| 1. |
- Walker, Anthony P, et al.
(författare)
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Horizon 2020 EuPRAXIA design study
- 2017
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Ingår i: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 874:1
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Tidskriftsartikel (refereegranskat)abstract
- The Horizon 2020 Project EuPRAXIA ("European Plasma Research Accelerator with eXcellence In Applications") is preparing a conceptual design report of a highly compact and cost-effective European facility with multi-GeV electron beams using plasma as the acceleration medium. The accelerator facility will be based on a laser and/or a beam driven plasma acceleration approach and will be used for photon science, high-energy physics (HEP) detector tests, and other applications such as compact X-ray sources for medical imaging or material processing. EuPRAXIA started in November 2015 and will deliver the design report in October 2019. EuPRAXIA aims to be included on the ESFRI roadmap in 2020.
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| 2. |
- Cukanovaite, E., et al.
(författare)
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Calibration of the mixing-length theory for structures of helium-dominated atmosphere white dwarfs
- 2019
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966. ; 490:1, s. 1010-1025
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Tidskriftsartikel (refereegranskat)abstract
- We perform a calibration of the mixing-length parameter at the bottom boundary of the convection zone for helium-dominated atmospheres of white dwarfs. This calibration is based on a grid of 3D DB (pure-helium) and DBA (helium-dominated with traces of hydrogen) model atmospheres computed with the CO5BOLD radiation-hydrodynamics code, and a grid of 1D DB and DBA envelope structures. The 3D models span a parameter space of hydrogen-to-helium abundances in the range -10.0 <= log (H/He) <= -2.0, surface gravities in the range 7.5 <= log g <= 9.0, and effective temperatures in the range 12 000K less than or similar to T-eff less than or similar to 34 000 K. The 1D envelopes cover a similar atmospheric parameter range, but are also calculated with different values of the mixing-length parameter, namely 0.4 <= ML2/alpha <= 1.4. The calibration is performed based on two definitions of the bottom boundary of the convection zone: the Schwarzschild and the zero convective flux boundaries. Thus, our calibration is relevant for applications involving the bulk properties of the convection zone including its total mass, which excludes the spectroscopic technique. Overall, the calibrated ML2/alpha is smaller than what is commonly used in evolutionary models and theoretical determinations of the blue edge of the instability strip for pulsating DB and DBA stars. With calibrated ML2/alpha we are able to deduce more accurate convection zone sizes needed for studies of planetary debris mixing and dredge-up of carbon from the core. We highlight this by calculating examples of metal-rich 3D DBAZ models and finding their convection zone masses. Mixing-length calibration represents the first step of in-depth investigations of convective overshoot in white dwarfs with helium-dominated atmospheres.
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| 3. |
- Manser, Christopher J., et al.
(författare)
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A planetesimal orbiting within the debris disc around a white dwarf star
- 2019
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 1095-9203 .- 0036-8075. ; 364:6435, s. 66-69
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Tidskriftsartikel (refereegranskat)abstract
- Many white dwarf stars show signs of having accreted smaller bodies, implying that they may host planetary systems. A small number of these systems contain gaseous debris discs, visible through emission lines. We report a stable 123.4-minute periodic variation in the strength and shape of the Ca II emission line profiles originating from the debris disc around the white dwarf SDSS J122859.93+104032.9. We interpret this short-period signal as the signature of a solid-body planetesimal held together by its internal strength.
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| 4. |
- Tremblay, P. -E, et al.
(författare)
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Convective overshoot and metal accretion onto white dwarfs.
- 2017
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Ingår i: MEMORIE della Società Astronomica Italiana. - 0037-8720 .- 1824-016X. ; 88, s. 104-
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- A large fraction of white dwarfs host evolved planetary systems and show evidence of accretion from planetary debris. The accretion-diffusion model is the preferred method to understand the metal pollution in these otherwise hydrogen- and helium-rich white dwarf atmospheres. In this scenario, the accreted material first settles on the atmosphere. If the outer stellar layers are unstable to convection, the metals are then rapidly mixed up within the convection zone. In the classical 1D approach, it is generally assumed that the convection zone has a sharp bottom boundary, below which microscopic diffusion is unhampered and slowly removes metals from the visible layers. More realistic 3D radiation-hydrodynamics simulations of white dwarfs with CO5BOLD demonstrate, however, that the bottom of the convection zone does not have a sharp boundary, and that velocities decay exponentially below the unstable convective layers with a velocity scale height of the order of one pressure scale height. This has a potentially dramatic effect on the inferred mass of accreted materiel, hence on the chemical composition and size of planetary debris around white dwarfs.
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