Kappa Andromedae

In traditional Arabic astronomy, the stars ι Andromedae, κ Andromedae, and λ Andromedae were known as Kaff al-Musalsala, the hand of the chained woman (i.e. Andromeda), and as Ra’s al-Nāqa, the head of the she-camel.^[13] The IAU Working Group on Star Names approved the name Kaffalmusalsala for κ Andromedae on 8 May 2025 and it is now so entered in the IAU Catalog of Star Names; the name Rasalnaqa was given to ι Andromedae.^[10]

In Chinese, 螣蛇 (Téng Shé), meaning Flying Serpent, refers to an asterism consisting of κ Andromedae, α Lacertae, 4 Lacertae, π² Cygni, π¹ Cygni, HD 206267, ε Cephei, β Lacertae, σ Cassiopeiae, ρ Cassiopeiae, τ Cassiopeiae, AR Cassiopeiae, 9 Lacertae, 3 Andromedae, 7 Andromedae, 8 Andromedae, λ Andromedae, ι Andromedae, and ψ Andromedae. Consequently, the Chinese name for κ Andromedae itself is 螣蛇二十一 (Téng Shé èrshíyī, English: the Twenty First Star of Flying Serpent).^[14]

The stellar classification of Kappa Andromedae is A0 V^[5] indicating that it is a main sequence star fusing hydrogen into helium at its core. The star has an estimated 2.8 times the mass of the Sun and is radiating 62.6 times the Sun's luminosity.^[7] It is spinning rapidly, with a projected rotational velocity of 162 km/s. Its true rotational velocity is 283.8 km/s, which is about 85% of its critical rotation rate (the rate at which it would break up).^[7] With such a rapid rotation rate, the star is deformed into an oblate spheroid, such that while the polar radius is 1.959 R_☉, the equatorial radius is significantly larger, at 2.303 R_☉.^[7] The outer envelope of the star is radiating energy into space with an effective temperature of 10,342 K at the equator and 12,050 K at its poles, producing a blue-white hue.^[7]

The age of Kappa Andromedae has been the subject of debate. The discovery paper for Kappa Andromedae b^[12] argued that the primary's kinematics are consistent with membership in the Columba Association, which would imply a system age of 20-50 million years, while a subsequent work derived an older age of 220±100 million years based on the star's position on the Hertzsprung-Russell diagram position assuming that the star is not a fast rotator viewed pole-on.^[15] Direct measurements of the star later showed that Kappa Andromedae A is in fact a rapid rotator viewed nearly pole-on and yield a best-estimated age of 47⁺²⁷
₋₄₀ million years.^[7] Modelling of the companion Kappa Andromedae b further constrains the age to 47±7 million years.^[8]

In November 2012, members of the Strategic Explorations of Exoplanets and Disks with Subaru (SEEDS) survey reported the discovery of a faint, directly-imaged companion Kappa Andromedae b.^[12] Follow-up photometry and spectroscopy of kappa And b with the Subaru Telescope, Keck Observatory, and Large Binocular Telescope constrained its mass to be about 13 Jupiter masses, temperature to be between 1700 K and 2150 K, and orbit to be highly eccentric with a semimajor axis likely greater than about 75 AU. The companion's spectrum shows evidence for water and carbon monoxide molecules and suggests the object has a low surface gravity.^[16][17][18]

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