Binary Origin of WR Stars

Many factors influence a star’s evolution, but a strong influential factor is the presence of a close companion. As shown in Figure 1.11, the binary fraction amongst Galactic O stars is thought to be high (Sana et al., 2012, 2013b). Therefore, knowledge of the star’s binary status is of great importance when trying to predict how the future will unfold.

Figure 1.11: Pie chart showing the fraction of O stars expected to evolve in isolation (blue) either as single stars or wide binaries, or with interaction from a companion (red). Approximate fractions have been indicated on the chart. Adapted from Sana et al.(2012) who used observations of young stellar clusters in the Milky Way and theoretical models to correct for observation bias and predict interaction type.

that is without interaction with a companion star (Sana et al.,2012). For these single stars the evolutionary path is likely to be dependent on the initial mass of the star, and follow the routes outlined in the Conti scenario, discussed in Section 1.5.1. The metallicity of the local environment governs the initial mass requires to enter to the WR phase. Table 1.2 compares the minimum masses needed for a single star to reach various milestones in massive star evolution, for MW and LMC metallicities. We see that metal-poor environments require higher mass progenitors to become WR stars, and this is a direct consequence of their weaker winds, as explained in Section 1.6.

Table 1.2: The minimum mass required for single stars to reach O star, WNL, WNE and WC phases at MW and LMC metallicities. Data taken fromGeorgy et al. (2012, 2015) for rotating stars.

Metallicity O star WNL WNE WC Z M M M M

MW 1 16 20 25 27

LMC 0.4 14 32 61 63

Binarity can add an additional level of complication to massive star evolution. For extremely wide separation binaries, the system will evolve as two single stars, but as the separation decreases, the stars will become within range such that at some point during their lifetimes the two stars will interact. This is especially relevant because, for the MW O star population,Sana et al. (2012) predict an intrinsic binary fraction of over 70%, shown in Figure 1.11, meaning the majority of massive stars evolve in a binary.

Within the ∼70% of interacting binaries there are varying degrees of interaction pre- dicted. 33% are thought to undergo envelope stripping, where the hydrogen envelope from the primary is stripped by the secondary through Roche lobe overflow (RLOF) (described below and illustrated in Figure 1.12). RLOF stripping can result in either conservative or non-conservative mass transfer, depending on whether the mass from the primary is accreted onto the secondary or lost to the system respectively. A further 14% of binaries experience accretion/common envelope (CE) following RLOF. The CE phase occurs when the secondary is incapable of accreting matter at the same rate the primary is losing it. This excess matter contributes to the filling of the secondarys Roche lobe, and subsequent RLOF. With both stars filling their Roche lobes, a common envelope of material surrounds the system (Iben & Livio, 1993; Podsiadlowski, 2001). Systems which undergo accretion experience an increase of the rotational velocity of the secondary, known as spin-up. Com-

ponents of a CE system however experience drag from the envelope of material they are orbiting through, and this causes the orbit to shrink. 24% of binaries will result in a merger, either whilst on the main sequence (case A) or for longer period binaries, between core hy- drogen and core helium burning (case B) or later phase (case C). For massive stars, the majority of mergers are expected to occur whilst on the main sequence (case A) (Paczy´nski,

1967; Sana et al., 2012).

Binary fractions for WR stars are lower than their O star counterparts. For a sample of 227 Galactic WR stars of varying spectral types, van der Hucht (2001) find a binary fraction of ∼40%. At lower metallicities, Bartzakos et al. (2001) investigate the binary population of the 23 known WO/WC stars within the LMC. They find an observed binary fraction of 13% when considering definite binaries, however when including an additional 5 potential binary systems, the binary fraction rises to 35%. The LMC WNE population was considered byFoellmi et al.(2003b), who derive a binary frequency of ∼30%, and similarly the WNL population revealed a 20% binary fraction (Schnurr et al., 2008). These results are reasonably consistent, suggesting binarity of WR stars is independent of metallicity. We should, however, note that the WR binary fractions discussed here are derived observa- tionally, and therefore represent lower limits since no adjustment has been made to correct for observational biases.

Figure 1.12 is a schematic outlining WR formation through the binary channel. To start we have two massive stars orbiting within the system, initially detached. As the primary evolves it will expand, filling its Roche lobe and triggering Roche lobe overflow (RLOF), which is when material from the expanding star flows onto the companion through an accretion disk (Paczy´nski, 1971). For the primary mass donor this results in a stripped, helium-rich star - a WR star. The secondary mass gainer has been rejuvenated, extending its lifetime on the main sequence and is spun-up, potentially altering its evolutionary path too.

Using a grid of spectral models G¨otberg et al. (2018) investigate the properties of stripped stars with initial masses in the range of 2-20 M. At solar metallicities they find

the lowest initial mass progenitor capable of producing a WR spectrum is ∼15 M, which

post-interaction results in a 5 M stripped star. Lower initial mass stars which undergo

mass transfer with a companion can also form a stripped helium star, however due to their weaker winds their (absorption line) spectra resemble hot subdwarfs.

It is thought both single and binary evolutionary routes to the WR phase are applicable, however the local environment can influence which route is dominant, as discussed in Section 1.6.

Figure 1.12: A schematic diagram showing the binary evolution to WR phase. (A) depicts two detached O stars in orbit with the primary on the left and secondary on the right. As the primary evolves it fills its Roche Lobe and the secondary begins to accrete material from the primary (B). After the envelope has been stripped from the primary, a hot helium rich core remains, the WR star, and the secondary is rejuvenated due to the accretion of more mass (C).