A sizable fraction of both young and old white dwarfs (WDs) are found to have metals in their atmospheres. This is puzzling because metals are expected to sink quickly below the WD's atmospheres compared their ages.
A widely accepted solution to this puzzle is that some WDs are accreting rocky material from an old planetary system. This is interesting as it links the metal lines with the presence of planets (or planetary debris) and can potentially teach us about planet formation and evolution in stars more massive than the sun (typically A and F stars).
However, the problem arises on the way that the planetary debris is being transported onto the WD's atmosphere because it has to satisfy the following conditions:
i) the material should survive the main sequence and giant phases of the host star;
ii) the material should be delivered at a high and steady state such that it is observed even in the oldest (>1 Gyr) white dwarfs.
No convincing solution has been proposed yet. In Petrovich & Muñoz 2017 we propose an alternative that at least alleviates some of the issues found in other previous works.
We proposed that the "polluting" material comes from a planetesimal disk that is delivered close to white dwarf by secular perturbations from a distant companion only after the main sequence and giant phases end. Once the rocks reach ~1 Solar radii, these are tidally disrupted and accreted onto the white dwarf.
How do we achieve this?
In our picture, there is sequence of events described in the sketch below:
a) during the main sequence there is a planetary system composed of planets and planetesimals. The planets shield the planetesimals from the secular perturbations from the distant pertuber(s).
b) the planets are engulfed during the giant phases (the radius of a star can reach out to several AU during the AGB phase).
c) mass loss expands the orbits of the surviving planetesimal and the distant pertuber(s).
d) the secular perturbations commence to excite extreme eccentricities such that the pericenter distance reaches ~1 Solar radii. Candidates for secular perturbations: Lidov-Kozai mechanism from a distant planet or star, secular chaos from distant planets.
Below there is an example of our proposal in which the perturber is distant star that forces the planetesimal to a radial orbit by the Lidov-Kozai mechanism