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| Paper: |
Evolved Planetary Systems around Very Cool and Old White Dwarfs |
| Volume: |
493, 19th European Workshop on White Dwarfs |
| Page: |
267 |
| Authors: |
Hollands, M. A.; Gänsicke, B. T.; Koester, D. |
| Abstract: |
We have spectroscopically identified 61 very cool (below 9000 K) and
old (1–7 Gyr) DZ white dwarfs from the Sloan Digital Sky Survey (SDSS).
These stars have evaded prior detection as the extremely broad Ca H/K lines in
the blue part of their spectra dramatically alter their colours, mixing them
into the colour-space of intermediate redshift quasars.
In most of these stars we detect photospheric Ca, Mg, Fe and Na.
The coolest of these has Teff ≲ 5000 K corresponding to a cooling age
of ∼ 7 Gyr.
The only mechanism that can explain the large amounts of metal in the
convection zones of these white dwarfs is accretion of planetary debris.
Hence, these stars provide a lower limit on the onset of the formation of rocky
material within the Milky Way, and, more generally, insight into the formation
of early terrestrial planets.
Additionally, we identify several of these DZ to have strong (0.6–10 MG)
magnetic fields leading to an observed incidence of magnetism of 13 %. |
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