Cool DZ white dwarfs in the SDSS

Abstract

Aims. We report the identification of 26 cool DZ white dwarfs that lie across and below the main sequence in the Sloan Digital Sky Survey (SDSS) (u − g) vs. (g − r) two-color diagram; 21 of these stars are new discoveries. Methods. The sample was identified by visual inspection of all spectra of objects that fall below the main sequence in the two-color diagram, as well as by an automated search for characteristic spectral features over a large area in color space that included the main sequence. The spectra and photometry provided by the SDSS project are interpreted with model atmospheres, including all relevant metals. Effective temperatures and element abundances are determined, while the surface gravity has to be assumed and was fixed at the canonical value of logg = 8. Results. These stars represent the extension of the well-known DZ sequence towards cooler temperatures and fill the gap around T eff = 6500 K present in a previous study. The metal abundances are similar to those in the hotter DZ, but the lowest abundances are missing, probably because of our selection procedures. The interpretation is complicated in terms of the accretion/diffusion scenario, because we do not know if accretion is still occurring or has ended long ago. Independent of that uncertainty, the masses of the metals currently present in the convection zones -and thus an absolute lower limit of the total accreted masses -of these stars are similar to the largest asteroids in our solar system.

A Common Origin of Magnetism from Planets to White Dwarfs

“…In volume-limited surveys-those that select stars within a maximum distance from the Sun-the incidence of magnetism is~20% (Kawka et al 2007;Giammichele et al 2012;Sion et al 2014), while for magnitude-limited surveys-those selecting stars brighter than a given apparent magnitude-it is~8% (Liebert et al 2003;Kepler et al 2013Kepler et al , 2016. Nevertheless, because a population of white dwarfs with magnetic fields weaker than ∼1 kG may exist, the fraction of magnetic white dwarfs could be larger (Koester et al 2011;Kawka & Vennes 2012;Ferrario et al 2015). Observations also suggest (Valyavin & Fabrika 1999;Liebert et al 2003;Kawka & Vennes 2014;Sion et al 2014;Valyavin et al 2014;Hollands et al 2015) that the fraction of white dwarfs with strong magnetic fields is larger at low effective temperatures.…”

Blue satellites on He lines due to He-He collisions

“…Because metals would rapidly settle, only H and He should appear in the spectra, unless there is dredging from the interior or accretion of material from a disk or remnant of a former exoplanetary system. The analysis of DBZ spectra is therefore a potentially valuable method to determine the existence in the stellar progenitor of exoplanetary systems and their chemical abundances (Farihi 2011;Koester et al 2011;Dufour et al 2012). In this work, we consider broadening of He by He to develop an accurate treatment of the resonance lines that enable correct calculations of radiative transport in DB white dwarfs.…”

Linking long-term planetaryN-body simulations with periodic orbits: application to white dwarf pollution

“…Further, the wide range of WD cooling ages (the time since the star became a WD) at which metal pollution is observed (up to 5 Gyr; see Farihi et al 2011 andKoester et al 2011) necessitate understanding the long-term evolution of planets and asteroids. For a recent review summarizing our current knowledge of the long-term behaviour of planetary systems, see (Davies et al 2014), and for post-main-sequence systems in particular, see (Veras 2016).…”