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Abstract We increase the spectroscopic completeness of the 100 pc white dwarf sample in the Sloan Digital Sky Survey footprint with 840 additional spectra. Our spectroscopy is 86% complete for white dwarfs hotter thanTeff = 5000 K, where Hαremains visible and provides reliable constraints on the atmospheric composition. We identify 2108 DA white dwarfs with pure hydrogen atmospheres, and show that ultramassive DA white dwarfs withM≥ 1.1M⊙are an order of magnitude less common below 10,000 K. This is consistent with a fraction of them getting stuck on the crystallization sequence due to22Ne distillation. In addition, there are no ultramassive DA white dwarfs withM≥ 1.1M⊙andTeff≤ 6000 K in our sample, likely because Debye cooling makes them rapidly fade away. We detect a significant trend in the fraction of He atmosphere white dwarfs as a function of temperature; the fraction increases from 9% at 20,000 K to 32% at 6000 K. This provides direct evidence of convective mixing in cool DA white dwarfs. Finally, we detect a relatively tight sequence of low-mass DQ white dwarfs in color–magnitude diagrams for the first time. We discuss the implications of this tight DQ sequence, and conclude with a discussion of the future prospects from the upcoming Ultraviolet Transient Astronomy Satellite mission and the large-scale multi-fiber spectroscopic surveys.
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The 100 pc White Dwarf Sample in the SDSS Footprint
We present follow-up spectroscopy of 711 white dwarfs within 100 pc, and present a detailed model atmosphere analysis of the 100 pc white dwarf sample in the SDSS footprint. Our spectroscopic follow-up is complete for 83% of the white dwarfs hotter than 6000 K, where the atmospheric composition can be constrained reliably. We identify 1508 DA white dwarfs with pure hydrogen atmospheres. The DA mass distribution has an extremely narrow peak at $$0.59~M_{\odot}$$, and reveals a shoulder from relatively massive white dwarfs with $M=0.7$$-$$0.9~M_{\odot}$$. Comparing this distribution with binary population synthesis models, we find that the contribution from single stars that form through mergers cannot explain the over-abundance of massive white dwarfs. In addition, the mass distribution of cool DAs shows a near absence of $$M>1~M_{\odot}$$ white dwarfs. The pile-up of 0.7-$$0.9~M_{\odot}$$ and the disappearance of $$M>1~M_{\odot}$$ white dwarfs is consistent with the effects of core crystallization. Even though the evolutionary models predict the location of the pile-up correctly, the delay from the latent heat of crystallization by itself is insufficient to create a significant pile-up, and additional cooling delays from related effects like phase separation are necessary. We also discuss the population of infrared-faint (ultracool) white dwarfs, and demonstrate for the first time the existence of a well defined sequence in color and magnitude. Curiously, this sequence is connected to a region in the color-magnitude diagrams where the number of helium-dominated atmosphere white dwarfs is low. This suggests that the infrared-faint white dwarfs likely have mixed H/He atmospheres.
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- Award ID(s):
- 1906379
- PAR ID:
- 10159642
- Date Published:
- Journal Name:
- The Astrophysical journal
- ISSN:
- 2471-4259
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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