Multiwavelength study of old open clusters: NGC 188 and M67

Abstract

The thesis focuses on understanding the single and binary stellar evolution in the old open clusters, M67 and NGC 188 using multiwavelength photometric data. We have used images from the Ultra-Violet Imaging Telescope (UVIT) on ASTROSAT, the first Indian space observatory and combined them with other photometric data from 0.14 - 11.5 μm wavelength range and spectra in the ultraviolet (UV) range. In NGC 188, using the first light data of UVIT, we have discovered a hot companion to a blue straggler star (BSS). The estimated fundamental parameters of the components

suggest that the BSS is formed as a result of a recent mass transfer (MT) from a post- AGB/HB star. A deep UV study of the M67 was performed using GALEX images

to reveal the presence of a large number of stars with far-UV (FUV) and/or near-UV (NUV) excess. Some of these stars were found to be in binaries and many are single

stars. These indicate that a large number of Sun-like stars in this cluster may be chromo- spherically active. We detect a few main-sequence + white dwarf (MS+WD) binaries

in M67, which could be progenitors of Cataclysmic variables. We created multiwave- length spectral energy distributions (SED)s of 45 interesting candidates and estimated

their fundamental parameters. We classified BSSs into three groups and suggest that they are formed between 400 Myr - 2 Gyr, more or less continuously. The multi-filter FUV images of M67 from UVIT were used to study 9 bright BSSs. The SEDs of 6 BSSs were found not to fit well with single star spectra, but fitted well with a hot + cool composite spectra, consistent with IUE and HST spectra. The estimated parameters suggest the hot companions to be WDs, and this is the first confirmed detection of WD companions to BSSs in M67. This study has thus demonstrated the capability of UVIT to detect and characterise binaries with WD companions, particularly to BSSs, with the discovery of 6 BSSs with WD companions in M67 and one BSS with post - AGB/HB companion in NGC 188. The study finds that a good fraction of BSSs are formed through binary MT in M67, apart from other pathways such as triple systems and/or collisions. The BSSs in NGC 188 are likely to be formed by binary MT.