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Both detailed and rapid population studies alike predict that binary black hole (BHBH) formation is orders of magnitude more efficient at low metallicity than high metallicity, while binary neutron star (NSNS) formation remains mostly flat with metallicity, and black hole-neutron star (BHNS) mergers show intermediate behavior. This finding is a key input to employ double compact objects as tracers of low-metallicity star formation, as spectral sirens, and for merger rate calculations. Yet, the literature offers various (sometimes contradicting) explanations for these trends. We investigate the dominant cause for the metallicity dependence of double compact object formation. We find that the BHBH formation efficiency at low metallicity is set by initial condition distributions, and conventional simulations suggest that about \textit{one in eight interacting binary systems} with sufficient mass to form black holes will lead to a merging BHBH. We further find that the significance of metallicities in double compact object formation is a question of formation channel. The stable mass transfer and chemically homogeneous evolution channels mainly diminish at high metallicities due to changes in stellar radii, while the common envelope channel is primarily impacted by the combined effects of stellar winds and mass-scaled natal kicks. Outdated giant wind prescriptions exacerbate the latter effect, suggesting BHBH formation may be much less metallicity dependent than previously assumed. NSNS formation efficiency remains metallicity independent as they form exclusively through the common envelope channel, with natal kicks that are assumed uncorrelated with mass. Forthcoming GW observations will provide valuable constraints on these findings., Comment: 15 pages, 8 Figures, Submitted to ApJ, Scripts and data to reproduce this work are at https://github.com/LiekeVanSon/ZdependentFormEff , and https://zenodo.org/records/13999532

Surveys in the Milky Way and Large Magellanic Cloud revealed that the majority of massive stars will interact with companions during their lives. However, knowledge of the binary properties of massive stars at low metallicity, which approaches the conditions of the Early Universe, remains sparse. We present the Binarity at LOw Metallicity (BLOeM) campaign - an ESO large programme designed to obtain 25 epochs of spectroscopy for 929 massive stars in the SMC - the lowest metallicity conditions in which multiplicity is probed to date (Z = 0.2 Zsun). BLOeM will provide (i) the binary fraction, (ii) the orbital configurations of systems with periods P < 3 yr, (iii) dormant OB+BH binaries, and (iv) a legacy database of physical parameters of massive stars at low metallicity. The stars are observed with the LR02 setup of the giraffe instrument of the Very Large Telescope (3960-4570A, resolving power R=6200; typical signal-to-noise ratio S/N=70-100). This paper utilises the first 9 epochs obtained over a three-month time. We describe the survey and data reduction, perform a spectral classification of the stacked spectra, and construct a Hertzsprung-Russell diagram of the sample via spectral-type and photometric calibrations. The sample covers spectral types from O4 to F5, spanning the effective temperature and luminosity ranges 6.5

Chemically homogeneous evolution (CHE) is a promising channel for forming massive binary black holes. The enigmatic, massive Wolf-Rayet (WR) binary HD 5980 A&B has been proposed to have formed through this channel. We investigate this claim by comparing its observed parameters with CHE models. Using MESA, we simulate grids of close massive binaries then use a Bayesian approach to compare them with the stars' observed orbital period, masses, luminosities, and hydrogen surface abundances. The most probable models, given the observational data, have initial periods ~3 days, widening to the present-day ~20 day orbit as a result of mass loss -- correspondingly, they have very high initial stellar masses ($\gtrsim$150 M$_\odot$). We explore variations in stellar wind-mass loss and internal mixing efficiency, and find that models assuming enhanced mass-loss are greatly favored to explain HD 5980, while enhanced mixing is only slightly favoured over our fiducial assumptions. Our most probable models slightly underpredict the hydrogen surface abundances. Regardless of its prior history, this system is a likely binary black hole progenitor. We model its further evolution under our fiducial and enhanced wind assumptions, finding that both stars produce black holes with masses ~19-37 M$_\odot$. The projected final orbit is too wide to merge within a Hubble time through gravitational waves alone. However, the system is thought to be part of a 2+2 hierarchical multiple. We speculate that secular effects with the (possible) third and fourth companions may drive the system to promptly become a gravitational-wave source., Comment: 23 pages, 9 figures, 2 tables, 2 appendices. Accepted for publication in ApJ

Current observations of binary black-hole ({BBH}) merger events show support for a feature in the primary BH-mass distribution at $\sim\,35\,\mathrm{M}_{\odot}$, previously interpreted as a signature of pulsational pair-instability (PPISN) supernovae. Such supernovae are expected to map a wide range of pre-supernova carbon-oxygen (CO) core masses to a narrow range of BH masses, producing a peak in the BH mass distribution. However, recent numerical simulations place the mass location of this peak above $50\,\mathrm{M}_{\odot}$. Motivated by uncertainties in the progenitor's evolution and explosion mechanism, we explore how modifying the distribution of BH masses resulting from PPISN affects the populations of gravitational-wave (GW) and electromagnetic (EM) transients. To this end, we simulate populations of isolated {BBH} systems and combine them with cosmic star-formation rates. Our results are the first cosmological BBH-merger predictions made using the \textsc{binary\_c} rapid population synthesis framework. We find that our fiducial model does not match the observed GW peak. We can only explain the $35\,\mathrm{M}_{\odot}$ peak with PPISNe by shifting the expected CO core-mass range for PPISN downwards by $\sim{}15\,\mathrm{M}_{\odot}$. Apart from being in tension with state-of-the art stellar models, we also find that this is likely in tension with the observed rate of hydrogen-less super-luminous supernovae. Conversely, shifting the mass range upward, based on recent stellar models, leads to a predicted third peak in the BH mass function at $\sim{}64\,\mathrm{M}_{\odot}$. Thus we conclude that the $\sim{}35\,\mathrm{M}_{\odot}$ feature is unlikely to be related to PPISNe., Comment: Accepted for publication in MNRAS. 19 pages, 8 figures includings appendices

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 $\sim9 \mathrm{M_{\odot}}$ 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 BH mass gap. The peak and cutoff mass are a consequence of 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 neutron star--BH mass gap in GW detections. This provides an alternative to the commonly adopted view that the gap emerges during BH formation., Comment: Accepted for publication in ApJ associated code is available at https://github.com/LiekeVanSon/LowMBH_and_StableChannel

New observational facilities are probing astrophysical transients such as stellar explosions and gravitational wave (GW) sources at ever increasing redshifts, while also revealing new features in source property distributions. To interpret these observations, we need to compare them to predictions from stellar population models. Such models require the metallicity-dependent cosmic star formation history ($\mathcal{S}(Z,z)$) as an input. Large uncertainties remain in the shape and evolution of this function. In this work, we propose a simple analytical function for $\mathcal{S}(Z,z)$. Variations of this function can be easily interpreted, because the parameters link to its shape in an intuitive way. We fit our analytical function to the star-forming gas of the cosmological TNG100 simulation and find that it is able to capture the main behaviour well. As an example application, we investigate the effect of systematic variations in the $\mathcal{S}(Z,z)$ parameters on the predicted mass distribution of locally merging binary black holes (BBH). Our main findings are: I) the locations of features are remarkably robust against variations in the metallicity-dependent cosmic star formation history, and II) the low mass end is least affected by these variations. This is promising as it increases our chances to constrain the physics that governs the formation of these objects., Comment: Accepted for publication in ApJ, 6 Figures, code available at https://github.com/LiekeVanSon/SFRD_fit, made using showyourwork

Population synthesis relies on semi-analytic formulae to determine masses of compact objects from the (helium or carbon-oxygen) cores of collapsing stars. Such formulae are combined across mass ranges that span different explosion mechanisms, potentially introducing artificial features in the compact object mass distribution. Such artifacts impair the interpretation of gravitational-wave observations. We propose a "top-down" remnant mass prescription where we remove mass from the star for each possible mass-loss mechanism, instead of relying on the fallback onto a "proto-compact-object" to get the final mass. For one of these mass-loss mechanisms, we fit the metallicity-dependent mass lost to pulsational-pair instability supernovae from numerical simulations. By imposing no mass loss in the absence of pulses, our approach recovers the existing compact object masses prescription at the low mass end and ensures continuity across the core-collapse/pulsational-pair-instability regime. Our remnant mass prescription can be extended to include other mass-loss mechanisms at the final collapse., Comment: to appear in RNAAS, made with showyourwork, code available at https://github.com/mathren/top_down_compact_obj_mass

Gravitational wave detectors are starting to reveal the redshift evolution of the binary black hole (BBH) merger rate, $R_{\mathrm{BBH}}(z)$. We make predictions for $R_{\mathrm{BBH}}(z)$ 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, characterised by whether the system has experienced a common envelope or not. We find that the CE channel preferentially produces BHs with masses below about $30\rm{M}_{\odot}$ and short delay times ($t_{\rm delay} \lesssim 1$Gyr), while the stable RLOF channel primarily forms systems with BH masses above $30\rm{M}_{\odot}$ and long delay times ($t_{\rm delay} \gtrsim 1$Gyr). We provide a new fit for the metallicity specific star-formation rate density based on the Illustris TNG simulations, and use this to convert the delay time distributions into a prediction of $R_{\mathrm{BBH}}(z)$. This leads to a distinct redshift evolution of $R_{\mathrm{BBH}}(z)$ for high and low primary BH masses. We furthermore find that, at high redshift, $R_{\mathrm{BBH}}(z)$ is dominated by the CE channel, while at low redshift it contains a large contribution ($\sim 40\%$) from the stable RLOF channel. Our results predict that, for increasing redshifts, BBHs with component masses above $30\rm{M}_{\odot}$ will become increasingly scarce relative to less massive BBH systems. Evidence of this distinct evolution of $R_{\mathrm{BBH}}(z)$ for different BH masses can be tested with future detectors., Comment: 20 pages + appendices, data is available at https://doi.org/10.5281/zenodo.5544170

Understanding common envelope (CE) evolution is an outstanding problem in binary evolution. Although the CE phase is not driven by gravitational-wave (GW) emission, the in-spiraling binary emits GWs that passively trace the CE dynamics. Detecting this GW signal would provide direct insight into the gas-driven physics. Even a non-detection might offer invaluable constraints. We investigate the prospects of detection of a Galactic CE by LISA. While the dynamical phase of the CE is likely sufficiently loud for detection, it is short and thus rare. We focus instead on the self-regulated phase that proceeds on a thermal timescale. Based on population synthesis calculations and the (unknown) signal duration in the LISA band, we expect $\sim 0.1-100$ sources in the Galaxy during the mission duration. We map the GW observable parameter space of frequency $f_\mathrm{GW}$ and its derivative $\dot f_\mathrm{GW}$ remaining agnostic on the specifics of the inspiral, and find that signals with $\mathrm{SNR}>10$ are possible if the CE stalls at separations such that $f_\mathrm{GW}\gtrsim2\times10^{-3}\,\mathrm{Hz}$. We investigate the possibility of misidentifying the signal with other known sources. If the second derivative $\ddot f_\mathrm{GW}$ can also be measured, the signal can be distinguished from other sources using a GW braking-index. Alternatively, coupling LISA with electromagnetic observations of peculiar red giant stars and/or infrared and optical transients might allow for the disentangling of a Galactic CE from other Galactic and extra-galactic GW sources., Comment: Accepted by ApJ -- author's version. Code available at: https://github.com/tcallister/LISA-and-CE-Evolution

The theory for single stellar evolution predicts a gap in the mass distribution of black holes (BHs) between approximately 45-130M$_{\odot}$, the so-called "pair-instability mass gap". We examine whether BHs can pollute the gap after accreting from a stellar companion. To this end, we simulate the evolution of isolated binaries using a population synthesis code, where we allow for super-Eddington accretion. Under our most extreme assumptions, we find that at most about 2% of all merging binary BH systems contains a BH with a mass in the pair-instability mass gap, and we find that less than 0.5% of the merging systems has a total mass larger than 90M$_{\odot}$. We find no merging binary BH systems with a total mass exceeding 100M$_{\odot}$. We compare our results to predictions from several dynamical pathways to pair-instability mass gap events and discuss the distinguishable features. We conclude that the classical isolated binary formation scenario will not significantly contribute to the pollution of the pair-instability mass gap. The robustness of the predicted mass gap for the isolated binary channel is promising for the prospective of placing constraints on (i) the relative contribution of different formation channels, (ii) the physics of the progenitors including nuclear reaction rates, and (iii), tentatively, the Hubble parameter., Comment: 20 pages, 9 Figures, to be published in ApJ

The present paper proposes a novel model for estimating the free-volume size of porous materials based on the analysis of various experimental ortho-positronium ($o$-Ps) lifetime data. The model is derived by combining the semi-classical (SE) physics model, which works in the region of large pores (pore size $R >$ 1 nm), with the conventional Tao-Eldrup (TE) model, which is applicable only for the small-pore region ($R <$ 1 nm). Thus, the proposed model, called the hybrid (HYB) model, is able to smoothly connect the $o$-Ps lifetimes in the two regions of the pore. Moreover, by introducing the $o$-Ps diffusion probability parameter ($D$), the HYB model has reproduced quite well the experimental $o$-Ps lifetimes in the whole region of pore sizes. It is even in a better agreement with the experimental data than the most up-to-date rectangular TE (RTE) and Tokyo models. In particular, by adjusting the value of $D$, the HYB model can also describe very well the two defined sets of experimental $o$-Ps lifetimes in the pores with spherical and channel geometries. The merit of the present model, in comparison with the previously proposed ones, is that it is applicable for the pore size in the universal range of $0.2 - 400$ nm for most of porous materials with different geometries., Comment: 18 pages, 4 figures, accepted in Radiation Physics and Chemistry

Focusing on stochastic systems arising in mean-field models, the systems under consideration belong to the class of switching diffusions, in which continuous dynamics and discrete events coexist and interact. The discrete events are modeled by a continuous-time Markov chain. Different from the usual switching diffusions, the systems include mean-field interactions. Our effort is devoted to obtaining laws of large numbers for the underlying systems. One of the distinct features of the paper is the limit of the empirical measures is not deterministic but a random measure depending on the history of the Markovian switching process. A main difficulty is that the standard martingale approach cannot be used to characterize the limit because of the coupling due to the random switching process. In this paper, in contrast to the classical approach, the limit is characterized as the conditional distribution (given the history of the switching process) of the solution to a stochastic McKean-Vlasov differential equation with Markovian switching., Comment: 34 pages

Liquids with quasi - chemical bonding between molecules are described in terms of vertex model. It is shown that this bonding results in liquid - liquid phase transition, which occurs between phases with different mean density of intermolecular bonds. The transition may be suggested to be a universal phenomena for those liquids., Comment: 12 pages, 3 figures

The pressure - temperature phase diagrams of Se and S are calculated. Both melting and polymorphous phase transition are described in the frames of statistical Patashinski model. The results are in good agreement with experimental data of Brazhkin et. al., Comment: 3 eps figures, will appear in Physica A, mail to first author ldson@mail.ru

While both La$_{0.7}$Sr$_{0.3}$MnO$_3$ and La$_{0.7}$Sr$_{0.3}$CoO$_3$ are ferromagnets with metallic conductivity below $T_\mathrm{c}$ (360K and 220K, respectively), partial substitutions of Co by Mn in La$_{0.7}$Sr$_{0.3}$Co$_{1-y}$Mn$_y$O$_3$ ($y < 0.1$) drastically suppress the ferromagnetic long-range order pre-established by Co-O-Co double exchange and tune the conductivity towards insulating behavior. Since Mn-O-Mn double-exchange interactions are avoidable at low Mn-substitution levels, the deterioration of ferromagnetism and conductivity thus provides evidence for no Mn-O-Co double exchange (but antiferromagnetic superexchange) in the present system. At $y = 0.1$, the ferromagnetism is no longer observed; the system becomes an insulating spin-glass with $T_\mathrm{g}\approx 62$K as estimated from the ac-susceptibility data using the conventional critical slowing-down scaling law. With further substitution ($y\geq 0.3$), the ferromagnetism is recovered ($T_\mathrm{c} = 165$K for $y = 0.3$ and 200K for $y = 0.5$) while the resistivity continues increasing and exhibits insulating behavior. These results indicate that the substitution is not simply a mixture of La$_{0.7}$Sr$_{0.3}$MnO$_3$ and La$_{0.7}$Sr$_{0.3}$CoO$_3$, but produces a more complicated scenario of spin states, interactions and disorder., Comment: RevTeX4, 5 pages, 4 figures, unpublished