We construct a new formulation that allows efficient exploration of steady-state accretion processes onto compact objects. Accretion onto compact objects is a common scenario in astronomy. These ...systems serve as laboratories to probe the nuclear burning of the accreted matter. Conventional stellar evolution codes have been developed to simulate in detail the nuclear reactions on the compact objects. In order to follow the case of steady burning, however, using these codes can be very expensive as they are designed to follow a time-dependent problem. Here we introduce our new code \(\textsc{StarShot}\), which resolves the structure of the compact objects for the case of stable thermonuclear burning, and is able to follow all nuclear species using an adaptive nuclear reaction network and adaptive zoning. Compared to dynamical codes, the governing equations can be reduced to time-independent forms under the assumption of steady-state accretion. We show an application to accreting low mass X-ray binaries (LMXBs) with accretion onto a neutron-star as compact object. The computational efficiency of \(\textsc{StarShot}\) allows us to explore the parameter space for stable burning regimes, and can be used to generate initial conditions for time-dependent evolution models.
We use the rapid binary population synthesis code COMPAS to investigate commonly used prescriptions for the determination of mass transfer stability in close binaries and the orbital separations ...after stable mass transfer. The degree of orbital tightening during non-conservative mass transfer episodes is governed by the poorly-constrained angular momentum carried away by the ejected material. Increased orbital tightening drives systems towards unstable mass transfer leading to a common envelope. We find that the fraction of interacting binaries that will undergo only stable mass transfer throughout their lives fluctuates between a few and \(\sim 20\%\) due to uncertainty in the angular momentum loss alone. If mass transfer is significantly non-conservative, stability prescriptions that rely on the assumption of conservative mass transfer under-predict the number of systems which experience unstable mass transfer and stellar mergers. This may substantially impact predictions about the rates of various transients, including luminous red novae, stripped-envelope supernovae, X-ray binaries, and the progenitors of coalescing compact binaries.
We carry out a systematic study of the response of companion stars in massive binaries after being impacted by supernova ejecta. A total of 720 1D stellar evolution calculations are performed to ...follow the inflation and contraction of the star in response to the energy injection and how it depends on various parameters. We find that the maximum luminosity achieved during the inflated phase is only dependent on the stellar mass and we derive an analytic formula to describe the relation. There is also a tight correlation between the duration of expansion and the intersected energy. These correlations will be useful to constrain pre-supernova binary parameters from future detections of inflated companions. We also discuss the possible outcomes of the binary system when the companion inflation is taken into account. Based on simple binary population synthesis, we estimate that \(\sim\)1-3% of stripped-envelope supernovae may have observable inflated companions. Finally, we apply our models to the observed companion of SN2006jc and place strong constraints on the possible pre-supernova binary parameters.
We chart the expected Galactic distribution of neutron stars and black holes. These compact remnants of dead stars -- the Galactic underworld -- are found to exhibit a fundamentally different ...distribution and structure to the visible Galaxy. Compared to the visible Galaxy, concentration into a thin flattened disk structure is much less evident with the scale height more than tripling to 1260 +- 30 pc. This difference arises from two primary causes. Firstly, the distribution is in part inherited from the integration over the evolving structure of the Galaxy itself (and hence the changing distribution of the parent stars). Secondly, an even larger effect arises from the natal kick received by the remnant at the event of its supernova birth. Due to this kick we find 30% of remnants have sufficient kinetic energy to entirely escape the Galactic potential (40% of neutron stars and 2% of black holes) leading to a Galactic mass loss integrated to the present day of ~ 0.4% of the stellar mass of the Galaxy. The black hole -- neutron star fraction increases near the Galactic centre: a consequence of smaller kick velocities in the former (the assumption made is that kick velocity is inversely proportional to mass). Our simulated remnant distribution yields probable distances of 19 pc and 21 pc to the nearest neutron star and black hole respectively, while our nearest probable magnetar lies at 4.2 kpc. Although the underworld only contains of order ~ 1% of the Galaxy's mass, observational signatures and physical traces of its population, such as microlensing, will become increasingly present in data ranging from gravitational wave detectors to high precision surveys from space missions such as Gaia.
Accreting main-sequence stars expand significantly when the mass accretion timescale is much shorter than their thermal timescales. This occurs during mass transfer from an evolved giant star onto a ...main-sequence companion in a binary system, and is an important phase in the formation of compact binaries including X-ray binaries, cataclysmic variables, and gravitational-wave sources. In this study, we compute 1D stellar models of main-sequence accretors with different initial masses and accretion rates. The calculations are used to derive semi-analytical approximations to the maximum expansion radius. We assume that mass transfer remains fully conservative as long as the inflated accretor fits within its Roche lobe, leading stars to behave like hamsters, stuffing excess material behind their expanding cheeks. We suggest a physically motivated prescription for the mass growth of such "hamstars", which can be used to determine mass-transfer efficiency in rapid binary population synthesis models. With this prescription, we estimate that progenitors of high-mass X-ray binaries and gravitational-wave sources may have experienced highly non-conservative mass transfer. In contrast, for low-mass accretors, the accretion timescale can exceed the thermal timescale by a larger factor without causing significant radial expansion.
We examine the sensitivity of neutrino emissions to stellar evolution models for a 15\(M_\odot\) progenitor, paying particular attention to a phase prior to the collapse. We demonstrate that the ...number luminosities in both electron-type neutrinos (\(\nu_e\)) and their anti-partners (\(\bar{\nu}_e\)) differ by more than an order of magnitude by changing spatial resolutions and nuclear network sizes on stellar evolution models. We also develop a phenomenological model to capture the essential trend of the diversity, in which neutrino luminosities are expressed as a function of central density, temperature and electron fraction. In the analysis, we show that neutrino luminosity can be well characterized by these central quantities. This analysis also reveals that the most influential quantity to the time evolution of \(\nu_e\) luminosity is matter density, while it is temperature for \(\bar{\nu}_e\). These qualitative trends will be useful and applicable to constrain the physical state of progenitors at the final stages of stellar evolution from future neutrino observations, although more detailed systematic studies including various mass progenitors are required to assess the applicability.
We conduct binary population synthesis to investigate the formation of wind-fed high-mass X-ray binaries containing black holes (BH-HMXBs). We evolve multiple populations of high-mass binary stars ...and consider BH-HMXB formation rates, masses, spins and separations. We find that systems similar to Cygnus X-1 likely form after stable Case A mass transfer (MT) from the main sequence progenitors of black holes, provided such MT is characterised by low accretion efficiency, \(\beta \lesssim 0.1\), with modest orbital angular momentum losses from the non-accreted material. Additionally, efficient BH-HMXB formation relies on a new simple treatment for Case A MT that allows donors to retain larger core masses compared to traditional rapid population-synthesis assumptions. At solar metallicity, our Preferred model yields \(\mathcal{O}(1)\) observable BH-HMXBs in the Galaxy today, consistent with observations. In this simulation, \(8\%\) of BH-HMXBs go on to merge as binary black holes or neutron star-black hole binaries within a Hubble time; however, none of the merging binaries have BH-HMXB progenitors with properties similar to Cygnus X-1. With our preferred settings for core mass growth, mass transfer efficiency and angular momentum loss, accounting for an evolving metallicity, and integrating over the metallicity-specific star formation history of the Universe, we find that BH-HMXBs may have contributed \(\approx2\)--\(5\) BBH merger signals to detections reported in the third gravitational-wave transient catalogue of the LIGO-Virgo-KAGRA Collaboration. We also suggest MT efficiency should be higher during stable Case B MT than during Case A MT.
Most neutron stars (NSs) and black holes (BHs) are believed to be the final
remnants in the evolution of massive stars. In this study, we propose a new
formation channel for the formation of BHs and ...peculiar NSs (specifically,
magnetars and Thorne-$\dot{\rm Z}$ytkow objects T$\dot{\rm Z}$Os), which we
refer to as the core merger-induced collapse (CMIC) model. This model involves
the merger at the end of a common-envelope phase of an oxygen/neon/magnesium
composition white dwarf and the core of a hydrogen-rich or helium-rich
non-degenerate star, leading to the creation of peculiar new types of objects.
The results of binary population synthesis simulations show that the CMIC
channel could make important contributions to the populations of (millisecond)
pulsars, T$\dot{\rm Z}$Os, magnetars and BHs. The possibility of superluminous
supernovae powered by T$\dot{\rm Z}$Os, magnetars and BHs formed through the
CMIC model is also being investigated. Magnetars with immediate matter
surroundings formed after the CMIC might be good sources for fast radio bursts.
The role of recombination during a common-envelope event has been long debated. Many studies have argued that much of hydrogen recombination energy, which is radiated in relatively cool and ...optically-thin layers, might not thermalise in the envelope. On the other hand, helium recombination contains 30% of the total recombination energy, and occurs much deeper in the stellar envelope. We investigate the distinct roles played by hydrogen and helium recombination in a common-envelope interaction experienced by a 12 solar mass red supergiant donor. We perform adiabatic, 3D hydrodynamical simulations that (i) include hydrogen, helium, and molecular hydrogen recombination, (ii) include hydrogen and helium recombination, (iii) include only helium recombination, and (iv) do not include recombination energy. By comparing these simulations, we find that the addition of helium recombination energy alone ejects 30% more envelope mass, and leads to a 16% larger post-plunge-in separation. Under the adiabatic assumption, adding hydrogen recombination energy increases the amount of ejected mass by a further 40%, possibly unbinding the entire envelope, but does not affect the post-plunge separation. Most of the ejecta becomes unbound at relatively high (>70%) degrees of hydrogen ionisation, where the hydrogen recombination energy is likely to expand the envelope instead of being radiated away.
Wolf-Rayet stars in close binary systems can be tidally spun up by their companions, potentially leaving behind fast-spinning highly-magnetized neutron stars, known as "magnetars", after core ...collapse. These newborn magnetars can transfer rotational energy into heating and accelerating the ejecta, producing hydrogen-poor superluminous supernovae (SLSNe). In this Letter, we propose that the magnetar wind of the newborn magnetar could significantly evaporate its companion star, typically a main-sequence or helium star, if the binary system is not disrupted by the SN kick. The subsequent heating and acceleration of the evaporated star material along with the SN ejecta by the magnetar wind can produce a post-peak bump in the SLSN lightcurve. Our model can reproduce the primary peaks and post-peak bumps of four example observed multiband SLSN lightcurves, revealing that the mass of the evaporated material could be \(\sim0.4-0.6\,M_\odot\) if the material is hydrogen-rich. We suggest that the magnetar could induce strongly enhanced evaporation from its companion star near the pericenter if the orbit of the post-SN binary is highly eccentric, ultimately generating multiple post-peak bumps in the SLSN lightcurves. This "magnetar-star binary engine" model offers a possible explanation for the evolution of polarization, along with the origin and velocity broadening of late-time hydrogen or helium broad spectral features observed in some bumpy SLSNe. The diversity in the lightcurves and spectra of SLSNe may be attributed to the wide variety of companion stars and post-SN binary systems.
