HR6819: a puffed-up stripped star system challenging stable mass transfer theory

We present Chandra and Swift X-ray observations of four extremely low-mass\n(ELM) white dwarfs with massive companions. We place stringent limits on X-ray\nemission from all four systems, indicating that neutron star companions are\nextremely unlikely and that the companions are almost certainly white dwarfs.\nGiven the observed orbital periods and radial velocity amplitudes, the total\nmasses of these binaries are greater than 1.02 to 1.39 Msun. The extreme mass\nratios between the two components make it unlikely that these binary white\ndwarfs will merge and explode as Type Ia or underluminous supernovae. Instead,\nthey will likely go through stable mass transfer through an accretion disk and\nturn into interacting AM CVn. Along with three previously known systems, we\nidentify two of our targets, J0811 and J2132, as systems that will definitely\nundergo stable mass transfer. In addition, we use the binary white dwarf sample\nfrom the ELM Survey to constrain the inspiral rate of systems with extreme mass\nratios. This rate, 0.00017/year, is consistent with the AM CVn space density\nestimated from the Sloan Digital Sky Survey. Hence, stable mass transfer double\nwhite dwarf progenitors can account for the entire AM CVn population in the\nGalaxy.\n

Many exoplanets have now been detected in orbits with ultra-short periods, very close to the Roche limit. Building upon our previous work, we study the possibility that mass loss through Roche lobe overflow (RLO) may affect the evolution of these planets, and could possibly transform a hot Jupiter into a lower-mass planet (hot Neptune or super-Earth). We focus here on systems in which the mass loss occurs slowly ("stable mass transfer" in the language of binary star evolution) and we compute their evolution in detail with the binary evolution code MESA. We include the effects of tides, RLO, irradiation and photo-evaporation of the planet, as well as the stellar wind and magnetic braking. Our calculations all start with a hot Jupiter close to its Roche limit, in orbit around a sun-like star. The initial orbital decay and onset of RLO are driven by tidal dissipation in the star. We confirm that such a system can indeed evolve to produce lower-mass planets in orbits of a few days. The RLO phase eventually ends and, depending on the details of the mass transfer and on the planetary core mass, the orbital period can remain around a few days for several Gyr. The remnant planets have a rocky core and some amount of envelope material, which is slowly removed via photo-evaporation at nearly constant orbital period; these have properties resembling many of the observed super-Earths and sub-Neptunes. For these remnant planets we also predict an anti-correlation between mass and orbital period; very low-mass planets ($M_{\rm pl}\,\lesssim\,5\,M_{\oplus}$) in ultra-short periods ($P_{\rm orb}$<1d) cannot be produced through this type of evolution.

The masses of the white dwarfs in a binary carry information about previous mass-transfer phases. The core mass -- radius relation of low-mass giants gives the size of the progenitor of a helium white dwarf at the moment it last filled its Roche lobe. Previously, we used this information for a few observed systems to propose a new mass-transfer type based on an angular momentum balance. Our aim is to investigate if stable mass transfer instead of the angular-momentum prescription is consistent with the observed double-helium white-dwarf masses. We reconstructed the progenitor evolution of observed double-helium white dwarfs using the core mass -- radius relation and evaluated if the periods at the start of the second phases of mass transfer are consistent with the outcome of stable mass transfer. More generally, we calculated the mass distribution of double-helium white dwarfs for three different progenitor scenarios: double common envelope (with parameter α łambda), angular-momentum prescription (with parameter γ), and stable mass transfer. We find that the observed systems are generally not consistent with stable mass transfer. Stable mass transfer leads to a tight correlation between the two white dwarf masses in a binary that is not consistent with the observed mass distribution. Double-common-envelope evolution is a particularly poor fit to the observations. The angular-momentum prescription can populate the observed mass distribution, but not perfectly. We conclude that the first phase of mass transfer initiated on the red giant branch in low-mass systems does not generally proceed as stable mass transfer nor as a common envelope, and thus it is poorly understood. This may be related to the fact that for many observed binaries that have finished the first phase of mass transfer the orbit is eccentric, which is an unexpected outcome of mass transfer.

Most close double helium white dwarfs will merge within a Hubble time due to orbital decay by gravitational wave radiation. However, a significant fraction with low mass ratios will survive for a long time as a consequence of stable mass transfer. Such stable mass transfer between two helium white dwarfs (HeWDs) provides one channel for the production of AM CVn binary stars. In previous calculations of double HeWD progenitors, the accreting HeWD was treated as a point mass. We have computed the evolution of 16 double HeWD models in order to investigate the consequences of treating the evolution of both components in detail. We find that the boundary between binaries having stable and unstable mass transfer is slightly modified by this approach. By comparing with observed periods and mass ratios, we redetermine masses of eight known AM CVn stars by our double HeWDs channel, i.e. HM Cnc, AM CVn, V406 Hya, J0926, J1240, GP Com, Gaia14aae and V396 Hya.We propose that central spikes in the triple-peaked emission spectra of J1240, GP Com and V396 Hya and the surface abundance ratios of N/C/O in GP Com can be explained by the stable double HeWD channel. The mass estimates derived from our calculations are used to discuss the predicted gravitational wave signal in the context of the Laser Interferometer Space Antenna (LISA) project.

A suggested model for quasi-periodic eruptions (QPEs) from galaxy nuclei invokes a white dwarf in an eccentric orbit about the central massive black hole. I point out that the extreme mass ratio allows the presence of strong Lindblad resonances in the accretion disc. These are important for the stability of mass transfer and may trigger the eruptions themselves by rapidly transferring angular momentum from the accretion disc (which is likely to be eccentric itself) to the orbiting WD companion at pericentre. I consider the effects of von Zeipel–Lidov–Kozai (ZLK) cycles caused by a perturber on a more distant orbit about the central black hole. I show that ZLK cycles can change the orbital periods of QPE systems on time-scales much shorter than the mass transfer time, as seen in ASASSN-14ko, and produce correlated short-term variations of their mass transfer rates and orbital periods, as recently observed in GSN 069. Further monitoring of these sources should constrain the parameters of any perturbing companions. This, in turn, may constrain the nature of the events creating QPE systems and perhaps give major insights into how the central black holes in low-mass galaxies are able to grow.

Most subdwarf B (sdB) + Helium white dwarf (He WD) binaries are believed to be formed from a particular channel. In this channel, the He WDs are produced first from red giants (RGs) with degenerate cores via stable mass transfer and sdB stars are produced from RGs with degenerate cores via common-envelope (CE) ejection. They are important for the studies of CE evolution, binary evolution, and binary population synthesis. However, the relation between WD mass and orbital period of sdB + He WD binaries has not been specifically studied. In this paper, we first use a semi-analytic method to follow their formation and find a WD mass and orbital period relation. Then we use a detailed stellar evolution code to model their formation from main-sequence binaries. We find a similar relation between the WD mass and orbital period, which is in broad agreement with observations. For most sdB + He WD systems, if the WD mass (orbital period) can be determined, the orbital period (WD mass) can be inferred with this relation and then the inclination angle can be constrained with the binary mass function. In addition, we can also use this relation to constrain the CE ejection efficiency and find that a relative large CE ejection efficiency is favoured. If both the WD and sdB star masses can be determined, the critical mass ratios of dynamically unstable mass transfer for RG binaries can also be constrained.

Absolute parameters of four W UMa stars with extreme low mass ratios

This study presents multi-band photometric observations and detailed period analysis of a totally eclipsing binary system exhibiting low photometric amplitude. The system exhibits characteristic W Ursae Majoris (EW)-type light curves with complete eclipses. In our light curve modeling, we tested two setups: one excluding third light and the other including it as a free parameter (accounting for a potential tertiary component). Photometric analysis reveals that ASASSN-V J171815.10+450432.9 (hereafter J171815) represents a marginal contact binary system with an extreme mass ratio (the more massive component is designated as the primary star), approaching the theoretical lower limit for stable contact configurations. Furthermore, our investigation of orbital period variations uncovers a long-term period increase at a rate of dPdt=(1.08±0.05)×10−6dayyr−1, which is likely attributable to ongoing mass transfer between components. This interpretation aligns with the system’s geometric configuration and observed light curve asymmetries. The unique characteristics presented by this binary system serve as a rare opportunity for in-depth research on the mass ratio theory, and also provide an important opportunity for testing the Thermal Relaxation Oscillation (TRO) theory.

A binary in which a slightly evolved star starts mass transfer to a neutron star can evolve towards ultra-short orbital periods under the influence of magnetic braking. This is called magnetic capture. We investigate in detail for which initial orbital periods and initial donor masses binaries evolve to periods less than 30–40 min within the Hubble time. We show that only small ranges of initial periods and masses lead to ultra-short periods, and that for those only a small time interval is spent at ultra-short periods. Consequently, only a very small fraction of any population of X-ray binaries is expected to be observed at ultra-short period at any time. If 2 to 6 of the 13 bright X-ray sources in globular clusters have an ultra-short period, as suggested by recent observations, their formation cannot be explained by the magnetic capture model.

Context. Massive binaries hosting a black hole (OB+BH) represent a critical phase in the production of BH mergers in the context of binary evolution. In spite of this, such systems have so far largely avoided detection. Single-lined spectroscopic (SB1) O-type binaries are ideal objects to search for elusive BH companions. Moreover, SB1 binaries hosting two main sequence stars probe a regime of more extreme mass ratios and longer periods compared to double-lined binaries (SB2), and they are thus valuable for establishing the natal mass ratio distribution of massive stars. Aims. We characterise the hidden companions in 51 SB1 O-type and evolved B-type binaries identified in the Large Magellanic Cloud (LMC) in the framework of the VLT-FLAMES Tarantula Survey (VFTS) and its follow-up, the Tarantula Massive Binary Monitoring (TMBM). The binaries cover periods between a few days to years (0 &lt; log P &lt; 3 [d]). Our goals are to hunt for BHs and sample the low-mass end of the mass-ratio distribution. Methods. To uncover the hidden companions, we implemented the shift-and-add grid disentangling algorithm using 32 epochs of spectroscopy acquired in the framework of TMBM with the FLAMES spectrograph, allowing us to detect companions contributing as little as ≈1–2% to the visual flux. We further analysed OGLE photometric data for the presence of eclipses or ellipsoidal variations. Results. Out of the 51 SB1 systems, 43 (84%) are found to have non-degenerate stellar companions, of which 28 are confident detections and 15 are less certain (SB1: or SB2:). Of these 43 targets, one is found to be a triple (VFTS 64), and two are found to be quadruples (VFTS 120, 702). Our sample includes a total of eight eclipsing binaries. The remaining eight targets (16%) retain an SB1 classification. We modelled the mass-ratio distribution as f (q) ∝ qκ, and derived k through a Bayesian approach. We used massratio constraints from previously known SB2 binaries, newly uncovered SB2 binaries, and SB1 binaries, while accounting for binary detection bias. We found k = 0.2 ± 0.2 for the entire sample and κ = −0.2 ± 0.2 when excluding binaries with periods shorter than 10 d. In contrast, k = 1.2 ± 0.5 was retrieved for tight binaries (P &lt; 10 d), and it is proposed here to be a consequence of binary interactions. Aside from the unambiguous O+BH binary VFTS 243, which was analysed in detail in a separate paper, we identified two additional OB+BH candidates: VFTS 514 and 779. Conclusions. Our study firmly establishes a virtually flat natal mass-ratio distribution (κ = 0) for O-type stars at LMC metallicity, covering the entire mass-ratio range (0.05 &lt; q &lt; 1) and periods in the range 0 &lt; log P &lt; 3 [d]. The nature of the OB+BH candidates should be verified through future monitoring, but the frequency of OB+BH candidates is generally in line with recent predictions at LMC metallicity.

Context. The formation history, progenitor properties, and expected rates of the binary black holes discovered by the LIGO-Virgo collaboration via the gravitational-wave emission during their coalescence are a topic of active research. Aims. We aim to study the progenitor properties and expected rates of the two lowest-mass binary black hole mergers, GW151226 and GW170608, detected within the first two Advanced LIGO-Virgo observing runs, in the context of the classical isolated binary-evolution scenario. Methods. We used the publicly available 1D-hydrodynamic stellar-evolution code MESA, which we adapted to include the black-hole formation and the unstable mass transfer developed during the so-called common-envelope phase. Using more than 60 000 binary simulations, we explored a wide parameter space for initial stellar masses, separations, metallicities, and mass-transfer efficiencies. We obtained the expected distributions for the chirp mass, mass ratio, and merger time delay by accounting for the initial stellar binary distributions. We predicted the expected merger rates and compared them with those of the detected gravitational-wave events. We studied the dependence of our predictions with respect to the (as yet) unconstrained parameters inherent to binary stellar evolution. Results. Our simulations for both events show that while the progenitors we obtained are compatible over the entire range of explored metallicities, they show a strong dependence on the initial masses of the stars, according to stellar winds. All the progenitors we found follow a similar evolutionary path, starting from binaries with initial separations in the 30−200 R⊙ range experiencing a stable mass transfer interaction before the formation of the first black hole, followed by a second unstable mass-transfer episode leading to a common-envelope ejection that occurs either when the secondary star crosses the Hertzsprung gap or when it is burning He in its core. The common-envelope phase plays a fundamental role in the considered low-mass range: only progenitors experiencing such an unstable mass-transfer phase are able to merge in less than a Hubble time. Conclusions. We find integrated merger-rate densities in the range 0.2–5.0 yr−1 Gpc−3 in the Local Universe for the highest mass-transfer efficiencies explored here. The highest rate densities lead to detection rates of 1.2–3.3 yr−1, which are compatible with the observed rates. The common-envelope efficiency αCE has a strong impact on the progenitor populations. A high-efficiency scenario with αCE = 2.0 is favoured when comparing the expected rates with the observations.

On 2019 April 25, the LIGO/Virgo Scientific Collaboration detected a compact binary coalescence, GW190425. Under the assumption of the binary neutron star (BNS), the total mass of $3.4^{+0.3}_{-0.1}\, \mathrm{M}_\odot$ lies five standard deviations away from the known Galactic population mean. In the standard common envelope scenario, the immediate progenitor of GW190425 is a close binary system composed of an NS and a He-rich star. With the detailed binary evolutionary modelling, we find that in order to reproduce GW190425-like events, super-Eddington accretion (e.g. $1,000\, \dot{M}_{\rm Edd}$) from a He-rich star onto the first-born NS with a typical mass of 1.33 M⊙ via stable Case BB mass transfer (MT) is necessarily required. Furthermore, the immediate progenitors should potentially have an initial mass of MZamsHe in a range of 3.0–3.5 M⊙ and an initial orbital period of Pinit from 0.08 d to 0.12 d, respectively. The corresponding mass accreted onto NSs via stable Case BB MT phase varies from $0.70\, \mathrm{M}_\odot$ to $0.77\, \mathrm{M}_\odot$. After the formation of the second-born NS, the BNSs are expected to be merged due to gravitational wave emission from ∼11 Myr to ∼190 Myr.

We explore properties of Type Ib and IIb SN progenitors that are produced by stable mass transfer in binary systems using a new grid of stellar evolution models from an initial primary mass in the range of at solar and Large Magellanic Cloud metallicities. We find that blue and yellow supergiant SN IIb progenitors (e.g., of SN 2008ax, SN 2011dh, and SN 2016gkg) have a hydrogen envelope mass less than about , mostly resulting from early Case B mass transfer with relatively low initial masses and/or low metallicity. Red supergiant (RSG) SN IIb progenitors (e.g., of SN 1993J, SN 2013df) are produced via late Case B mass transfers and have a more massive hydrogen envelope ( ). SN Ib progenitors are predominantly produced by early Case B mass transfers. Our models predict that SN IIb progenitors are systematically more luminous in the optical ( ) than the majority of SN Ib progenitors ( ) for our considered initial mass range. However, the optically bright progenitor of SN Ib iPTF13bvn (i.e., ) can be well explained by a relatively low-mass progenitor with a final mass of . The event rate of blue and yellow SN IIb progenitors would increase as metallicity decreases, while the event rate of SN Ib progenitors would decrease instead. By contrast, the population of RSG SN IIb progenitors would not be significantly affected by metallicity.

Context.The stability of mass transfer is critical in determining pathways towards various kinds of compact binaries, such as compact main-sequence white-dwarf binaries, and transients, such as double white-dwarf mergers and luminous red novae. Despite its importance, very few systematic studies of the stability of mass transfer exist.Aims.We study the behaviour of mass-losing donor stars in binary systems in a systematic way. We focus on identifying and understanding the parameter space for stable mass transfer in low- and intermediate-mass binaries with post-main-sequence donor stars as well as the properties of ultimately unstable binary systems at the onset of the instabilities.Methods.We employed the 1D stellar evolution code MESA to simulate the mass-transfer evolution of 1404 binary systems with donor-star masses between 1 M⊙and 8 M⊙. We studied the behaviour of the binaries during mass transfer, without assuming that the donor star responds adiabatically to mass loss. We treated the accretor as a point mass, which we do not evolve, and assumed the mass transfer is conservative.Results.We considered several criteria to define when unstable mass transfer occurs. We find that the criterion that best predicts the onset of runaway mass transfer is based on the transition to an effectively adiabatic donor response to mass loss. Using this quasi-adiabatic criterion, we determine the location of the stability boundary to within a relative uncertainty of five per cent in the mass ratio at the onset of mass transfer. Defining this critical mass ratio (qqad) in terms of accretor mass over donor mass, we find thatqqad ∼ 0.25 for stars with radiative envelopes that cross the Hertzsprung gap, while for convective giantsqqaddecreases from ∼1 at the base of the red giant branch to ∼0.1 at the onset of thermal pulses on the asymptotic giant branch. Compared with recent similar studies, we find increased stability of mass transfer from convective giants. This is because an effectively adiabatic response of the donor star only occurs at a very high critical mass-transfer rate due to the short local thermal timescale in the outermost layers of a red giant. Furthermore, we find that forq &gt; qqadmass transfer is self-regulated, but that for evolved giants the resulting mass-transfer rates can be so high that the evolution becomes dynamical and/or the donor can overflow its outer lobe.Conclusions.Our results indicate that mass transfer is stable for a wider range of binary parameter space than typically assumed in rapid binary population synthesis. Moreover, we find a systematic dependence of the critical mass ratio on the donor star mass and radius, which may have significant consequences for predictions of post-mass-transfer populations.

Context. Numerous studies have established that in main-sequence binary systems, mass transfer generally proceeds from the initially more massive star to its less massive companion, thereby inducing rejuvenation in the latter. However, in certain massive close binary systems with orbital periods on the order of a few days, a reversed mass transfer scenario emerges: mass flows from the initially less massive component to the more massive one, during which the two stars simultaneously overflow their Roche lobes. This configuration potentially facilitates stellar rejuvenation in both members of the binary system. Aims. The phenomenon of reverse mass transfer in binary systems is closely linked to the efficiency of stellar rejuvenation in the mass accretor. However, the physical mechanism driving the rejuvenation of the accretor remains poorly understood. In this work, we employ the Schwarzschild criterion to define the boundaries of convective regions at the solar metallicity, as opposed to the Ledoux criterion typically used for convective conditions at low metallicities. Our study aims to investigate how mass transfer significantly influences the rejuvenation process of the mass-accreting star. Furthermore, we aim to systematically investigate how initial binary parameters (particularly the mass ratio and orbital period) regulate the initiation of reverse mass transfer from the secondary to the primary component. Methods. We constructed a new set of detailed, grid-based binary evolution models, systematically varying initial orbital periods and mass ratios, q, within a parameter space. Results. Our results show that for systems with initial primary masses of 16 M⊙ and initial mass ratios of qini= M2M1 ≥ 0.4, those with shorter initial orbital periods, Porb &lt; 1.8 days, are statistically more prone to evolving into contact binaries that exhibit reverse mass transfer. The results further demonstrate that stellar rejuvenation significantly influences the overall evolution of binary systems, notably extending the main-sequence lifetime of the mass-gaining star. Specifically, rejuvenation can induce increases in both the radius and luminosity of the accretor, potentially triggering reverse mass transfer. The transfer causes the mass of accretor’s convective core to increase, facilitating the mixing of fresh fuel from outer layers into the central nuclear-burning region. Following mass transfer, the evolutionary state of the accretor is closer to the zero age main sequence. Due to the high efficiency of rejuvenation, the mass-gaining star can even outpace the mass-losing star in terms of evolution. Meanwhile, the initially more massive star can also undergo rejuvenation via reverse mass transfer, as it must rapidly adapt to its newly increased mass.