EK_Eridani
EK Eridani
Star in the constellation Eridanus
EK Eridani is a single variable star[3] in the equatorial constellation of Eridanus.[1] 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.[1] 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.[6]
From South Africa in 1964, R. Lake reported a variation of 0.7 in the visual magnitude of this star.[9] 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.[10] F. M. Walter and S. Bowyer detected X-ray emission from this star in 1973, another indicator of magnetic activity in the chromosphere.[11] 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.[12] By 1990, this period was revised upward to 335 days.[3]
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.[3] 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.[13] 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.[14]
This is an evolving subgiant star with a very slow rotation period of 308.8 days.[7] It is classified as a BY Draconis variable[5] that changes in luminosity as star spots rotate across the visible surface of the star.[7] 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.[15] 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.[7]