Formation of Extremely Low-Mass WDs in Double Degenerates. (arXiv:1812.07226v1 [astro-ph.SR])

Formation of Extremely Low-Mass WDs in Double Degenerates. (arXiv:1812.07226v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Li_Z/0/1/0/all/0/1">Zhenwei Li</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Chen_X/0/1/0/all/0/1">Xuefei Chen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Chen_H/0/1/0/all/0/1">Hai-Liang Chen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Han_Z/0/1/0/all/0/1">Zhanwen Han</a>

Extremely low-mass white dwarfs (ELM WDs) are helium white dwarf with mass
less than $sim$$0.3rm;M_odot$. Most ELM WDs are found in double degenerates
(DDs) in the ELM Survey leading by Brown and Kilic. These systems are supposed
to be significant gravitational wave sources in mHz frequency. In this paper,
we firstly analyzed the observational characteristics of ELM WDs and found that
there are two distinct groups in the ELM WD mass and orbital period plane,
indicating two different formation scenarios of such objects, i.e. stable Roche
lobe overflow channel (RL channel) and common envelope ejection channel (CE
channel). We then systematically investigated the formation of ELM WDs in DDs
by the combination of detailed binary evolution calculation and binary
population synthesis. Our study shows that the majority of ELM WDs with mass
less than $0.22rm;M_odot$ are formed from the RL channel. The most common
progenitor mass in this way is in the range of $1.25-1.45rm;M_odot$ and the
resulted ELM WDs have a peak around $0.18rm;M_odot$ when selection effects
are taken into account, consistent with observations. The ELM WDs with mass
larger than $0.22rm;M_odot$ are more likely from the CE channel, and have a
peak of ELM WD mass around $0.27rm;M_odot$ which needs to be confirmed by
future observations. By assuming a constant star formation rate of
2$rm;M_odot yr^{-1}$ for a Milky Way-like galaxy, the birthrate and local
density are $5times10^{-4}rm;yr^{-1}$ and $1500rm;kpc^{-3}$ for DDs with
ELM WD mass less than $0.25rm;M_odot$, respectively.

Extremely low-mass white dwarfs (ELM WDs) are helium white dwarf with mass
less than $sim$$0.3rm;M_odot$. Most ELM WDs are found in double degenerates
(DDs) in the ELM Survey leading by Brown and Kilic. These systems are supposed
to be significant gravitational wave sources in mHz frequency. In this paper,
we firstly analyzed the observational characteristics of ELM WDs and found that
there are two distinct groups in the ELM WD mass and orbital period plane,
indicating two different formation scenarios of such objects, i.e. stable Roche
lobe overflow channel (RL channel) and common envelope ejection channel (CE
channel). We then systematically investigated the formation of ELM WDs in DDs
by the combination of detailed binary evolution calculation and binary
population synthesis. Our study shows that the majority of ELM WDs with mass
less than $0.22rm;M_odot$ are formed from the RL channel. The most common
progenitor mass in this way is in the range of $1.25-1.45rm;M_odot$ and the
resulted ELM WDs have a peak around $0.18rm;M_odot$ when selection effects
are taken into account, consistent with observations. The ELM WDs with mass
larger than $0.22rm;M_odot$ are more likely from the CE channel, and have a
peak of ELM WD mass around $0.27rm;M_odot$ which needs to be confirmed by
future observations. By assuming a constant star formation rate of
2$rm;M_odot yr^{-1}$ for a Milky Way-like galaxy, the birthrate and local
density are $5times10^{-4}rm;yr^{-1}$ and $1500rm;kpc^{-3}$ for DDs with
ELM WD mass less than $0.25rm;M_odot$, respectively.

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