Blue Straggler Formation in Clusters (original) (raw)
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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
Stellar Collisions and Blue Straggler Formation
We review recent 3D hydrodynamic calculations of stellar collisions using the Smoothed Particle Hydrodynamics (SPH) method, and we discuss the implications of the results for the formation and evolution of blue stragglers in globular clusters. We also discuss the construction of simple analytic models for merger remnants, approximating the mass loss, shock heating, mixing, and angular momentum transport during a collision with simple algorithms that can be calibrated using our 3D hydrodynamic results. The thermodynamic and chemical composition profiles predicted by these simple models are compared with those from our SPH simulations, demonstrating that our new models provide accurate representations of true collisional merger remnants.
2021
Blue straggler stars (BSSs) are the most massive stars in a cluster formed via binary or higher-order stellar interactions. Though the exact nature of such formation scenarios is difficult to pin down, we provide observational constraints on the different possible mechanism. In this quest, we first produce a catalogue of BSSs using Gaia DR2 data. Among the 670 clusters older than 300 Myr, we identified 868 BSSs in 228 clusters and 500 BSS candidates in 208 clusters. In general, all clusters older than 1 Gyr and massive than 1000 M⊙ have BSSs. The average number of BSSs increases with cluster age and mass, and there is a power-law relation between the cluster mass and the maximum number of BSSs in the cluster. We introduce the term fractional mass excess ( mathcalMe\\mathcal {M}_{e}mathcalMe) for BSSs. We find that at least 54 per cent of BSSs have mathcalMe\\mathcal {M}_{e}mathcalMe < 0.5 (likely to have gained mass through a binary mass transfer (MT)), 30 per cent in the 1.0 < mathcalMe\\mathcal {M}_{e}mathcalMe <0.5 range...
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.
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.
On Blue Straggler Formation by Direct Collisions of Main-Sequence Stars
1995
We report the results of new SPH calculations of parabolic collisions between main-sequence (MS) stars. The stars are assumed to be close to the MS turn-off point in a globular cluster and are therefore modeled as n=3, Γ=5/3 polytropes. We find that the high degree of central mass concentration in these stars has a profound effect on the hydrodynamics. In particular, very little hydrodynamic mixing occurs between the dense, helium-rich inner cores and the outer envelopes. As a result, and in contrast to what has been assumed in previous studies, blue stragglers formed by direct stellar collisions are not necessarily expected to have anomalously high helium abundances in their envelopes or to have their cores replenished with fresh hydrogen fuel.
Stellar Collisions and the Interior Structure of Blue Stragglers
The Astrophysical Journal, 2002
Collisions of main sequence stars occur frequently in dense star clusters. In open and globular clusters, these collisions produce merger remnants that may be observed as blue stragglers. Detailed theoretical models of this process require lengthy hydrodynamic computations in three dimensions. However, a less computationally expensive approach, which we present here, is to approximate the
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.