arXiv subjects

J. C. Lombardi

Publications and source records attributed to J. C. Lombardi.

3 recordsLinked to original sources

arXiv2012

On the origin of the metallicity dependence in dynamically formed extragalactic low-mass X-ray binaries

Globular clusters (GCs) effectively produce dynamically-formed low-mass X-ray binaries (LMXBs). Observers detect ~100 times more LMXBs per stellar mass in GCs compared to stars in the fields of galaxies. It has also been observationally established that metal-rich GCs are about 3 times more likely to contain an X-ray source than their metal-poor counterparts. Recent observations have shown that this ratio holds in extragalactic GCs for all bright X-ray sources with Lx between 2X10^{37} and 5X10^{38} erg/s. In this Letter, we propose that the observed metallicity dependence of LMXBs in extragalactic GCs can be explained by the differences in the number densities and average masses of red giants in populations of different metallicities. Red giants serve as seeds for the dynamical production of bright LMXBs via two channels - binary exchanges and physical collisions - and the increase of the number densities and masses of red giants boost LMXB production, leading to the observed difference. We also discuss a possible effect of the age difference in stellar populations of different metallicities.

astro-ph.HE↗

arXiv2007

Mixing in massive stellar mergers

The early evolution of dense star clusters is possibly dominated by close interactions between stars, and physical collisions between stars may occur quite frequently. Simulating a stellar collision event can be an intensive numerical task, as detailed calculations of this process require hydrodynamic simulations in three dimensions. We present a computationally inexpensive method in which we approximate the merger process, including shock heating, hydrodynamic mixing and mass loss, with a simple algorithm based on conservation laws and a basic qualitative understanding of the hydrodynamics of stellar mergers. The algorithm relies on Archimedes' principle to dictate the distribution of the fluid in the stable equilibrium situation. We calibrate and apply the method to mergers of massive stars, as these are expected to occur in young and dense star clusters. We find that without the effects of microscopic mixing, the temperature and chemical composition profiles in a collision product can become double-valued functions of enclosed mass. Such an unphysical situation is mended by simulating microscopic mixing as a post-collision effect. In this way we find that head-on collisions between stars of the same spectral type result in substantial mixing, while mergers between stars of different spectral type, such as type B and O stars ($\sim$10 and $\sim$40\msun respectively), are subject to relatively little hydrodynamic mixing.

astro-ph↗

arXiv1995

Collisions of Main-Sequence Stars and the Formation of Blue Stragglers in Globular Clusters

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 $M/M_{TO}=0.2,0.5,0.75$, and $1$, where $M_{TO}$ is the cluster turnoff mass (typically about $0.8\,M_\odot$). Low-mass stars (with $M=0.2 M_{TO}$ or $0.5 M_{TO}$) are assumed to be fully convective and are therefore modeled as $n=1.5$ polytropes. Stars at the turnoff (with $M= M_{TO}$) are assumed to be mostly radiative and are modeled as $n=3$ polytropes. Intermediate-mass stars (with $M=0.75 M_{TO}$) 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. In contrast to what has been done in previous studies, blue stragglers formed by direct stellar collisions should not necessarily be assumed to have initially homogeneous composition profiles.

astro-ph↗