Evolution Of Low-mass Binaries Research Articles

Influence of the initial orbital period and accretion efficiency on the low-mass binary evolution

This paper presents detailed evolutionary models of low-mass binary systems (1.25 + 1 M?) with initial orbital periods of 10, 50 and 100 days and accretion efficiency of 10%, 20%, 50%, and a conservative assumption. All models are calculated with the MESA (Modules for Experiments in Stellar Astrophysics) evolutionary code. We show that such binary systems can evolve via a stable Case B mass transfer into long period helium white dwarf systems.

Gone with the wind: the impact of wind mass transfer on the orbital evolution of AGB binary systems

In low-mass binary systems, mass transfer is likely to occur via a slow and dense stellar wind when one of the stars is in the asymptotic giant branch (AGB) phase. Observations show that many binaries that have undergone AGB mass transfer have orbital periods of 1–10 yr, at odds with the predictions of binary population synthesis models. In this paper we investigate the mass-accretion efficiency and angular-momentum loss via wind mass transfer in AGB binary systems and we use these quantities to predict the evolution of the orbit. To do so, we perform 3D hydrodynamical simulations of the stellar wind lost by an AGB star in the time-dependent gravitational potential of a binary system, using the AMUSE framework. We approximate the thermal evolution of the gas by imposing a simple effective cooling balance and we vary the orbital separation and the velocity of the stellar wind. We find that for wind velocities higher than the relative orbital velocity of the system the flow is described by the Bondi-Hoyle-Lyttleton approximation and the angular-momentum loss is modest, which leads to an expansion of the orbit. On the other hand, for low wind velocities an accretion disk is formed around the companion and the accretion efficiency as well as the angular-momentum loss are enhanced, implying that the orbit will shrink. We find that the transfer of angular momentum from the binary orbit to the outflowing gas occurs within a few orbital separations from the centre of mass of the binary. Our results suggest that the orbital evolution of AGB binaries can be predicted as a function of the ratio of the terminal wind velocity to the relative orbital velocity of the system, v∞/vorb. Our results can provide insight into the puzzling orbital periods of post-AGB binaries. The results also suggest that the number of stars entering into the common-envelope phase will increase, which can have significant implications for the expected formation rates of the end products of low-mass binary evolution, such as cataclysmic binaries, type Ia supernovae, and double white-dwarf mergers.

Evolution of low-mass binaries with black-hole components

We consider evolutionary models for the population of short-period ( P orb ≲ 10 h) low-mass black-hole binaries (LMBHB) and compare them with observations of soft X-ray transients (SXT). Evolution of LMBHB is determined by nuclear evolution of the donors and/or orbital angular momentum loss due to magnetic braking by the stellar wind of the donors and gravitational wave radiation. We show that the absence of observed stable luminous LMBHB implies that upon RLOF by the low-mass donor angular momentum losses are substantially reduced with respect to the Verbunt and Zwaan “standard” prescription for magnetic braking. Under this assumption masses and effective temperatures of the model secondaries of LMBHB are in a satisfactory agreement with the masses and effective temperatures (as inferred from their spectra) of the observed donors in LMBHB. Theoretical mass-transfer rates in SXTs are consistent with the observed ones if one assumes that accretion discs in these systems are truncated (“leaky”). We find that the population of short-period SXT is formed mainly by systems which had unevolved or slightly evolved ( X c ≳ 0.35) donors at the Roche-lobe overflow. Longer period ( P orb ≃ (0.5–1) day) SXT might descend from systems with initial donor mass about 1 M ⊙ and X c ≲ 0.35. It is unnecessary to invoke donors with almost hydrogen-depleted cores to explain the origin of LMBHB. Our models suggest that a very high efficiency of common-envelopes ejection is necessary to form LMBHB, unless currently commonly accepted empirical estimates of mass-loss rates by winds for pre-WR and WR-stars are significantly over-evaluated.

Helium core white dwarf evolution - including white dwarf companions to neutron stars

We present a detailed calculation of the evolution of low-mass helium white dwarfs. These white dwarfs are formed via long-term, low-mass binary evolution. After detachment from the Roche lobe, the hot helium cores have a rather thick hydrogen layer with masses between 0.001 to 0.06 M⊙. We found that the majority of our computed models experience one or two hydrogen shell flashes. The duration of the flashes is between a few ×106 y to a few ×107 y. In several flashes the white dwarf radius will increase so much that it forces the model to fill its Roche lobe again. Our calculations show that the cooling history of the helium white dwarf depends dramatically on the thickness of the hydrogen layer. The presence of low-mass helium white dwarf secondaries in millisecond pulsar binaries allows to determine the age of the systems independently of the rotational history of the pulsars. The same method may be applied to double degenerate systems. We discuss the cooling history of the low-mass, helium core white dwarfs in short orbital period millisecond pulsars.

Cooling curves and initial models for low-mass white dwarfs (<0.25 M ) with helium cores

We present a detailed calculation of the evolution of low-mass (<0.25 M⊙) helium white dwarfs. These white dwarfs (the optical companions to binary millisecond pulsars) are formed via long-term, low-mass binary evolution. After detachment from the Roche lobe, the hot helium cores have a rather thick hydrogen layer with mass between 0.01 and 0.06 M⊙. As a result of mixing between the core and outer envelope, the surface hydrogen content (Xsurf) is 0.5–0.35, depending on the initial value of the heavy element Z and the initial secondary mass. We found that the majority of our computed models experience one or two hydrogen shell flashes. We found that the mass of the helium dwarf in which the hydrogen shell flash occurs depends on the chemical composition. The minimum helium white dwarf mass in which a hydrogen flash takes place is 0.213 M⊙ (Z=0.003), 0.198 M⊙ (Z=0.01), 0.192 M⊙ (Z=0.02) or 0.183 M⊙ (Z=0.03). The duration of the flashes (independent of chemical composition) is between a few ×106 and a few ×107 yr. In several flashes the white dwarf radius will increase so much that it forces the model to fill its Roche lobe again. Our calculations show that the cooling history of the helium white dwarf depends dramatically on the thickness of the hydrogen layer. We show that the transition from a cooling white dwarf with a temporarily stable hydrogen-burning shell to a cooling white dwarf in which almost all residual hydrogen is lost in a few thermal flashes (via Roche lobe overflow) occurs between 0.183 and 0.213 M⊙ (depending on the heavy element value).

Evolution of binaries with ultra-short periods: Systematic study

A detailed investigation of the evolution of low-mass binaries is performed for the case when the secondary fills its Roche lobe at the stage of core hydrogen exhaustion. The obtained results are compared with observational data for ultra-short periodic X-ray systems MXB 1820-30 and MXB 1916-05. In the frame of the proposed evolutionary scenario it is possible to obtain for MXB 1820-30 its periodP=11.4 min twice (see Figure 2). In the first case the parameters of the system are:M2 ≈ 0.13−0.15M⨀,X≈0.05–0.13, |P/P| ≈ (3.6−6.2) } 10−7 yr−1, M2 ≈ (4.1−9.6) } 10−9M⨀ yr−1, for the second:M2 ≈ 0.08−0.09M⨀,X= 0, |P/P| ≈ (1.3−1.5) } 10−7 yr−1, M2 ≈ (1.4−1.8) } 10−8M⨀ yr−1. It is suggested that MXB 1916-05 is the progenitor of the system MXB 1820-30 (M2 = 0.1M⨀,X ≃ 0.221,M2 ≃ 1.8 × 10−10M⨀ yr−1).

Evolution of Low Mass Binaries under the Influence of Gravitational Radiation

Evolutionary changes of masses and periods under the influence of gravitational radiation (GR) are computed for binaries with main-sequence or degenerate hydrogen-helium (H), helium (He) and carbon (C) secondaries. Tracks in the P-M2 and P-M2 planes are determined. The orbital period of systems P with a non-degenerate dwarf filling its Roche lobe decreases until the time-scale of GR (~ 109−1010yrs) becomes shorter than the thermal time-scale τKH of the mass losing component M2. When M2 becomes ~ 0.1 M⊙, P begins to increase. This could account for the existence of the minimal P ≈ 1.3h and for the accumulation of observed cataclysmic binaries (CB) below P ≤ 2h. The GR can be the driving force for evolution of cataclysmic binaries: TT Ari, OY Car, Z Cha, and WZ Sge as their P, M2 and indicate. The rate of mass exchange driven by GR is ~ 10−9−10−10 M⊙/yrs. It is enough to feed the X-ray bursters and related objects.

Problems of low-mass binary evolution

Some problems connected with low-mass binary evolution (from contact binaries to cataclysmic variables and origin of bursters) are considered. Most attention is given to contact W UMa-stars and to (still unclear) scenarios where the angular momentum loss by magnetic braking may, at least partly, control the contact binary evolution.

Gravitational Radiation and the Evolution of Low Mass Binaries

Gravitational radiation of energy and angular momentum can modify and in some cases, control the evolution of a close binary system. The region of interest is briefly delineated. Recent work of the author and colleagues of relevance to this area is discussed, including theoretical studies of accretion, mass loss and mass transfer, and an observational study of a system, HZ 29 where gravitational radiation may dictate its behaviour.