Close Binary Evolution Research Articles

THE PROGENITOR MASS OF THE MAGNETAR SGR1900+14

Magnetars are young neutron stars with extreme magnetic fields (B ≳ 1014–1015 G). How these fields relate to the properties of their progenitor stars is not yet clearly established. However, from the few objects associated with young clusters it has been possible to estimate the initial masses of the progenitors, with results indicating that a very massive progenitor star (Mprog> 40 M) is required to produce a magnetar. Here, we present adaptive-optics assisted Keck/NIRC2 imaging and Keck/NIRSPEC spectroscopy of the cluster associated with the magnetar SGR 1900+14, and report that the initial progenitor star mass of the magnetar was a factor of 2 lower than this limit, Mprog = 17 ± 2 M. Our result presents a strong challenge to the concept that magnetars can only result from very massive progenitors. Instead, we favor a mechanism which is dependent on more than just initial stellar mass for the production of these extreme magnetic fields, such as the "fossil-field" model or a process involving close binary evolution.

A new extremely low-mass white dwarf in the NLTT catalogue

We report on the discovery of the extremely low-mass, hydrogen-rich white dwarf, NLTT 11748. Based on measurements of the effective temperature (8540+/-50 K) and surface gravity (log g = 6.20+/-0.15) obtained by fitting the observed Balmer line profiles with synthetic spectra, we derive a mass of 0.167+/-0.005 M_solar. This object is one of only a handful of white dwarfs with masses below 0.2 M_solar that are believed to be the product of close binary evolution with an episode of Roche lobe overflow onto a degenerate companion (neutron star or white dwarf). Assuming membership in the halo population, as suggested by the kinematics and adopting a cooling age of 4.0 - 6.3 Gyrs for the white dwarf, we infer a progenitor mass of 0.87 - 0.93 M_solar. The likely companion has yet to be identified, but a search for radial velocity variations may help constrain its nature.

White dwarf post common envelope binaries from the SDSS

Post common envelope binaries (PCEBs) consisting of a white dwarf and a main sequence star are ideal systems to calibrate current theories of angular momentum loss in close compact binary stars. The potential held by PCEBs for further development of close binary evolution could so far not be exploited due to the small number of known systems and the inhomogeneity of the sample. The Sloan Digital Sky Survey is changing this scene dramatically, as it is very efficient in identifying white dwarf/main sequence (WDMS) binaries, including both wide systems whose stellar components evolve like single stars and - more interesting in the context of close binary evolution - PCEBs. We pursue a large-scale follow-up survey to identify and characterise the PCEBs among the WDMS binaries that have been found with SDSS. We here present preliminary results of our survey and derive first constraints on current models of white dwarf binary evolution.

Mass transfer in eccentric binaries: the new oil-on-water smoothed particle hydrodynamics technique

To measure the onset of mass transfer in eccentric binaries, we have developed a two-phase smoothed particle hydrodynamics (SPH) technique. Mass transfer is important in the evolution of close binaries, and a key issue is to determine the separation at which mass transfer begins. The circular case is well understood and can be treated through the use of the Roche formalism. To treat the eccentric case, we use a newly developed two-phase system. The body of the donor star is made up from high-mass water particles, whilst the atmosphere is modelled with low-mass oil particles. Both sets of particles take part fully in SPH interactions. To test the technique, we model circular mass-transfer binaries containing a 0.6 M-circle dot donor star and a 1 M-circle dot white dwarf; such binaries are thought to form cataclysmic variable ( CV) systems. We find that we can reproduce a reasonable CV mass-transfer rate, and that our extended atmosphere gives a separation that is too large by approximately 16 per cent, although its pressure scale height is considerably exaggerated. We use the technique to measure the semimajor axis required for the onset of mass transfer in binaries with a mass ratio of q = 0.6 and a range of eccentricities. Comparing to the value obtained by considering the instantaneous Roche lobe at pericentre, we find that the radius of the star required for mass transfer to begin decreases systematically with increasing eccentricity.

Merging of components in close binaries: Type Ia supernovae, massive white dwarfs, and Ap stars

The "Scenario Machine" (a computer code designed for studies of the evolution of close binaries) was used to carry out a population synthesis for a wide range of merging astrophysical objects: main-sequence stars with main-sequence stars; white dwarfs with white dwarfs, neutron stars, and black holes; neutron stars with neutron stars and black holes; and black holes with black holes.We calculate the rates of such events, and plot the mass distributions for merging white dwarfs and main-sequence stars. It is shown that Type Ia supernovae can be used as standard candles only after approximately one billion years of evolution of galaxies. In the course of this evolution, the average energy of Type Ia supernovae should decrease by roughly 10%; the maximum and minimum energies of Type Ia supernovae may differ by no less than by a factor of 1.5. This circumstance should be taken into account in estimations of parameters of acceleration of the Universe. According to theoretical estimates, the most massive - as a rule, magnetic - white dwarfs probably originate from mergers of white dwarfs of lower mass. At least some magnetic Ap and Bp stars may form in mergers of low-mass main sequence stars (<1.5 mass of the Sun) with convective envelopes.

Circumbinary disk, an efficient medium extracting orbital angular momentum in close binaries

The loss of orbital angular momentum plays an important role in the mass transfer and orbital evolution of close binaries. The traditional mechanisms of orbital angular momentum loss consist of gravitational wave radiation, mass loss and magnetic braking. However, a small fraction of the mass outflow may form a thin circumbinary disk (CB disk) located in the orbital plane of the binary during mass exchange. The tide torques caused by the gravitational interaction between a CB disk and a binary system brake binary effectively, and extract the orbital angular momentum from the binary system. In this study, numerical calculations for the evolution of the white dwarf binary show that a CB disk is an efficient medium extracting orbital angular momentum even if the mass loss is very small. Finally, some theoretical research and observational progress on CB disks are presented.

Post-common-envelope binaries from SDSS - V. Four eclipsing white dwarf main-sequence binaries

We identify SDSS 011009.09+132616.1, SDSS 030308.35+005444.1, SDSS 143547.87+ 373338.5 and SDSS 154846.00+405728.8 as four eclipsing white dwarf plus main-sequence (WDMS) binaries from the Sloan Digital Sky Survey (SDSS), and report on follow-up observations of these systems. SDSS 0110+1326, SDSS 1435+3733 and SDSS 1548+4057 contain DA white dwarfs, while SDSS 0303+0054 contains a cool DC white dwarf. Orbital periods and ephemerides have been established from multiseason photometry. SDSS 1435+3733, with Porb= 3 h has the shortest orbital period of all known eclipsing WDMS binaries. As for the other systems, SDSS 0110+1326 has Porb= 8 h, SDSS 0303+0054 has Porb= 3.2 h and SDSS 1548+4057 has Porb= 4.4 h. Time-resolved spectroscopic observations have been obtained and the Hα and Ca ii λλ8498.02, 8542.09, 8662.14 triplet emission lines, as well as the Na i λλ8183.27, 8194.81 absorption doublet were used to measure the radial velocities of the secondary stars in all four systems. A spectral decomposition/fitting technique was then employed to isolate the contribution of each of the components to the total spectrum, and to determine the white dwarf effective temperatures and surface gravities, as well as the spectral types of the companion stars. We used a light-curve modelling code for close binary systems to fit the eclipse profiles and the ellipsoidal modulation/reflection effect in the light curves, to further constrain the masses and radii of the components in all systems. All three DA white dwarfs have masses of MWD∼ 0.4–0.6 M⊙, in line with the expectations from close binary evolution. The DC white dwarf in SDSS 0303+0054 has a mass of MWD≳ 0.85 M⊙, making it unusually massive for a post-common-envelope system. The companion stars in all four systems are M dwarfs of spectral type M4 and later. Our new additions raise the number of known eclipsing WDMS binaries to 14, and we find that the average white dwarf mass in this sample is 〈MWD〉=0.57 ± 0.16 M⊙, only slightly lower than the average mass of single white dwarfs. The majority of all eclipsing WDMS binaries contain low-mass (<0.6 M⊙) secondary stars, and will eventually provide valuable observational input for the calibration of the mass–radius relations of low-mass main-sequence stars and of white dwarfs.

The VLT-FLAMES survey of massive stars: constraints on stellar evolution from the chemical compositions of rapidly rotating Galactic and Magellanic Cloud B-type stars

Aims. We have previously analysed the spectra of 135 early B-type stars in the Large Magellanic Cloud (LMC) and found several groups of stars that have chemical compositions that conflict with the theory of rotational mixing. Here we extend this study to Galactic and Small Magellanic Cloud (SMC) metallicities. Methods. We provide chemical compositions for ~50 Galactic and ~100 SMC early B-type stars and compare these to the LMC results. These samples cover a range of projected rotational velocities up to ~300 km s-1 and hence are well suited to testing rotational mixing models. The surface nitrogen abundances are utilised as a probe of the mixing process since nitrogen is synthesized in the core of the stars and mixed to the surface. Results. In the SMC, we find a population of slowly rotating nitrogen-rich stars amongst the early B type core-hydrogen burning stars, which is comparable to that found previously in the LMC. The identification of non-enriched rapid rotators in the SMC is not possible due to the relatively high upper limits on the nitrogen abundance for the fast rotators. In the Galactic sample we find no significant enrichment amongst the core hydrogen-burning stars, which appears to be in contrast with the expectation from both rotating single-star and close binary evolution models. However, only a small number of the rapidly rotating stars have evolved enough to produce a significant nitrogen enrichment, and these may be analogous to the non-enriched rapid rotators previously found in the LMC sample. Finally, in each metallicity regime, a population of highly enriched supergiants is observed, which cannot be the immediate descendants of core-hydrogen burning stars. Their abundances are, however, compatible with them having gone through a previous red supergiant phase. Together, these observations paint a complex picture of the nitrogen enrichment in massive main sequence and supergiant stellar atmospheres, where age and binarity cause crucial effects. Whether rotational mixing is required to understand our results remains an open question at this time, but could be answered by identifying the true binary fraction in those groups of stars that do not agree with single-star evolutionary models.

A Model for Adiabatic Mass-loss

AbstractWe describe our work on the development and application of a stellar structure code to compute model sequences representing donor stars in interacting binaries subject to rapid (adiabatic) mass-loss. The donor star is assumed to remain in hydrostatic equilibrium, but no heat flow is allowed. These sequences can be used to define bifurcation sequences in close binary evolution, and to circumscribe possible survivors of common envelope evolution.

Blue Stragglers from Primordial Binary Evolution

AbstractBinaries exist in all clusters and much evidence suggests that close-binary evolution makes an important contribution to the blue straggler population, at least in some clusters as well as in the field. Here we present different channels to blue stragglers from primordial binary evolution and examine their contributions to the integrated spectral energy distribution of the host clusters in theory via binary population synthesis.

Complexity in close binary evolution

AbstractThe complex nature of close binary evolution is briefly reviewed. Population synthesis calculations of zero‐age cataclysmic variable stars (ZACVs) are presented using a common envelope (CE) efficiency parameter, αCE, that is a function of the core mass of the primary. Specifically, we investigate the possibility that αCE is different for CE evolution involving red giant branch (RGB) primaries than for asymptotic giant branch (AGB) primaries. We calculate a sequence of model orbital period distributions in present‐day ZACVs for 11 different combinations of efficiency parameters (αRGB, αAGB). We find that if CE evolution is much less efficient for RGB primaries than for AGB primaries (αRGB ∼ 0.1 or less), the number of CVs that form below 2 h is significantly decreased compared with standard constant αCE models. © 2008 Wiley Periodicals, Inc. Complexity, 2008.

DN Bootis: A low mass-ratio W UMa-type contact binary

New photoelectric BVR light curves and radial velocity curves were obtained for the HIPPARCOS discovery DN Boo at the TÜBİTAK 1 TÜBİTAK: The Scientific and Technical Research Council of Turkey. 1 National Observatory of Turkey (TUG) and Dominion Astrophysical Observatory (DAO), respectively, to determine physical nature of the variable. The character of the obtained light curves and double-lined spectroscopic structures in the obtained spectra are revealed that DN Boo is a genuine EW type eclipsing binary. During the analysis of our new observations a simultaneous solution were derived for the photometric and spectroscopic data by using the Wilson–Devinney code and orbital parameters with absolute dimensions of the system were determined for the first time. Finally, the importance of very low mass-ratio contact binaries in the late stages of close binary evolution was discussed.

Observational Evidence for Tidal Interaction in Close Binary Systems

This paper reviews the rich corpus of observational evidence for tidal effects, mostly based on photometric and radial-velocity measurements. This is done in a period when the study of binaries is being revolutionized by large-scaled photometric surveys that are detecting many thousands of new binaries and tens of extrasolar planets. We begin by examining the short-term effects, such as ellipsoidal variability and apsidal motion. We next turn to the long-term effects, of which circularization was studied the most: a transition period between circular and eccentric orbits has been derived for eight coeval samples of binaries. The study of synchronization and spin-orbit alignment is less advanced. As binaries are supposed to reach synchronization before circularization, one can expect finding eccentric binaries in pseudo-synchronization state, the evidence for which is reviewed. We also discuss synchronization in PMS and young stars, and compare the emerging timescale with the circularization timescale. We next examine the tidal interaction in close binaries that are orbited by a third distant companion, and review the effect of pumping the binary eccentricity by the third star. We elaborate on the impact of the pumped eccentricity on the tidal evolution of close binaries residing in triple systems, which may shrink the binary separation. Finally we consider the extrasolar planets and the observational evidence for tidal interaction with their parent stars. This includes a mechanism that can induce radial drift of short-period planets, either inward or outward, depending on the planetary radial position relative to the corotation radius. Another effect is the circularization of planetary orbits, the evidence for which can be found in eccentricity-versus-period plot of the planets already known. Whenever possible, the paper attempts to address the possible confrontation between theory and observations, and to point out noteworthy cases and observations that can be performed in the future and may shed some light on the key questions that remain open.

Rotation and Massive Close Binary Evolution

AbstractWe review the role of rotation in massive close binary systems. Rotation has been advocated as an essential ingredient in massive single star models. However, rotation clearly is most important in massive binaries where one star accretes matter from a close companion, as the resulting spin-up drives the accretor towards critical rotation. Here, we explore our understanding of this process, and its observable consequences. When accounting for these consequences, the question remains whether rotational effects in massive single stars are still needed to explain the observations.

Late stages of the evolution of close compact binaries: Type I supernovae, gamma-ray bursts, and supersoft X-ray sources

We consider the evolution of close binaries resulting in the most intensive explosive phenomena in the stellar Universe—Type Ia supernovae and gamma-ray bursts. For Type Ia supernovae, which represent thermonuclear explosions of carbon-oxygen dwarfs whose masses reach the Chandrasekhar limit during the accretion of matter from the donor star, we derive the conditions for the accumulation of the limiting mass by the degenerate dwarf in the close binary. Accretion onto the degenerate dwarf can be accompanied by supersoft X-ray radiation with luminosity 1–104 L ⊙. Gamma-ray bursts are believe to accompany the formation and rapid evolution of compact accretion-decretion disks during the formation of relativistic objects—black holes and neutron stars. The rapid (∼1 M ⊙/s) accretion of matter from these disks onto the central compact relativistic star results in an energy release of ∼0.1 M ⊙ c 2 ∼ 1053 erg in the form of gamma-rays and neutrinos over a time of 0.1–1000 s. Such disks can form via the collapse of the rapidly rotating cores of Type Ib, Ic supernovae, which are components in extremely close binaries, or alternately due to the collapse of accreting oxygen-neon degenerate dwarfs with the Chandrasekhar mass into neutron stars, or the merging of neutron stars with neutron stars or black holes in close binaries. We present numerical models of the evolution of some close binaries that result in Type Ia supernovae, and also estimate the rates of these supernovae (∼0.003/year) and of gamma-ray bursts (∼10−4/year) in our Galaxy for various evolutionary scenarios. The collimation of the gamma-ray burst radiation within an opening angle of several degrees “matches” the latter estimate with the observed rate of these events, ∼10−7–10−8/year calculated for a galaxy with the mass of our Galaxy.

Efficiency of mass transfer in massive close binaries

Aims.One of the major uncertainties in close binary evolution is the efficiency of mass transfer β: the fraction of transferred mass that is accreted by a secondary star. We attempt to constrain the mass-transfer efficiency for short-period massive binaries undergoing case A mass transfer.

A Photometric and Spectroscopic Search for White Dwarfs in the Open Clusters NGC 6633 and NGC 7063

We present photometric and spectroscopic studies of the white dwarf (WD) populations in the intermediate-age open clusters NGC 6633 and NGC 7063 as part of the ongoing Lick-Arizona White Dwarf Survey (LAWDS). Using wide-field CCD imaging, we locate 41 candidate WDs in the two cluster fields: 32 in NGC 6633, and 9 in NGC 7063. Spectroscopic observations confirm 13 of these candidates to be bona-fide WDs. We describe in detail our Balmer line fitting technique for deriving effective temperatures and surface gravities from optical DA WD spectra and apply the technique to the 11 DA WDs in the sample. Of these, only two DA WDs are at the cluster distance moduli, one in each cluster. Two more DAs lie 0.75 mag foreground to NGC 6633, raising the possibility that these are double degenerate systems in the cluster. If nearly equal-mass binaries, both of these systems likely have combined masses above the Chandrasekhar limit. One DB WD is found to be consistent with membership in NGC 6633, which would make this the third confirmed He-atmosphere WD in an open cluster, though further data are needed to confirm cluster membership. The WD consistent with membership in the cluster NGC 7063 has a low mass (~0.4 Mo), suggesting it may be a He-core WD resulting from close binary evolution. Three of the eleven hydrogen-atmosphere WDs in this study are observed to have Ca II absorption; the number of DAZs in this study is consistent with previous observations that ~25% of field WDs are DAZs.

Evolving to Type Ia Supernovae with Long Delay Times

Recent investigations on the delay time of Type Ia supernovae have set useful constraints on the progenitors of Type Ia supernovae. Here we have calculated the evolution of close binaries consisting of a white dwarf and a main-sequence or subgiant companion. We assume that, once Roche lobe overflow occurs, a small fraction of the lost mass from the system forms a circumbinary disk, which extracts the orbital angular momentum from the system through tidal torques. Our calculations indicate that the existence of a circumbinary disk can enhance the mass transfer rate and cause secular orbital shrinkage. The white dwarf can grow in mass efficiently to trigger Type Ia supernovae even with relatively low mass (2 M☉) donor stars. Thus, this scenario suggests a new possible evolutionary channel to those Type Ia supernovae with long delay times ~1-3 Gyr.

Ultraviolet Studies of Interacting Binaries

Interacting Binaries consist of a variety of stellar objects in different stages of evolution and those containing accreting compact objects still represent a major challenge to our understanding of not only close binary evolution but also of the chemical evolution of the Galaxy. These end-points of binary star evolution are ideal laboratories for the study of accretion and outflow processes, and provide insight on matter under extreme physical conditions. One of the key-questions of fundamental relevance is the nature of SNIa progenitors. The study of accreting compact binary systems relies on observations over the entire electromagnetic spectrum and we outline here those unresolved questions for which access to the ultraviolet range is vital, as they cannot be addressed by observations in any other spectral region.

Angular momentum evolution of Algol binaries

We have compiled the well-determined absolute parameters of Algol-type binaries. The lists contain the parameters of 74 detached and 61 semidetached close binaries. The double-lined eclipsing binaries provide not only the most accurate determinations of stellar mass, radius and temperatures but also distance-independent luminosity for each of their individual components. The distributions of the primary and secondary masses of detached binaries (DBs) are similar, whilst the secondary masses of the semidetached binaries (SDBs) are mostly smaller than 2 M ⊙ with a peak in the M 2 -bin (0.21-1.0). The components of the DBs are almost all located ' in the main-sequence band. On the contrary, the secondary components of the SDBs have larger radii and luminosity with respect to the same mass and the same effective temperature of main-sequence counterparts. They occupy a region of the Hertzsprung-Russell diagram between terminal-age main sequence and giants. Moreover, the total angular momenta and specific angular momenta are larger for the SDBs of orbital periods with P > 5 d than those of the shorter period ones. The specific angular momenta of SDBs with periods longer than 5 d are 65 per cent greater than that of the short period group with the same mass. The DBs and the SDBs with orbital periods longer and shorter than 5 d are separated into three groups in the J/M 5/3 - q diagram. The SDBs with mass ratios greater than 0.3 and P > 5 d have almost the same angular momentum to those of DBs. However, the SDBs with short periods have the smallest angular momentum even though they have the same mass ratios. This result reveals that angular momentum loss (AML) considerably affects the evolution of close binary systems. Recently, Chen, Li & Qian suggested that, in addition to magnetic braking, a circumbinary disc may play an important role in AML from Algol-type binaries. Their calculations indicated that the evolution of Algol-type binaries can be significantly affected by the circumbinary disc. Our results show that the evolution of close binaries begins as a DB and losing angular momentum, first via stellar wind and then magnetic braking plus circumbinary disc the period is shortened and orbit shrinks. Thereafter, the evolution of the system is accelerated and mass transfer rates are enhanced which result in a smaller mass ratios.