In young dense clusters repeated collisions between massive stars may lead to the formation of a very massive star (above 100 Msun). In the past the study of the long-term evolution of merger remnants has mostly focussed on collisions between low-mas
s stars (up to about 2 Msun) in the context of blue-straggler formation. The evolution of collision products of more massive stars has not been as thoroughly investigated. In this paper we study the long-term evolution of a number of stellar mergers formed by the head-on collision of a primary star with a mass of 5-40 Msun with a lower mass star at three points in its evolution in order to better understand their evolution. We use smooth particle hydrodynamics (SPH) calculations to model the collision between the stars. The outcome of this calculation is reduced to one dimension and imported into a stellar evolution code. We follow the subsequent evolution of the collision product through the main sequence at least until the onset of helium burning. We find that little hydrogen is mixed into the core of the collision products, in agreement with previous studies of collisions between low-mass stars. For collisions involving evolved stars we find that during the merger the surface nitrogen abundance can be strongly enhanced. The evolution of most of the collision products proceeds analogously to that of normal stars with the same mass, but with a larger radius and luminosity. However, the evolution of collision products that form with a hydrogen depleted core is markedly different from that of normal stars with the same mass. They undergo a long-lived period of hydrogen shell burning close to the main-sequence band in the Hertzsprung-Russell diagram and spend the initial part of core helium burning as compact blue supergiants.
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 frac
tion of all cluster stars. One of the proposed formation channels for blue straggler stars is the physical collision and merger of two stars. In the context of multiple populations, collisions between stars with different helium abundances should occur and contribute to the observed blue straggler population. This will affect the predicted blue straggler colour and luminosity function. We quantify this effect by calculating models of mergers resulting from collisions between stars with different helium abundances and using these models to model a merger population. We then compare these results to four observed clusters, NGC 1851, NGC 2808, NGC 5634 and NGC 6093. As in previous studies our models deviate from the observations, particularly in the colour distributions. However, our results are consistent with observations of multiple populations in these clusters. In NGC 2808, our best fitting models include normal and helium enhanced populations, in agreement with helium enhancement inferred in this cluster. The other three clusters show better agreement with models that do not include helium enhancement. We discuss future prospects to improve the modelling of blue straggler populations and the role that the models we present here can play in such a study.
In the cores of young dense star clusters repeated stellar collisions involving the same object can occur, which has been suggested to lead to the formation of an intermediate-mass black hole. In order to verify this scenario we compute the detailed
evolution of the merger remnant of three sequences. We follow the evolution until the onset of carbon burning and estimate the final remnant mass to determine the ultimate fate of a runaway merger sequence. We use a detailed stellar evolution code to follow the evolution of the collision product. At each collision, we mix the two colliding stars, taking account of mass loss during the collision. During the stellar evolution we apply mass loss rates from the literature, as appropriate for the evolutionary stage of the merger remnant. We compute models for high ($Z=0.02$) and low ($Z=0.001$) metallicity to quantify metallicity effects. We find that the merger remnant becomes a Wolf-Rayet star before the end of core hydrogen burning. Mass loss from stellar winds dominates over the mass increase due to repeated mergers for all three merger sequences that we consider. In none of our high metallicity models an intermediate-mass black hole is formed, instead our models have a mass of 10--14 Msun{} at the onset of carbon burning. For low metallicity we expect the final remnant of the merger sequence to explode as a pair creation supernova. We find that our metal-rich models become inflated as a result of developing an extended low-density envelope. This may increase the probability of further collisions, but self-consistent $N$-body calculations with detailed evolution of runaway mergers are required to verify this.
In a companion paper we studied the detailed evolution of stellar collision products that occurred in an $N$-body simulation of the old open cluster M67 and compared our detailed models to simple prescriptions. In this paper we extend this work by st
udying the evolution of the collision products in open clusters as a function of mass and age of the progenitor stars. We calculated a grid of head-on collisions covering the section of parameter space relevant for collisions in open clusters. We create detailed models of the merger remnants using an entropy-sorting algorithm and follow their subsequent evolution during the initial contraction phase, through the main sequence and up to the giant branch with our detailed stellar evolution code. We compare the location of our models in a colour-magnitude diagram to the observed blue straggler population of the old open clusters M67 and NGC 188 and find that they cover the observed blue straggler region of both clusters. For M67, collisions need to have taken place recently. Differences between the evolution tracks of the collision products and normal main sequence stars can be understood quantitatively using a simple analytic model. We present an analytic recipe that can be used in an $N$-body code to transform a precomputed evolution track for a normal star into an evolution track for a collision product.
Stellar collisions are an important formation channel for blue straggler stars in globular and old open clusters. Hydrodynamical simulations have shown that the remnants of such collisions are out of thermal equilibrium, are not strongly mixed and ca
n rotate very rapidly. Detailed evolution models of collision products are needed to interpret observed blue straggler populations and to use them to probe the dynamical history of a star cluster. We expand on previous studies by presenting an efficient procedure to import the results of detailed collision simulations into a fully implicit stellar evolution code. Our code is able to evolve stellar collision products in a fairly robust manner and allows for a systematic study of their evolution. Using our code we have constructed detailed models of the collisional blue stragglers produced in the $N$-body simulation of M67 performed by Hurley emph{et al.} in 2005. We assume the collisions are head-on and thus ignore the effects of rotation in this paper. Our detailed models are more luminous than normal stars of the same mass and in the same stage of evolution, but cooler than homogeneously mix
When two stars collide and merge they form a new star that can stand out against the background population in a starcluster as a blue straggler. In so called collision runaways many stars can merge and may form a very massive star that eventually for
ms an intermediate mass blackhole. We have performed detailed evolution calculations of merger remnants from collisions between main sequence stars, both for lower mass stars and higher mass stars. These stars can be significantly brighter than ordinary stars of the same mass due to their increased helium abundance. Simplified treatments ignoring this effect give incorrect predictions for the collision product lifetime and evolution in the Hertzsprung-Russell diagram.
The evolution of stellar collision products in cluster simulations has usually been modelled using simplified prescriptions. Such prescriptions either replace the collision product with an (evolved) main sequence star, or assume that the collision pr
oduct was completely mixed during the collision. It is known from hydrodynamical simulations of stellar collisions that collision products are not completely mixed, however. We have calculated the evolution of stellar collision products and find that they are brighter than normal main sequence stars of the same mass, but not as blue as models that assume that the collision product was fully mixed during the collision.
