White Dwarfs, Neutron stars, and fast radio bursts
Having ejected nearly all their BHs, the centers of core-collapsed globular clusters are dominated by the next most massive stellar populations: neutron stars and, especially, massive white dwarfs (see Figure 1). In Kremer et al. (2021b), we showed the presence of white dwarfs in globular clusters leads naturally to the dynamical formation of merging white dwarf+white dwarf binaries that are observable as gravitational wave sources at milli- and decihertz frequencies as they inspiral. Notably, we showed that over 90% of these mergers have a total mass greater than the Chandrasekhar limit as a result of mass segregation in the cluster. Thus, if the merger remnants are not destroyed completely in an explosive transient, they likely undergo collapse (e.g., Nomoto et al. 1984), potentially creating young pulsars/magnetars.
Figure 1: Adapted from Kremer et al. 2021b. Enclosed mass versus radius for various stellar populations for our best-fit model for the cluster NGC 6397. The vertical dashed black line denotes the cluster-centric distance within which Vitral & Mamon 2021 constrained a diffuse subsystem of roughly 1000 solar masses in NGC 6397. We argue that, in NGC 6397, white dwarfs are the most plausible explanation for this massive dark population.
A recent result connected to this topic is the repeating fast radio burst observed in a globular cluster in the nearby galaxy M81 (Bhardwaj et al. 2021; Kirsten et al. 2021). The origin of fast radio bursts remains unclear, however the Galactic fast radio burst found coincident with a known magnetar (e.g., Bochenek et al. 2020) provides clear evidence that some fast radio bursts are magnetar-powered. Magnetars are traditionally expected to form in association with massive stellar evolution in part due to the association of a number of Galactic magnetars with supernova remnants (e.g., Kaspi & Beloborodov 2017). However, in old (t>10 Gyr) globular clusters, neutron stars formed through massive star evolution have been inactive for billions of years. Thus, alternative models are necessary to explain the M81 fast radio burst. In Kremer et al. (2021c), I explored dynamical formation scenarios for fast radio bursts in old globular clusters and showed that young highly-magnetized neutron stars formed through white dwarf mergers similar to those in Kremer et al.\,(2021b) are the most natural mechanism. This channel operates at a rate consistent with the fast radio burst rate inferred from the M81 detection. Also, properties of neutron stars expected to form through WD mergers (e.g., magnetic fields of roughly 10^11 G and spin periods of 10 ms; Schwab 2021) produce burst energetics consistent with the M81 burst.
Figure 2: From Kremer et al. 2021c. Overlaid colored curves denote the active fast radio burst lifetime required to reproduce the inferred volumetric event rate inferred for the M81 FRB (5×10^6 per Gpc^3). Different colors denote various FRB progenitor formation channels in globular clusters. The dashed black curves show the isotropic equivalent time-averaged luminosity for the M81 FRB assuming efficiency factors of 10^−8, 10^−6, and 1 for creation of coherent radio emission. The hatched gray band denotes the allowable parameter space for a magnetically-powered source with radio emission efficiency of 10^−4. For reference, we include in blue all radio sources in the ATNF Pulsar Catalog (Manchester et al. 2005) and magnetars from the McGill Magnetar Catalog (Olausen & Kaspi 2014).
In addition to my dynamics work, I have studied mass transfer in close double WD binaries. In Kremer et al. 2015, I examined the stability of such systems as they undergo direct-impact accretion and in Kremer et al. 2017, I explored the implications of these accreting white dwarf binaries for LISA, predicting a unique class of gravitational wave sources: mass-transferring white dwarf binaries that exhibit reverse gravitational chirps, i.e., systems that evolve toward lower gravitational wave frequencies as a consequence of mass-transfer.