WASP-21

WASP-21 is a G-type star (spectral type G3V) that is reaching the end of its main sequence lifetime^[4][5] approximately 850 light years from Earth in the constellation of Pegasus. The star is relatively metal-poor, having 40% of heavy elements compared to the Sun.^[5] Kinematically, WASP-21 belongs to the thick disk of the Milky Way.^[1] It has an exoplanet named WASP-21b. ^[6]

WASP 21 / Tangra

Size comparison between Sun and WASP-21
Observation data Epoch J2000      Equinox J2000
Constellation	Pegasus
Right ascension	23h 09m 58.25s[1]
Declination	+18° 23′ 45.9″[1]
Apparent magnitude (V)	11.58 ± 0.08[1]
Characteristics
Spectral type	G3V
Astrometry
Radial velocity (Rv)	-89.45 [1] km/s
Proper motion (μ)	RA: 17.597 ± 0.074 [2] mas/yr Dec.: 18.867 ± 0.05 [2] mas/yr
Parallax (π)	3.8412 ± 0.0427 mas[2]
Distance	849 ± 9 ly (260 ± 3 pc)
Absolute magnitude (MV)	4.97
Details
Mass	0.89 ± 0.071[3] M☉
Radius	1.136 ± 0.049[3] R☉
Surface gravity (log g)	4.277 ± 0.025[3] cgs
Temperature	5800 ± 100[1] K
Metallicity	-0.4 ± 0.1 [1]
Metallicity [Fe/H]	-0.46 ± 0.11 [1] dex
Rotational velocity (v sin i)	1.5 ± 0.6 [1] km/s
Age	12 ± 5[1] Gyr
Other designations
Tangra, GSC 01715-00679, 2MASS J23095825+1823459, Gaia DR2 2831084391023184128
Database references
SIMBAD	data

The survey in 2012 have failed to find any stellar companions to WASP-21.^[7]

Naming

In 2019 the WASP-21 system was chosen as part of the NameExoWorlds campaign organised by the International Astronomical Union, which assigned each country a star and planet to be named. WASP-21 was assigned to Bulgaria. The winning proposal named the star Tangra after a deity worshipped by the early Bulgars, and the planet Bendida after a deity worshipped by the Thracians.^[8]

Planetary System

In 2010 WASP-21 was discovered to host a hot Jupiter type planet by the Wide Angle Search for Planets (WASP).^[1] and confirmed by radial velocity by the WASP team in 2010.

Transit-timing variation analysis in 2015 did not find an additional planets in the system.^[5]

In 2020, spectroscopic analysis has found the WASP-21 b atmosphere is mostly cloudless and contains sodium.^[9]

The WASP-21 planetary system^[3][5]

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
b / Bendida	0.276 ± 0.018 MJ	0.0499 ± 0.0013	4.322482	0.0 [1]	86.97 ± 0.33°	1.162 RJ

References

1. Bouchy, F.; Hebb, L.; Skillen, I.; Collier Cameron, A.; Smalley, B.; Udry, S.; Anderson, D. R.; Boisse, I.; Enoch, B.; Haswell, C. A.; Hébrard, G.; Hellier, C.; Joshi, Y.; Kane, S. R.; Maxted, P. F. L.; Mayor, M.; Moutou, C.; Pepe, F.; Pollacco, D.; Queloz, D.; Ségransan, D.; Simpson, E. K.; Smith, A. M. S.; Stempels, H. C.; Street, R.; Triaud, A. H. M. J.; West, R. G.; Wheatley, P. J. (2010). "WASP-21b: A hot-Saturn exoplanet transiting a thick disc star". Astronomy and Astrophysics. 519: A98. arXiv:1006.2605. Bibcode:2010A&A...519A..98B. doi:10.1051/0004-6361/201014817. S2CID 119182669.
2. 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.
3. Ciceri, S.; Mancini, L.; Southworth, J.; Nikolov, N.; Bozza, V.; Bruni, I.; Calchi Novati, S.; d'Ago, G.; Henning, Th. (2013). "Simultaneous follow-up of planetary transits: Revised physical properties for the planetary systems HAT-P-16 and WASP-21". Astronomy & Astrophysics. 557: A30. arXiv:1307.5874. Bibcode:2013A&A...557A..30C. doi:10.1051/0004-6361/201321669. S2CID 55192357.
4. Barros, S. C. C.; Pollacco, D. L.; Gibson, N. P.; Howarth, I. D.; Keenan, F. P.; Simpson, E. K.; Skillen, I.; Steele, I. A. (2011). "A lower mass for the exoplanet WASP-21b". Monthly Notices of the Royal Astronomical Society. 416 (4): 2593–2599. arXiv:1106.2118. Bibcode:2011MNRAS.416.2593B. doi:10.1111/j.1365-2966.2011.19210.x. S2CID 56165266.
5. Seeliger, M.; Kitze, M.; Errmann, R.; Richter, S.; Ohlert, J. M.; Chen, W. P.; Guo, J. K.; Göğüş, E.; Güver, T.; Aydın, B.; Mottola, S.; Hellmich, S.; Fernandez, M.; Aceituno, F. J.; Dimitrov, D.; Kjurkchieva, D.; Jensen, E.; Cohen, D.; Kundra, E.; Pribulla, T.; Vaňko, M.; Budaj, J.; Mallonn, M.; Wu, Z.-Y.; Zhou, X.; Raetz, St.; Adam, C.; Schmidt, T. O. B.; Ide, A.; et al. (2015), "Ground-based transit observations of the HAT-P-18, HAT-P-19, HAT-P-27/WASP40 and WASP-21 systems", Monthly Notices of the Royal Astronomical Society, 451 (4): 4060–4072, arXiv:1508.06215, Bibcode:2015MNRAS.451.4060S, doi:10.1093/mnras/stv1187, S2CID 56034663
6. "Exoplanet-catalog". Exoplanet Exploration: Planets Beyond our Solar System. Retrieved 2022-12-10.
7. Ginski, C.; Mugrauer, M.; Seeliger, M.; Eisenbeiss, T. (2012), "A lucky imaging multiplicity study of exoplanet host stars", Monthly Notices of the Royal Astronomical Society, 421 (3): 2498–2509, arXiv:1202.4586, Bibcode:2012MNRAS.421.2498G, doi:10.1111/j.1365-2966.2012.20485.x, S2CID 118573795
8. "Bulgaria | NameExoworlds". Name Exoworlds. 2019-12-17. Retrieved 2019-12-30.
9. Chen, G.; Casasayas-Barris, N.; Pallé, E.; Welbanks, L.; Madhusudhan, N.; Luque, R.; Murgas, F. (2020), "Detection of Na in WASP-21b's lower and upper atmosphere", Astronomy & Astrophysics, 642: A54, arXiv:2007.13429, Bibcode:2020A&A...642A..54C, doi:10.1051/0004-6361/202038661, S2CID 220793336