A binary origin for 'blue stragglers' in globular clusters (original) (raw)
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The origins of blue stragglers and binarity in globular clusters
Monthly Notices of the Royal Astronomical Society, 2013
Two basic formation channels have been proposed for blue straggler stars in globular clusters: binary star evolution and stellar collisions. We recently showed that the number of blue stragglers found in the core of a globular cluster is strongly correlated with the total stellar mass of the core, but not with the collision rate in the core. This result strongly favoured binary evolution as the dominant channel for blue straggler formation. Here, we use newly available empirical binary fractions for globular clusters to carry out a more direct test of the binary evolution hypothesis, but also of collisional channels that involve binary stars. More specifically, using the correlation between blue straggler numbers and core mass as a benchmark, we test for correlations with the number of binary stars, as well as with the rates of single-single, single-binary, and binary-binary encounters. We also consider joint models, in which blue straggler numbers are allowed to depend on star/binary numbers and collision rates simultaneously.
An analytic model for blue straggler formation in globular clusters
Monthly Notices of the Royal Astronomical Society, 2011
We present an analytic model for blue straggler formation in globular clusters. We assume that blue stragglers are formed only through stellar collisions and binary star evolution, and compare our predictions to observed blue straggler numbers taken from the catalogue of Leigh, Sills & Knigge (2011). We can summarize our key results as follows: (1) Binary star evolution consistently dominates blue straggler production in all our best-fitting models.
Collisions of Main-Sequence Stars and the Formation of Blue Stragglers in Globular Clusters
The Astrophysical Journal, 1996
We report the results of new SPH calculations of parabolic collisions between two main-sequence stars in a globular cluster. Such collisions are directly relevant to the formation of blue stragglers. In particular, we consider parent stars of mass M/M T O = 0.2, 0.5, 0.75, and 1, where M T O is the cluster turnoff mass (typically about 0.8 M ⊙). Low-mass stars (with M = 0.2M T O or 0.5M T O) are assumed to be fully convective and are therefore modeled as n = 1.5 polytropes. Stars at the turnoff (with M = M T O) are assumed to be mostly radiative and are modeled as n = 3 polytropes. Intermediate-mass stars (with M = 0.75M T O) are modeled as composite polytropes consisting of a radiative core with polytropic index n = 3 and a convective envelope with n = 1.5. We focus our study on the question of hydrodynamic mixing of helium and hydrogen, which plays a crucial role in determining the observable characteristics of blue stragglers. In all cases we find that there is negligible hydrodynamic mixing of helium into the outer envelope of the merger remnant. The amount of hydrogen mixed into the core of the merger depends strongly on the entropy profiles of the two colliding stars. For two stars with nearly equal masses (and hence entropy profiles) very little hydrodynamic mixing occurs at all, especially if they are close to the turnoff point. This is because the hydrogen-rich material from both stars maintains, on average, a higher specific entropy than the helium-rich material. If the two parent stars are close to turnoff, very little hydrogen is present at the center of the merger remnant and the main-sequence lifetime of the blue straggler could be very short. In contrast, during a collision between two stars of sufficiently different masses (mass ratio q ∼ < 0.5), the hydrogen-rich material
Multiple stellar populations and their influence on blue stragglers
Monthly Notices of the Royal Astronomical Society, 2010
It has become clear in recent years that globular clusters are not simple stellar populations, but may host chemically distinct sub-populations, typically with an enhanced helium abundance. These helium-rich populations can make up a substantial fraction of all cluster stars.
Double Blue Straggler Sequences in Globular Clusters: The Case of NGC 362
The Astrophysical Journal, 2013
We used high-quality images acquired with the WFC3 on board the HST to probe the blue straggler star (BSS) population of the Galactic globular cluster NGC 362. We have found two distinct sequences of BSS: this is the second case, after M 30, where such a feature has been observed. Indeed the BSS location, their extension in magnitude and color and their radial distribution within the cluster nicely resemble those observed in M 30, thus suggesting that the same interpretative scenario can be applied: the red BSS sub-population is generated by mass transfer binaries, the blue one by collisions. The discovery of four new W UMa stars, three of which lying along the red-BSS sequence, further supports this scenario. We also found that the inner portion of the density profile deviates from a King model and is well reproduced by either a mild power-law (α ∼ −0.2) or a double King profile. This feature supports the hypothesis that the cluster is currently undergoing the core collapse phase. Moreover, the BSS radial distribution shows a central peak and monotonically decreases outward without any evidence of an external rising branch. This evidence is a further indication of the advanced dynamical age of NGC 362: in fact, together with M 30, NGC 362 belongs to the family of dynamically old clusters (Family III) in the "dynamical clock" classification proposed by . The observational evidence presented here strengthens the possible connection between the existence of a double BSS sequence and a quite advanced dynamical status of the parent cluster.
A Blue Straggler Binary with Three Progenitors in the Core of a Globular Cluster?
The Astrophysical Journal, 2006
We show that the X-ray source W31 in the core of the globular cluster 47 Tucanae is physically associated with the bright blue straggler BSS-7. The two sources are astrometrically matched to 0.061 ′′ , with a chance coincidence probability of less than 1%. We then analyse optical time-series photometry obtained with the Hubble Space Telescope (HST) and find that BSS-7 displays a 1.56 day periodic signal in the I band. We also construct a broad-band (far-ultraviolet through far-red) spectral energy distribution for BSS-7 and fit this with single and binary models. The binary model is a better fit to the data, and we derive the corresponding stellar parameters.
Blue Straggler Formation in Clusters
AIP Conference Proceedings, 2010
Blue stragglers are thought to be formed from the merger or coalescence of two stars, but the details of their formation in clusters has been difficult to disentangle. We discuss the two main formation mechanisms for blue stragglers (stellar collisions or mass transfer in a binary system). We then look at the additional complications caused by the stars living in the dynamically active environment of a star cluster. We review the recent observational and theoretical work which addresses the question "which mechanism dominates?" and conclude that the most likely answer is that both mechanisms are at work, although with different importances in different environments and at different times in the cluster lifetime. We conclude with a short discussion of some avenues for future work.
The Evolution of Blue Stragglers Formed via Stellar Collisions
The Astronomical Journal, 1998
We have used the results of recent smoothed particle hydrodynamic simulations of colliding stars to create models appropriate for input into a stellar evolution code. In evolving these models, we find that little or no surface convection occurs, precluding angular momentum loss via a magnetically-driven stellar wind as a viable mechanism for slowing rapidly rotating blue stragglers which have been formed by collisions. Angular momentum transfer to either a circumstellar disk (possibly collisional ejecta) or a nearby companion are plausible mechanisms for explaining the observed low rotation velocities of blue stragglers Under the assumption that the blue stragglers seen in NGC 6397 and 47 Tuc have been created solely by collisions, we find that the majority of blue stragglers cannot have been highly mixed by convection or meridional circulation currents at anytime during their evolution. Also, on the basis of the agreement between the predictions of our non-rotating models and the observed blue straggler distribution, the evolution of blue stragglers is apparently not dominated by the effects of rotation.
2013
We present the first dynamical study of Blue Straggler Stars (BSSs) in three Galactic globular clusters, NGC 3201, NGC 5139 (ωCen), and NGC 6218, based on medium-resolution spectroscopy (R 10000) obtained with IMACS. Our BSS candidate selection technique uses HST/ACS and ESO/WFI photometric data out to >4.5 r_c. We use radial velocity measurements to discard non-members and achieve a success rate of ∼93%, which yields a sample of 116 confirmed BSSs. Using the penalized pixel fitting method (pPXF) we measure the vsin(i) values of the sample BSSs and find their distribution functions peaked at slow velocities with a long tail towards fast velocities in each globular cluster. We find that the BSSs in NGC 3201 and NGC 6218 which show vsin(i)>50 km s^-1 are all found in the central cluster regions, inside a projected 2 r_c, of their parent clusters. We find a similar result in ωCen for BSSs with vsin(i)>70 km s^-1 which are all, except for two, concentrated inside 2 r_c. In all ...
On the triple origin of blue stragglers
Astrophysical Journal Letters, 2009
Blue straggler stars (BSSs) are stars observed to be hotter and bluer than other stars with the same luminosity in their environment. As such they appear to be much younger than the rest of the stellar population. Two main channels have been suggested to produce such stars: (1) collisions between stars in clusters or (2) mass transfer between, or merger of, the components of primordial short-period binaries. Here we suggest a third scenario, in which the progenitor of BSSs are formed in primordial (or dynamically formed) hierarchical triple stars. In such configurations the dynamical evolution of the triples through the Kozai mechanism and tidal friction can induce the formation of very close inner binaries. Angular momentum loss in a magnetized wind or stellar evolution could then lead to the merger of these binaries (or to mass transfer between them) and produce BSSs in binary (or triple) systems. We study this mechanism and its implications and show that it could naturally explain many of the characteristics of the BSS population in clusters, most notably the large binary fraction of long period BSS binaries; their unique period-eccentricity distribution (with typical periods>700 days); and the typical location of these BSSs in the color-magnitude diagram, far from the cluster turn-off point of their host clusters. We suggest that this scenario has a major (possibly dominant) role in the formation of BSSs in open clusters and give specific predictions for the the BSSs population formed in this manner. We also note that triple systems may be the progenitors of the brightest planetary nebulae in old elliptical galaxies, which possibly evolved from BSSs.