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Thorne–Żytkow objects (TŻO) are potential end products of the merger of a neutron star with a non-degenerate star. In this work, we have computed the first grid of evolutionary models of TŻOs with the MESA stellar evolution code. With these models, we predict several observational properties of TŻOs, including their surface temperatures and luminosities, pulsation periods, and nucleosynthetic products. We expand the range of possible TŻO solutions to cover $3.45 \lesssim \rm {\log \left(T_{eff}/K\right)}\lesssim 3.65$ and $4.85 \lesssim \rm {\log \left(L/L_{\odot }\right)}\lesssim 5.5$. Due to the much higher densities our TŻOs reach compared to previous models, if TŻOs form we expect them to be stable over a larger mass range than previously predicted, without exhibiting a gap in their mass distribution. Using the GYRE stellar pulsation code we show that TŻOs should have fundamental pulsation periods of 1000–2000 d, and period ratios of ≈0.2–0.3. Models computed with a large 399 isotope fully coupled nuclear network show a nucleosynthetic signal that is different to previously predicted. We propose a new nucleosynthetic signal to determine a star’s status as a TŻO: the isotopologues $\mathrm{^{44}Ti} \rm {O}_2$ and $\mathrm{^{44}Ti} \rm {O}$, which will have a shift in their spectral features as compared to stable titanium-containing molecules. We find that in the local Universe (∼SMC metallicities and above) TŻOs show little heavy metal enrichment, potentially explaining the difficulty in finding TŻOs to-date.

 

Abstract Common envelope (CE) evolution is an outstanding open problem in stellar evolution, critical to the formation of compact binaries including gravitational-wave sources. In the “classical” isolated binary evolution scenario for double compact objects, the CE is usually the second mass transfer phase. Thus, the donor star of the CE is the product of a previous binary interaction, often stable Roche lobe overflow (RLOF). Because of the accretion of mass during the first RLOF, the main-sequence core of the accretor star grows and is “rejuvenated.” This modifies the core-envelope boundary region and decreases significantly the envelope binding energy for the remaining evolution. Comparing accretor stars from self-consistent binary models to stars evolved as single, we demonstrate that the rejuvenation can lower the energy required to eject a CE by ∼42%–96% for both black hole and neutron star progenitors, depending on the evolutionary stage and final orbital separation. Therefore, binaries experiencing first stable mass transfer may more easily survive subsequent CE events and result in possibly wider final separations compared to current predictions. Despite their high mass, our accretors also experience extended “blue loops,” which may have observational consequences for low-metallicity stellar populations and asteroseismology. 

Future searches for gravitational waves from space will be sensitive to double compact objects in our Milky Way. We present new simulations of the populations of double black holes (BHBHs), BH neutron stars (BHNSs), and double neutron stars (NSNSs) that will be detectable by the planned space-based gravitational-wave detector called Laser Interferometer Space Antenna (LISA). For our estimates, we use an empirically informed model of the metallicity-dependent star formation history of the Milky Way. We populate it using an extensive suite of binary population-synthesis predictions for varying assumptions relating to mass transfer, common-envelope, supernova kicks, remnant masses, and wind mass-loss physics. For a 4(10) yr LISA mission, we predict between 30–370(50–550) detections over these variations, out of which 6–154 (9–238) are BHBHs, 2–198 (3–289) are BHNSs, and 3–35 (4–57) are NSNSs. We expect that about 50% (60%) can be distinguished from double white dwarf sources based on their mass or eccentricity and localization. Specifically, for about 10% (15%), we expect to be able to determine chirp masses better than 10%. For 13% (13%), we expect sky-localizations better than 1°. We discuss how the variations in the physics assumptions alter the distribution of properties of the detectable systems, even when the detection rates are unchanged. We further discuss the possibility of multimessenger observations of pulsar populations with the Square Kilometre Array and assess the benefits of extending the LISA mission.

 

Gravitational-wave (GW) detections are starting to reveal features in the mass distribution of double compact objects. The lower end of the black hole (BH) mass distribution is especially interesting as few formation channels contribute here and because it is more robust against variations in the cosmic star formation than the high-mass end. In this work we explore the stable mass transfer channel for the formation of GW sources with a focus on the low-mass end of the mass distribution. We conduct an extensive exploration of the uncertain physical processes that impact this channel. We note that, for fiducial assumptions, this channel reproduces the peak at ∼9M_☉in the GW-observed binary BH mass distribution remarkably well and predicts a cutoff mass that coincides with the upper edge of the purported neutron star–black hole (NS–BH) mass gap. The peak and cutoff mass are a consequence of the unique properties of this channel; namely (1) the requirement of stability during the mass transfer phases, and (2) the complex way in which the final compact object masses scale with the initial mass. We provide an analytical expression for the cutoff in the primary component mass and show that this adequately matches our numerical results. Our results imply that selection effects resulting from the formation channel alone can provide an explanation for the purported NS–BH mass gap in GW detections. This provides an alternative to the commonly adopted view that the gap emerges during BH formation.

 

Gravitational-wave detectors are starting to reveal the redshift evolution of the binary black hole (BBH) merger rate,R_BBH(z). We make predictions forR_BBH(z) as a function of black hole mass for systems originating from isolated binaries. To this end, we investigate correlations between the delay time and black hole mass by means of the suite of binary population synthesis simulations,COMPAS. We distinguish two channels: the common envelope (CE), and the stable Roche-lobe overflow (RLOF) channel, characterized by whether the system has experienced a common envelope or not. We find that the CE channel preferentially produces BHs with masses below about 30M_⊙and short delay times (t_delay≲ 1 Gyr), while the stable RLOF channel primarily forms systems with BH masses above 30M_⊙and long delay times (t_delay≳ 1 Gyr). We provide a new fit for the metallicity-dependent specific star formation rate density based on the Illustris TNG simulations, and use this to convert the delay time distributions into a prediction ofR_BBH(z). This leads to a distinct redshift evolution ofR_BBH(z) for high and low primary BH masses. We furthermore find that, at high redshift,R_BBH(z) is dominated by the CE channel, while at low redshift, it contains a large contribution (∼40%) from the stable RLOF channel. Our results predict that, for increasing redshifts, BBHs with component masses above 30M_⊙will become increasingly scarce relative to less massive BBH systems. Evidence of this distinct evolution ofR_BBH(z) for different BH masses can be tested with future detectors.