A Search for Low-Mass Neutron Stars in the Third Observing Run of Advanced LIGO and Virgo

Most observed neutron stars have masses around 1.4 $M_\odot$, consistent with current formation mechanisms. To date, no sub-solar mass neutron star has been observed. Observing a low-mass neutron star would be a significant milestone, providing crucial constraints on the nuclear equation of state, unveiling a new population of neutron stars, and advancing the study of their formation processes and underlying mechanisms. We present the first targeted search for tidally deformed sub-solar mass binary neutron stars (BNS), with primary masses ranging from 0.1 to 2 $M_\odot$ and secondary masses from 0.1 to 1 $M_\odot$, using data from the third observing run of the Advanced LIGO and Advanced Virgo gravitational-wave detectors. We account for the tidal deformabilities of up to $O(10^4)$ of these systems, as low-mass neutron stars are more easily distorted by their companions' gravitational forces. Previous searches that neglect tidal deformability lose sensitivity to low-mass sources, potentially missing more than $\sim30\%$ of detectable signals from a system with a chirp mass of 0.6 $M_\odot$ binaries. No statistically significant detections were made. In the absence of a detection, we place a $90\%$ confidence upper limit on the local merger rate for sub-solar mass BNS systems, constraining it to be $< 6.4\times10^4$ Gpc$^{-3}$Yr$^{-1}$ for a chirp mass of 0.2 $M_\odot$ and $< 2.2\times 10^3$ Gpc$^{-3}$Yr$^{-1}$ for 0.7 $M_\odot$. With future upgrades to detector sensitivity, development of next-generation detectors, and ongoing improvements in search pipelines, constraints on the minimum mass of neutron stars will improve, providing the potential to constrain the nuclear equation of state, reveal new insights into neutron star formation channels, and potentially identify new classes of stars.