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تسجيل مستخدم جديدsdA in SDSS DR12 are Overwhelmingly Not Extremely Low-Mass (ELM) White Dwarfs
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In a search for new white dwarfs in DR12 of the Sloan Digital Sky Survey, Kepler et al. (2016) found atmospheric parameters for thousands of objects with effective temperatures below 20,000 K and surface gravities between 5.5 < log(g) < 6.5. They classified these objects as cool subdwarfs -- sdA -- and speculated that many may be extremely low-mass (ELM) white dwarfs (helium-core white dwarfs with masses below 0.3 Msun). We present evidence -- using radial velocities, photometric colors, and reduced proper motions -- that the vast majority (>99%) of these objects are unlikely to be ELM white dwarfs. Their true identity remains an interesting question.
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Extremely low mass (ELM) white dwarfs (WDs) with masses <0.25 Msun are rare objects that result from compact binary evolution. Here, we present a targeted spectroscopic survey of ELM WD candidates selected by color. The survey is 71% complete and has
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l-wave sources in the 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. a 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 a 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 $1.15-1.45rm;M_odot$ and the resulting ELM WDs have a peak around $0.18rm;M_odot$ when selection effects are taken into account, consistent with observations. The ELM WDs with a mass larger than $0.22rm;M_odot$ are more likely to be from the CE channel and have a peak of ELM WD mass around $0.25rm;M_odot$ 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 birth rate and local density are $5times10^{-4}rm;yr^{-1}$ and $1500rm;kpc^{-3}$, respectively, for DDs with an ELM WD mass less than $0.25rm;M_odot$.
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