Binary Stars In Globular Clusters Research Articles

Models for metal-poor stars with different initial abundances of C, N, O, Mg, and Si. II. Application to the colour–magnitude diagrams of the globular clusters 47 Tuc, NGC 6362, M 5, M 3, M 55, and M92

ABSTRACTStellar models for −2.5 ≤ [Fe/H] ≤−0.5 that have been computed for variations in the C:N:O abundance ratio (for two different values of [CNO/Fe]) are compared with HST Wide Field Camera 3 (WFC3) observations of the globular clusters (GCs) 47 Tuc, NGC 6362, M 5 M 3, M 55, and M 92. The bolometric corrections (BCs) used to transpose the models to the observed planes are based on new MARCS synthetic spectra that incorporate improved treatments of molecules that involve atoms of C, N, and O. On the assumption of well-supported distance moduli and reddenings, isochrones for [O/Fe] = 0.6 and [m/Fe] = 0.4 for the other α elements, which are favoured by binary stars in GCs, generally reproduce the main features of observed colour–magnitude diagrams (CMDs) to within ∼0.03 mag. In particular, they appear to match the spreads in the observed (MF336W − MF438W)0 colours that are spanned by CN-weak and CN-strong stars along the lower giant branch quite well, but not the bluest giants, which are suspected to be N-poor ([N/Fe] ≲ −0.5). Both the absolute (MF438W − MF606W)0 colours and the variations in these colours at a given MF606W magnitude on the giant branch are difficult to explain unless the reddest stars are C-rich ([C/Fe] ≳ +0.5). Allowing for moderate He abundance variations ($\delta \, Y \sim 0.05$) improves the fits to the observations.

The binary content of multiple populations in NGC 3201

We investigate the binary content of the two stellar populations that coexist in the globular cluster NGC 3201. Previous studies of binary stars in globular clusters have reported higher binary fractions in their first populations (P1, having field-like abundances) compared to their second populations (P2, having anomalous abundances). This is interpreted as evidence for the latter forming more centrally concentrated. In contrast to previous studies, our analysis focusses on the cluster centre, where comparable binary fractions between the populations are predicted because of short relaxation times. However, we find that even in the centre of NGC 3201, the observed binary fraction of P1 is higher (23.1 ± 6.2)% compared to (8.2 ± 3.5)% in P2. Our results are difficult to reconcile with a scenario in which the populations only differ in their initial concentrations, but instead suggests that the populations also formed with different fractions of binary stars.

The evolution of binary fractions in globular clusters

We study the evolution of binary stars in globular clusters using a new Monte Carlo approach combining a population synthesis code (StarTrack), and a simple treatment of dynamical interactions in the dense cluster core using a new tool for computing 3-body and 4-body interactions (Fewbody). We find that the combination of stellar evolution and dynamical interactions (binary–single and binary–binary) leads to a rapid depletion of the binary population in the cluster core. The maximum binary fraction today in the core of a typical dense cluster like 47 Tuc, assuming an initial binary fraction of 100%, is only about 5–10%. We show that this is in good agreement with recent HST observations of close binaries in the core of 47 Tuc, provided that a realistic distribution of binary periods is used to interpret the results. Our findings also have important consequences for the dynamical modeling of globular clusters, suggesting that “realistic models” should incorporate much larger initial binary fractions than has usually been done in the past.

Binary formation within globular clusters: X-ray clues

We have investigated the effect of the number of primordial binaries on the relationship between the total number of detected binaries within globular cluster and its collision rate. We have used simulated populations of binary stars in globular clusters : primordial binaries and binaries formed through gravitational interactions. We show that the initial number of primordial binaries influences the relationship between the number of detected sources and the collision rate, which we find to be a power law. We also show that observing an incomplete sample provides the same results as those obtained with a complete sample. We use observations made by XMM-Newton and Chandra to constrain the formation mechanism of sources with X-ray luminosities larger than 10^{31} erg/s, and show that some of the cataclysmic variables within globular clusters should be primordial objects. We point out a possibly hidden population of neutron stars within high mass globular clusters with a low collision rate.

W UMa-Type Binary Stars in Globular Clusters

A sample of 86 contact binary systems in 14 globular clusters with available color index data in B-V or in V-I has been analyzed. A large fraction of all systems (at least one-third) are numerous foreground Galactic disk projections over long lines of sight to the clusters. Since the selection of the cluster members has been based on the MV(log P,color) calibrations, the matter of a metallicity correction required particular attention with the result that such a correction is apparently not needed at the present level of accuracy. Analysis of the color-magnitude and period-color relations shows that globular cluster members have different properties from the Galactic disk contact systems: They are underluminous mainly because of the smaller sizes and, consequently, have shorter orbital periods; the color-index effect of the diminished blanketing is relatively less important, especially for V-I. Among the class 1 members (deviations in MV smaller than 0.5 mag), the most common are blue straggler (BS) systems. The variability amplitudes for the BS systems show a significantly different distribution from that for systems below the turn-off point (TOP): The BS systems in the sample have only small amplitudes (which may be an indication of small mass ratios), while the distribution for the systems below the TOP is peculiar in containing only large-amplitude systems. This difference may be linked to the relatively small number of the detected main-sequence contact systems below the TOP as resulting from an observational selection effect due to the rapidly increasing measurement difficulties below the TOP. As a consequence, efforts at determining the frequency of occurrence of the contact systems below the TOP have been judged to be premature. The frequency among the BS stars could be moderately well established at about 45 ± 10 BS stars per one contact BS binary; thus, contact binaries are about 3 times more common among the BS stars than among the disk population dwarfs. Contact binary systems with periods longer than 0.6 days are absent in the sample, possibly because the more massive stars have left the contact binary domain.

The Optical Counterparts of the Luminous X‐Ray Binary Stars in Globular Clusters

Ten percent of our GalaxyIs luminous ergs (L X 1036 s~1) X-ray point sources are located in globular clusters (GCs), but GCs contribute a far smaller fraction of normal stars to the Galaxy. X-ray bursts have been observed from nearly all GC sources, indicating that they are low-mass X-ray binary (LMXB) systems containing a neutron star and a low-mass companion star from which matter is being transferred. The understanding of LMXB overabundance in GCs may well lead to important insights into the formation and evolution of these exotic binary systems as well as the dynamics of GCs themselves. The goals of this dissertation are to identify the optical counterparts to some GC LMXBs lacking candidates, bring together and compare in the most homogeneous fashion all available Hubble Space T elescope (HST ) optical observations of the current group of GC LMXB counterparts, and discuss the implications for cluster dynamics and LMXB systems in general. In this work, candidates for three additional optical counterparts to luminous X-ray binaries in GCs are presented, thereby doubling the number of optical counterpart candidates. Two, those in NGC 1851 and NGC 6441, are very likely correct although they require additional work to conÐrm the identiÐcations, while the third, in NGC 6652, remains somewhat tentative because of the positional discrepancy with the X-ray coordinates and the fact that the entire error circle is not surveyed. A homogeneous set of HST photometric measurements for all the counterparts identiÐed thus far is presented, and their optical properties are compared with those of Ðeld LMXBs. The mean color of the cluster sources is (U[B)0 identical to that of the Ðeld sources, and the mean is M B0 similar in range, but fainter on average than bursters in the Ðeld. The ratio of optical to X-ray Nux of cluster sources seems to show a signiÐcantly larger dispersion than that of the Ðeld sources. The GC LMXBs seem to conform well to the previously suggested optical luminosity, X-ray luminosity, orbital period relation (J. van Paradijs & J. McClintock, A&A, 290, 133 [1994]). If the source in NGC 1851Efor which no orbital period has yet been determinedEfollows this pattern, then the period will likely be less than 0.8 hr. Similarly, if the object advanced here in NGC 6652 is indeed the correct optical counterpart, this relation suggests that its orbital period will also prove to be less than 1 hr. New and archival HST spectra and imaging data are analyzed to intercompare the UV/optical spectral energy distributions (SEDs) of GC LMXBs. A set of simple model SEDs is introduced and compared with the observations to infer accretion rates, disk diameters, and other properties of these systems. The GC LMXBs are particularly well suited for SED modeling as the distance and reddening of these systems is quite well understood, unlike most Ðeld LMXBs. The sources in NGC 6441 and M15 are found to be optically quite luminous, with comparatively shallow SEDs. The faint sources in NGC 6712 and NGC 1851 have signiÐcantly steeper SEDs and are most likely underluminous because their orbital separations and thus accretion disks are small. The case of NGC 6624 appears slightly di†erent from the others, with an average optical luminosity, yet with the steepest SED slope. This is most likely due to its high accretion rate isD10 times higher than any of the other (L X GC LMXBs) and extremely small size. From its luminosity, it is inferred that the source in NGC 6652 will have similar system parameters and SED to the one in NGC 1851. If these inferences prove correct, it will mean that twothirds of GC LMXBs for which orbital periods are known have periods less than 1 hr and are therefore quite likely double degenerate ultracompact binaries. Such a high fraction of ultracompact systems would be remarkable and may indicate that the processes responsible for creating such short-period systems are more common than previously thought.

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).

Three-dimensional hydrodynamical simulations of colliding stars . III - Collisions and tidal captures of unequal-mass main-sequence stars

A 3D smooth-particle hydrodynamic code with 7000 particles is used to simulate collisions between lower main sequence (MS) stars whose masses differ by a factor of five. Collisions between point-mass intruders and MS stars are simulated to study how the physics of the encounters depends on the finite radius of the smaller MS stars. It is found that the two MS stars become gravitationally bound in a physical collision if their relative velocity at infinity is less than a critical velocity Vd. Vd decreases from 1000 km/s in a head-on collision to 150 km/s in a grazing one. If the less massive star is replaced by a point mass, Vd remains the same for grazing collision and tidal encounters but drops to about 600 km/s in head-on collisions. The collision and coalescence of two MS stars resets the nuclear clock of the coalesced star, so it contracts to the MS. Tidal capture of binary stars in globular clusters can only occur in encounters in which the closest approach of the two stars to their center of mass is less than 2.0 times the sum of their radii.

Did the binary stars in globular clusters come to an early end by being too eccentric?

The orbits of hard binaries in globular clusters shrink and develop high eccentricities in encounters with other stars. The most eccentric binaries suffer the physical coalescence of their two components. Slightly less eccentric binaries shrink down to very tightly bound binaries as a result of dissipation (tidal, gravitational radiation) at periastron. Visual binaries in dense globular clusters have either been dissociated by stellar encounters or they have shrunk by these mechanisms to form a combination of coalesced stars and very close binaries. These mechanisms may contribute significantly to the observed scarcity of binaries among globular cluster giants, but at the same time, they form and allow the retention of the close binaries that are needed to produce the x-ray sources in globular clusters.

Main-sequence binaries, contact binaries, and blue stragglers in globular clusters

The existence of binary stars in globular clusters has a long history of controversy1–3. At present the only evidence for binaries in globular clusters comes from binaries in extreme states: cataclysmic variables4,5 and X-ray sources (see refs 6 and 7 for reviews, and ref. 8 for new observations of very weak X-ray sources). The existence of the latter is customarily ascribed to tidal capture of red dwarf companions by neutron stars (or possibly solar-mass black holes)9. In fact, tidal capture should be at least as effective in creating close binaries comprising two main-sequence stars9,10. Subsequent evolution of such binaries is very likely to lead to mass exchange between the two, and possibly merger. In conventional calculations, gravitational radiation drives the binary orbital evolution11,12; recently, it has been shown that in the denser globular clusters, where tidal capture is most effective, gravitational encounters with passing field stars also have an important role, both by slow, cumulative compression of the binary orbit, and by occasional catastrophic physical collisions13. On the basis of these mechanisms, we predict that globular clusters should contain substantial numbers of: close binaries; low-mass contact binaries, perhaps similar to the somewhat more massive WUMa stars; and ‘blue stragglers’, stars lying on the extrapolated main sequence beyond the turn-off. These objects should be most common in the densest regions of the Galaxy's globular cluster system.

On the incidence of close binary stars in globular clusters and the nature of the cluster X-ray sources

{ '1§i7I~Ev'§€6I lie:-I Mon. Not. R. astr. Soc. (1977) 178, 335~343 On the incidence of close binary stars in globular clusters and the nature of the cluster X-ray sources Virginia * Department of Physics, University of California, Irvine, California 92 71 7, USA, and Astronomy Program, University of Maryland, College Park, Maryland 20 742, USA Received 1976 July 22; in original fonn 1976 May 17 Summary. Recent calculations by Hills suggest that the globular clusters could not have formed with more than 20 per cent of the normal Population I fraction of their stars in binary systems. The fact that the clusters have more than their fair share of novae and U Geminorum stars (three each out of ~ 200 of each known, while the clusters contain only about 10'“ of the mass and 10’3 of the luminosity of the galaxy) therefore becomes surprising. The hypo- thesis of binary capture within cluster cores suggested by Fabian, Pringle & Rees to account for the clusters’ high X-ray luminosity provides a few extra systems, but neither it nor any of the similar encounter or capture mechanisms suggested can account for the novae and U Gem stars, which remain puzzling. The numbers of Algol-type and WUMa eclipsing binaries predicted by these hypotheses do not conflict with data presently available, but careful searches for them would constitute a critical test of the theories. Background The identification of five strong, variable X-ray sources with the globular clusters NGC 1851, 6440, 6441, 6624 and 7078 (Giacconi et al. 1974; Clark, Markert & Li 1975; Markert et al. 1975) implies that the ratio of X-ray luminosity to optical luminosity (or mass) for the clusters is about 100 times that for our Galaxy as a whole (Katz 1975). The sorts of systems thought to be responsible for most galactic X-ray sources (neutron stars accreting matter supplied by normal companions in close binary systems) caxmot have survived in globular clusters from the time of their formation 10 billion or so years ago until the present (Clark 1975). Thus, some other mechanism is required. The clusters involved are richer and more centrally condensed (or, alternatively, have larger escape velocities and smaller core relaxation times) than typical globular clusters (Arp 1965; Kukarkin 1974; Peterson & King 1975), although not all rich, condensed clusters are X-ray sources (Uhner, Murray & Gursky 1976a). The mechanisms suggested have, there- fore, focused on processes that would be favoured by high stellar density and large escape * Alfred P. Sloan Foundation Research Fellow, 1972-74. © Royal Astronomical Society 0 Provided by the NASA Astrophysics Data System