Low-mass Binary Systems Research Articles

Discovery of Two Very Low Mass Binaries: Final Results of an Adaptive Optics Survey of Nearby M6.0–M7.5 Stars

We present updated results of a high-resolution, magnitude-limited (Ks < 12 mag) imaging survey of nearby low-mass M6.0-M7.5 field stars. The observations were carried out using adaptive optics at the Gemini North, VLT, Keck II, and Subaru telescopes. Our sample of 36 stars consists predominantly of nearby (≲30 pc) field stars, five of which we have resolved as binaries. Two of the binary systems, 2MASSI J0429184-312356 and 2MASSI J1847034+552243, are presented here for the first time. All five of the discovered binary systems have separations between 008 and 053 (2-9 AU) with similar mass ratios (q > 0.8, ΔKs < 1 mag). This result supports the hypothesis that wide (a > 20 AU), very low mass (VLM; Mtot < 0.19 M☉) binary systems are rare. The projected semimajor axis distribution of these systems peak at ~5 AU, and we report a sensitivity-corrected binary fraction of 9 % for stars with primaries of spectral type M6.0-M7.5 with separations ≳3 AU and mass ratios q ≳ 0.6. Within these instrumental sensitivities, these results support the overall trend that both the semimajor axis distribution and binary fraction are a function of the mass of the primary star and decrease with decreasing primary mass. These observations provide important constraints for low-mass binary star formation theories.

X-Ray Binaries: A Laboratory for Frontier Physics

The goal of this paper is to discuss the behaviour of the X-ray binary systems, in order to present to the readers an updated panorama of this important class of X-ray sources. They have in common the binary nature, but rather different characteristics: millisecond pulsations in low mass binary systems, seconds-hundreds seconds pulsations in transient high mass X-ray systems, pulsations limited in a narrow ∼6-12s band in the enigmatic class of systems named anomalous X-ray pulsars. However, all these systems are characterized by a neutron star as collapsed object. A few words will be devoted also to those systems having a black hole as collapsed object. Some comments on radio pulsars-SNRs and X-ray pulsars-SNRs associations will be given too.

An Adaptive Optics Survey of M6.0–M7.5 Stars: Discovery of Three Very Low Mass Binary Systems Including Two Probable Hyades Members

A survey of 30 nearby M6.0–M7.5 dwarfs with Ks 12 and 0>29 (3–10 AU) with similar mass ratios (q > 0:8, DKs 20 AU), very low mass (Mtot 3 AU. This binary frequency is less than the � 32% measured among early M dwarfs over the same separation range. Two of the low-mass binaries are probable Hyades open cluster members based on proper motions, cluster membership probabilities, radial velocities, and near-IR photometry. LP 415-20 has the distinction of being the tightest (3.6 AU) multiple system ever spatially resolved in the cluster, and the companions of LP 415-20 and LP 475-855 are among the least massive objects ever resolved in the Hyades, with estimated masses of 0:081 þ0:009 � 0:010 and 0:082 þ0:009 � 0:009 M� . Subject headings: binaries: general — instrumentation: adaptive optics — open clusters and associations: individual (Hyades) — stars: individual (LP 415-20, LP 475-855, 2MASSW J1750129+442404) — stars: low-mass, brown dwarfs

Visual orbit for the low-mass binary Gliese 22 AC from speckle interferometry

Based on 14 data points obtained with near-infrared speckle interferometry and covering an almost entire revolution, we present a first visual orbit for the low-mass binary system Gliese 22 AC. The quality of the orbit is largely improved with respect to previous astrometric solutions. The dynamical system mass is $0.592\pm 0.065\,M_{\odot}$, where the largest part of the error is due to the Hipparcos parallax. A comparison of this dynamical mass with mass-luminosity relations on the lower main sequence and theoretical evolutionary models for low-mass objects shows that both probably underestimate the masses of M dwarfs. A mass estimate for the companion Gliese 22 C indicates that this object is a very low-mass star with a mass close to the hydrogen burning mass limit.

An Astrometric Study of the Low-Mass Binary Star Ross 614

Long accepted as the quintessential low-mass star, the secondary of the nearby diminutive astrometric binary Ross 614 has attracted considerable astrophysical interest. Unfortunately, the orbital period of 16.6 yr exceeds the duration of the mission-limited studies of most space-borne or instrumental-proving observational programs. As with most such binaries, the only full-orbit studies are based on photographic materials. The last extended study of this system was based upon the plate collections of the McCormick and Sproul Observatories. The work reported here combines data from the Multichannel Astrometric Photometer, the Hipparcos Intermediate Astrometric Data, the previously unmeasured photographic plates of the Allegheny Observatory, published observations of the visual binary, and recently published radial velocities of the system. Together, these data span more than three orbits of the low-mass binary system. Limiting our analysis to the most recent observations of the binary, and five older observations that are in fair agreement with them, we find masses of 0.2228 ± 0.0055 and 0.1107 ± 0.0028 M⊙ for the primary and secondary, respectively, with the largest source of error being the visual separations of the system. We find a parallax of 244.07 ± 0.73 mas, a period of 16.595 ± 0.0077 yr, and an increased estimate of the semimajor axis of 1101.2 ± 8.2 mas. The latter led to a significant increase in the computed masses. All other aspects of the orbital elements and astrometry are in excellent agreement with those found in the independent study of the McCormick and Sproul plates. The importance of long-term astrometric coverage is pointed out by the fact that the orbital motion of the system only resulted in an acceleration during the compilation of the Hipparcos Catalogue. No orbital parameters or mass estimates can be discerned from these high-precision but short-term data.

Tidal interaction of a rotating 1 $\vec{M_\odot}$ star with a binary companion

We calculate the tidal interaction of a uniformly rotating 1 star with an orbiting companion at various phases of core hydrogen burning from the ZAMS to core hydrogen exhaustion. By using the traditional approximation we reduce the solution of the non-adiabatic oscillation equations for the tidal forcing of a rotating star to a one dimensional problem by solving a separate eigenvalue problem for the angular dependence of the tidal perturbations. The radial oscillation equations are then solved by using finite differencing on a fine grid with an implicit matrix inversion method like for stellar evolution calculations. We are able to identify resonances with gravity and quasi-toroidal with up to 1000 radial nodes in the more evolved stellar models. The resulting tidal torque is calculated down to low forcing frequencies close to corotation. For low tidal frequencies we find significant response due to inertial in the convective envelope. The inertial modes are damped by turbulent dissipation in the envelope and generate a relatively high torque-level in the low frequency region where the (retrograde) high radial order g-mode resonances become tidally inefficient due to their rotational confinement to the stellar equator and strong damping by radiative losses. For still lower retrograde forcing frequencies we find a large number of closely spaced weakly damped quasi-toroidal q-mode resonances. Our results indicate that effects related to stellar rotation can considerably enhance the speed of tidal evolution in low-mass binary systems. hydrodynamics – stars: binaries: generas – stars: rotation – stars: oscillation

Short-Period Noncontact Close Binary Systems. VI. V392 Orionis

Abstract Photometric and spectroscopic observations in the optical region of the short-period eclipsing binary system V392 Ori (A5 V, $P = 0.6593 \,\mathrm{d}$) were performed and light and radial velocity curves were obtained. The light curves taken at various times are slightly different from one another. Analyses of the light and radial velocity curves yielded photometric and spectroscopic elements of the system. The derived mass function is quite small, $f(M) = 0.0108 \,{{{M}_{\odot}}}$. The masses of the primary and secondary components were found to be 1.2${M}_{\odot}$ and 0.3${M}_{\odot}$, respectively, and the radii are 1.7${R}_{\odot}$ and 0.9${R}_{\odot}$. V392 Ori was found to belong to a group of low-mass binary systems. Due to the semidetached nature of the system, the loss of mass/angular momentum from the system may occasionally occur, inducing a probable effect on the light curves of the system. We compare the system with other low-mass binary systems and discuss the evolutionary status of the system.

Changing Frequency Separation of Kilohertz Quasi‐periodic Oscillations in the Sonic‐Point Beat‐Frequency Model

Previous work on the sonic-point beat-frequency (SPBF) model of the kilohertz quasi-periodic oscillations (QPOs) observed in the X-ray flux from neutron stars in low-mass binary systems has shown that it naturally explains many properties of these QPOs. These include the existence of just two principal QPOs in a given source, the commensurability of the frequency separation of the two kilohertz QPOs and the spin frequency \nus inferred from burst oscillations, and the high frequencies, coherence, and amplitudes of these QPOs. Here we show that the SPBF model predicts that \Dnu is less than but close to \nus, consistent with the observed differences between \Dnu and \nus. It also explains naturally the decrease in \Dnu with increasing QPO frequency seen in some sources and the plateau in the QPO frequency--X-ray flux observed in 4U 1820-30. The model fits well the QPO frequency behavior observed in Sco X-1, 4U 1608-52, 4U 1728-34, and 4U 1820-30 (chisqdof = 0.4-2.1), giving masses ranging from 1.59 to 2.0 Msun and spin rates ranging from 279 to 364 Hz. In the SPBF model, the kilohertz QPOs are effects of strong-field gravity. Thus, if the model is validated, the kilohertz QPOs can be used not only to determine the properties of neutron stars but also to explore quantitatively general relativistic effects in the strong-field regime.

The Discovery of a Companion to the Very Cool Dwarf Gliese 569B with the Keck Adaptive Optics Facility.

We report observations obtained with the Keck adaptive optics facility of the nearby (d=9.8 pc) binary Gl 569. The system was known to be composed of a cool primary (dM2) and a very cool secondary (dM8.5) with a separation of 5&arcsec; (49 AU). We have found that Gl 569B is itself double with a separation of only 0&farcs;101+/-0&farcs;002 (1 AU). This detection demonstrates the superb spatial resolution that can be achieved with adaptive optics at Keck. The difference in brightness between Gl 569B and the companion is approximately 0.5 mag in the J, H, and K&arcmin; bands. Thus, both objects have similarly red colors and very likely constitute a very low mass binary system. For reasonable assumptions about the age (0.12-1.0 Gyr) and total mass of the system (0.09-0.15 M middle dot in circle), we estimate that the orbital period is approximately 3 yr. Follow-up observations will allow us to obtain an astrometric orbit solution and will yield direct dynamical masses that can constrain evolutionary models of very low mass stars and brown dwarfs.

Recycling NSs to ultrashort periods: a statistical analysis of their evolution in the μ – P plane

AbstractWe investigate the statistical evolution of magnetic neutron stars recycled in Low Mass Binary (LMB) systems, simulating synthetic populations. Irrespective to the details of the physical models, we find to be significant the fraction of neutron stars spinning close to their mass shedding limit relative to the millisecond pulsar population. The simulated neutron stars show a tail in their period distribution at periods shorter than 1.558 ms, the minimum detected so far. Crustal magnetic field decay models predict also the existence of massive rapidly spinning neutron stars with very low magnetic moment.

On the Formation of Helium Double Degenerate Stars and Pre–Cataclysmic Variables

The evolution of low-mass (M 2.5 binaries through the common envelope phase has been M _ ) studied for systems in which one member is on its —rst ascent of the red giant branch. Three-dimensional hydrodynamical simulations have been carried out for a range of red giant masses (1¨2 with degen- M _ ) erate helium cores (0.28¨0.45 and companions (0.1¨0.45 for initial orbital periods ranging from M _ ) M _ ) D15 to 1000 days. The results suggest that these low-mass binary systems can survive the common envelope phase provided that the helium degenerate core is more massive than about 0.2¨0.25 and M _ that the mass of the red giant progenitor is Speci—c applications are made to observed double (2 M _ . helium degenerate systems, precataclysmic variables, and subdwarf B stars in order to place constraints on progenitor systems evolving through the common envelope phase. For the observed short-period double degenerate systems, it is found that evolutionary scenarios involving two phases of common envelope evolution are not likely and that a scenario involving an Algol-like phase of mass transfer fol- lowed by a common envelope phase is viable, suggesting that the —rst-formed white dwarf is often reheated by nuclear burning on its surface. A formation mechanism for two subdwarf B stars observed in eclipsing short-period binaries with low-mass main-sequence stars is also described. Subject headings: binaries: closecircumstellar matterhydrodynamics ¨ novae, cataclysmic variablesstars: evolutionstars: interiors

Binary stars in young clusters: models versus observations of the Trapezium Cluster

The frequency of low-mass pre-main sequence binary systems is significantly lower in the Trapezium Cluster than in Taurus-Auriga. We investigate if this difference can be explained through stellar encounters in dense clusters. To this effect, a range of possible models of the well observed Trapezium Cluster are calculated using Aarseth's direct N-body code, which treats binaries accurately. The results are confronted with observational constraints. The range of models include clusters in virial equilibrium, expanding clusters as a result of instantaneous mass loss, as well as collapsing clusters. In all cases the primordial binary proportion is larger than 50 per cent, with initial period distributions as observed in Taurus-Auriga and the Galactic field. It is found that the expanding model, with an initial binary population as in the Galactic field, is most consistent with the observational constraints. This raises the possibility that the primordial group of OB stars may have expelled the cluster gas roughly 50 000 yr ago. The cluster's bulk expansion rate is thus a key observable that needs to be determined. The other models demonstrate that the rapidly decreasing binary proportion, its radial dependence and the form of the period distribution, together with structural and kinematical data, are very useful diagnostics on the present and past dynamical state of a young cluster. In particular, kinematical cooling from the disruption of wide binaries is seen for the first time.

STRANGE STARS IN LOW-MASS BINARY PULSAR SYSTEMS

Based on observational facts and a variety of theoretical arguments we discuss in this work the possibility that pulsars in Low-Mass Binary Pulsar systems could be strange stars rather than neutron stars. It is shown that, although subject to reasonable uncertainties, the consideration of the physics of the SQM core and thin normal crusts leads to the prediction of several observed features of the magnetic field history of these systems within this working hypothesis.

Flux Expulsion and Field Evolution in Neutron Stars

Models for the evolution of magnetic fields of neutron stars are constructed, assuming the field is embedded in the proton superconducting core of the star. The rate of expulsion of the magnetic flux out of the core, or equivalently the velocity of outward motion of flux-carrying proton-vortices is determined from a solution of the Magnus equation of motion for these vortices. A force due to the pinning interaction between the proton-vortices and the neutron-superfluid vortices is also taken into account in addition to the other more conventional forces acting on the proton-vortices. Alternative models for the field evolution are considered based on the different possibilities discussed for the effective values of the various forces. The coupled spin and magnetic evolution of single pulsars as well as those processed in low-mass binary systems are computed, for each of the models. The predicted lifetimes of active pulsars, field strengths of the very old neutron stars, and distribution of the magnetic fields versus orbital periods in low-mass binary pulsars are used to test the adopted field decay models. Contrary to the earlier claims, the buoyancy is argued to be the dominant driving cause of the flux expulsion, for the single as well as the binary neutron stars. However, the pinning is also found to play a crucial role which is necessary to account for the observed low field binary and millisecond pulsars.

Rapid X-ray variability of neutron stars in low-mass binary systems

The dramatic discovery with the Rossi X-Ray Timing Explorer satellite of remarkably coherent ∼300–1200 Hz oscillations in the X-ray brightness of some sixteen neutron stars in low-mass binary systems has spurred theoretical modeling of these oscillations and investigation of their implications for the neutron stars and accretion flows in these systems. High-frequency oscillations are observed both during thermonuclear X-ray bursts and during intervals of accretion-powered emission and appear to be a characteristic feature of disk-accreting neutron stars with weak magnetic fields. In this review we focus on the high-frequency quasi-periodic oscillations (QPOs) seen in the accretion-powered emission. We first summarize the key properties of these kilohertz QPOs and then describe briefly the models that have been proposed to explain them. The existing evidence strongly favors beat-frequency models. We mention several of the difficulties encountered in applying the magnetospheric beat-frequency model to the kilohertz QPOs. The most fully developed and successful model is the sonic-point beat-frequency model. We describe the work on this model in some detail. We then discuss observations that could help to distinguish between models. We conclude by noting some of the ways in which study of the kilohertz QPOs may advance our understanding of dense matter and strong gravitational fields.

Sonic‐Point Model of Kilohertz Quasi‐periodic Brightness Oscillations in Low‐Mass X‐Ray Binaries

Quasi-periodic brightness oscillations (QPOs) with frequencies ranging from ~300 to ~1200 Hz have been discovered in the X-ray emission from 14 neutron stars in low-mass binary systems and from another neutron star in the direction of the Galactic center. These kilohertz QPOs are very strong, with rms relative amplitudes ranging up to ~15% of the total X-ray count rate, and are remarkably coherent, with frequency-to-FWHM ratios as large as ~200. Two simultaneous kilohertz QPOs differing in frequency by ~250-350 Hz have been detected in 12 of the 15 sources. Here we propose a model for these QPOs. In this model, the X-ray source is a neutron star with a surface magnetic field ~107-1010 G and a spin frequency of a few hundred hertz, accreting gas via a Keplerian disk. Some of the accreting gas is channeled by the stellar magnetic field but some remains in a Keplerian disk flow that penetrates to within a few kilometers of the stellar surface. The frequency of the higher frequency QPO in a kilohertz QPO pair is the Keplerian frequency at a radius near the sonic point at the inner edge of the Keplerian flow, whereas the frequency of the lower frequency QPO is the difference between the Keplerian frequency at a radius near the sonic point and the fundamental or first overtone of the stellar spin frequency. The difference between the frequencies of the pair of QPOs is therefore close to (but not necessarily equal to) the stellar spin frequency. The amplitudes of the QPOs at the sonic-point Keplerian frequency and at the beat frequency depend on the strength of the neutron star's magnetic field and the accretion rate, and hence one or both of these QPOs may sometimes be undetectable. Oscillations at the stellar spin frequency and its overtones are expected to be weak but may sometimes be detectable. This model is consistent with the magnetic field strengths, accretion rates, and scattering optical depths inferred from previous modeling of the X-ray spectra and rapid X-ray variability of the atoll and Z sources. It explains naturally the frequencies of the kilohertz QPOs and the similarity of these frequencies in sources with different accretion rates and magnetic fields. The model also explains the high coherence and large amplitudes of the kilohertz QPOs and the steep increase of QPO amplitude with photon energy. The increase in QPO frequency with inferred accretion rate seen in many sources is also understandable in this model. We show that if the frequency of the higher frequency QPO in a pair is an orbital frequency, as in the sonic-point model, the frequencies of these QPOs place interesting upper bounds on the masses and radii of the neutron stars in the kilohertz QPO sources and provide new constraints on the equation of state of matter at high densities. Further observations of these QPOs may provide compelling evidence for the existence of a marginally stable orbit, confirming a key prediction of general relativity in the strong-field regime.

ASCA Observations of Transient X-Ray Sources in Quiescence

Soft X-ray transients are most probably low-mass binary systems containing either a neutron star (NS-LMXB) or a black hole (BH-LMXB). We have systematically investigated thirteen soft X-ray transients (four NS-LMXBs and nine BH-LMXBs) in the quiescent state using the available ASCA data. The luminosities of the observed NS-LMXBs in quiescence are on the order of ∼ 1032−33 erg s−1, whereas those of the BH-LMXBs are found to be systematically lower, most of them giving upper limits in the range below 1032−33 erg s−1 . The quiescent luminosity of LMXBs may be explained in terms of an optically thin, advection-dominated accretion disk, for which most of the thermal and kinetic energies of accreting matter are advected into the compact object with little X-ray emission. However, this scenario predicts a much larger luminosity for NS-LMXBs than observed, if the advected energies are liberated at the neutron-star surface, which is absent in BH-LMXBs. Mass accretion onto the neutron-star surface may be suppressed through some mechanism, such as the so-called propeller effect. The soft component observed in the NS-LMXBs is most probably emission from the magnetic pole regions of the neutron star.

PSR J1012+5307: younger than it looks?

Lorimer et al. have recently reported that the spin-down age (∼7 × 109 yr) of the low-mass binary pulsar PSR J1012+5307 is much higher than the cooling age (3 × 108 yr) of its white dwarf companion. The proposed solutions for this discrepancy are outlined and discussed. In particular, the revised cooling age estimate proposed by Alberts et al. agrees with data from other low-mass binary pulsar systems if a transition to the ‘classical’ cooling regime occurs between ∼0.14 and ∼0.28 M⊙. If this transition is excluded, PSR J1012+5307 seems to have finished its accretion phase far from the spin-up line.

Lense‐Thirring Precession and Quasi‐periodic Oscillations in X‐Ray Binaries

It has recently been suggested that gravitomagnetic precession of the inner part of the accretion disk, possibly driven by radiation torques, may be responsible for some of the quasi-periodic X-ray brightness oscillations (QPOs) and other spectral features with frequencies between 20 and 300 Hz observed in the power spectra of some low-mass binary systems containing accreting neutron stars and black hole candidates. We have explored the free and driven normal modes of geometrically thin disks in the presence of gravitomagnetic and radiation warping torques. We have found a family of low-frequency gravitomagnetic (LFGM) modes with precession frequencies that range from the lowest frequency allowed by the size of the disk up to a certain critical frequency ωcrit, which is ~1 Hz for a compact object of solar mass. The lowest frequency (lowest order) LFGM modes are similar to the previously known radiation warping modes, extend over much of the disk, and have damping rates 10 times their precession frequencies. The highest frequency LFGM modes are tightly wound spiral corrugations of the disk that extend to ~10 times its inner radius and have damping rates 103 times their precession frequencies. A radiation warping torque can cause a few of the lowest frequency LFGM modes to grow with time, but even a strong radiation warping torque has essentially no effect on the LFGM modes with frequencies 10-4 Hz. We have also discovered a second family of high-frequency gravitomagnetic (HFGM) modes with precession frequencies that range from ωcrit up to slightly less than the gravitomagnetic precession frequency ωgm,i of a particle at the inner edge of the disk, which is 30 Hz if the disk extends inward to the innermost stable circular orbit around a 2 M☉ compact object with dimensionless angular momentum cJ/GM2 = 0.2. The lowest frequency HFGM modes are very strongly damped and have warp functions and precession frequencies very similar to those of the highest frequency LFGM modes. In contrast, the highest frequency (lowest order) HFGM modes are very localized spiral corrugations of the inner disk and are weakly damped, with Q-values of ~2-50. We discuss the implications of our results for the observability of Lense-Thirring precession in X-ray binaries.

The evolution of short-period binary pulsars: a systematic study

We present evolutionary sequences for low-mass close binary systems in which a low-mass (1.0–1.5 M⊙) secondary star transfers mass to a neutron star. Roche lobe filling occurs when the secondary is a turn-off main-sequence star (having a small helium core). We assume loss of angular momentum owing to gravitational wave radiation and magnetic braking. We have found that the loss (and the mechanism of loss) of mass and angular momentum from the system is the main factor determining the value of the bifurcation period (Pbif). The bifurcation period separates the formation of the converging systems from the diverging systems. Variations in the initial chemical composition, and in the initial mass of the secondary, lead only to minor changes in Pbif. We have also investigated how changes in the chemical composition influence the initial orbital period (Pi) versus final orbital period (Pf) relation. The initial chemical composition has a more significant effect on this relation for shorter Pi than for longer Pi. We have found systematic differences for the Pf versus white dwarf mass relation for various chemical compositions. For converging systems, we have found that there is a boundary orbital period (Pb) such that if Pi < Pb, a system will evolve through the period gap (there are no low-mass X-ray binaries with orbital periods between one and three hours) with a Roche lobe overflowing secondary, but the accretion on to the neutron star is forbidden owing to the ‘propeller effect’. The systems will end their evolution as ultra-short period and very bright, low-mass X-ray binaries. If Pb < Pi < Pbif, then short orbital period millisecond binary pulsar systems will be formed.