The Chemical Composition of an Extrasolar Kuiper-Belt-Object*

Abstract

The Kuiper Belt of our solar system is a source of short-period comets that may have delivered water and other volatiles to Earth and the other terrestrial planets. However, the distribution of water and other volatiles in extrasolar planetary systems is largely unknown. We report the discovery of an accretion of a Kuiper-Belt-Object analog onto the atmosphere of the white dwarf WD 1425+540. The heavy elements C, N, O, Mg, Si, S, Ca, Fe, and Ni are detected, with nitrogen observed for the first time in extrasolar planetary debris. The nitrogen mass fraction is ∼2%, comparable to that in comet Halley and higher than in any other known solar system object. The lower limit to the accreted mass is ∼10 22 g, which is about one hundred thousand times the typical mass of a shortperiod comet. In addition, WD 1425+540 has a wide binary companion, which could facilitate perturbing a Kuiper-Belt-Object analog into the white dwarf's tidal radius. This finding shows that analogs to objects in our Kuiper Belt exist around other stars and could be responsible for the delivery of volatiles to terrestrial planets beyond the solar system.

H–He collision-induced satellite in the Lyman α profile of DBA white dwarf stars

“…1. As described in Xu et al (2017), there are two unusual features of the Lyman-α profile in WD 1425+540. First, the line profile is very asymmetric exhibiting an extend blue wing with the satellite feature as noted.…”

“…We have shown there that the extension of the red wing of the Lyman-α line seen in DZ white dwarf spectra depends strongly on the stellar temperature, while it is not dependent on the helium density. We also predicted a blue satellite which only very recently has been observed in Hubble Space Telescope Cosmic Origins Spectrograph (HST COS ) observations (Xu et al 2017). The importance of a correct determination of the blue wing of Lyman-α line to interpret the asymmetrical shape of the Lyman-α line observed with COS is presented in Sect.…”

“…WD 1425+540 (T=14,490 K, log g=7.95) is the prototype of DBA white dwarfs and it is a helium-dominated white dwarf that also has a large amount of hydrogen in its atmosphere (Bergeron et al 2011). It was observed with HST COS un-der program 13453, and the details of observation and data reduction strategy were reported by Xu et al (2017). Here, we focus on the spectrum of segment B of the G130M grating, which covers 1130-1270Å, as shown in Fig.…”

“…The asymmetry also could not be produced by white dwarf models of Xu et al (2017) because the opacity data used for the Lyman-α profile did not take into account the quasi-molecular line satellite predicted in Allard & Christova (2009). Once this feature is included, the observed asymmetry is reproduced (Gänsicke et al 2018).…”

“…The fundamental theory of calculating the spectral line profile (Allard et al 1999) requires knowledge of molecular potentials with high accuracy because the shape and strength of the line profile are very sensitive to the details of the molecular potential curves describing the atom-atom collisions. In Allard & Christova (2009) we made an exhaustive study of the red wing of Lyman-α line perturbed by H-He collisions, where we used the potentials and electric dipole transition (Xu et al 2017). The strong emission at the center of Lyman-α is from Earth's geocoronal hydrogen above the HST orbit.…”