Angular momentum drives proton-rich nucleosynthesis in hyperaccreting neutron stars in common envelopes

Abstract

Interacting binaries can produce a wide range of exotic systems, including X-ray binaries and merging neutron stars, through a mass transfer phase called Common Envelope (CE) evolution. A CE phase can occur during rapid expansion as a star as it moves off the main sequence. If the engulfed star is a compact object (e.g. neutron star), a CE phase can lead to hyperaccretion onto the neutron star. Previous work focused on systems in which the accreting material has low angular momentum, studying turbulent outflows. This study investigates the impact of angular momentum on accreting material leading to the formation of an accretion disk. Disk accretion systems lead to very different nuclear burning conditions. This paper presents the results of nucleosynthesis modelling of material ejected from an accretion disk surrounding a 1.5 M$_{\odot}$ neutron star in a CE with a 15 M$_{\odot}$ companion. As material is accreted towards the neutron star, sufficient heating will occur to eject a fracton of the material back into the surrounding envelope, producing a nucleosynthetic yield signature that differs from other explosions. We find that significant mass fractions of rp-process products are synthesised, thereby providing another mechanism for rp-process contribution to galactic chemical evolution, following ejection of the CE. Furthermore, later stages of the CE evolution the accrete helium leading to alpha-rich, supernova-like nucleosynthesis, producing $^{44}$Ti and $^{56}$Ni. Further work on modelling both the accretion disk wind, and the companion envelope ejection, is vital to understand the contributions of these scenarios to chemical evolution.

Figures (17)

• Figure 1: The temperature evolution of three different accretion rate trajectories; $1\times10^{-5}$ M$_{\odot}~$s$^{-1}$ (green), $32\times10^{-5}$ M$_{\odot}~$s$^{-1}$ (orange) and $512\times10^{-5}$ M$_{\odot}~$s$^{-1}$ (purple). These are split into three phases; the infall phase (rapid increase at $t$ = 0), the disk evolution (plateau and increase to peak), and the ejection phase (rapidly cooling post-peak). The temperature evolution of material ejected near the disk edge is shown in dotted lines and the material ejected close to the neutron star is shown in solid lines.
• Figure 2: Evolution of the 15 M$_{\odot}$ companion. The x-axis shows the time remaining until core collapse and the y-axis shows the radius of the companion. The markers show the time during the companion evolution at which we modelled the common envelope events. The companion ages used are: $1.256\times10^{7}$, $1.257\times10^{7}$ and $1.258\times10^{7}$ years
• Figure 3: Composition of a 15 M$_{\odot}$ ZAMS star, when it has expanded to 20.3 R$_{\odot}$. The mass fraction (left axis) for $^{1}$H, $^{4}$He, $^{12}$C, $^{14}$N and $^{16}$O shown as a function of radial distance from the companion core. Also shown is the accretion rate (right axis) in solar masses per second vs radial distance. Black asterisks indicate accretion rates used in current study.
• Figure 4: Final isotopic mass fractions for a 1.5 M$_{\odot}$ neutron star accreting 32$\times10^{-5} \textrm{M}_{\odot}\textrm{s}^{-1}$ from a 15 M$_{\odot}$ companion. The top shows the resulting isotopic distribution when using the initial composition from 2018MNRAS.480..538R and including all solar seed nuclei (389 isotopes between $^{1}\textrm{H}$ and $^{209}\textrm{Bi}$). The bottom figure shows the resulting isotopic distribution when using an initial composition taken from 2018MNRAS.480..538R MESA models (170 isotopes between $^{1}\textrm{H}$ and $^{62}\textrm{Ni}$).
• Figure 5: Average undecayed final isotopic distribution from the $1\times10^{-5} \textrm{M}_\odot~\textrm{s}^{-1}$ accretion rate trajectory. Average over disk ejection radius ($1.0422\times10^{6} - 4.9695\times10^{6}$ cm). Stable nuclei shown in black boxes.