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.

Modeling Circumstellar Gas Emission around a White Dwarf Using cloudy

“…They described a "Z II" region model, which is akin to an H II region, and the gas is heated by UV light from the white dwarf and cools by optically thick emission lines. As mentioned in Steele et al (2021), CLOUDY computes a self-consistent temperature profile, which is an advantage compared to other disk modeling papers assuming an isothermal disk (e.g., Fortin-Archambault et al 2020; Budaj et al 2022).…”

Studying of exoasteroids orbiting around WD 1145+017

“…First of all, we ignore the solid core of exoasteroid and suppose that the dust almost covers the whole disc of the star. Properties of the dust were determined by Budaj et al (2022):it is composed mostly of silicates and mean grain size is about 5 microns. We also use dust opacities calculated by Budaj et al (2015).…”

Asynchronous accretion can mimic diverse white dwarf pollutants I: core and mantle fragments

“…Asynchronous core-mantle accretion may also occur in the very different scenario where large objects circularize before they disrupt. Such a scenario might play out around some white dwarfs, considering the transits of disintegrating planetesimals seen in some systems (Vanderburg et al 2015;Manser et al 2019;Farihi et al 2022;Budaj et al 2022). In the example of WD 1145+017, these transits are best modelled by differentiated planetesimals, whose mantles are shedding material, while their denser cores remain intact initially (Veras et al 2017;Duvvuri et al 2020).…”

“…The standard interpretation is that these stars are sampling the remains of old planetary systems (Jura 2003;Jura & Young 2014;Veras 2021), whose planets and asteroid belts beyond a few AU survived the expansion of the star on the red and asymptotic giant branches (Duncan & Lissauer 1998;Villaver & Livio 2009;Veras et al 2011;Veras & Tout 2012;Veras 2016) and were subsequently scattered close to the white dwarf via a range of mechanisms (Bonsor et al 2011;Debes et al 2012;Antoniadou & Veras 2016;Mustill et al 2018;Maldonado et al 2020;Veras et al 2021). In some systems, this circumstellar material emits a detectable infrared excess (Rocchetto et al 2015;Wilson et al 2019;Rogers et al 2020), and there are several instances where the existence of circumstellar material is implied E-mail: mgb52@cam.ac.uk by the detection of transits (Vanderburg et al 2015;Manser et al 2019;Vanderbosch et al 2020Vanderbosch et al , 2021Guidry et al 2021;Farihi et al 2022;Budaj et al 2022). Recently, ongoing accretion onto a white dwarf was confirmed via the detection of X-rays (Cunningham et al 2022).…”