Decihertz Gravitational Waves from Double White Dwarf Merger Remnants

Close binaries of double white dwarfs (DWDs) inspiral and merge by emitting gravitational waves (GWs). Orbital motion of some of these binaries is expected to be observed at a low-frequency band by space-borne laser interferometric detectors of GWs. The merger remnant may suffer thermonuclear runaway and explode as SNe Ia if they are massive enough. As GW sources, the remnants have so far been scarcely studied. Here we propose a new mechanism of GW emission from DWD merger remnants that may be observed by planned GW detectors in a decihertz frequency band. A remnant is temporarily expected to have a high degree of differential rotation as a consequence of the merger process. It is then unstable to oscillation modes whose azimuthal pattern speed coincides with the stellar rotation. We solve the eigenvalue problem of differentially rotating remnants and identify unstable eigenmodes, which may be categorized to inertial modes. The estimate of characteristic strain of GWs shows that they may be detectable within the distance of the Virgo cluster by planned gravitational wave observatories targeting the decihertz band.