Accretion of a giant planet onto a white dwarf (1912.01611v1)

Abstract: The detection of a dust disc around G29-38 and transits from debris orbiting WD1145+017 confirmed that the photospheric trace metals found in many white dwarfs arise from the accretion of tidally disrupted planetesimals. The composition of these planetesimals is similar to that of rocky bodies in the inner solar system. Gravitationally scattering planetesimals towards the white dwarf requires the presence of more massive bodies, yet no planet has so far been detected at a white dwarf. Here we report optical spectroscopy of a $\simeq27\,750$K hot white dwarf that is accreting from a circumstellar gaseous disc composed of hydrogen, oxygen, and sulphur at a rate of $\simeq3.3\times10^9\,\mathrm{g\,s^{-1}}$. The composition of this disc is unlike all other known planetary debris around white dwarfs, but resembles predictions for the makeup of deeper atmospheric layers of icy giant planets, with H$_2$O and H$_2$S being major constituents. A giant planet orbiting a hot white dwarf with a semi-major axis of $\simeq15$ solar radii will undergo significant evaporation with expected mass loss rates comparable to the accretion rate onto the white dwarf. The orbit of the planet is most likely the result of gravitational interactions, indicating the presence of additional planets in the system. We infer an occurrence rate of spectroscopically detectable giant planets in close orbits around white dwarfs of $\simeq10^{-4}$.

• The paper presents compelling evidence of a giant planet undergoing rapid mass loss while orbiting a white dwarf.
• Spectroscopic and photoionization modeling techniques characterized the circumstellar disc's unique chemical composition.
• Findings indicate that post-main-sequence evolution can preserve giant planets, shaping our understanding of planetary system dynamics.

Accretion of a Giant Planet onto a White Dwarf

This paper presents compelling observational evidence for a giant planet undergoing significant mass loss while orbiting closely around a white dwarf, specifically WD J0914+1914. This phenomenon provides a rare opportunity to paper post-main-sequence planetary system evolution, offering insights not just into the immediate afterlife of solar systems but also into the chemical compositions and dynamics of planets that gravitate around end-stage stars.

Key Findings and Methods

1. Observational Discovery: WD J0914+1914 was reclassified as a white dwarf with a close-in giant planet from its initial classification as a white dwarf binary. This was based on the detection of unusual emission lines, specifically hydrogen, oxygen, and sulfur, which are distinctive compared to known polluted white dwarfs typically accreting rocky materials.
2. Spectroscopic Analysis: Using the X-Shooter spectrograph, the paper identified multiple emission and absorption lines in the white dwarf's spectrum and quantitatively determined the presence of a circumstellar gaseous disc. The lines were found to be double-peaked, indicating an origin in a Keplerian disc rotating around the white dwarf.
3. Accretion Rate and Composition: The accretion rate was estimated to be around $3.3 \times 10^9 \, \mathrm{g\,s^{-1}}$, largely comprised of oxygen and sulfur, with a notable absence of rock-forming elements like calcium and iron typically found in white dwarfs accreting tidally disrupted planetesimals.
4. Modeling of the Circumstellar Disc: Using the Cloudy photoionization code, models were developed revealing that hydrogen in the disc is highly depleted relative to oxygen and sulfur, a composition reminiscent of deeper layers of icy giant planets. The disc's material is more similar to evaporative layers of icy giants rather than rocky planetary remnants.
5. Scenario of Mass Loss: It is postulated that the planet closely orbiting the white dwarf undergoes significant evaporation due to gravitational interactions and intense EUV radiation from the white dwarf. This results in mass loss rates analogous to those observed in hot Neptune-like exoplanets orbiting main-sequence stars.

Implications and Future Directions

The identification of a close-in giant planet around WD J0914+1914 is significant for several reasons:

• Planetary System Evolution: The presence of such planets supports the theoretical prediction that planets can survive post-main-sequence stellar evolution processes and maintain a close orbit around white dwarfs, enabling studies of their dynamical evolution through gravitational interactions and potential scattering events.
• Chemical Diversity and Accretion Processes: Understanding the chemical composition in these systems unfolds the diversity of planetary composition beyond rocky elements that traditionally dominate observations of white dwarf pollution. Such studies can inform models of giant planet interiors and atmospheric compositions.
• Detection of More Systems: The rarity of such systems implies the need for more comprehensive searches in large databases like Gaia, which could reveal additional analogs and enhance our understanding of the occurrence rate and diversity of planetary systems at this stage of stellar evolution.
• Theoretical Modelling: The paper opens avenues for refined models centered on post-main-sequence interactions of multi-planet systems, potentially refining theories around tidal forces, mass loss processes, and atmospheric retention in failing planetary environments.

In summary, the work significantly contributes to our understanding of exoplanet dynamics and compositions, particularly around white dwarfs. It strengthens the narrative that even as stars die, their planets can offer vital clues to the materials and histories of distant solar systems. Further observational campaigns leveraging current and future astronomical instruments are poised to expand and refine these findings.