PhD student LI Zhenwei and his collaborators, from Yunnan Observatories of Chinese Academy of Sciences, reported the gravitational-wave (GW) radiation of double degenerates with extremely low-mass white dwarf (ELM WD) companions in The Astrophysical Journal on April 8.

ELM WD is a kind of helium core white dwarf with mass less than 0.3 solar mass. In general, ELM WDs have thick hydrogen-rich envelope with the comparison of massive white dwarfs (e.g. CO WDs). The residual hydrogen burning in the envelope can sustain a relatively high luminosity for a long time, which leads ELM WDs to be detected by the electromagnetic (EM) telescope more easily.

Recently, ELM Survey (a targeted survey of ELM WDs) found many double degenerates with ELM WD companions. The orbital periods for these binary systems are very short, and several compact systems are supposed to be detected by the future space borne GW detector (e.g. the Laser Interferometer Space Antenna; LISA) at mHz waveband. As a result, double degenerates with ELM WD companions are expected to be detected with the combination of EM and GW observations.

Researchers systematically investigated the formation of ELM WDs in double degenerates by a combination of detailed binary evolution calculation and binary population synthesis. Then they obtained the populations of double degenerates with ELM WD companions in the Galaxy. The total number of such binary systems at present is about 20 million. Researchers analyzed the GW radiation of double degenerates with ELM WD companions. They found that most of the Galactic halo binaries containing ELM WDs are produced with long orbital periods, which are hardly detectable by LISA.

The main contribution of LISA-detectable sources is in the galactic disk and bulge. About 6000 sources are supposed to be solely detected by LISA, including about 2000 chirping sources (the time variations of GW frequency for these sources can be measured). With the combination of EM and GW observations, about 100 LISA-detectable sources are expected to be detected by Gaia.

LI and his cooperators found that more than half of LISA-detectable sources are in the southern sky, indicating that more unbiased sources at low Galactic latitudes are expected to be detected in the future.

Contact:
LI Zhenwei, YNAO, CAS
lizw@ynao.ac.cn