| Observation data EpochJ2000 EquinoxJ2000 | |
|---|---|
| Constellation | Cygnus |
| Right ascension | 20h 33m 49.805s[1] |
| Declination | +32° 29′ 01.21″[1] |
| Characteristics | |
| Evolutionary stage | White dwarf |
| Astrometry | |
| Proper motion (μ) | RA: 3.479mas/yr[1] Dec.: -6.971mas/yr[1] |
| Parallax (π) | 2.4525±0.4364 mas[1] |
| Distance | approx. 1,300 ly (approx. 410 pc) |
| Details[2] | |
| Mass | 1.2 to 1.27 M☉ |
| Radius | 3400+700 −600 km |
| Temperature | 34900+1300 −1500 K (hydrogen side) 36700+1300 −1600 K (helium side) K |
| Rotation | 14.97 minutes |
| Other designations | |
| |
| Database references | |
| SIMBAD | data |
Janus, also known by its nameZTF J203349.8+322901.1, is a transitioningwhite dwarf located more than 1,300 light-years (400 pc) away[3] in the constellationCygnus, discovered in 2019 by theZwicky Transient Facility (ZTF), located at thePalomar Observatory, while looking for periodically variable white dwarfs.[2] Subsequent observations using the Low-Resolution Imaging Spectrometer (LRIS) on theW. M. Keck Observatory discovered its two-faced nature, with one hemisphere dominated by hydrogen, and the other dominated by helium.[4]
ZTF J203349.8+322901.1 is located more than 1,300 light-years (400 pc) away in the constellationCygnus, with a mass between 1.2 M☉ and 1.27 M☉ (1.21 for an oxygen–neon core and 1.27 for a carbon–oxygen core), a radius of3400+700
−600 km and a surface temperature of approximately 35,000 Kelvin. It is rare in that it has two hemispheres of different gases, one dominated by hydrogen, and the other dominated by helium. Another star,GD 323, shares this feature, albeit much more subtle.[2]
Janus' rotation period was observed by using CHIMERA, a high-speed imaging photometer, and HiPERCAM, a quintuple-beam imager (data collected on the nights of 6 and 9 September 2021 for a total of 2.1 hours), both located on theGran Telescopio Canarias, which revealed a period of 14.97 minutes, which is much faster than what is usually observed in white dwarfs (hours to days). As it rotates, its spectrum transitions from only hydrogen lines to only helium lines at phases ≈ 0 and 0.5, respectively. There was noZeeman splitting observed. The two hemispheres were measured to be at different temperatures, with the hydrogen side at34900+1300
−1500 K and the helium side at36700+1300
−1600 K.[2][5]
There are some theories as to why ZTF J203349.8+322901.1's hemispheres are so starkly defined and of different compositions.[2][6][5]
The first theory revolves around the belief that white dwarfs undergo an evolutionary phase, where helium sinks towards the bottom and hydrogen rises towards the top due to their masses. It is theorised that Janus was observed in an intermediate stage of this phase, and is exiting the DB gap on its way to becoming aDB white dwarf.[2][5]
The second theory is based on asymmetric magnetic fields:[2] If one hemisphere has a stronger magnetic field than the other, then the magnetic pressure at the pole will be higher, causing the hydrogen to diffuse towards the pole due to the ion pressure gradient, requiring a magnetic field of at least tens ofkGs,[5] and less than a few MG.[2]
The third theory revolves around ZTF J203349.8+322901.1 being the result of a merger of two white dwarfs, due to its large mass and short rotation period.[2][5]