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EK Eridani
	; A visual band light curve for EK Eridani, adapted from Dall et al. (2010)
Observation data; Epoch J2000.0 Equinox J2000.0
Constellation	Eridanus
Right ascension	04h 20m 38.642s
Declination	−06° 14′ 45.58″
Apparent magnitude (V)	6.15
Characteristics
Spectral type	G8 IV-III or K0 III
B−V color index	0.901±0.002
Variable type	BY Dra
Astrometry
Radial velocity (Rv)	6.866±0.0002 km/s
Proper motion (μ)	RA: +92.020 mas/yr ; Dec.: −55.192 mas/yr
Parallax (π)	15.6213 ± 0.0236 mas
Distance	208.8 ± 0.3 ly ; (64.02 ± 0.10 pc)
Absolute magnitude (MV)	2.22
Details
Mass	1.84±0.12 M☉
Radius	4.96±0.14 R☉
Luminosity	15.07±0.35 L☉
Surface gravity (log g)	3.39±0.12 cgs
Temperature	5,135±60 K
Metallicity [Fe/H]	+0.02±0.04 dex
Rotation	308.8±2.5 d
Rotational velocity (v sin i)	< 1.6±0.4 km/s
Other designations
	EK Eri, NSV 1563, BD−06° 875, GC 5264, HD 27536, HIP 20263, HR 1362, SAO 131129, PPM 186813
Database references
SIMBAD	data

EK Eridani is a single variable star^[4] in the equatorial constellation of Eridanus.^[2] It has the designation HR 1362 from the Bright Star Catalogue; EK Eridani is the variable star designation, abbreviated EK Eri. This star is dimly visible to the naked eye with a brightness that fluctuates around 6.15.^[2] Based on parallax measurements, it is located at a distance of 209 light years from the Sun. The star is drifting further away with a heliocentric radial velocity of 6.9 km/s.^[7]

From South Africa in 1964, R. Lake reported a variation of 0.7 in the visual magnitude of this star.^[10] In 1973, W. P. Bidelman and D. J. MacConnell placed HR 1362 on a list of brighter stars of astrophysical interest because it displayed emission in the H and K lines, although they were uncertain of this finding.^[11] F. M. Walter and S. Bowyer detected X-ray emission from this star in 1973, another indicator of magnetic activity in the chromosphere.^[12] A series of measurements of the brightness of this star were made from 1979 until 1984, demonstrating it is variable with a 154-day period. At the time, this was the longest period known for a chromospherically active star.^[13] By 1990, this period was revised upward to 335 days.^[4]

K. G. Strassmeier and associates found the stellar spectrum and color indices to be consistent with a stellar classification of G8 III-IV for HR 1362. They confirmed the moderately strong H and K emission lines as being overactive by over an order of magnitude compared to other cool giant stars.^[4] The level of magnetic activity for this star is unusually high for its rotation period, which in 1993 led K. Stępień to suggest it was a strongly magnetic Ap star while on the main sequence.^[14] The mean strength of the magnetic field was determined to be 270 Gauss, which is comparable to the typical field strength of RS CVn or FK Com type variable stars.^[15]

This is an evolving subgiant star with a very slow rotation period of 308.8 days.^[8] It is classified as a BY Draconis variable^[6] that changes in luminosity as star spots rotate across the visible surface of the star.^[8] The lingering magnetic field may be the result of an interaction between the remnant field from an Ap progenitor star and a deep convection zone. At a 60° axial tilt, the field can be successfully modeled as a simple dipole magnet with a persistent cool spot at the magnetic pole.^[16] Asteroseismological measurements show EK Eri has 1.84 times the mass of the Sun and 5 times the Sun's radius. It is radiating 15 times the luminosity of the Sun from its enlarged photosphere at an effective temperature of 5,135 K.^[8]

References

^ Dall, T. H.; Bruntt, H.; Stello, D.; Strassmeier, K. G. (May 2010). "Solar-like oscillations and magnetic activity of the slow rotator EK Eridani" (PDF). Astronomy and Astrophysics. 514: A25. arXiv:1003.0433. Bibcode:2010A&A...514A..25D. doi:10.1051/0004-6361/200913710. Retrieved 21 September 2024.
^ ^a ^b ^c ^d ^e ^f Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters, 38 (5): 331, arXiv:1108.4971, Bibcode:2012AstL...38..331A, doi:10.1134/S1063773712050015, S2CID 119257644.
^ ^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 Strassmeier, Klaus G.; et al. (February 1990), "HR 1362: A Test Case for Stellar Dynamo Theories", Astrophysical Journal, 350: 367, Bibcode:1990ApJ...350..367S, doi:10.1086/168390.
^ Houk, N.; Swift, C. (1999), "Michigan catalogue of two-dimensional spectral types for the HD Stars", Michigan Spectral Survey, 5, Bibcode:1999MSS...C05....0H.
^ ^a ^b Samus', N. N; et al. (2017), "General catalogue of variable stars", Astronomy Reports, GCVS 5.1, 61 (1): 80, Bibcode:2017ARep...61...80S, doi:10.1134/S1063772917010085, S2CID 125853869.
^ ^a ^b Soubiran, C.; et al. (2018), "Gaia Data Release 2. The catalogue of radial velocity standard stars", Astronomy and Astrophysics, 616: A7, arXiv:1804.09370, Bibcode:2018A&A...616A...7S, doi:10.1051/0004-6361/201832795, S2CID 52952408.
^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k Bonanno, A.; et al. (August 2019), "Acoustic oscillations and dynamo action in the G8 sub-giant EK Eridani", Astronomy and Astrophysics, 628 (A106): A106, arXiv:1907.01338, Bibcode:2019A&A...628A.106B, doi:10.1051/0004-6361/201935834.
^ "EK Eri", SIMBAD, Centre de données astronomiques de Strasbourg, retrieved 2024-01-15.
^ Lake, R. (1964), "Photoelectric Magnitudes and Colours for 100 Southern Stars (Fifth List)", Monthly Notes of the Astronomy Society of Southern Africa, 23: 136, Bibcode:1964MNSSA..23..136L.
^ Bidelman, W. P.; MacConnell, D. J. (October 1973), "The brighter stars astrophysical interest in the southern sky", Astronomical Journal, 78: 687–733, Bibcode:1973AJ.....78..687B, doi:10.1086/111475. See table VII.
^ Walter, F. M.; Bowyer, S. (April 1981), "On the coronae of rapidly rotating stars. I. The relation between rotation and coronal activity in RS CVn systems", Astrophysical Journal, 245: 671–676, Bibcode:1981ApJ...245..671W, doi:10.1086/158842.
^ Boyd, L. J.; et al. (March 1985), "HR 1362: a Chromospherically Active Variable with a 5-month Period", Information Bulletin on Variable Stars, 2696 (1): 1, Bibcode:1985IBVS.2696....1B.
^ ^a ^b Stepien, K. (October 1993), "HR 1362: The Evolved 53 Camelopardalis", Astrophysical Journal, 416: 368, Bibcode:1993ApJ...416..368S, doi:10.1086/173240.
^ Aurière, M.; et al. (November 2008), "EK Eridani: the tip of the iceberg of giants which have evolved from magnetic Ap stars", Astronomy and Astrophysics, 491 (2): 499–505, arXiv:0810.2228, Bibcode:2008A&A...491..499A, doi:10.1051/0004-6361:200810502.
^ Aurière, M.; et al. (October 2011), "A dominant magnetic dipole for the evolved Ap star candidate EK Eridani", Astronomy & Astrophysics, 534, id. A139, arXiv:1109.5570, Bibcode:2011A&A...534A.139A, doi:10.1051/0004-6361/201117502.