Binary neutron star mergers with a subsolar mass star

Abstract

While there are a number of proposed formation channels for subsolar mass compact objects, including black holes formed primordially, or neutron stars that form in collapsar disks, there have yet to be any conclusive observations of such objects. Motivated by the possibility that, if such objects exist, gravitational waves from binary mergers may reveal them, we study binary neutron star mergers where one star has a subsolar-mass in order to determine how well such systems are described by current models, and when they could be distinguished from a system with a subsolar-mass black hole. We perform fully general-relativistic simulations of a $1.7\ M_{\odot}$ star merging with a $0.8\ M_{\odot}$ star, leading to tidal deformabilities of up to $\mathcal{O}(10^4)$ for the latter, and quantify how this affects the merger dynamics and associated gravitation and electromagnetic signals. In this regime, we find mass transfer between the stars, as well as significantly lower disruption frequencies. Though this is not captured by current gravitational waveform models, we conclude that this does not significantly impact the sensitivity of current gravitational wave detectors to these sources. Assuming design sensitivity of the LIGO and Virgo detectors, we find no biases in the recovered intrinsic parameters for signal-to-noise ratios $\lesssim 100$. We also find that the large deformabilities lead to a significant increase in the amount of dynamically ejected matter compared to equal mass systems, exceeding the predictions of current phenomenological models.

Figures (11)

• Figure 1: Rest-mass density profile in the equatorial plane on a logarithmic scale at different times for the BSk21 simulation. Note that the bottom row is zoomed out (by a factor of 2) compared to the top row.
• Figure 2: Change in the total rest mass surrounding the subsolar neutron star for the BSk21 simulation as a function of time. We also show the corresponding gravitational wave frequency at the top. We only show the mass evolution from $\sim 28$ ms onward as the mass does not vary much before then.
• Figure 3: Left: The amount of rest mass outside a given coordinate radius from the center of mass for the post-merger remnant for mergers of BNSs with various EOSs. The solid curves show the total amount, while the points show just the unbound rest mass. Right: The amount of unbound rest mass binned by the velocity at infinity, with each bin of width 0.05c.
• Figure 4: We show the plus polarization of the gravitational wave radiation (top) of the BNS mergers when observed face-on at 100 Mpc for two equations of state. The bottom panel shows the corresponding gravitational wave frequency. The waveforms are aligned in phase and time at a reference frequency of 500 Hz.
• Figure 5: Hybridization of the BSk21 configuration with the SEOBNRv5_ROM_NRTidalv3 model. The alignment interval is marked by vertical dashed lines. The SEOBNRv5_ROM_NRTidalv3 EOB waveform is shown as a purple dashed curve and the NR waveform as a green solid curve. The final hybrid combines the long inspiral from the EOB waveform, which includes several hundred cycles (not shown in the figure), and the late inspiral, as well as the post-merger phase of the NR waveform