Stellar Collisions and the Interior Structure of Blue Stragglers

Abstract

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 merger process (including shock heating, hydrodynamic mixing, mass ejection, and angular momentum transfer) with simple algorithms based on conservation laws and a basic qualitative understanding of the hydrodynamics. These algorithms have been fine-tuned through comparisons with the results of our previous hydrodynamic simulations. We find that the thermodynamic and chemical composition profiles of our simple models agree very well with those from recent SPH (smoothed particle hydrodynamics) calculations of stellar collisions, and the subsequent stellar evolution of our simple models also matches closely that of the more accurate hydrodynamic models. Our algorithms have been implemented in an easy-to-use software package, which we are making publicly available. 4 This software could be used in combination with realistic dynamical simulations of star clusters that must take into account stellar collisions.

The evolution of runaway stellar collision products

“…satisfies the Ledoux stability criterion, Kippenhahn & Weigert 1990). An algorithm for doing this was first developed by Lombardi et al (2002) for low-mass stars, for which it is sufficient to sort the mass shells in order of increasing entropy and then integrate the equation of hydrostatic equilibrium. For massive stars where radiation pressure is important this does not necessarily produce a stable configuration and some mass shells need to be moved again after the equation of hydrostatic equilibrium has been integrated.…”

Monte Carlo Simulations of Globular Cluster Evolution. IV. Direct Integration of Strong Interactions

“…For relative speeds greater than the escape speed from the surface of the star (v 1 k v esc % 500 km s À1 for a typical solar-mass main-sequence star), which can occur in galactic nuclei, a collision typically results in a large fraction of the total mass lost from the system (see, e.g., Freitag & Benz [2005], and references therein). For v 1 P v esc , which is satisfied for globular clusters, the result is typically a clean merger, with a negligible amount of mass lost from the system (e.g., Benz & Hills 1987;Lombardi et al 2002). Since we are concerned with the latter case in this paper, we treat physical single-single collisions using the sticky sphere approximation, which assumes that if the radii of stars touch during a strong interaction, they merge with no mass loss.…”

Models of Individual Blue Stragglers

“…Since material that has a higher mean molecular weight has a lower entropy, any material that is helium-rich will fall to the centre of the collision product. This "sort by entropy" prescription is the basis for the codes Make Me A Star (MMAS) [29] and Make Me A Massive Star (MMAMS) [12], which provide detailed stellar structure profiles of collision products for collisions between low mass and high mass stars respectively.…”