YZ Ceti

YZ Ceti is a red dwarf star in the constellation Cetus. Although it is relatively close to the Sun at just 12 light years,[1] this star cannot be seen with the naked eye. It is classified as a flare star that undergoes intermittent fluctuations in luminosity. YZ Ceti is about 13 percent the mass of the Sun and 17% of its radius.

YZ Ceti

Location of YZ Ceti, click for larger image.
Observation data Epoch J2000      Equinox J2000
Constellation	Cetus
Right ascension	01^h 12^m 30.6368^s[1]
Declination	−16° 59′ 56.3613″[1]
Apparent magnitude (V)	12.03 - 12.18[2]
Characteristics
Spectral type	M4.0Ve[3]
U−B color index	+1.430[4]
B−V color index	+1.811[4]
Variable type	Flare star[5]
Astrometry
Radial velocity (Rv)	+28.09[6] km/s
Proper motion (μ)	RA: 1205.176±0.170[1] mas/yr Dec.: 637.758±0.120[1] mas/yr
Parallax (π)	269.3628 ± 0.0785[1] mas
Distance	12.108 ± 0.004 ly (3.712 ± 0.001 pc)
Absolute magnitude (MV)	14.30[3]
Details[7]
Mass	0.130 M☉
Radius	0.168 R☉
Temperature	3,056 K
Metallicity [Fe/H]	−0.26 dex
Age	5.0 Gyr
Other designations
GCTP 248.01, GJ 54.1, G 268-135, LHS 138, LTT 670, L 725-32, HIP 5643, 2MASS J01123052-1659570, Gaia DR2 2358524597030794112
Database references
SIMBAD	data

This star is unusually close to Tau Ceti, a star of spectral class G8. The two are only about 1.6 light years apart,[8] a little more than a third of the distance from the Sun to the Solar System's nearest neighbor, Proxima Centauri.

Variability

YZ Ceti is a variable star designation: the star shows occasional rapid and brief increases in brightness, sometimes reaching magnitude 12.03, caused by eruptions from the surface. This type of variable star is known as a UV Ceti star after its first member, or more colloquially as a flare star. It also shows small periodic variations in brightness caused by starspots or chromospheric features moving as the star rotates. This class of variable stars are known as BY Draconis variables.[2] The periodic variations allow the rotational period of the star to be measured at 68.3 days, although modelling of its planetary system gives a rotational period for the star of 83 days.[9]

Planetary system

On 10 August 2017 three planets were announced to have been discovered around YZ Ceti and a possible fourth sub-Earth planet candidate, still needing confirmation, with 0.472±0.096 Earth masses at an orbital period of 1.04 days. The orbits of the three confirmed planets were determined to be too close to YZ Ceti to be within the star's habitable zone, with equilibrium temperatures ranging from 347–491 K (74–218 °C; 165–424 °F), 299–423 K (26–150 °C; 79–302 °F), and 260–368 K (−13–95 °C; 8–203 °F) for planets b, c, and d, respectively.[7]

An August 2018 study reexamined the discovery measurements, confirming the orbit of YZ Ceti d, but finding a possibly marginally longer orbital period of YZ Ceti b of 2.02 days rather than 1.97 days, and additionally finding that YZ Ceti c probably orbits in only 0.75 days rather than 3.06 days. If the latter is true, YZ Ceti c would have a mass of only 0.58 Earth masses and a roughly 10% chance of transiting YZ Ceti.

The YZ Ceti planetary system

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
e (unconfirmed)	0.472 M⊕	0.01018	1.04	~0	—	—
b	0.75 ± 0.13 M⊕	0.01557 ± 0.00052	1.96876 ± 0.00021	0.00 ± 0.12	—	—
c	0.98 ± 0.14 M⊕	0.02090 ± 0.00070	3.06008 ± 0.00022	0.04 ± 0.11	—	—
d	1.14 ± 0.17 M⊕	0.02764 ± 0.00093	4.65627 ± 0.00042	0.129 ± 0.096	—	—

See also

• List of nearest stars

References

1. Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
2. Watson, C. L. (2006). "The International Variable Star Index (VSX)". The Society for Astronomical Sciences 25th Annual Symposium on Telescope Science. Held May 23–25. 25: 47. Bibcode:2006SASS...25...47W.
3. Davison, Cassy L; White, R. J; Henry, T. J; Riedel, A. R; Jao, W.-C; Bailey, J. I; Quinn, S. N; Cantrell, J. R; Subasavage, J. P; Winters, J. G (2015). "A 3D Search for Companions to 12 Nearby M Dwarfs". The Astronomical Journal. 149 (3): 106. arXiv:1501.05012. Bibcode:2015AJ....149..106D. doi:10.1088/0004-6256/149/3/106. S2CID 9719725.
4. Koen, C; Kilkenny, D; Van Wyk, F; Marang, F (2010). "UBV(RI)C JHK observations of Hipparcos-selected nearby stars". Monthly Notices of the Royal Astronomical Society. 403 (4): 1949. Bibcode:2010MNRAS.403.1949K. doi:10.1111/j.1365-2966.2009.16182.x.
5. Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S. 1. Bibcode:2009yCat....102025S.
6. Nidever, David L; Marcy, Geoffrey W; Butler, R. Paul; Fischer, Debra A; Vogt, Steven S (2002). "Radial Velocities for 889 Late-Type Stars". The Astrophysical Journal Supplement Series. 141 (2): 503–522. arXiv:astro-ph/0112477. Bibcode:2002ApJS..141..503N. doi:10.1086/340570. S2CID 51814894.
7. Astudillo-Defru, N; Díaz, R. F; Bonfils, X; Almenara, J. M; Delisle, J.-B; Bouchy, F; Delfosse, X; Forveille, T; Lovis, C; Mayor, M; Murgas, F; Pepe, F; Santos, N. C; Ségransan, D; Udry, S; Wünsche, A (2017). "The HARPS search for southern extra-solar planets. XLII. A system of Earth-mass planets around the nearby M dwarf YZ Ceti". Astronomy & Astrophysics. 605: L11. arXiv:1708.03336. Bibcode:2017A&A...605L..11A. doi:10.1051/0004-6361/201731581. S2CID 119393757.
8. Page of Tau Ceti, see the chapter of Closest Neighbors, for YZ Ceti.
9. Jayasinghe, T; Stanek, K. Z; Kochanek, C. S; Holoien, T. W.-S; Shields, J. V; Thompson, T. A; Shappee, B. J; Prieto, J. L; Dong, Subo (2017). "ASAS-SN V-band Light Curve of Multi-Planet M-dwarf Host YZ Cet Reveals a Rotation Period of 68 Days". The Astronomer's Telegram. 0643: 1. Bibcode:2017ATel10643....1J.

External links

• ARICNS