Context. Massive stars often evolve in binary systems, and the interactions of these systems significantly affect their evolution. Massive stars in the Galaxy serve as valuable test beds for such interactions due to their proximity. Aims. We computed the evolution of more than 38 000 galactic binary systems with initial primary star masses of 5 − 100 M⊙. In this paper we aim to investigate the surface properties of post-mass-transfer mass donor and mass-gainer stars through core hydrogen burning, core helium burning, and for the pre-supernova stage. Methods. The models were computed with MESA, incorporating detailed stellar and binary physics, including internal differential rotation, magnetic angular momentum transport, mass-dependent overshooting, stellar wind mass-loss, mass and angular momentum transfer, and tidal interaction. They incorporate a new extensive nuclear network for hydrogen burning, which allow us to track the full range of hydrogen burning nucleosynthesis products, from the light elements to aluminum. The widest non-interacting binary models in our grid effectively serve as single-star models. Results. We find that mass gainers and donors may evolve through long-lived blue and yellow supergiant stages during core helium burning, where single stars of the same mass remain red supergiants. Furthermore, some of our mass gainers evolve into more luminous yellow and blue supergiants prior to core collapse than single stars, while some mass donors end their life as red or yellow supergiants, showing a rich diversity in supernova progenitors. We show that the surface elemental and isotopic abundances carry valuable information about a star’s evolutionary history and can be used to distinguish binary interaction products from single stars. Conclusions. Our binary model grid may serve as a tool for identifying post-mass-transfer stars and supernovae. It also holds potential for population studies, supernova modeling, and guidance of future observations.

The evolution of massive stars above 8 M_⊙ depends critically on the amount of mixing above the convective core during the main sequence. However, current models typically extrapolate results from lower-mass stars, where constraints from asteroseismology and eclipsing binary systems are more readily available. A new opportunity to study the evolution of massive stars and their distribution in the Hertzsprung–Russell diagram arises by combining the IACOB spectroscopic sample of over 900 Galactic OB-type stars with Gaia distances. We use this homogeneously analyzed sample to place population-level constraints on main-sequence evolution. We analyze the data by forward modeling stellar evolution tracks with MESA and applying Bayesian inference. This enables us to 1) determine a physically-motivated, data-driven location of the terminal-age main sequence, 2) constrain convective boundary mixing and resulting core masses, and 3) provide a set of massive star models calibrated against modern data. We explore how boundary mixing depends on mass and find that it is well described by a constant overshooting parameter in the mass range of 12 to 40 M_⊙, with α_ov = 0.33 ± 0.02, or f_ov = 0.028 ± 0.003 in the step and exponential overshooting schemes respectively. We find evidence against a continuation of the trend to increase with mass that is found at lower masses. Instead, the data does not exclude a decreasing trend at the high mass end. We find that the resulting helium core masses are 10 to 40

Binary evolution plays a central role in producing rapidly rotating stars. Previous studies have shown that mass gainers in binaries can reach critical rotation after accreting only modest amounts of material, particularly during thermal-timescale Case B mass transfer, in which tidal spin-down is ineffective due to wide orbits. However, such rapid accretion often drives the mass gainer out of thermal equilibrium, and its subsequent spin evolution during thermal relaxation has not been analyzed in depth. In this study, we construct a suite of accreting detailed single-star models with different accretion prescriptions, which inflate and spin up to critical rotation during the accretion. After the accretion has ended, the models relax thermally and deflate. We find that the ratio of surface to critical angular velocity decreases to subcritical values during thermal contraction, with the magnitude of this decrease correlating with the degree of thermal disequilibrium at the end of accretion. This reduction in fractional critical rotation is even stronger when internal angular momentum transport is inefficient. Detailed binary models show the same trend, indicating that the results from our toy single-star models also apply to real binary evolution. Our results highlight that binary mass transfer does not always produce critically rotating stars, but instead may yield a wide range of spin rates depending on the mass transfer and accretion history. Our findings offer new insights into the rotational properties of mass gainers in binaries, stellar merger products, and newly formed massive stars following accretion.

Mass transfer in binary systems is the key process in the formation of various classes of objects, including merging binary black holes (BBHs) and neutron stars. The orbital evolution that occurs during mass transfer depends on how much mass is accreted and how much angular momentum is lost – two of the main uncertainties in binary evolution. This poses a challenge for obtaining reliable predictions from binary channels. Here, we demonstrate that despite these unknowns, a fundamental limit exists to how close binary systems can become via stable mass transfer (SMT) that is robust against uncertainties in orbital evolution. Based on detailed evolutionary models of interacting systems with a BH accretor and a massive-star companion, we show that the post-interaction orbit is always wider than ∼10 R⊙, even when extreme shrinkage due to L2 outflows is assumed. Systems evolving toward tighter orbits become dynamically unstable and result in stellar mergers. This separation limit has direct implications for the properties of BBH mergers, including long delay times (≳1 Gyr) and an absence of high BH spins from the tidal spin-up of helium stars. At high metallicity, the SMT channel may be severely quenched due to Wolf-Rayet winds. We predict BBH mergers from ∼10 M⊙ to 90 M⊙, with case A mass transfer dominating above 40 M⊙. The reason for the separation limit lies in the stellar structure, not in binary physics. If the orbit becomes too narrow during mass transfer, a dynamical instability is triggered by a rapid expansion of the remaining donor envelope due to its near-flat entropy profile. The closest separations can be achieved from core-He burning (∼8−15 R⊙) and Main Sequence donors (∼15−30 R⊙), while Hertzsprung gap donors lead to wider orbits (≳30−50 R⊙) and non-merging BBHs. These outcomes and mass transfer stability are determined by the entropy structures, which are governed by internal composition profiles. Consequently, the formation of BBH mergers and other compact binaries via SMT is a sensitive probe of chemical mixing in stars, and it may help address open questions of stellar astrophysics, such as the blue supergiant problem. Finally, we propose a new simplified treatment of mass transfer stability that more accurately reproduces detailed results and remains flexible under varying assumptions for orbital evolution.

The detected Gaia systems hosting compact objects challenge standard models of binary star evolution. In particular, if the observed black hole (BH) systems evolved in isolation, they are expected to have undergone a mass transfer phase. Given their highly unequal masses, such mass transfer is dynamically unstable within standard models, leading to a stellar merger or a short-period binary. In contrast, the observed systems have much wider orbits than predicted, making their formation within conventional evolutionary frameworks difficult to reconcile. Using detailed binary evolution calculations, we test whether non-conservative mass transfer, in which most of the mass is lost from the system carrying the specific angular momentum of the donor's center of mass, can explain the properties of two Gaia BH systems. This mass-loss geometry differs from standard isotropic re-emission from the accretor's vicinity. We find that our mass-loss geometry model reproduces the orbital periods of the two Gaia BH systems remarkably well over a wide range of initial conditions, offering a plausible formation pathway. We speculate this may point to enhanced eruptive mass loss, potentially driven by high-opacity subsurface layers in the donor prior to Roche-lobe overflow, consistent with preferentially bipolar outflows observed in luminous blue variables. Alternatively, it may indicate the need for more sophisticated mass-transfer prescriptions that account for highly unequal Roche-lobe sizes, sub-synchronous rotation, and possible self-accretion. Similar mechanisms may operate in other post-mass-transfer systems facing analogous evolutionary challenges, including Gaia neutron-star and white-dwarf binaries, stripped-envelope Wolf-Rayet stars, and low-mass X-ray binaries.

Context. Planets orbiting one of the two stars in a binary are vulnerable to gravitational perturbations from the other star. Particularly, highly eccentric companion stars risk disrupting planetary orbits, such as in the extreme system TOI 4633 where close encounters between the companion and a gas giant planet in the habitable zone make it one of the most fragile systems discovered so far. Aims. We assess whether the planet in TOI 4633 can remain stable over the system's lifetime and identify which orbital configurations are compatible with long-term survival, thereby constraining the system's formation and subsequent dynamical history. Methods. We carry out direct N-body integrations of TOI 4633 over Gigayear timescales across observationally allowed orbital parameters to identify regions of dynamical stability. Results. We report that TOI4633's planet likely survived the encounters from the companion star throughout the system's age by orbiting in the opposite direction (retrograde) than the binary, whereby it is stabilised through the Coriolis force. We show it otherwise tends to collide with the binary stars or becomes free-floating after getting ejected. A retrograde planetary orbit has profound implications for TOI 4633's formation and evolution, suggesting an extraordinary history where its stellar companion was perturbed to large eccentricity or randomly captured after planet formation. Alternatively, if stars and planet are born in situ from the same gas clump, we show the planet must have formed at sub-snow-line distances, contrary to the conventional core-accretion model. Conclusions. Our study highlights the importance of considering the long-term stability (greater than or similar to Gyr) of planets in eccentric binaries and demonstrates that the mere existence in such dynamically hostile environments places strong constraints on their orbital configuration and formation.

Type Ib and Ic supernovae (SNe Ib/Ic) are the bright finale of massive stars that have lost their hydrogen envelopes, making them powerful probes of mass stripping in massive star evolution. The advent of modern large photometric and spectroscopic surveys presents a unique opportunity to investigate systematic differences between these two kinds of SNe. In this study, we analyze a large, homogeneous sample of SNe Ib/Ic light curves from the Zwicky Transient Facility. We find a systematic difference in their apparent optical colors at peak: SNe Ib are, on average, bluer than SNe Ic, with two-sample tests confirming that the distributions differ ( p < 0.05). The difference in their host galaxy reddening, as currently constrained, is too small to fully explain it. The color difference therefore most likely has an intrinsic origin, reflecting progenitors with different degrees of stripping – helium-rich for SNe Ib and helium-poor for SNe Ic. In addition, we find that SNe Ib/Ic with narrow lines (SNe Ibn/Icn) are bluer than those without, which might originate from circumstellar matter interaction, with potential connection to fast blue optical transients. We demonstrate that SN colors offer a promising probe of mass stripping in massive stars, potentially providing a useful tool for analyzing large photometric data and improving predictions for the final outcomes of stripped massive stars.

Radiation is crucial not only for observing astrophysical objects, but also for transporting energy and momentum. However, accurate on-the-fly radiation transport in astrophysical simulations is challenging and computationally expensive. Here we introduce AREPO-IDORT (implicit discrete ordinate radiation transport), a scheme coupled to the explicit magnetohydrodynamic (MHD) solver in the 3D moving-mesh code AREPO. The discrete ordinate scheme means that we directly solve the specific intensities in discrete directions. We solve the time-dependent relativistic radiation transport equation via an implicit Jacobi-like iterative finite-volume solver, which overcomes the small radiation time-steps needed by explicit methods. Compared to commonly used moment-based methods, such as flux-limited diffusion (FLD) or M1 closure, this scheme has the advantage of correctly capturing the directions of radiation in both optically thick and thin regions. It is based on the scheme developed for the adaptive mesh refinement code ATHENA++, but we generalise the scheme to support (1) an unstructured moving-mesh, (2) local time-stepping, and (3) general equations of state. We show various test problems that commonly used moment-based methods fail to reproduce accurately. To apply the scheme to a real astrophysics problem, we show the first global 3D radiation hydrodynamic simulation of the entire convective envelope of a red supergiant star. We even marginally resolve the photosphere, which is a known challenge for global 3D simulations of stars. For this problem, the radiation module only takes less than half of the total computational cost. Our current scheme assumes grey radiation, is first-order accurate in both time and space, and is memory-intensive (especially for large cosmological simulations), but we discuss potential avenues for future improvements. We expect that our scheme will enable more accurate multi-scale radiation MHD simulations involving supersonic bulk motions, ranging from planet formation in protoplanetary disks, stars and associated transients, to accretion flows near black holes.

Current explanations of the mass-loss mechanism for stripped-envelope supernovae remain divided between single and binary progenitor systems. Here we obtain deep ultraviolet (UV) imaging with the Hubble Space Telescope (HST) of the Type Ic SN 2012fh to search for the presence of a surviving companion star to the progenitor. We synthesize these observations with archival HST imaging, ground-based spectroscopy, and previous analyses from the literature to provide three independent constraints on the progenitor system. We fit the color-magnitude diagram of the surrounding population to constrain the most likely age of the system to be <20 Myr. Analysis of spectra of SN 2012fh provide an estimate of the He core mass of the progenitor star, >5.6 M_⊙. We analyze deep HST images at the precise location after the SN faded to constrain the luminosity of any remaining main-sequence binary companion to be log(L/L_⊙) ≲ 3.35. Combining observational constraints with current binary population synthesis models excludes the presence of a faint stellar companion to SN 2012fh at the ≲10% level. The progenitor was therefore either effectively isolated at the time of explosion or orbited by a black-hole companion. The latter scenario dominates if we only consider models that produce successful supernovae.

Context. The blue supergiant (BSG) domain contains a large variety of stars whose past and future evolutionary paths are still highly uncertain. Since binary interaction plays a crucial role in the fate of massive stars, investigating the multiplicity among BSGs helps shed light on the fate of such objects. Aims. We aim to estimate the binary fraction of a large sample of BSGs in the Small Magellanic Cloud (SMC) within the Binarity at LOw Metallicity (BLOeM) survey. In total, we selected 262 targets with spectral types B0-B3 and luminosity classes I-II. Methods. This work is based on spectroscopic data collected by the FLAMES instrument, mounted on the Very Large Telescope, which gathered nine epochs over three months. Our spectroscopic analysis for each target includes the individual and peak-to-peak radial velocity measurements, an investigation of the line profile variability, and a periodogram analysis to search for possible short- and long-period binaries. Results. By applying a 20 km s(-1) threshold on the peak-to-peak radial velocities above which we would consider the star to be binary, the resulting observed spectroscopic binary fraction for our BSG sample is 23 +/- 3%. An independent analysis of line profile variability reveals 11 (plus 5 candidates) double-lined spectroscopic binaries and 32 (plus 41 candidates) single-lined spectroscopic binaries. Based on these results, we estimated the overall observed binary fraction in this sample to be 34 +/- 3%, which is close to the computed intrinsic binary fraction of 40 +/- 4%. In addition, we derived reliable orbital periods for 41 spectroscopic binaries and potential binary candidates, among which there are 17 eclipsing binaries, including 20 SB1 and SB2 systems with periods of less than 10 days. We reported a significant drop in the binary fraction of BSGs with spectral types later than B2 and effective temperatures less than 18 kK, which could indicate the end of the main sequence phase in this temperature regime. We found no metallicity dependence in the binary fraction of BSGs, compared to existing spectroscopic surveys of the Galaxy and Large Magellanic Cloud.

Some stars orbiting supermassive black holes (SMBHs) are expected to undergo a gravitational wave (GW)–driven inspiral and initiate mass transfer on nearly circular orbits. However, the stability and duration of such phases remain unexplored. In this work, we focus on the evolution of a low-mass, radiative-envelope subgiant star being stripped by an SMBH. We find that such systems can undergo a long-lasting, stable mass transfer phase, even if none of the angular momentum of the transferred material returns to the orbit to counterbalance the GW-driven decay. We show an example where a 2 M ⊙ subgiant is stripped before entering the Laser Interferometer Space Antenna (LISA) band and loses almost its entire hydrogen envelope. The remaining helium core undergoes a prolonged GW-driven inspiral, becoming a loud LISA source. If formed in our Galaxy, the system would be detectable for several hundred thousand years, ultimately reaching extreme signal-to-noise ratios of a million. Hydrogen shell flashes in the residual envelope cause temporary radial expansions of the stripped star. As a result, a few additional phases of rapid mass transfer occur at orbital periods of 20–30 hr. Eventually, the core possibly undergoes circular partial tidal disruption at an orbital period of ∼10 minutes, corresponding to a GW emission frequency of a few mHz. We estimate a chance of about 1% that such a detectable LISA source exists in our own Galactic center. The loud final GW transient may lead to a few detections reaching as far as ∼1 Gpc, including, e.g., the Abell clusters.

Binary stars and their interactions shape the formation of compact binaries, supernovae, and gravitational-wave sources. The efficiency of mass transfer—the fraction of mass retained by the accretor during binary interaction—is a critical parameter that significantly impacts the final fates of these systems. However, this parameter is observationally poorly constrained, due to the scarcity of well-characterized post-mass-transfer binaries. Be+sdOB binaries, consisting of a rapidly rotating Be star and a stripped hot subdwarf companion, are particularly valuable for studying mass transfer, since they represent clear examples of past binary interaction. Recently, a significantly expanded observational sample of 16 Be+sdOB binaries with well-constrained masses was obtained through combined spectroscopic and interferometric observations. In this work, we compile and analyze this sample, to provide robust constraints on the mass-transfer efficiency in binaries that underwent stable mass transfer during the donor’s hydrogen-shell-burning phase. Our analysis reveals that the mass transfer was predominantly conservative: half of the systems require mass-transfer efficiencies above 50%. This challenges the commonly adopted assumptions of highly nonconservative mass transfer in binary evolution modeling. Our findings are inconsistent with models that account for spinup and limit accretion due to a centrifugal barrier. We also find tension with a commonly used mass-transfer model in rapid population synthesis that limits accretion based on the thermal timescale of the accretor. These results have strong implications for almost all products of binary evolution, including a variety of supernovae, white dwarfs, blue stragglers, runaway stars, X-ray binaries, and gravitational-wave sources.

One of the main challenges in modeling massive stars to the onset of core collapse is the computational bottleneck of nucleosynthesis during advanced burning stages. The number of isotopes formed requires solving a large set of fully coupled stiff ordinary differential equations, making the simulations computationally intensive and prone to numerical instability. To overcome this barrier, we design a nuclear neural network (NNN) framework with multiple hidden layers to emulate nucleosynthesis calculations and conduct a proof of concept to evaluate its performance. The NNN takes the temperature, density, and composition of a burning region as input and predicts the resulting isotopic abundances along with the energy generation and loss rates. We generate training sets for initial conditions corresponding to oxygen core depletion and beyond using large nuclear reaction networks, and compare the predictions of the NNNs to results from a commonly used small net. We find that the NNNs improve the accuracy of the electron fraction by 280%–660%, the average atomic and mass numbers by 150%–360%, and the nuclear energy generation by 250%–750%, consistently outperforming the small network across all time steps. They also achieve significantly better predictions of neutrino losses on relatively short timescales, with improvements ranging from 100% to 1,000,000%. While further work is needed to enhance their accuracy and applicability to different stellar conditions, integrating NNN-trained models into stellar evolution codes is promising for facilitating the large-scale generation of core-collapse supernova progenitors with higher physical fidelity.

Current explanations of the mass-loss mechanism for stripped-envelope supernovae (SNe) remain divided between single and binary progenitor systems. Here we obtain deep ultraviolet (UV) imaging with the Hubble Space Telescope (HST) of the Type Ic SN 2012fh to search for the presence of a surviving companion star to the progenitor. We synthesize these observations with archival HST imaging, ground-based spectroscopy, and previous analyses from the literature to provide three independent constraints on the progenitor system. We fit the color-magnitude diagram of the surrounding population to constrain the most likely age of the system to be <20 Myr. Analysis of spectra of SN 2012fh provides an estimate of the He core mass of the progenitor star, >5.6 M-circle dot. We analyze deep HST images at the precise location after the SN faded to constrain the luminosity of any remaining main-sequence binary companion to be log(L/L-circle dot)less than or similar to 3.35 . Combining observational constraints with current binary population synthesis models excludes the presence of a faint stellar companion to SN 2012fh at the less than or similar to 10% level. The progenitor was therefore either effectively isolated at the time of explosion or orbited by a black-hole companion. The latter scenario dominates if we only consider models that produce successful SNe.

Among the over 200 gravitational wave detections reported so far, GW231123 is a remarkable event that not only holds the record for the most massive black hole merger but also exhibits extreme spins. Its origin is actively debated. Proposed scenarios include dynamical formation through hierarchical mergers, Population III stars, accretion in active galactic nucleus disks and also more exotic explanations including primordial black holes and cosmic strings, each facing different challenges. Recent work showed that the incoming black holes of GW231123 could form from massive, rapidly rotating collapsing helium stars. Here, we address the question how such stars can be formed in very close binary systems and explore chemically homogeneous evolution (CHE) involving progenitors with masses above the pair-instability mass gap. We compute a grid of detailed massive binary models with the stellar evolution code MESA to follow the early evolution of binary progenitors and show that (i) very massive ( M _i > 140 M _⊙ ) CHE binaries at low metallicity ( Z = 10 ^−5 ) naturally produce rapidly rotating progenitors with high masses and high spins matching the properties of the black holes in GW231123 and (ii) the maximum spin of the progenitors is bound by their critical rotation rate, producing a tight correlation between the dimensionless spin and mass, a ∝ M ^−0.9 , in models without any hydrogen left. We conclude that the CHE channel appears to be a viable and natural scenario to produce progenitors. We compare and discuss the differences with earlier studies and comment on the large uncertainties in the final fate and collapse.

Context. The majority of massive stars are born with a close binary companion. How this affects their evolution and fate is still largely uncertain, especially at low metallicity. Aims. We derive synthetic populations of massive post-interaction binary products and compare them with corresponding observed populations in the Small Magellanic Cloud (SMC). Methods. We analyse 53298 detailed binary evolutionary models computed with MESA. Our models include the physics of rotation, mass and angular momentum transfer, magnetic internal angular momentum transport, and tidal spin-orbit coupling. They cover initial primary masses of 5-100 M-circle dot, initial mass ratios of 0.3-0.95, and all initial periods for which interaction is expected, 1-3162 d. They are evolved through the first mass transfer and the donor star death, and a a possible ensuing Be X-ray binary phase, and they end when the mass gainer leaves the main sequence. Results. In our fiducial synthetic population, 8% of the OB stars in the SMC are post-mass-transfer systems, and 7% are merger products. In many of our models, the mass gainers are spun up and expected to form Oe/Be stars. While our model underpredicts the number of Be X-ray binaries in the SMC, it reproduces the main features of their orbital period distribution and the observed number of SMC binary WR stars. We further expect similar to 50 OB+BH binaries below and similar to 170 above the 20 d orbital period. The long-period OB+BH binaries might produce merging double black holes. However, their progenitors, the predicted long-period WR+OB binaries, are not observed. Conlcusions. While the comparison with the observed SMC stars supports many physics assumptions in our high-mass binary models, a better match for the large number of observed OBe stars and Be X-ray binaries likely requires a lower merger rate and/or a higher mass transfer efficiency during the first mass transfer. The fate of the initially wide O star binaries remains particularly uncertain.

Red supergiants (RSGs) represent a late evolutionary stage of massive stars. Recent observations reveal that the observed luminosity range of RSGs in young open clusters is wider than expected from single-star evolution models. Binary evolution effects have been suggested as a possible explanation. Here, we analyze 3670 detailed binary-evolution models, as well as corresponding single-star models, to probe the contribution of binary mass transfer and binary mergers to the luminosity distribution of RSGs in star clusters with ages up to 100 Myr. We confirm that the expected luminosity range of RSGs in a coeval population can span a factor of 10, as a consequence of mergers between two main-sequence stars, which reproduces the observed RSG luminosity ranges in rich clusters well. While the luminosity increase as consequence of mass transfer is more limited, it may help to increase the number of overluminous RSGs. However, our results also demonstrate that binary effects alone are insufficient to account for the number of RSGs found with luminosities of up to 3 times those predicted by current single-star models. We discuss observational accuracy, rotational mixing, age spread, and intrinsic RSG variability as possible explanations. Further observations of RSGs in young open clusters, in particular studies of their intrinsic brightness variability, appear crucial for disentangling these effects.

Historically, various methods have been employed to understand the origin of the elements, including observations of elemental abundances which have been compared to Galactic Chemical Evolution (GCE) models. It is also well known that 1D Local Thermodynamic Equilibrium (LTE) measurements fail to accurately capture elemental abundances. Non-LTE (NLTE) effects may play a significant role, and neglecting them leads to erroneous implications in galaxy modelling. In this paper, we calculate 3D NLTE abundances of seven key iron-peak and neutron-capture elements (Mn, Co, Ni, Sr, Y, Ba, Eu) based on carefully assembled 1D LTE literature measurements, and investigate their impact within the context of the OMEGA+ GCE model. Our findings reveal that 3D NLTE abundances are significantly higher for iron-peak elements at [Fe/H]< -3, with (for the first time ever) [Ni/Fe] and (confirming previous studies) [Co/Fe] on average reaching 0.6-0.8 dex, and [Mn/Fe] reaching -0.1 dex, which current 1D core-collapse supernova (CCSN) models cannot explain. We also observe a slightly higher production of neutron-capture elements at low metallicities, with 3D NLTE abundances of Eu being higher by +0.2 dex at [Fe/H]= -3. 3D effects are most significant for iron-peak elements in the very metal-poor regime, with average differences between 3D NLTE and 1D NLTE reaching up to 0.15 dex. Thus, ignoring 3D NLTE effects introduces significant biases, so including them should be considered whenever possible.

Mass transfer is crucial in binary evolution, yet its theoretical treatment has long relied on analytic models whose key assumptions remain debated. We present a direct and systematic evaluation of these assumptions using high-resolution 3D hydrodynamical simulations including the Coriolis force. We simulate streams overflowing from both the inner and outer Lagrangian points, quantify mass transfer rates, and compare them with analytic solutions. We introduce scaling factors, including the overfilling factor, to render the problem dimensionless. The donor-star models are simplified, with either an isentropic initial stratification and adiabatic evolution or an isothermal structure and evolution. However, the scalability of this formulation allows us to extend the results for a mass-transferring system to arbitrarily small overfilling factors for the adiabatic case. We find that the Coriolis force - often neglected in analytic models - strongly impacts the stream morphology: breaking axial symmetry, reducing the stream cross section, and shifting its origin toward the donor's trailing side. Contrary to common assumptions, the sonic surface is not flat and does not always intersect the Lagrangian point: instead, it is concave and shifted, particularly toward the accretor's trailing side. Despite these structural asymmetries, mass transfer rates are only mildly suppressed relative to analytic predictions and the deviation is remarkably small - within a factor of two (ten) for the inner (outer) Lagrangian point over seven orders of magnitude in mass ratio. We use our results to extend the widely used mass-transfer rate prescriptions by Ritter (1988, A&A, 202, 93) and Kolb & Ritter (1990, A&A, 236, 385), for both the inner and outer Lagrangian points. These extensions can be readily adopted in stellar evolution codes like MESA, with minimal changes where the original models are already in use.

Mass transfer in binary systems is the key process in the formation of various classes of objects, including merging binary black holes (BBHs) and neutron stars. Orbital evolution during mass transfer depends on how much mass is accreted and how much angular momentum is lost - two of the main uncertainties in binary evolution. Here, we demonstrate that, despite these unknowns, a fundamental limit exists to how close binary systems can get via stable mass transfer (SMT), that is robust against uncertainties in orbital evolution. Based on detailed evolutionary models of interacting systems with a BH accretor and a massive star companion, we show that the post-interaction orbit is always wider than ∼10R_⊙, even with extreme shrinkage due to L2 outflows. Systems evolving towards tighter orbits become unstable and result in stellar mergers. This separation limit has direct implications for the properties of BBH mergers: long delay times (≳1 Gyr), and no high BH spins from the tidal spin-up of helium stars. At high metallicity, the SMT channel may be severely quenched due to Wolf-Rayet winds. The reason for the separation limit lies in the stellar structure, not in binary physics. If the orbit gets too narrow during mass transfer, a dynamical instability is triggered by a rapid expansion of the remaining donor envelope due to its near-flat entropy profile. The closest separations can be achieved from core-He burning (∼8-15R_⊙) and Main Sequence donors (∼15-30R_⊙), while Hertzsprung Gap donors lead to wider orbits (≳30-50R_⊙) and non-merging BBHs. These outcomes and mass transfer stability are governed by the internal composition profiles of donor stars. Thus, the formation of compact binaries is a sensitive probe of chemical mixing in stars. We propose a simple treatment of mass transfer stability to reproduce the detailed results.