Detection of extreme low mass white dwarfs in globular clusters M3 and M13

Abstract

Different evolutionary scenario of stars affects their WD population. Globular clusters harbour single as well as binary stellar populations in their different evolutionary phases which lead to end up their WD population at redder than their “normal cooling sequence” in H-R diagram. We have studied twin Galactic globular clusters, M3 and M13 using far-UV observations of the Ultraviolet Imaging Telescope (UVIT) on-board the AstroSat satellite. We could find each 24 WD probable candidates in M3 and M13, respectively. We used spectral energy distribution from far-ultraviolet (13-18 nm) to near-infrared (𝑢𝑝 𝑡𝑜 100 𝑛𝑚) photometric fluxes to determine their effective temperature, bolometric luminosities and radius. To determine their evolutionary status, we compared them with the WD cooling sequences obtained from single, binary, and massive stars evolution. The cooling sequences of extremely low mass (ELM) He-core WDs (0.15 ≤ M/𝑀⊙ ≤ 0.43) are taken from Althaus et al. (2013) which are generated by computing the non-conservative evolution of a binary system consisting of an initially 1.0 𝑀⊙ zero-age main-sequence (ZAMS) star and a 1.4 𝑀⊙ neutron star for various initial orbital periods. The cooling tracks CO-core WDs were taken from Renedo et al. (2010) for WD mass range 0.50 − 0.86 𝑀⊙. These cooling tracks are derived from normal single stellar evolution. The cooling tracks for massive WDs (1.06 − 1.28 𝑀⊙) were obtained from Althaus et al. (2007) which is computed under partial degenerate conditions of core carbon burning of WDs. There are seven WDs of M3 and 12 WDs of M13 lying within the ELM WD cooling sequence (𝑀𝑊𝐷 ≤ 0.43 𝑀⊙). 15 WDs of M3 and M13 are lying in-between 0.43 − 0.50 𝑀⊙ WD cooling sequence where we do not have any confirmed evolutionary sequence (i.e., they might be formed due to binary evolution or single star evolution). There are five WDs in M3 lying even bluer than 0.86 M WD cooling sequence. We found at least two of them could be lying near 0.86 𝑀⊙, but bluer side, and one was lying near massive WD cooling sequence (𝑀𝑊𝐷 = 1.1 𝑀⊙; Althaus et al. 2007) which burn carbon in partially degenerate conditions. The rest two WDs of M3 were found far bluer (at log(Teff /K) ∼ 5.0) than any WD cooling sequence to be fitted.