is an indication of the star being in a binary orbit. A simulation of possible binary systems with the observed RVs suggests a binary period of either 39.12 days or 17.83 days with mass ratio q\(\geq\)1.0. The [Fe/H] derived using the high-resolution spectra is \(\sim -\)0.7 dex, which closely matches the cluster metallicity. The spectroscopic and photometric measurements suggest \Teff\ and \(\log g\) of the star as 17\,500\(\pm\)1\,000~K and 2.37\(\pm\)0.20~dex, respectively. Various PAGB evolutionary tracks on the Hertzsprung--Russell (H-R) diagram suggest a current mass of the star in the range 0.51\(-\)0.55 \Msun. The star is enriched with C and O abundances, showing similar CNO abundances compared to the other PAGB stars in GCs with the evidence of the third dredge-up on the AGB phase.