Abstract

Blue stragglers are stars that are brighter, hotter, and bluer than the main-sequence turnoff, and therefore should have evolved into giant stars long ago. These stars are commonly found in stellar binaries paired with a white dwarf companion, which suggests they formed via mass transfer in a binary system. The future evolution of blue straggler-white dwarf binaries may result in interesting evolutionary stages including common envelope evolution and the formation of X-ray binaries or double white dwarf mergers. In this work we use the binary population synthesis code COSMIC to explore the impact of both stellar metallicity and common envelope ejection efficiency on the final evolutionary outcome of blue straggler-white dwarf binaries. Using COSMIC, we model a grid of blue straggler-white dwarf binaries, varying the blue straggler mass, white dwarf mass, initial orbital period, and common envelope efficiency. We vary the blue straggler mass from 0.8 to 2.2 solar masses, and the orbital period from 10 to 10000 days. We run both a low metallicity ([Fe/H] = -2.0) and a solar-metallicity grid of models. We find that low-mass double white dwarf mergers may be much more common than the rate predicted using typical population synthesis assumptions about common envelope efficiency. We also conclude that lower metallicity blue stragglers cause fewer blue straggler binaries to evolve into merging white dwarves, as opposed to those with solar metallicity. These models show that blue straggler evolutionary paths likely lead to a variety of astrophysically important outcomes including X-ray binaries, double white binaries, and explosive transients caused by the merging of white dwarfs.

Introduction

In order to form a blue straggler star, a binary of main sequence stars undergo mass transfer. One star will give mass to the other, causing the star to evolve brighter, hotter, and bluer than most. This bluer star is the blue straggler star, and its companion becomes a white dwarf. In this new binary, another round of mass transfer can occur, however, a common envelope may form around the binary. As the binary’s orbital period shrinks dramatically due to the envelope, the released orbital energy is used to eject the common envelope surrounding the binary. The extent to which the orbital energy ejects the common envelope is governed by the common envelope efficiency (or CE efficiency). A common envelope results in orbital decay possibly to the point of a binary merger, with a lower CE efficiency predicting more mergers. Here we investigate the impact of common envelope efficiency and stellar metallicity on the predicted merger rate of a observed sample of blue straggler-white dwarf binaries in binary evolution models.

Methods

COSMIC (Breivik et al. 2020) is a stellar evolution code used to conduct population synthesis simulations. In terms of large groups of stars, COSMIC uses arrays of initial conditions to evolve stars for a certain length of time. Since COSMIC focuses on population sythesis, it does not do well simulating individual stars in detail. COSMIC also assumes that any mass transfer occuring in a binary or tertiary sytem is conservative, meaning all of the material lost by one object in the system is accreted by the other object in the system. However, mass loss and accretion is rarely perfectly conservative, instead mass transfer is more likely to be non-conservative. The distinction made may change the outcome of the simulation (an outcome which is not possible in COSMIC due to certain assumptions).

Starting with a massive main sequence star and a white dwarf of varying preliminary parameters (mainly primary mass, secondary mass, and orbital period), the COSMIC stellar evolution code is used to evolve the initial binary model for a total of 14 Gigayears for two metallicities (solar metallicity and a lower metallicity indicative of blue stragglers found in the field). The results of the simulated evolution are graphed in Figures 1 and 2 by the initial orbital period of the blue straggler white dwarf simulated binary and the initial mass of the blue straggler in that simulation, where the color of the bar represents the results of the simulation.

Results

Many simulated blue straggler and white dwarf binary mergers are not accounted for in simulated models, because CE efficiency is set to 1.0 instead of 0.2-0.4, as suggested by observations (Zorotovic et al. 2010).

According to Figures 1 and 2, blue straggler-white dwarf binaries are more likely to merge when they have a higher metallicity. All low metallicity binaries do not merge, while around 7 of the solar metallicity binaries are predicted to merge out of the 23 mapped on the grid in Figure 1.

Figures 1 and 2 are grids which can be used to predict the outcomes of observed blue straggler and white dwarf binaries. The black squares in Figures 1 and 2 represent observed data sets of blue straggler-white dwarf binaries with solar and low metallicity respectfully which were obtained from Carney et al. 2001, Carney et al. 2005, Preston & Sneden 2000, Leiner et al. 2019, Latham & Milone 1996, and Geller et al. 2009. The former three contain solar metallicity blue straggler binary data, the latter three contain low metallicity blue straggler binary data for binaries found in the field.

Discussion

Blue straggler stars often evolve through a common envelope, forming close double white dwarf binaries or mergers. These double white dwarf mergers may be detectable as transients such as calcium rich supernovae (Kasliwal et al 2012), R Coronae Borealis stars (Schwab 2019) or some other kind of sub-luminous supernova. The preceding orbital decay may also be emit gravitational wave radiation that might be detected by future space-based interferometers.

Figure 1: The figure shows a grid of simulated binaries, evolved at a constant CE efficiency of 0.25 and at a constant solar metallicity, whose evolutionary results are color-coded based on the stellar types of the objects in the binary. The figure is a grid plotting the simulated binaries based on their initial period and the initial mass of the blue straggler in the binary. The black squares show observed blue stragglers whose data was collected from telescopes.

Figure 2: The figure shows a grid of simulated binaries, evolved at a constant CE efficiency of 0.25 and at a constant low metallicity of z = 0.002, whose evolutionary results are color-coded based on the stellar types of the objects in the binary. The figure is a grid plotting the simulated binaries based on their initial period and the initial mass of the blue straggler in the binary. The black squares show observed blue stragglers whose data was collected from telescopes.