# Extreme evaporation of planets in hot thermally unstable protoplanetary discs: the case of FU Ori

**Authors:** Sergei Nayakshin, James E. Owen, Vardan Elbakyan  
**arXiv:** [2305.03392](https://arxiv.org/abs/2305.03392)  
**Date:** 2023-05-05  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/2305.03392v1/

## Key points

- Protostar FU Ori has experienced a sudden increase in disc accretion rate hundreds of times greater than its previous levels, which has remained elevated to this day.
- This phenomenon challenges existing models of FU Ori and prompted the development of a new theory called Extreme Evaporation (EE).
- EE occurs when young gas giant planets in discs with midplane temperatures exceeding 30,000 K undergo thermal instability bursts, causing them to evaporate at rates of approximately 10^-5 Msun/yr.
- If this exceeds the background disc accretion activity, the system enters a planet-sourced mode where the planet becomes the dominant source of matter for the star for around O(100) years.
- Researchers found that dusty gas giants born in outer self-gravitating discs reach the innermost disc within approximately 10,000 years with radii around 10 R_J.
- A six Jupiter mass planet evaporating in a disc fed at an average rate of approximately 10^-6 Msun/yr appears to explain all that is currently known about FU Ori's accretion outburst.
- More massive planets and/or planets in older less massive discs do not experience EE process.
- Future modeling may constrain planet internal structure and evolution of earliest discs.
- Simple estimates suggest that EE can occur only if there are enough gas giants formed in outer self-gravitating parts of protoplanetary disks; however, current observational constraints on such populations are still weak.
- This study provides new insights into how extreme evaporation processes can impact protoplanetary disks and contribute to our understanding of FU Ori's unusual behavior.

## Summary

The protostar FU Ori has experienced a sudden increase in disc accretion rate hundreds of times greater than its previous levels, which has remained elevated to this day. This phenomenon challenges existing models of FU Ori and prompted the development of a new theory called Extreme Evaporation (EE). EE occurs when young gas giant planets in discs with midplane temperatures exceeding 30,000 K undergo thermal instability bursts, causing them to evaporate at rates of approximately 10^-5 Msun/yr. If this exceeds the background disc accretion activity, the system enters a planet-sourced mode where the planet becomes the dominant source of matter for the star for around O(100) years. Using a 1D time-dependent code, researchers found that dusty gas giants born in outer self-gravitating discs reach the innermost disc within approximately 10,000 years with radii around 10 R_J. The team used disc viscosity constrained by simulations and observations of dwarf novae instabilities and constrained planet properties with a stellar evolution code. They found that a six Jupiter mass planet evaporating in a disc fed at an average rate of approximately 10^-6 Msun/yr appears to explain all that is currently known about FU Ori's accretion outburst. More massive planets and/or planets in older less massive discs do not experience EE process. Future modeling may constrain planet internal structure and evolution of earliest discs. Simple estimates suggest that EE can occur only if there are enough gas giants formed in outer self-gravitating parts of protoplanetary disks; however, current observational constraints on such populations are still weak. Overall, this study provides new insights into how extreme evaporation processes can impact protoplanetary disks and contribute to our understanding of FU Ori's unusual behavior.