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Abstract We present follow-up spectroscopy and a detailed model atmosphere analysis of 29 wide double white dwarfs, including eight systems with a crystallized C/O core member. We use the state-of-the-art evolutionary models to constrain the physical parameters of each star, including the total age. Assuming that the members of wide binaries are coeval, any age difference between the binary members can be used to test the cooling physics for white dwarf stars, including potential delays due to crystallization and22Ne distillation. We use our control sample of 14 wide binaries with noncrystallized members to show that this method works well; the control sample shows an age difference of only ΔAge = −0.03 ± 0.15 Gyr between its members. For the eight crystallized C/O core systems we find a cooling anomaly of ΔAge = Gyr. Even though our results are consistent with a small additional cooling delay (∼1 Gyr) from22Ne distillation and other neutron-rich impurities, the large uncertainties make this result not statistically significant. Nevertheless, we rule out cooling delays longer than 3.6 Gyr at the 99.7% (3σ) confidence level for 0.6–0.9M⊙white dwarfs. Further progress requires larger samples of wide binaries with crystallized massive white dwarf members. We provide a list of subgiant + white dwarf binaries that could be used for this purpose in the future.
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Distillation of 56 Fe in Ultramassive O–Ne White Dwarfs
Abstract When white dwarfs freeze, the plasma mixtures inside them undergo separation processes that can produce radical changes in the composition profile of the star. The abundance of neutron-rich elements, such as22Ne or56Fe, determines whether or not the first crystals are more or less dense than the surrounding fluid and thus whether they sink or float. These processes have now been studied for C–O–Ne and C–O–Fe mixtures, finding that distillation and precipitation processes are possible in white dwarfs. In this work, we calculate the phase diagram of more complicated O–Ne–Fe mixtures and make predictions for the internal structure of the separated white dwarf. There are two possible outcomes determined by a complicated interplay between the Ne abundance, the22Ne fraction, and the56Fe abundance. Either Fe distills to form an inner core because the first O–Ne solids are buoyant, or an O–Ne inner core forms and Fe accumulates in the liquid until Fe distillation begins and forms an Fe shell. In the case of an Fe shell, a Rayleigh–Taylor instability may arise and overturn the core. In either case, Fe distillation may only produce a cooling delay of order 0.1 Gyr, as these processes occur early at high white dwarf luminosities. Fe inner cores and shells may be detectable through asteroseismology and could enhance the yield of neutron-rich elements such as55Mn and58Ni in supernovae.
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- Award ID(s):
- 1927130
- PAR ID:
- 10404995
- Publisher / Repository:
- DOI PREFIX: 10.3847
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 946
- Issue:
- 2
- ISSN:
- 0004-637X
- Format(s):
- Medium: X Size: Article No. 78
- Size(s):
- Article No. 78
- Sponsoring Org:
- National Science Foundation
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