Prospects for detecting neutron star-white dwarf mergers with decihertz gravitational-wave observatories

Based on different neutron star-white dwarf (NS-WD) population models, we investigate the prospects of gravitational-wave (GW) detections for NS-WD mergers, with the help of early warnings from two space-borne decihertz GW observatories, DO-Optimal and DECIGO. We not only give quick assessments of the GW detection rates for NS-WD mergers with the two decihertz GW detectors, but also report systematic analyses on the characteristics of GW-detectable merger events using the method of Fisher matrix. With a sufficient 1-d early-warning time, the yearly GW detection number for DO-Optimal is in the range of (1.5-1.9) x 10(3), while it is (3.3-4.6) x 10(4) for DECIGO. More importantly, our results show that most NS-WD mergers can be localized with an uncertainty of O(10(-2))deg(2). Given the NS-WD merger as a possible origin for a peculiar long-duration gamma-ray burst, GRB 211211A, followed with kilonova-like emissions, we further suggest that the GW early-warning detection would allow future electromagnetic telescopes to get prepared to follow up transients after some special NS-WD mergers. Based on our analyses, we emphasize that such a feasible 'wait-for' pattern can help to firmly identify the origin of GRB 211211A-like events in the future and bring excellent opportunities for the multimessenger astronomy.