White dwarfs with planetary remnants in the era of Gaia - I. Six emission line systems

White dwarfs with emission lines from gaseous debris discs are among the rarest examples of planetary remnant hosts, but at the same time they are key objects for studying the final evolutionary stage of planetary systems. Making use of the large number of white dwarfs identified in Gaia Data Release 2 (DR2), we are conducting a survey of planetary remnants and here we present the first results of our search: six white dwarfs with gaseous debris discs. This first publication focuses on the main observational properties of these objects and highlights their most unique features. Three systems in particular stand out: WD J084602.47+570328.64 displays an exceptionally strong infrared excess that defies the standard model of a geometrically thin, optically thick dusty debris disc; WD J213350.72+242805.93 is the hottest gaseous debris disc host known with \mbox{T_{\mathrm{eff}}}=29\,282 K; and WD J052914.32-340108.11 in which we identify a record number of 51 emission lines from five elements. These discoveries shed light on the underlying diversity in gaseous debris disc systems and bring the total number of these objects to 21. With these numbers we can now start looking at the properties of these systems as a class of objects rather than on a case-by-case basis

First detection of a disk free of volatile elements around a young A-type star: A possible sign of collisions between rocky planets

Aims. We present the first detailed analysis of the astrophysical parameters of the poorly studied Sco-Cen member HD 152384 and its circumstellar environment. Methods. We analyse newly obtained optical-near-IR X-shooter spectra, as well as archival TESS data, of HD 152384. In addition, we use literature photometric data to construct a detailed spectral energy distribution (SED) of the star. Results. The photospheric absorption lines in the spectrum of HD 152384 are characteristic of an A0 V star, for which we derive a stellar mass of 2.1 ± 0.1 M⊙ and a stellar age > 4.5 Myr. Superimposed on the photospheric absorption, the optical spectrum also displays double-peaked emission lines of Ca II, Fe I, Mg I, and Si I, typical of circumstellar disks. Notably, all hydrogen and helium lines appear strictly in absorption. A toy model shows that the observed emission line profiles can be reproduced by emission from a compact (radius < 0.3 au) disk seen at an inclination of ∼24°. Further evidence for the presence of circumstellar material comes from the detection of a moderate IR excess in the SED, similar to those found in extreme debris disk systems. Conclusions. We conclude that HD 152384 is surrounded by a tenuous circumstellar disk that, although rich in refractory elements, is highly depleted of volatile elements. To the best of our knowledge, such a disk is unique among young stars. However, it is reminiscent of the disks seen in some white dwarfs, which have been attributed to the disruption of rocky planets. We suggest that the disk around HD 152384 may have a similar origin and may be due to collisions in a newly formed planetary system

A Volume-limited Sample of Cataclysmic Variables from Gaia DR2: Space Density and Population Properties

We present the first volume-limited sample of cataclysmic variables (CVs), selected using the accurate parallaxes provided by the second data release (DR2) of the European Space Agency Gaia space mission. The sample is composed of 42 CVs within 150 pc, including two new systems discovered using the Gaia data, and is (77±10) per cent complete. We use this sample to study the intrinsic properties of the Galactic CV population. In particular, the CV space density we derive, ρ=(4.8+0.6−0.8)×10−6pc−3⁠, is lower than that predicted by most binary population synthesis studies. We also find a low fraction of period bounce CVs, seven per cent, and an average white dwarf mass of ⟨MWD⟩=(0.83±0.17)M⊙⁠. Both findings confirm previous results, ruling out the presence of observational biases affecting these measurements, as has been suggested in the past. The observed fraction of period bounce CVs falls well below theoretical predictions, by at least a factor of five, and remains one of the open problems in the current understanding of CV evolution. Conversely, the average white dwarf mass supports the presence of additional mechanisms of angular momentum loss that have been accounted for in the latest evolutionary models. The fraction of magnetic CVs in the 150 pc sample is remarkably high at 36 per cent. This is in striking contrast with the absence of magnetic white dwarfs in the detached population of CV progenitors, and underlines that the evolution of magnetic systems has to be included in the next generation of population models

Alkali metals in white dwarf atmospheres as tracers of ancient planetary crusts

White dwarfs that accrete the debris of tidally disrupted asteroids1 provide the opportunity to measure the bulk composition of the building blocks, or fragments, of exoplanets2. This technique has established a diversity of compositions comparable to what is observed in the Solar System3, suggesting that the formation of rocky planets is a generic process4. The relative abundances of lithophile and siderophile elements within the planetary debris can be used to investigate whether exoplanets undergo differentiation5, yet the composition studies carried out so far lack unambiguous tracers of planetary crusts6. Here we report the detection of lithium in the atmospheres of four cool (<5,000 K) and old (cooling ages of 5–10 Gyr ago) metal-polluted white dwarfs, of which one also displays photospheric potassium. The relative abundances of these two elements with respect to sodium and calcium strongly suggest that all four white dwarfs have accreted fragments of planetary crusts. We detect an infrared excess in one of the systems, indicating that accretion from a circumstellar debris disk is ongoing. The main-sequence progenitor mass of this star was 4.8 ± 0.2 M⊙, demonstrating that rocky, differentiated planets may form around short-lived B-type stars