Spectral analysis of ultra-cool white dwarfs polluted by planetary debris

Title	Spectral analysis of ultra-cool white dwarfs polluted by planetary debris
Authors	Abbigail K. Elms, Pier-Emmanuel Tremblay, Boris T. Gänsicke, Detlev Koester, Mark A. Hollands, Nicola P. Gentile Fusillo, Tim Cunningham, Kevin Apps
Date	06/10/2022
Introduction	This study identifies two exceptionally cool, metal-polluted (DZ) white dwarfs, WDJ2147−4035 and WDJ1922+0233, as the coolest and second coolest DZ stars known, with effective temperatures around 3050 K and 3340 K, respectively. WDJ1922+0233 exhibits an unusually blue colour for its low temperature, attributed to strong atmospheric collision-induced absorption (CIA) that suppresses red optical and infrared flux. Conversely, WDJ2147−4035 shows moderate infrared CIA but possesses the reddest optical colours for a DZ white dwarf. Improvements to the microphysics of non-ideal effects and CIA opacities in our model atmosphere code provide consistent explanations for observations of these ultra-cool stars. WDJ2147−4035 has a cooling age exceeding 10 Gyr, the largest known for a DZ white dwarf, while WDJ1922+0233 is slightly younger at 9 Gyr. Kinematic calculations from precise Gaia EDR3 astrometry indicate these ultra-cool DZ stars are likely members of the Galactic disc, suggesting their importance for future studies constraining an upper age limit for the Milky Way's disc. We present intermediate-resolution spectroscopy for both objects, including the first spectroscopic observations of WDJ2147−4035. Detections of sodium and potassium are made in both white dwarfs, in addition to calcium in WDJ1922+0233 and lithium in WDJ2147−4035. The magnetic nature of WDJ2147−4035 is apparent from Zeeman splitting in the lithium line, and a tentative detection of carbon is also made, leading to its classification as DZQH. WDJ1922+0233 likely accreted planetary crust material, while the exact composition of the debris polluting WDJ2147−4035 remains undetermined.
Quote	Abbigail K. Elms, Pier-Emmanuel Tremblay and Boris T. Gänsicke et al. Spectral analysis of ultra-cool white dwarfs contaminated by planetary debris. 2022.
Element	Sodium (Na) , Potassium (K) , Calcium (Ca) , Lithium (Li) , Carbon (C)
Industry	Space , Research & Laboratory