WD 1145+017: Alternative models of the atmosphere, dust clouds, and gas rings

Abstract

Context. WD 1145+017 (WD1145) is the first white dwarf known to be orbited by disintegrating exoasteroids. It is a DBZ-type white dwarf with strongly variable broad circumstellar lines and variable shallow ultraviolet (UV) transits. Various models of the dust clouds and gaseous rings have been proposed as an explanation for this behavior. Aims. We aim to revisit these observations and propose alternative or modified models of the atmosphere of this white dwarf, its dust clouds, and gas rings. Methods. The simple radiative transfer code Shellspec was modified for this purpose and used for testing the new dust cloud and gas disk models. We used modified TLUSTY and SYNSPEC codes to calculate atmosphere models assuming the local thermodynamical equilibrium (LTE) or nonLTE (NLTE), and to calculate the intrinsic spectrum of the star. We then used these atmosphere models to estimate the mass of the radiative and convective zones and NLTE spectrum synthesis to estimate their chemical composition. Results. We offer an alternative explanation of some (not all) shallow UV transits. These may be naturally caused by the optical properties of the dust grains: opacities and mainly phase functions as a result of the forward scattering. The latter is much stronger in UV compared to the optical region, leaving more UV photons in the original direction during the transit. We also developed an alternative model of the gaseous disk, consisting of an inner, hotter, and almost circular disk and an outer, cooler, and eccentric disk. The structure precesses with a period of 3.83 ± 0.12 yr. We demonstrate that it fits the observed circumstellar lines reasonably well. These alternative models solve a few drawbacks that might be associated with the previous models, but they also have their own disadvantages. We confirm that the chemical composition of the atmosphere is similar to that of CI chondrites but carbon, nitrogen, and sulfur are significantly underabundant and much closer to the bulk Earth composition. This is a strong argument that the star has recently encountered and accreted material from a body of Earth-like composition.