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Massive stars mainly form in close binaries, where their mutual interactions can profoundly alter their evolutionary paths. Evolved binaries consisting of a massive OB-type main-sequence star with a stripped helium star or a compact companion represent a crucial stage in the evolution towards double compact objects, whose mergers are (potentially) detectable via gravitational waves. The recent detection of X-ray quiet OB+black hole binaries and OB+stripped helium star binaries has set the stage for discovering more of these systems in the near future. In this work, based on 3670 detailed binary-evolution models and using empirical distributions of initial binary parameters, we compute the expected population of such evolved massive binaries in coeval stellar populations, including stars in star clusters and in galaxies with starburst activities, for ages up to 100 Myr. Our results are vividly illustrated in an animation that shows the evolution of these binaries in the color-magnitude diagram over time. We find that the number of OB+black hole binaries peaks around 10 Myr, and OB+neutron star binaries are most abundant at approximately 20 Myr. Both black holes and neutron stars can potentially be found in populations with ages up to 90 Myr. Additionally, we analyze the properties of such binaries at specific ages. We find that OB+helium stars and OB+black hole binaries are likely to be identifiable as single-lined spectroscopic binaries. Our research serves as a guide for future observational efforts to discover such binaries in young star clusters and starburst environments., Comment: 32 pages, 15 figures. Accepted for Publication in Astrophysical Journal Letters (ApJL)

The growing database of gravitational-wave (GW) detections with the binary black holes (BHs) merging in the distant Universe contains subtle insights into their formation scenarios. One of the puzzling properties of detected GW sources is the possible (anti)correlation between mass ratio q of BH-BH binaries and their effective spin. We use rapid binary evolution models to demonstrate that the isolated binary evolution followed by efficient tidal spin-up of stripped helium core produces a similar pattern in Xeff vs q distributions of BH-BH mergers. In our models, the progenitors of unequal BH-BH systems in the stable mass transfer formation scenario are more likely to efficiently shrink their orbits during the second Roche-lobe overflow than the binaries that evolve into nearly equal-mass component systems. This makes it easier for unequal-mass progenitors to enter the tidal spin-up regime and later merge due to GW emission. Our results are, however, sensitive to some input assumptions, especially, the stability of mass transfer and the angular momentum loss during non-conservative mass transfer. We note that mass transfer prescriptions widely adopted in rapid codes favor the formation of BH-BH merger progenitors with unequal masses and moderate separations. We compare our results with detailed stellar model grids and find reasonable agreement after appropriate calibration of the physics models. We anticipate that future detections of unequal-mass BH-BH mergers could provide valuable constraints on the role of the stable mass transfer formation channel. A significant fraction of BH-BH detections with mass ratio q in range (0.4 - 0.7) would be consistent with the mass ratio reversal scenario during the first, relatively conservative mass transfer, and a non-enhanced angular momentum loss during the second, highly non-conservative mass transfer phase., Comment: Version accepted for publication in A&A

For binary systems with an unseen primary and a luminous secondary, the astrometric wobble of the secondary could be used to study the primary. With Gaia, it is possible to measure the mass of the black hole or neutron star with a luminous companion (hereafter BH/NS-LC). Our aim is to provide a method for predicting Gaia's ability in measuring the mass of BH/NS-LCs. We also tried to estimate the number of solvable BH/NS-LCs using Gaia. We used a realistic Markov chain Monte Carlo simulation of mock Gaia observations to obtain a relation between the uncertainty of mass measurement of the primary in BH/NS-LCs with the observable variables of the secondary astrometric orbit. Furthermore, we used the MOBSE code to evolve a Galactic BH/NS-LC sample with a combined Milky Way model. Our relation is applied to this sample to estimate the number of solvable BH/NS-LCs. We derived a good relation between the mass uncertainty and the binary parameters. For the first time, we show the quantitive influence of the period P, inclination i, eccentricity e, and ecliptic latitude $\beta$ to the mass measurement. Our results suggest that $48^{+7}_{-7}$ BH-LCs and $102^{+11}_{10}$ NS-LCs are solvable during a 5 yr Gaia mission. We also give the distribution of the distance and apparent magnitude of the Gaia solvable BH/NS-LCs. This solvable sample would be increased by additional spectroscopic data or a prolonged Gaia mission. The mass uncertainty relation could be used in future simulations of BH/NS-LCs observed by Gaia. The prediction of the solvable BH/NS-LCs is not only influenced by the process in generating the Galactic BH/NS-LC sample, but is also affected by our uncertainty relation. In particular, the relations of parameters such as $[P, e, i, \beta]$ are very useful to correct the selection effect in the statistic results of the future BH/NS-LC sample observed by Gaia.

Properties of the to-date-observed binary black hole (BBH) merger events suggest a preference towards spin-orbit aligned mergers. Naturally, this has caused widespread interest and speculations regrading implications on various merger formation channels. Here we show that (i) not only the BBH-merger population from isolated binaries, but also (ii) BBH population formed in young massive clusters (YMC) would possess an asymmetry in favour of aligned mergers, in the distribution of the events' effective spin parameter ($\chi_{\rm eff}$). In our analysis, we utilize BBH-merger outcomes from state-of-the-art N-body evolutionary models of YMCs and isolated binary population synthesis. We incorporate, for the first time in such an analysis, misalignments due to both natal kicks and dynamical encounters. The YMC $\chi_{\rm eff}$ distribution has a mean (an anti-aligned merger fraction) of $\langle\chi_{\rm eff}\rangle\leq0.04$ ($f_X-\approx40\%$), which is smaller (larger) than but consistent with the observed asymmetry of $\langle\chi_{\rm eff}\rangle\approx0.06$ ($f_X-\approx28\%$) as obtained from the population analysis by the LIGO-Virgo-KAGRA collaboration. In contrast, isolated binaries alone tend to produce a much stronger asymmetry; for the tested physical models, $\langle\chi_{\rm eff}\rangle\approx0.25$ and $f_X-\lesssim7\%$. Although the YMC $\chi_{\rm eff}$ distribution is more similar to the observed counterpart, none of the channels correctly reproduce the observed distribution. Our results suggest that further extensive model explorations for both isolated-binary and dynamical channels as well as better observational constraints are necessary to understand the physics of 'the symmetry breaking' of the BBH-merger population., Comment: 20 pages including Appendix, 7 figures, 3 tables. Extended results and discussions; main conclusions are unchanged. Accepted for publication in ApJ

Our understanding of the convective-engine paradigm driving core-collapse supernovae has been used for 2 decades to predict the remnant mass distribution from stellar collapse. These predictions improve as our understanding of this engine increases. In this paper, we review our current understanding of convection (in particular, the growth rate of convection) in stellar collapse and study its effect on the remnant mass distribution. We show how the depth of the mass gap between neutron stars and black holes can help probe this convective growth. We include a study of the effects of stochasticity in both the stellar structure and the convective seeds caused by stellar burning. We study the role of rotation and its effect on the pair-instability mass gap. Under the paradigm limiting stellar rotation to those stars in tight binaries, we determine the effect of rotation on the remnant mass distribution., Comment: 17 pages, accepted by ApJ