Study of UV bright stellar populations in the globular cluster NGC 1261 using AstroSat

TL;DR

This study uses UVIT on ASTROSAT to analyze UV-bright stellar populations in globular cluster NGC 1261, revealing detailed properties of horizontal branch stars and their formation pathways.

Contribution

It provides the first comprehensive UV photometry of NGC 1261's HB stars, including EHB stars, and constrains their formation mechanisms using multi-wavelength data.

Findings

Detected the full horizontal branch in UV, including EHB stars.

Estimated Teff, luminosities, and radii of HB stars, identifying the G-jump at ~11500 K.

EHB stars have Teff around 31,000 K and suggest formation via extreme mass loss or early hot-flash.

Abstract

We present the UV photometry of the globular cluster NGC 1261 using images acquired with the Ultraviolet Imaging Telescope (UVIT) on-board ASTROSAT. We performed PSF photometry on four near-UV (NUV) and two far-UV (FUV) images and constructed UV colour-magnitude diagrams (CMDs), in combination with HST, Gaia, and ground-based optical photometry for member stars. We detected the full horizontal branch (HB) in NUV, blue HB in the FUV and identified two extreme HB (EHB) stars. HB stars have a tight sequence in UV-optical CMDs well-fitted with isochrones generated (12.6 Gyr age, [Fe/H] = -1.27 metallicity) using updated BaSTI-IAC models. Effective temperatures (Teff), luminosities and radii of bright HB stars were estimated using spectral energy distribution. As we detect the complete sample of UV bright HB stars, the hot end of the HB distribution is found to terminate at the G-jump (Teff ~ 11500 K). The two EHB stars, fitted well with single spectra, have Teff= 31,000 K and a mass = 0.495Msun and follow the same Teff-Radius relation of the blue HB stars. We constrain the formation pathways of these EHB stars to extreme mass loss in the RGB phase (either due to rotation or enhanced Helium), OR early hot-flash scenario.