White Dwarf Subsystems in Core-Collapsed Globular Clusters

Abstract

Numerical and observational evidence suggests that massive white dwarfs dominate the innermost regions of corecollapsed globular clusters by both number and total mass. Using NGC 6397 as a test case, we constrain the features of white dwarf populations in core-collapsed clusters, both at present day and throughout their lifetimes. The dynamics of these white dwarf subsystems have a number of astrophysical implications. We demonstrate that the collapse of globular cluster cores is ultimately halted by the dynamical burning of white dwarf binaries. We predict that core-collapsed clusters in the local universe yield a white dwarf merger rate of -- 10 Gpc yr 3 1 ( ) , roughly 0.1%-1% of the observed Type Ia supernova rate. We show that prior to merger, inspiraling white dwarf binaries will be observable as gravitational-wave sources at millihertz and decihertz frequencies. Over 90% of these mergers have a total mass greater than the Chandrasekhar limit. We argue that the merger/collision remnants, if not destroyed completely in an explosive transient, may be observed in core-collapsed clusters either as young neutron stars/pulsars/magnetars (in the event of accretion-induced collapse) or as young massive white dwarfs offset from the standard white dwarf cooling sequence. Finally, we show that collisions between white dwarfs and main-sequence stars, which may be detectable as bright transients, occur at a rate of -- 100 Gpc yr 3 1 ( ) in the local universe. We find that these collisions lead to depletion of blue straggler stars and main-sequence star binaries in the centers of core-collapsed clusters.

Stellar graveyards: Clustering of compact objects in globular clusters NGC 3201 and NGC 6397

“…We also display a dashed transparent-blue line at our best-fit Re scale radius (cf., Table A1). The lower panels show the contribution in mass (M ) from each compact object (remnant) as a function of the 3D distance from the cluster's centre (in pc), obtained from CMC models Kremer et al (2019aKremer et al ( , 2021a, in addition to a grey line depicting our best MAMPOSST-PM mass fit of a sub-cluster of unseen objects (i.e., M CUO ). The correspondences for each remnant are solid black: black holes; loosely dotted brown: neutron stars; dashed green: [ONeMg] white dwarfs; dashed-dotted red: [CO] white dwarfs; densely dotted purple: [He] white dwarfs.…”

“…As explained before, core-collapse is thought to occur once the black hole original population of the cluster has been almost entirely ejected, and therefore no strong energy input can be provided to delay the core from collapsing (e.g., Merritt et al 2004;Mackey et al 2007;Breen & Heggie 2013;Wang et al 2016;Askar et al 2018;Kremer et al 2019aKremer et al , 2020a. Recent studies have then proposed that the dark population detected by VM21 is most likely composed of segregated massive white dwarfs, which form a subcluster in the internal regions of the GC (Rui et al 2021a;Kremer et al 2021a).…”

“…For NGC 3201, we use the CMC model presented in Kremer et al (2019a). For NGC 6397, we use the models published in Kremer et al (2021a) and also compute a few additional models in effort to more accurately match the compact object distributions inferred from our analysis. In both clusters, CMC starts with isotropic stellar orbits (e.g., assumes standard King profiles as initial conditions; King 1966), and despite 3-body encounters and natal kicks, the models remain roughly isotropic over time.…”

“…Ultimately ( 10 Gyr timescales), the clustered black hole population becomes negligible, allowing other luminous stellar components to sink, as well as less massive compact objects such as neutron stars and white dwarfs. When these more luminous components collapse in the centre, forming the characteristic core-collapse inner cusp, stellar and white dwarf binary-burning effectively halts further shrinking of the core (Kremer et al 2021a). Complementary observational constraints are required in order to validate these various theoretical predictions.…”

Searching for new observational signatures of the dynamical evolution of star clusters

“…The main differences between the values of A 5 , P 5 , and S 2.5 in simulations with and without primordial binaries are expected during the advanced phases of the evolution, towards CC and post-CC, when the milder contraction of the clusters with primordial binaries might lead to a different and/or less extreme evolution of these parameters. The study presented here will also be further extended to explore the effects of different retention fractions of dark remnants (neutron stars and black holes; see, e.g., Alessandrini et al 2016;Giersz et al 2019;Kremer et al 2020Kremer et al , 2021Gieles et al 2021, for some studies on the dynamical effects of dark remnants). Finally, a forthcoming paper will be dedicated to build nCRDs and determine the three parameters here defined in a sample of observed star clusters.…”