AI Phoenicis
AI Phoenicis is a variable star in the constellation of Phoenix. An Algol-type eclipsing binary, its apparent magnitude is constant at 8.58 for most of the time, sharply dropping to 9.35 during primary eclipse and to 8.89 during secondary eclipse. The system's variability was discovered by W. Strohmeier in 1972. From parallax measurements by the Gaia spacecraft, the system is located at a distance of from Earth, in agreement with earlier estimates based on its luminosity (173 ± 11 parsecs).

The primary star is a K-type subgiant with a spectral type of K0IV and an effective temperature of 5,000 K, while the secondary is an F-type main sequence star with a spectral type of F7V and a temperature of 6,300 K. The primary component, while visually fainter, is slightly more luminous than the secondary due to its higher infrared output. The primary is at the end of its main sequence life and is likely in the short contraction phase known as a hook, where core hydrogen fusion has ceased but shell burning has not yet started, before ascending towards the red giant branch. Photometric and spectroscopic observations have allowed the direct determination of the parameters of the stars with extreme precision, and this system is frequently used to test stellar evolution models. The masses of the stars, 1.247 for the primary and 1.197 for the secondary, are known to a precision of just 0.3%, while the radii of 2.91 and 1.84 have uncertainties of 0.8% and 0.5% respectively. Stellar evolution models show the stars have a common age of about 4.4 billion years.

The orbit of AI Phoenicis has a period of 24.59248 days and a moderate eccentricity of 0.1821 ± 0.0051. The observation of eclipses is allowed by its 88.5° inclination to the plane of the sky. Times of minimum light show the orbital period of the system is not constant, which can be caused by a third star in the system. An analysis of the alignment of the system by the Rossiter–McLaughlin effect suggests that the secondary star rotation axis is not aligned with the orbital axis, with an angle of 87 ± 17° between them, which also indicates interactions with a third star.

Investigations continue with the TESS observatory in 2020. See Arxiv astro-ph.SR/
2003.09295 
Q6243
variable star
Concept
Q8928
constellation
Concept
Q10441
Phoenix
CEL
Q24452
Algol
Concept
Q50053
eclipsing binary
Concept
Q124313
apparent magnitude
Concept
Q165074
parallax
Concept
Q767805
Gaia
MISC
Q6227
subgiant
Concept
Q25377588
spectral type
Concept
Q854050
effective temperature
Concept
Q11579
K
Concept
Q1353952
F-type main sequence star
Concept
Q11388
infrared
Concept
Q3450
main sequence
Concept
Q6227
hook
Concept
Q3929897
red giant branch
Concept
Q6472
stellar evolution
Concept
Q37640
period
Concept
Q208474
eccentricity
Concept
Q4112212
inclination
Concept
Q1763739
Rossiter–McLaughlin effect
Concept
Q1323537
TESS
MISC
Q61746010
AI Phoenicis
CEL
Q61746010
AI Phoenicis
CEL
1972-01-01T00:00:00Z^^http://www.w3.org/2001/XMLSchema#dateTime
1972
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2020-01-01T00:00:00Z^^http://www.w3.org/2001/XMLSchema#dateTime
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constellation
type of variable star
instance of
follows