AstroSat study of the globular cluster NGC 2298: probable evolutionary scenarios of hot HB stars

TL;DR

This study uses UVIT on AstroSat to analyze the hot horizontal branch stars in globular cluster NGC 2298, revealing their properties and possible evolutionary paths, including helium enrichment and late-stage evolution scenarios.

Contribution

First detailed UV photometry of NGC 2298's HB stars combining multiple datasets, with analysis suggesting helium enhancement and evolutionary scenarios for hot HB stars.

Findings

Hot HB stars are helium enhanced compared to BHB stars.

Three hot HB stars have temperatures around 35,000-40,000 K, one reaches 100,000 K.

Evolutionary scenarios include progeny of EHB stars and late-stage evolution into white dwarfs.

Abstract

We present the far-UV (FUV) photometry of images acquired with UVIT on AstroSat to probe the horizontal branch (HB) population of the Galactic globular cluster NGC 2298. UV-optical color-magnitude diagrams (CMDs) are constructed for member stars in combination with HST UV Globular Cluster Survey (HUGS) data for the central region and Gaia and ground-based photometric data for the outer region. Blue HB (BHB) sequence with a spread and four hot HB stars are detected in all FUV-optical CMDs and are compared with theoretical updated BaSTI isochrones and synthetic HB models with a range in helium abundance, suggesting that the hot HB stars are helium enhanced when compared to the BHB. The estimated effective temperature, radius, and luminosity of HB stars, using best SED fits, were compared with various HB models. BHB stars span a temperature range from 7,500-12,250 K. The three hot HB stars have 35,000-40,000 K, whereas one star has around 100,000K. We suggest the following evolutionary scenarios: two stars are likely to be the progeny of extreme HB (EHB) stars formed through an early hot-flasher scenario; one is likely to be an EHB star with probable helium enrichment, the hottest HB star is about to enter the WD cooling phase, could have evolved from BHB phase. Nevertheless, these are interesting spectroscopic targets to understand the late stages of evolution.