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Register a new userEvolution of the Binary Fraction in Dense Stellar Systems
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Using our recently improved Monte Carlo evolution code, we study the evolution of the binary fraction in globular clusters. In agreement with previous N-body simulations, we find generally that the hard binary fraction in the core tends to increase with time over a range of initial cluster central densities for initial binary fractions <~ 90%. The dominant processes driving the evolution of the core binary fraction are mass segregation of binaries into the cluster core and preferential destruction of binaries there. On a global scale, these effects and the preferential tidal stripping of single stars tend to roughly balance, leading to overall cluster binary fractions that are roughly constant with time. Our findings suggest that the current hard binary fraction near the half-mass radius is a good indicator of the hard primordial binary fraction. However, the relationship between the true binary fraction and the fraction of main-sequence stars in binaries (which is typically what observers measure) is non-linear and rather complicated. We also consider the importance of soft binaries, which not only modify the evolution of the binary fraction, but can drastically change the evolution of the cluster as a whole. Finally, we describe in some detail the recent addition of single and binary stellar evolution to our cluster evolution code.
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Populations of massive stars are directly reflective of the physics of stellar evolution. Counting subtypes of massive stars and ratios of massive stars in different evolutionary states have been used ubiquitously as diagnostics of age and metallicity effects. While the binary fraction of massive stars is significant, inferences are often based upon models incorporating only single-star evolution. In this work, we utilize custom synthetic stellar populations from the Binary Population and Stellar Synthesis (BPASS) code to determine the effect of stellar binaries on number count ratios of different evolutionary stages in both young massive clusters and galaxies with massive stellar populations. We find that many ratios are degenerate in metallicity, age, and/or binary fraction. We develop diagnostic plots using these stellar count ratios to help break this degeneracy, and use these plots to compare our predictions to observed data in the Milky Way and the Local Group. These data suggest a possible correlation between the massive star binary fraction and metallicity. We also examine the robustness of our predictions in samples with varying levels of completeness. We find including binaries and imposing a completeness limit can both introduce $gtrsim0.1$ dex changes in inferred ages. Our results highlight the impact that binary evolution channels can have on the massive star population.
We study the dynamical evolution of globular clusters containing primordial binaries, including full single and binary stellar evolution using our Monte Carlo cluster evolution code updated with an adaptation of the single and binary stellar evolution codes SSE/BSE from Hurley et. al (2000, 2002). We describe the modifications we have made to the code. We present several test calculations and comparisons with existing studies to illustrate the validity of the code. We show that our code finds very good agreement with direct N-body simulations including primordial binaries and stellar evolution. We find significant differences in the evolution of the global properties of the simulated clusters using stellar evolution compared to simulations without any stellar evolution. In particular, we find that the mass loss from stellar evolution acts as a significant energy production channel simply by reducing the total gravitational binding energy and can significantly prolong the initial core contraction phase before reaching the binary-burning quasi steady state of the cluster evolution as noticed in Paper IV. We simulate a large grid of clusters varying the initial cluster mass, binary fraction, and concentration and compare properties of the simulated clusters with those of the observed Galactic globular clusters (GGCs). We find that our simulated cluster properties agree well with the observed GGC properties. We explore in some detail qualitatively different clusters in different phases of their evolution, and construct synthetic Hertzprung-Russell diagrams for these clusters.
Upon their formation, dynamically cool (collapsing) star clusters will, within only a few million years, achieve stellar mass segregation for stars down to a few solar masses, simply because of gravitational two-body encounters. Since binary systems are, on average, more massive than single stars, one would expect them to also rapidly mass segregate dynamically. Contrary to these expectations and based on high-resolution Hubble Space Telescope observations, we show that the compact, 15-30 Myr-old Large Magellanic Cloud cluster NGC 1818 exhibits tantalizing hints at the >= 2 sigma level of significance (> 3 sigma if we assume a power-law secondary-to-primary mass-ratio distribution) of an increasing fraction of F-star binary systems (with combined masses of 1.3-1.6 Msun) with increasing distance from the cluster center, specifically between the inner 10 to 20 (approximately equivalent to the clusters core and half-mass radii) and the outer 60 to 80. If confirmed, this will offer support of the theoretically predicted but thus far unobserved dynamical disruption processes of the significant population of soft binary systems---with relatively low binding energies compared to the kinetic energy of their stellar members---in star clusters, which we have access to here by virtue of the clusters unique combination of youth and high stellar density.
Stellar fundamental properties (masses, radii, effective temperatures) can be extracted from observations of eclipsing binary systems with remarkable precision, often better than 2%. Such precise measurements afford us the opportunity to confront the validity of basic predictions of stellar evolution theory, such as the mass-radius relationship. A brief historical overview of confrontations between stellar models and data from eclipsing binaries is given, highlighting key results and physical insight that have led directly to our present understanding. The current paradigm that standard stellar evolution theory is insufficient to describe the most basic relation, that of a stars mass to its radius, along the main sequence is then described. Departures of theoretical expectations from empirical data, however, provide a rich opportunity to explore various physical solutions, improving our understanding of important stellar astrophysical processes.
We compile observations of the surface mass density profiles of dense stellar systems, including globular clusters in the Milky Way and nearby galaxies, massive star clusters in nearby starbursts, nuclear star clusters in dwarf spheroidals and late-type disks, ultra-compact dwarfs, and galaxy spheroids spanning the range from low-mass cusp bulges and ellipticals to massive core ellipticals. We show that in all cases the maximum stellar surface density attained in the central regions of these systems is similar, Sigma_max ~ 10^11 M_sun/kpc^2 (~20 g/cm^2), despite the fact that the systems span 7 orders of magnitude in total stellar mass M_star, 5 in effective radius R_e, and have a wide range in effective surface density M_star/R_e^2. The surface density limit is reached on a wide variety of physical scales in different systems and is thus not a limit on three-dimensional stellar density. Given the very different formation mechanisms involved in these different classes of objects, we argue that a single piece of physics likely determines Sigma_max. The radiation fields and winds produced by massive stars can have a significant influence on the formation of both star clusters and galaxies, while neither supernovae nor black hole accretion are important in star cluster formation. We thus conclude that feedback from massive stars likely accounts for the observed Sigma_max, plausibly because star formation reaches an Eddington-like flux that regulates the growth of these diverse systems. This suggests that current models of galaxy formation, which focus on feedback from supernovae and active galactic nuclei, are missing a crucial ingredient.
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