Primordial Binaries Research Articles

Origin and dynamics of comets and star formation

Several important consequences that would arise for the solar system's comet population if the Sun formed in an embedded star cluster are outlined. Owing to the higher stellar number density and larger tidal fields in this environment, formation of the comet cloud would proceed differently from that envisaged in a usually assumed isolated solar nebula scenario. In particular, early planetesimal scattering by the giant planets may lead to a substantial population of free-floating intracluster comets. Furthermore, the potentially important influence the primordial binaries may have had for cometary dynamics is emphasized. Long-period binaries are likely to accelerate planetesimal growth, and short-period binaries increase the capture cross-section of intracluster comets. Some of the intracluster comets might have been captured by the Sun and remained bound to it until after the cluster dispersed. If captured comets were in stable orbits over the remaining age of the solar system, it is expected they will constitute a second class of comets in addition to the comets scattered outwards by the planets.

Orbital Characteristics of Binary Systems after Asymmetric Supernova Explosions

We present an analytical method for studying the changes of the orbital characteristics of binary systems with circular orbits due to a kick velocity imparted to the newborn neutron star during a supernova (SN) explosion. Assuming a Maxwellian distribution of kick velocities we derive analytical expressions for the distribution functions of orbital separations and eccentricities immediately after the explosion, of orbital separations after circularization of the post-SN orbits, and of systemic velocities of binaries that remain bound after the explosion. These distributions of binary characteristics can be used to perform analytical population synthesis calculations of various types of binaries, the formation of which involves a supernova explosion. We study in detail the dependence of the derived distributions on the kick velocity and the pre-SN characteristics, we identify all the limits imposed on the post-SN orbital characteristics, and we discuss their implications for the population of X-ray binaries and double neutron star systems. We show that large kick velocities do not necessarily result in large systemic velocities; for typical X-ray binary progenitors the maximum post-SN systemic velocity is comparable to the relative orbital velocity prior to the explosion. We also find that, unless accretion-induced collapse is a viable formation channel, X-ray binaries in globular clusters have most probably been formed by stellar dynamical interactions only and not directly from primordial binaries.

Close approach during hard binary-binary scattering

It is now clear that there is a substantial population of primordial binaries in Galactic globular clusters and that binary interactions are a major influence on globular cluster evolution. Collisional interactions involving stars in binaries may provide a significant channel for the formation of various stellar exotica, such as blue stragglers, X-ray binaries and millisecond pulsars. We report on an extensive series of numerical experiments of binary-binary scattering, analysing the cross-section for close approach during interactions for a range of hard binary parameters of interest in globular cluster cores. We consider the implied rate for tidal interactions for different globular clusters and compare our results with previous, complementary estimates of stellar collision rates in globular clusters. We find that the collision rate for binary-binary encounters dominates in low-density clusters if the binary fraction in the cluster is larger than 0.2 for wide main-sequence binaries. In dense clusters, binary-single interactions dominate the collision rate and the core binary fraction must be ≾0.1 per decade in semimajor axis, or too many collisions take place compared with observations. The rates are consistent if binaries with semimajor axes ∼ 100 au are overabundant in low-density clusters, or if breakup and ejection substantially lower the binary fraction in denser clusters. Given reasonable assumptions about fractions of binaries in the cores of low-density clusters such as NGC 5053, we cannot account for all the observed blue stragglers by stellar collisions during binary encounters, suggesting that a substantial fraction may be due to coalescence of tight primordial binaries.

Truncation of the Binary Distribution Function in Globular Cluster Formation

Recent observational searches suggest that the frequency of primordial binaries in globular clusters may reach ~ 10% (see Hut et al. 1992 for a review). Several different treatments conclude that primordial binaries are effective in halting core collapse, supporting the core and driving the post-core collapse expansion phase (Goodman & Hut 1989, McMillan et al. 1990, 1991, Gao et al. 1991, Heggie & Aarseth 1992, McMillan & Hut 1994). The abundance and binding energy distribution have a direct impact on observable characteristics of globular clusters such as the size of the core radius (Vesperini & Chernoff 1994). In this analysis we have investigated how the initial binary distribution function may be altered in the formation of a cluster; the key question we have addressed is whether it is possible for binaries, assumed to be primordial, to survive the birth of the cluster. A detailed description of our results is in Vesperini & Chernoff (1995).

Truncation of the Binary Distribution Function in Globular Cluster Formation

Recent observational searches suggest that the frequency of primordial binaries in globular clusters may reach ~ 10% (see Hut et al. 1992 for a review). Several different treatments conclude that primordial binaries are effective in halting core collapse, supporting the core and driving the post-core collapse expansion phase (Goodman & Hut 1989, McMillan et al. 1990, 1991, Gao et al. 1991, Heggie & Aarseth 1992, McMillan & Hut 1994). The abundance and binding energy distribution have a direct impact on observable characteristics of globular clusters such as the size of the core radius (Vesperini & Chernoff 1994). In this analysis we have investigated how the initial binary distribution function may be altered in the formation of a cluster; the key question we have addressed is whether it is possible for binaries, assumed to be primordial, to survive the birth of the cluster. A detailed description of our results is in Vesperini & Chernoff (1995).

The Role of Binaries in the Dynamical Evolution of the Core of a Globular Cluster

The size of the core is one of the main diagnostics of the evolutionary state of a globular cluster. Much has been learned over the last few years about the behavior of the core radius during and after core collapse, under a variety of different conditions related to the presence or absence of large numbers of binaries. An overview is presented of the basic physical principles that can be used to estimate the core radius. Four different situations are discussed, and expressions are presented for the ratio rc/rh of core radius to half mass radius. The regimes are: deep collapse in the absence of primordial binaries; steady post-collapse evolution after primordial binaries have been burned up; chaotic post-collapse evolution under the same conditions; and post-collapse evolution in the presence of primordial binaries. In addition, modifications to all of these cases are indicated for the more realistic situation where effects of the galactic tidal field are taken into account.

Searches for Binary Stars in Globular Clusters

Primordial binaries in globular clusters are important both because their properties are an integral part of the description of the stellar population and because they can strongly influence the dynamical evolution of the cluster (see Hut, this volume).

Dynamics and Binary (Trans)formation in Globular Clusters

Globular clusters form ideal laboratories for studying the interactions between stellar evolution and stellar dynamics. In the past, highly exceptional systems such as X-ray binaries and later millisecond pulsars have provided us with useful diagnostic tools. However, the fate of the bulk of the more normal stars has remained less clear. At present, rapid progress is being made in our understanding of the distributions of normal stars and primordial binaries, as well as their most abundant reaction products: blue stragglers and binaries that are produced through exchange encounters with other single stars or binaries. The complexity of the network of exchange reactions is illustrated through some specific examples, such as a formation scenario for the hierarchical triple system containing the millisecond pulsar PSR B1620-26 in M4, the first triple star system ever detected in a globular cluster.

A stellar audit: the computation of encounter rates for 47 Tucanae and ω Centauri

Using King-Mitchie Models, we compute encounter rates between the various stellar species in the globular clusters $\omega$ Cen, and 47 Tuc. We also compute event rates for encounters between single stars and a population of primordial binaries. Using these rates, and what we have learnt from hydrodynamical simulations of encounters performed earlier, we compute the production rates of objects such as low-mass X-ray binaries (LMXBs), smothered neutron stars and blue stragglers (massive main-sequence stars). If 10\% of the stars are contained in primordial binaries, the production rate of interesting objects from encounters involving these binaries is as large as that from encounters between single stars. For example, encounters involving binaries produce a significant number of blue stragglers in both globular cluster models. The number of smothered neutron stars may exceed the number of low-mass X-ray binaries (LMXBs) by a factor of 5-20, which may help explain why millisecond pulsars are observed to outnumber LMXBs in globular clusters.

Blue Stragglers and Other Stellar Anomalies: Implications for the Dynamics of Globular Clusters

There are several observed phenomena in globular clusters that are thought to be the result of dynamical processes or binary star evolution. This review examines these manifestations of the interaction between globular cluster dynamics and stellar evolution. Blue stragglers may be formed by the evolution of primordial binaries or by collisions. Current evidence suggests that both processes are likely to occur, and that the observed blue straggler sequences can place dynamically interesting limits on rates. Color gradients in globular clusters are thought to be caused by the stripping of giants by collisions, although the creation of blue subdwarfs by the same process may also be required to explain the observations. The observed X-ray sources and radio pulsars are apparently also made by a variety of dynamical processes that are still not fully understood.

On the luminosity function, lifetimes, and origin of blue stragglers in globular clusters

We compute theoretical evolutionary tracks of blue stragglers created by mergers. Two formation scenarios are considered: mergers of primordial binaries, and stellar collisions. These two scenarios predict strikingly different luminosity functions, which are potentially distinguishable observationally. Tabulated theoretical luminosity functions and lifetimes are presented for blue stragglers formed under a variety of input conditions. We compare our results with observations of the blue straggler sequences in 47 Tucanae and M3. In the case of 47 Tuc, the luminosity function and the formation rate are compatible with the hypothesis that the blue stragglers formed through the collision of single stars. Mergers of primordial binaries are only marginally cosistent with the data, and a significant enhancement of the collision cross section by binary-single-star encounters appears to be ruled out. In the case of M3, we find that the innermost blue stragglers have a luminosity function significantly different from that of the outer stragglers, thus confirming earlier suggestions that there are two distinct populations of blue stragglers in this cluster. The inner stragglers are preferentially brighter and bluer, as would be expected if they were made by collisions, but there are so many of them that the collision rate would need to be enhanced by interactions involving wide binaries. The luminosity function of the outer stragglers is almost identical to the predictions of mergers from primordial binaries and is inconsistent with the collision hypothesis.

Primordial main equence binary stars in the globular cluster M71

We report the identification of five short-period variables near the center of the metal-rich globular cluster M71. Our observations consist of multiepoch VI charge coupled device (CCD) images centered on the cluster and covering a 6.3 min x 6.3 min field. Four of these variables are contact eclipsing binaries with periods between 0.35 and 0.41 days; one is a detached or semidetached eclipsing binary with a period of 0.56 days. Two of the variables were first identified as possible eclipsing binaries in an earlier survey by Hodder et al. (1992). We have used a variety of arguments to conclude that all five binary stars are probable members of M71, a result that is consistent with the low number (0.15) of short-period field binaries expected along this line of sight. Based on a simple model of how contact binaries evolve from initially detached binaries, we have determined a lower limit of 1.3% on the frequency of primordial binaries in M71 with initial orbital periods in the range 2.5 - 5 days. This implies that the overall primordial binary frequency, f, is 22(sup +26)(sub -12)% assuming df/d log P = const ( the 'flat' distribution), or f = 57(sup +15)(sub -8)% for df/d log P = 0.032 log P + const as observed for G-dwarf binaries in the solar neighborhood (the 'sloped' distribution). Both estimates of f correspond to binaries with initial periods shorter than 800 yr since any longer-period binaries would have been disrupted over the lifetime of the cluster. Our short-period binary frequency is in excellent agreement with the observed frequency of red-giant binaries observed in globulars if we adopt the flat distribution. For the sloped distribution, our results significantly overestimate the number of red-giant binaries. All of the short-period M71 binaries lie within 1 mag of the luminosity of the cluster turnoff in the color-magnitude diagram despite the fact we should have easily detected similar eclipsing binaries 2 - 2.5 mag fainter than this. We discuss the implications of this on our estimates of the binary frequency in M71 and on the formation of blue stragglers.

The range of core size of postcollapse clusters supported by primordial binaries

We investigate how the core size of globular clusters in a postcollapse phase driven by primordial binaries depends upon the parameters of the distribution of the binary binding energy and the abundance of the binaries. We show that the variation of these quantites increases the plausible range for the core size.

Star cluster evolution with primordial binaries. 3: Effect of the Galactic tidal field

We present the results of N-body simulations of tidally limited star clusters with an initial population of 0%-20% binaries. We find that (1) if enough binaries are initially present, the binary fraction may fall to a minimum value, then increase at late times; (2) the cluster evaporation timescale is quite insensitive to the details of the initial binary distribution; (3) the cluster core radius stabilizes at a few percent of the half-mass radius when binaries are present, just as in the case of isolated clusters; and (4) there may be a marked difference between the spatial distribution of low-energy and high-energy binaries as the cluster evolves. Specifically, the spatial distribution of the lower energy systems is often substantially more extended than that of the more tightly bound pairs. At no time are our simulated clusters well described by simple dynamical models that neglect the close coupling between the binding energies and the center-of-mass energies of the binaries they contain.

Formation and evolution of luminous supersoft X-ray sources

Luminous supersoft X-ray sources, with characteristic luminosities of approximately 10<SUP>38</SUP> ergs/s and temperatures, kT, of approximately 35 eV, have been established as a new and distinct class of X-ray source through recent Roentgen Satellite (ROSAT) observations. Several possible physical models have been proposed for these sources. One promising scenario (van den Heuvel et al. 1992) involves mass transfer, which is unstable on a thermal timescale, from a main-sequence or subgiant donor star onto the surface of a white dwarf. For a narrow range of accretion rates, steady nuclear burning of the accreted matter can take place. This process can provide the high luminosities and the correct range of temperatures observed in the supersoft sources. However, given the limited range of mass transfer rates that are consistent with this phenomenon, it is far from obvious that a sufficient population of such systems exists in galaxies such as our own, M31, and the Magellanic Clouds, in order to account for the large number of supersoft sources which can be inferred from present observations. This work addresses the population question in detail, through a Monte Carlo simulation of the formation and evolution of such systems, which starts with zero-age primordial binaries. In order to evolve into close binary systems which contain a white dwarf component and a companion transferring mass at a rate within the requisite narrow range, a binary system must undergo a specific progression of evolutionary steps. We find that a sufficient subset of our initial binaries evolve to become systems with the requisite properties, so that they can account for the population of supersoft sources that is inferred from observations. In particular, we find that there should be more than 1000 systems in the Galaxy today with properties that very closely match those of the observed supersoft sources. From our models, we find expected luminosities, white dwarf effective temperatures, and orbital periods in the ranges of 10<SUP>37</SUP> - 10<SUP>38</SUP> ergs/s, (1-5) x 10<SUP>5</SUP> K, and 8 hr-1.4 days, respectively. The masses of the white dwarf and donor star are expected to lie in the range of 0.7-1.05 solar mass and 1.3-2.7 solar mass. Finally, we discuss the role that supersoft X-ray sources may play as the progenitors of Type Ia supernovae, and estimate the rate of production via this channel.

Binary--Single Star Interactions in Globular Clusters

An extensive series of three-body interactions involving hard binaries and single stars are calculated by direct integration of ∼10 5 encounters. Unlike previous calculations, the stars have different mass ratios, chosen to be representative of the stellar and remnant population in a globular cluster, and most effort was devoted to the difficult case of hard binaries, the most likely primordial binaries to survive in a cluster. Graphs, tables and analytical fits to differential and integral cross sections for properties of different interaction channels are presented and discussed

NGC 6397: a case study in the resolution of post-collapse globular cluster cores

Model surface brightness profiles based on Fokker-Planck simulations have been used to assess the high resolution surface brightness profile of the globular cluster NGC 6397 of Lauzeral et al. (1992). The profile is well fitted by the model in the maximum-expansion phase of a gravothermal oscillation with a core radius of 0.06 pc (6''), but an unresolved core cannot be ruled out. A core this size is also expected in highly evolved clusters including significant numbers of primordial binaries. A more massive cluster will provide better statistics, as would star counts. The greater distance of the other globular clusters and the requirements of star counting will demand very high resolution observations to definitively resolve cores this size.

Does the coalescence of white dwarfs produce millisecond pulsars in globular clusters?

view Abstract Citations (20) References (37) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Does the Coalescence of White Dwarfs Produce Millisecond Pulsars in Globular Clusters? Chen, Kaiyou ; Leonard, Peter J. T. Abstract Isolated main-sequence stars with initial masses in the range from about 1 to 8 or 9 solar masses will evolve into C + O white dwarfs with masses up to about 1.1 solar mass, and those main-sequence stars with initial masses in the range from about 8 or 9 to 10 or 11 solar masses may produce O + Ne(+Mg) white dwarfs with masses up to about 1.4 solar mass or less. Many such white dwarfs should exist in globular clusters, and pairs of them should coalesce frequently via collisional processes, especially if primordial binaries are present. It is quite possible that most of the short-period millisecond pulsars in the precore-collapse globular clusters have been produced via mergers of white dwarf pairs with total masses exceeding the Chandrasekhar limit. Publication: The Astrophysical Journal Pub Date: July 1993 DOI: 10.1086/186916 Bibcode: 1993ApJ...411L..75C Keywords: Coalescing; Globular Clusters; Pulsars; Stellar Evolution; White Dwarf Stars; Neutron Stars; Supernovae; Astrophysics; GALAXY: GLOBULAR CLUSTERS: GENERAL; STARS: SUPERNOVAE: GENERAL; STARS: NEUTRON; STARS: WHITE DWARFS full text sources ADS | data products SIMBAD (7)

Canada-France-Hawaii Telescope observations of globular cluster cores - Blue straggler stars in M3 (NGC 5272, GC 1339 + 286)

view Abstract Citations (47) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Canada-France-Hawaii Telescope Observations of Globular Cluster Cores: Blue Straggler Stars in M3 (NGC 5272, GC 1339+286) Bolte, Michael ; Hesser, James E. ; Stetson, Peter B. Abstract We have obtained a complete inventory of stars brighter than the main-sequence turnoff inside 5 core radii, r(c), in M3, which reveals a blue straggler star (BSS) population in the central regions of this intermediate-density cluster. The data are derived from CCD frames with FWHM below 0.6 arcsec. In the 2 x 2 arcmin region surveyed, the BSSs are marginally more centrally concentrated than giants in the same V-magnitude range. The specific frequency of BSSs inside 5 r(c) is about 2.8 and about 3.9 times lower than that measured in the outskirts of M3 and in the low-density cluster NGC 5053, respectively. This result suggests that the production of BSSs through direct stellar collisions or the formation of hard binaries is not significant for central stellar densities up to that of the core of M3. Rather, we may be seeing the effects of a higher destruction rate for primordial binaries in the core of M3 compared with its outer regions, or compared with the lower density cluster NGC 5053. Publication: The Astrophysical Journal Pub Date: May 1993 DOI: 10.1086/186838 Bibcode: 1993ApJ...408L..89B Keywords: Binary Stars; Globular Clusters; Spatial Distribution; Stellar Cores; Stellar Magnitude; Main Sequence Stars; Stellar Luminosity; Astrophysics; GALAXY: GLOBULAR CLUSTERS: GENERAL; STARS: BINARIES: GENERAL; STARS: KINEMATICS full text sources ADS | data products SIMBAD (1)

Dynamical effects of primordial binaries in star clusters - I. Equal masses

The evolution of an idealized star cluster in which all stars have the same mass and initially a modest fraction of the stars form hard binaries is described. The treatment ranges from simple theory based on considerations of time-scales and energetics, through continuum models which include the effects of segregation of binaries (as they have twice the mass of the single stars), to N-body simulations of systems with a few thousand stars. Particular consideration is given to the circumstances at the close of core collapse and its immediate aftermath