The major goals of this project were to advance our understanding of the planetary system around the white dwarfs - "dead' stars, which are the end products of stellar evolution. This project was guided by recent exciting discoveries of various signatures of planetary material around the white dwarfs, that must have survived through the post-main sequence evolution stage of the central star. These discoveries include (1) pollution of the white dwarf atmospheres with heavy elements ("metals"), (2) compact debris disks composed of particles, which reprocess starlight and emit in the infrared, accessible to Spitzer, as well as (3) compact gaseous discs spatially overlapping with particulate disks in many systems. These observed components show rich and yet unexplained dynamics, study of which was one of the proposal's goals. In particular, the majority of gaseous disks around the white dwarfs exhibit signatures of non-axisymmetric features - eccentric distortions and spiral arms - which evolve in time on year-long intervals.

Moreover, recent discovery of semi-regular transit events around WD1145 by the Kepler mission provided direct evidence for the picture of tidal disruption of the planetary objects by the white dwarfs, resulting in the formation of the observed disks. How such tidal disruptions operate in the case of solid objects was one of the questions addressed in this proposal.

Finally, some additional goals emerged as the project was in progress. In October 2017 Pan-STARRS survey detected the first interstellar minor object - 'Oumuamua - passing through the Solar System on a hyperbolic orbit. A number of reasons indicate that the origin of this object may be linked to tidal disruptions of the planetary objects by the white dwarfs, as we show in our work.

As a result of carrying out the proposed research program, we developed a theory for explaining the observed precession of gaseous debris disks has been developed, including the effects of pressure-driven precession as well as the general relativistic precession. Characteristic precession timescales have been linked to other observables of the disks (inner radii) and shown to broadly agree with observations (Fig. 1). Also, a framework for computing the potential of an apsidally-aligned, eccentric disk has been developed, for arbitrary radial profiles of the disk eccentricity and mass distribution (Fig. 2). Results of this work allow one to understand the conditions under which an eccentric disk (gaseous or particulate) around a white dwarf can coherently precess under its own self-gravity alone. We also numerically explored the dynamical effect of the spiral arms (inferred to be present in some systems) evolving into shocks on the underlying disk in which the arms propagate.

We identified the key features of the tidal disruption of solid objects (planetoids) by analogy with stellar tidal disruption events by the supermassive black holes. Important role of the collisional fragmentation caused by vertical compression of the debris in shaping the spectrum of fragments has been identified. We have also sketched the overall picture of how the highly eccentric debris resulting from the disruption event circularizes to produced the observed compact debris disks. 

We have persuasively linked the origin of the interstellar minor object 'Oumuamua to tidal disruption events of planetoids by the white dwarfs (Fig. 3). This production channel explains many surprising features of this newly discovered object - its unusual shape, size, mass budget in the Galaxy, and so on. Also, strong arguments have been advanced to suggest that the anomalous acceleration of 'Oumuamua cannot be caused by its cometary activity, rendering the cometary interpretation unlikely (Fig. 4). As a byproduct of this study, a direct connection between the rate of spin evolution and  the non-gravitational acceleration has been established observationally for the Solar System comets (Fig. 5). 

Results of this project have been presented in several conferences and colloquia/seminars and documented in several publications. Some of them led to press reports in popular science outlets such as Sky & Telescope, Quanta, AAS Nova. An important broader impct of this project was training of the young generation of scientists, including women in STEM fields. 

Last Modified: 01/15/2019
Modified by: Roman R Rafikov