Nucleosynthetic yields from neutron stars accreting in binary common envelopes

Abstract

Massive-star binaries can undergo a phase where one of the two stars expands during its advanced evolutionary stage as a giant and envelops its companion, ejecting the hydrogen envelope and tightening its orbit. Such a common envelope phase is required to tighten the binary orbit in the formation of many of the observed X-ray binaries and merging compact binary systems. In the formation scenario for neutron star binaries, the system might pass through a phase where a neutron star spirals into the envelope of its giant star companion. These phases lead to mass accretion onto the neutron star. Accretion onto these common-envelope-phase neutron stars can eject matter that has undergone burning near to the neutron star surface. This paper presents nucleosynthetic yields of this ejected matter, using population synthesis models to study the importance of these nucleosynthetic yields in a galactic chemical evolution context. Depending on the extreme conditions in temperature and density found in the accreted material, both proton-rich and neutron-rich nucleosynthesis can be obtained, with efficient production of neutron rich isotopes of low Z material at the most extreme conditions, and proton rich isotopes, again at low Z, in lower density models. Final yields are found to be extremely sensitive to the physical modeling of the accretion phase. We show that neutron stars accreting in binary common envelopes might be a new relevant site for galactic chemical evolution, and therefore more comprehensive studies are needed to better constrain nucleosynthesis in these objects.

The R-Process Alliance: Chemodynamically Tagged Groups of Halo r-process-enhanced Stars Reveal a Shared Chemical-evolution History

“…Recent observations of highly r-process-enhanced (hereafter RPE) stars in the satellite ultrafaint dwarf (UFD) galaxy Reticulum II (Ji et al 2016(Ji et al , 2019Roederer et al 2016), coupled with the gravitational-wave detection of an NSM by LIGO/VIRGO (GW170817; Abbott et al 2017), with subsequent photometric and spectroscopic analysis of its associated kilonova (SSS17a; e.g., Drout et al 2017;Kilpatrick et al 2017;Shappee et al 2017;Watson et al 2019), provided strong evidence for two key pieces of the puzzle for the astrophysical origin of the r-process elements: first, that NSMs can plausibly be the dominant source of the heavy (Z > 52) r-process elements (e.g., Hotokezaka et al 2018;Horowitz et al 2019)-although it is important to note that theoretical predictions of the NSM yields and frequencies vary, leading to significant uncertainties in the predicted amounts of expected r-process elements produced (e.g., Côté et al 2019;Keegans et al 2019;Siegel et al 2019)-and second, that dwarf spheroidal (dSph) galaxies and UFDs may be the primary birthplaces of the metal-poor RPE stars in the halo of the Milky Way, which were later distributed throughout it when their parent dwarfs were disrupted during accretion (see, e.g., Brauer et al 2019). 14 This opens the exciting possibility of grouping RPE stars on the basis of the similarity in their dynamical properties, which are expected to be approximately conserved even after their parent galaxy (or globular cluster) is destroyed (see, e.g., Roederer et al 2018a).…”

Modelling Neutron Star-Black Hole Binaries: Future Pulsar Surveys and Gravitational Wave Detectors

“…Due to the uncertainty, we also present results for a model where no accretion of mass or angular momentum is allowed during CE evolution (see Section 2.4). NSs accreting during CE evolution may also eject processed material (Keegans et al 2019). Barkov & Komissarov (2011) study a scenario in which pulsars are recycled during CE evolution leading to a dramatic increase in their magnetic field strength to magnetar levels (𝐵 10 15 G) powering a supernova-like explosion.…”

Evolution of Accreting Neutron Star Common Envelopes and Their Nucleosynthesis Possible P-nuclide Production

“…In the scenario of interest here, the accretor is a neutron star and the expanding star is entering its red giant phase. Once inside the CE, the neutron star will slowly inspiral towards the core of the red giant while accreting hydrogen rich material at hypercritical rates [1], [3]. The angular momentum of the accreting material will cause it to hang up in an accretion disk.…”

“…The angular momentum of the accreting material will cause it to hang up in an accretion disk. If the accretion rate is high, the temperature of this accreting material will become high enough to drive nuclear burning in the accretion disk as material approaches the neutron star [3]. A proportion of this material will then be ejected from the system, current hydrodynamical simulations say up to 25% [2].…”