EK Eridani

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]

EK Eridani

A visual band light curve for EK Eridani, adapted from Dall et al. (2010)[1]
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Eridanus[2]
Right ascension	04h 20m 38.642s[3]
Declination	−06° 14′ 45.58″[3]
Apparent magnitude (V)	6.15[2]
Characteristics
Spectral type	G8 IV-III[4] or K0 III[5]
B−V color index	0.901±0.002[2]
Variable type	BY Dra[6]
Astrometry
Radial velocity (Rv)	6.866±0.0002[7] km/s
Proper motion (μ)	RA: +92.020 mas/yr[3] Dec.: −55.192 mas/yr[3]
Parallax (π)	15.6213±0.0236 mas[3]
Distance	208.8 ± 0.3 ly (64.02 ± 0.10 pc)
Absolute magnitude (MV)	2.22[2]
Details
Mass	1.84±0.12[8] M☉
Radius	4.96±0.14[8] R☉
Luminosity	15.07±0.35[8] L☉
Surface gravity (log g)	3.39±0.12[8] cgs
Temperature	5,135±60[8] K
Metallicity [Fe/H]	+0.02±0.04[8] dex
Rotation	308.8±2.5 d [8]
Rotational velocity (v sin i)	< 1.6±0.4[8] km/s
Other designations
EK Eri, NSV 1563, BD−06° 875, GC 5264, HD 27536, HIP 20263, HR 1362, SAO 131129, PPM 186813[9]
Database references
SIMBAD	data

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]

See also

• 53 Camelopardalis – a star with properties similar to the EK Eri progenitor.^[14]