[Paper Review] Death by Dynamics: Planetoid-Induced Explosions on White Dwarfs

R. Di Stefano, Robert Fisher+2 more|arXiv (Cornell University)|Jan 30, 2015

Gamma-ray bursts and supernovae3 citations

TL;DR

This paper proposes that planetoid collisions with white dwarfs (WDs) can trigger detectable explosive events and, in rare cases, initiate Type Ia supernovae (SNe Ia) via 'death-by-dynamics'—a novel channel where repeated impacts enrich the WD's atmosphere, potentially enabling nuclear burning. Simulations show these collisions produce short-lived X-ray and longer optical emissions, with even massive impacts detectable in external galaxies.

ABSTRACT

At intervals as short as ten thousand years, each white dwarf (WD) passes within a solar radius of a planetoid, i.e., a comet, asteroid, or planet. Gravitational tidal forces tear the planetoid apart; its metal-rich debris falls onto the WD, enriching the atmosphere. A third of WDs exhibit atmospheric "pollution". For roughly every hundred planetoid disruptions, a planetoid collides with a WD. We simulate a small number of collisions, in which "death-by-dynamics" refers to the fate of the planetoid. We also compute the energies and likely durations of a broad sample of collision events, and identify detection strategies at optical and X-ray wavelengths. Collisions with the most massive planetoids can be detected in external galaxies. Some may trigger nuclear burning. If one in $\sim 10^7-10^8$ of WD-planetoid collisions creates the conditions needed for a Type Ia supernova (SN~Ia), "death-by-dynamics" would also refer to the fate of the WD, and could provide a novel channel for the production of SN~Ia. We consider the circumstances under which the rate of SNe~Ia can be increased by interactions with planetoids.

Motivation & Objective

• To investigate whether planetoid impacts on white dwarfs can produce observable explosive events detectable at optical and X-ray wavelengths.
• To assess the feasibility of planetoid-induced nuclear burning as a pathway to Type Ia supernovae, independent of binary companions.
• To quantify the energy release, timescales, and detectability of WD-planetoid collision events across a range of planetoid masses.
• To explore how planetary system interactions could influence the rate and diversity of SNe Ia, particularly in isolated WDs.
• To provide testable predictions for future surveys on the properties of SN Ia progenitors and circumstellar environments.

Proposed method

• Simulated planetoid impacts using the FLASH hydrodynamical code, modeling collisions at 45° angles with WDs of solar mass and radius.
• Calculated impact energies and timescales using tidal radius $ r_t = R_{\text{WD}} (\bar{\rho}_{\text{WD}} / \bar{\rho}_{\text{pl}})^{1/3} $, with free-fall time $ t_{\text{ff}} \sim 10 \text{ min} $.
• Modeled energy deposition and thermalization in WD atmospheres, estimating X-ray and optical luminosities over time.
• Assessed detectability by comparing peak luminosities and durations to survey sensitivities of Pan-STARRS, PTF, SkyMapper, and GAIA.
• Evaluated nuclear burning potential by analyzing the effect of heavy-element admixtures on $\alpha$-capture rates and ignition timescales.
• Explored synergy with standard SN Ia models (SD and DD), considering how planetoid-induced enrichment alters mass transfer stability and explosion conditions.

Experimental results

Research questions

• RQ1 Can WD-planetoid collisions produce detectable optical and X-ray transients?
• RQ2 What is the energy budget and timescale of WD-planetoid impact events across different planetoid masses?
• RQ3 Under what conditions can repeated planetoid impacts trigger nuclear burning and lead to Type Ia supernovae?
• RQ4 How do planetoid-induced enrichments affect the viability of SN Ia production in isolated white dwarfs?
• RQ5 What observational signatures would distinguish 'death-by-dynamics' SN Ia progenitors from standard binary models?

Key findings

• Collisions of 4 km planetoids with white dwarfs produce X-ray emission lasting ~10–100 ms and optical emission over longer timescales (~100 s to hours).
• Impacts of more massive planetoids (e.g., 400 km radius) release energies sufficient to be detectable in external galaxies, especially in deep optical surveys.
• A single impact heats the WD surface and atmosphere to several million Kelvin, with ejecta expanding and cooling rapidly.
• The inclusion of even a small fraction of heavy nuclei in a helium envelope can drastically reduce ignition timescales due to enhanced $\alpha$-capture rates.
• One in $ \sim 10^7 $ to $ 10^8 $ WD-planetoid collisions may create conditions conducive to a Type Ia supernova, suggesting a viable alternative progenitor channel.
• Death-by-dynamics models predict early-time SN Ia light curves and chemical diversity, consistent with observed SN Ia data, and may explain isolated SNe Ia without close companions.