WOCS 4540: Detailed Analysis of a Very Long Orbital Period Blue Straggler

TL;DR

This paper presents a detailed evolutionary analysis of WOCS 4540, the longest orbital period blue straggler-white dwarf pair in NGC 188, combining observations with stellar modeling to understand its formation history.

Contribution

It introduces a comprehensive modeling approach that accounts for multiple mass transfer mechanisms to explain the properties of WOCS 4540, a unique long-period BSS-WD system.

Findings

The best-fit model matches observational data of the BSS and WD.

All three mass transfer mechanisms contribute to the BSS's luminosity.

Mass transfer efficiency is approximately 55%.

Abstract

WOCS 4540 is the longest orbital period ($P_{\rm orb}=3030$ days) blue straggler (BSS) - white dwarf (WD) pair in the old open cluster NGC 188 . It also contains one of the most luminous BSS in the cluster. Prior \textit{Hubble Space Telescope} COS spectroscopy measured a WD mass of 0.53 $M_{\odot}$, indicative of a carbon-oxygen WD and suggesting previous mass transfer from an asymptotic branch giant (AGB). Detailed modeling of the system evolution, including red giant branch phase wind mass transfer, AGB wind Roche-lobe overflow and regular Roche-lobe overflow, is done with Modules for Experiments in Stellar Astrophysics. The best-fit model produces excellent agreement with a wide array of observational constraints on the BSS, the WD and the binary system. To produce the observed luminosity and effective temperature of the BSS, all three donor mass-transfer mechanisms contribute similarly to build a 1.5 $M_{\odot}$ BSS. The overall mass-transfer efficiency is 55%. Regular Roche-lobe overflow occurs only during the largest AGB thermal pulse, but yields a very high accretion rate at 75% efficiency and briefly (less than 1 Myr) a very high luminosity boost from the accretor.