UOCS. V. UV study of the old open cluster NGC 188 using AstroSat

TL;DR

This study uses UVIT on ASTROSAT to analyze the old open cluster NGC 188, revealing UV-bright stars, constructing the first NUV-optical CMDs, and estimating stellar parameters through SED fitting.

Contribution

First NUV-optical CMDs of NGC 188 constructed, combined with spectral energy distribution fitting to characterize UV-bright stars and cluster properties.

Findings

Detected hot BSSs, a hot subdwarf, and a WD candidate in FUV images.

Constructed and fitted the first NUV-optical CMDs with isochrones and WD cooling sequences.

Estimated temperatures, radii, and luminosities of UV-bright stars using SED fitting.

Abstract

We present the UV photometry of the old open cluster NGC188 obtained using images acquired with Ultraviolet Imaging Telescope (UVIT) on board the ASTROSAT satellite, in two far-UV (FUV) and one near-UV (NUV) filters. UVIT data is utilised in combination with optical photometric data to construct the optical and UV colour-magnitude diagrams (CMDs). In the FUV images, we detect only hot and bright blue straggler stars (BSSs), one hot subdwarf, and one white dwarf (WD) candidate. In the NUV images, we detect members up to a faintness limit of ~22 mag including 21 BSSs, 2 yellow straggler stars (YSSs), and one WD candidate. This study presents the first NUV-optical CMDs, and are overlaid with updated BaSTI-IAC isochrones and WD cooling sequence, which are found to fit well to the observed CMDs. We use spectral energy distribution (SED) fitting to estimate the effective temperatures, radii, and luminosities of the UV-bright stars. We find the cluster to have an HB population with three stars (Teff = 4750 - 21000 K). We also detect two yellow straggler stars, with one of them with UV excess connected to its binarity and X-ray emission.