# A transiting brown dwarf in a 2 hour orbit

**Authors:** Kareem El-Badry, Kevin B. Burdge, Jan van Roestel, Antonio C. Rodriguez  
**arXiv:** [2307.15729](https://arxiv.org/abs/2307.15729)  
**Date:** 2023-07-28  
**Summary page:** https://www.summarizepaper.com/en/paper/2307.15729v1/

## Key points

- ZTF J2020+5033 is a high-mass brown dwarf transiting a low-mass star with an orbital period of 1.90 hours.
- It is the shortest-period known transiting brown dwarf by more than a factor of 7.
- The system was studied using spectroscopy, light curves, and astrometry from Gaia.
- The brown dwarf has a mass of $M_{\rm BD} = 80.1\pm 1.6\ M_{\rm J}$ and a radius of $R_{\rm BD} = 1.05 \pm 0.02 \ R_{\rm J}$.
- Its temperature is estimated to be approximately 1700 K.
- The system has a high tangential velocity and thick disk-like Galactic orbit, indicating it is old.
- Both components of ZTF J2020+5033 were larger in their youth, suggesting significant orbital shrinkage since formation.
- Magnetic braking is proposed as the explanation for this phenomenon.
- Further observations were conducted using CHIMERA and WISE satellite, revealing additional data on primary and secondary eclipses and ellipsoidal variation.
- This discovery contributes to our understanding of brown dwarfs in short-period orbits, stellar evolution, binary systems, and magnetic braking mechanisms in low mass stars and brown dwarfs.

## Summary

We report the discovery of ZTF J2020+5033, a high-mass brown dwarf (BD) transiting a low-mass star with an incredibly short orbital period of 1.90 hours. This finding is significant as ZTF J2020+5033 is now the shortest-period known transiting brown dwarf by more than a factor of 7. The system was studied using phase-resolved spectroscopy, optical and infrared light curves, and precise astrometry from Gaia, allowing for the determination of masses, radii, and temperatures with few-percent precision. The researchers inferred a brown dwarf mass of $M_{\rm BD} = 80.1\pm 1.6\ M_{\rm J}$, placing it almost exactly at the boundary between stellar and substellar objects. The brown dwarf also exhibited a moderately inflated radius of $R_{\rm BD} = 1.05 \pm 0.02 \ R_{\rm J}$. The temperature of the transiting object was estimated to be approximately 1700 K based on the depth of the infrared secondary eclipse, strongly suggesting that it is indeed a brown dwarf. The system's high tangential velocity ($v_\perp = 98 \ \rm km \ s^{-1}$) and thick disk-like Galactic orbit indicate that it is old. Additionally, its relatively close distance ($d \approx 140$ pc) suggests that brown dwarfs in short-period orbits may be relatively common. Interestingly, both components of ZTF J2020+5033 must have been considerably larger in their youth as they would comfortably fit inside the Sun's current size; this implies that the binary system has experienced significant orbital shrinkage since its formation. The researchers propose that magnetic braking continues to operate efficiently in at least some low-mass stars and brown dwarfs as the simplest explanation for this phenomenon. Further observations of ZTF J2020+5033 were conducted using the Caltech HIgh-speed Multi-color camERA (CHIMERA) and the Wide-field Infrared Survey Explorer (WISE) satellite. The CHIMERA observations captured simultaneous data in the Sloan g- and zs-bands, revealing a slightly deeper primary eclipse in the g-band and evidence of ellipsoidal variation and hints of a secondary eclipse in the zs-band data. The WISE satellite provided regular light curve measurements for ZTF J2020+5033 too. Overall, this discovery provides valuable insights into properties and behavior of brown dwarfs - particularly those in short period orbits - contributing to our understanding of stellar evolution, binary systems and magnetic braking mechanisms operating in low mass stars and brown dwarfs alike..