It is a member of theHyadesopen cluster. As such its age is well constrained at 625 million years.[2] It is claimed to be the heaviest among planet-harboring stars with reliable initial masses.[2] Given its large mass, this star, though presently ofspectral type K0 III, was formerly of spectral type A that has now evolved off themain sequence into the giant phase. It is regarded as ared clump giant; that is, a core-helium burning star.[2]
Since Epsilon Tauri lies near the plane of theecliptic, it is sometimesocculted by theMoon and (very rarely) byplanets.
It has an 11th magnitude companion 182arcseconds from the primary, although this is an unrelated background star.[11]
The star bore the traditional nameAin (Arabicعين for "eye") and was given the nameOculus Boreus (Latin for "Northern eye") byJohn Flamsteed.[12][9] In 2016, theInternational Astronomical Union organized aWorking Group on Star Names (WGSN)[13] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[14] included a table of the first two batches of names approved by the WGSN; which includedAin for this star.
In July 2014, theInternational Astronomical Union launchedNameExoWorlds, a process for giving proper names to certain exoplanets.[15] The process involved public nomination and voting for the new names.[16] In December 2015, the IAU announced the winning name wasAmateru for this planet.[17]
The winning name was based on that submitted by the Kamagari Astronomical Observatory ofKure,Hiroshima Prefecture,Japan: namely 'Amaterasu', the Shinto goddess of the Sun, born from the left eye of the godIzanagi. The IAU substituted 'Amateru' – which is a common Japanese appellation forshrines when they enshrine Amaterasu – because 'Amaterasu' is already used for an asteroid (10385 Amaterasu).[18]
In 2007, a massiveexoplanet was reported orbiting the star with a period of 1.6 years in a somewhateccentric orbit. It was the first planet ever discovered in an open cluster.[2] A 2023 study updated this planet's parameters, and detected additionalradial velocity variations that are likely caused by stellar activity.[8]
^abBöhm-Vitense, Erika; et al. (December 2000), "Ultraviolet Emission Lines in BA and Non-BA Giants",The Astrophysical Journal,545 (2):992–999,Bibcode:2000ApJ...545..992B,doi:10.1086/317850.
