Janus, also known by its name ZTF J203349.8+322901.1, is a transitioning white dwarf located more than 1,300 light-years (400 pc) away[3] in the constellation Cygnus, discovered in 2019 by the Zwicky Transient Facility (ZTF), located at the Palomar Observatory, while looking for periodically variable white dwarfs.[2] Subsequent observations using the Low-Resolution Imaging Spectrometer (LRIS) on the W. M. Keck Observatory discovered its two-faced nature, with one hemisphere dominated by hydrogen, and the other dominated by helium.[4]
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| 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: 3.479 mas/yr[1] Dec.: -6.971 mas/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 |
Properties
editZTF J203349.8+322901.1 is located more than 1,300 light-years (400 pc) away in the constellation Cygnus, 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 of 3400+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 the Gran 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 no Zeeman splitting observed. The two hemispheres were measured to be at different temperatures, with the hydrogen side at 34900+1300
−1500 K and the helium side at 36700+1300
−1600 K.[2][5]
Theories
editThere 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 a DB 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 of kGs,[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]
References
edit- ^ a b c d Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
- ^ a b c d e f g h i Caiazzo, Ilaria; Burdge, Kevin B.; Tremblay, Pier-Emmanuel; Fuller, James; Ferrario, Lilia; Gänsicke, Boris T.; Hermes, J. J.; Heyl, Jeremy; Kawka, Adela; Kulkarni, S. R.; Marsh, Thomas R.; Mróz, Przemek; Prince, Thomas A.; Richer, Harvey B.; Rodriguez, Antonio C.; van Roestel, Jan; Vanderbosch, Zachary P.; Vennes, Stéphane; Wickramasinghe, Dayal; Dhillon, Vikram S.; Littlefair, Stuart P.; Munday, James; Pelisoli, Ingrid; Perley, Daniel; Bellm, Eric C.; Breedt, Elmé; Brown, Alex J.; Dekany, Richard; Drake, Andrew; Dyer, Martin J.; Graham, Matthew J.; Green, Matthew J.; Laher, Russ R.; Kerry, Paul; Parsons, Steven G.; Riddle, Reed L.; Rusholme, Ben; Sahman, Dave I. (14 August 2023). "A rotating white dwarf shows different compositions on its opposite faces" (PDF). Nature. 620 (7972): 61–66. arXiv:2308.07430. Bibcode:2023Natur.620...61C. doi:10.1038/s41586-023-06171-9. PMID 37468630. S2CID 259993565. Retrieved 9 March 2024.
- ^ Cooper, Keith (19 July 2023). "Two-faced white dwarf star leaves astronomers puzzled". Physics World. Retrieved 11 March 2024.
- ^ Devlin, Hannah (20 July 2023). "Two-faced star with helium and hydrogen sides baffles astronomers". The Guardian. Retrieved 9 March 2024.
- ^ a b c d e Moss, Adam; Bergeron, P.; Kilic, Mukremin; Jewett, Gracyn; Brown, Warren R.; Kosakowski, Alekzander; Vincent, Olivier (12 December 2023). "Discovery of a magnetic double-faced DBA white dwarf". Monthly Notices of the Royal Astronomical Society. 527 (4): 10111–10122. arXiv:2312.05749. doi:10.1093/mnras/stad3825. Retrieved 11 March 2024.
- ^ Ravisetti, Monisha (21 July 2023). "Strange two-faced dying star 'Janus' baffles scientists in cosmic oddity". Space.com. Retrieved 9 March 2024.