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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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