I will talk about two topics for binary studies for gravitational wave astronomy.

1. Remnants of first stars for gravitational wave sources

Using our population synthesis code, we found that the typical chirp mass of binary
black holes (BH-BHs) whose origin is the first star (Pop III) is ~30 Msun with the total
mass of ~60 Msun so that the inspiral chirp signal as well as quasi normal mode (QNM) of
the merging black hole are interesting targets of LIGO, VIRGO, and KAGRA (Kinugawa et
al.2014, 2016). The detection rate of the coalescing Pop III BH-BHs is ~180 events/yr in
our standard model. Furthermore, we found that the chirp mass has a peak at ~30Msun in
most of parameters and distribution functions (Kinugawa et al. 2016). This result
predicted the gravitational wave events like GW150914. LIGO paper said recently
predicted BBH total masses agree astonishingly well with GW150914 and can have
sufficiently long merger times to occur in the nearby universe (Kinugawa et al. 2014;
Abbot et al. 2016). Thus, there is a good chance to check indirectly the existence of
Pop III massive stars by gravitational waves.

2. Probe for Type Ia supernova progenitor in decihertz gravitational wave astronomy.

A binary WD (WD-WD) merger is a possible progenitor of Type Ia supernovae. Space-based
gravitational wave (GW) detectors with great sensitivity in the decihertz range like
DECIGO can observe WD-WD mergers directly. Therefore, access to the deci-Hz band of GWs
would enable multi-messenger observations of Type Ia supernovae to constrain their
progenitor and explosion mechanism. In this talk, we consider the event rate of WD-WD
mergers and minimum detection range to observe one WD-WD merger per year, using nearby
galaxy catalog and the relation between the Ia supernova and the host
galaxy. Furthermore, we calculate the DECIGO's ability to localize WD-WD mergers and to
determine the masses of binary mergers.