UV photometry of spotted stars in the horizontal branch of the globular cluster NGC 2808 using AstroSat

TL;DR

This study analyzes hot extreme horizontal branch stars in globular cluster NGC 2808 using UVIT on AstroSat, revealing a correlation between rotation period and UV brightness of spotted stars, and identifying fainter variable EHB stars.

Contribution

It presents UV photometry of EHB stars in NGC 2808, combining multiple datasets, and suggests a potential link between stellar rotation and UV variability in these stars.

Findings

FUV-optical CMDs reveal hot EHB stars fainter than ZAHB.

Faintest variable EHB stars have longer periods.

Tentative correlation between rotation period and UV brightness.

Abstract

A recent study of hot (20,000 to 30,000 K) extreme horizontal branch (EHB) stars in globular clusters (GCs) has led to the discovery of their variability. It is suggested that this variability is driven by the projected rotation of magnetic spots on the stellar surfaces and is expected to have higher amplitudes at shorter wavelengths. Here, we present the analysis of such hot stars in the massive GC NGC 2808 using the Ultraviolet Imaging Telescope (UVIT), aboard AstroSat. We use the UVIT data in combination with the Hubble Space Telescope UV Globular Cluster Survey (HUGS) data for the central region (within $\sim$ $2.7'$ $\times$ $2.7'$) and ground-based optical photometry for the outer parts of the cluster. We generate the far-UV (FUV) - optical colour-magnitude diagrams (CMDs) and in these we find a population of hot EHB stars fainter than the zero-age horizontal branch (ZAHB) model. A comparison of our FUV magnitudes of the already reported variable EHB stars (vEHBs) shows that the longest period vEHBs are the faintest, along with a tentative correlation between rotation period and UV magnitude of spotted stars. In order to firmly establish any correlation, further study is essential.