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XX Trianguli
Star system in the constellation Taurus From Wikipedia, the free encyclopedia
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XX Trianguli, abbreviated XX Tri, is a variable star in the northern constellation of Triangulum, about 1.5° to the WNW of Beta Trianguli along the constellation border with Andromeda.[7] It is classified as a RS Canum Venaticorum variable and ranges in brightness from magnitude 8.1 down to 8.7,[2] which is too faint to be visible to the naked eye. The system is located at a distance of approximately 642 light years from the Sun based on parallax,[1] but is drifting closer with a radial velocity of −26 km/s.[4]
This is a single-lined spectroscopic binary with an orbital period of 23.96924 days.[3] The visible component is an orange-hued K-type giant star with a stellar classification of K0 III,[3] indicating it has exhausted the supply of hydrogen at its core then cooled and expanded off the main sequence. It is around eight billion years old with 26% more mass than the Sun and has expanded to 11 times the Sun's radius. It is radiating roughly 30 times the luminosity of the Sun from its photosphere at an effective temperature of 4,620 K.[5]
The star is "covered with large high-latitude and even polar spots and with occasional small equatorial spots".[5] XX Tri is notable for having a huge starspot larger than the diameter of the Sun, discovered using Doppler imaging.[9] For its size, the star has a rapid rotation rate of about 24 days. It has a weak, Sun-like differential rotation. The star appears to show a magnetic activity cycle of 26±6 years, although only a single cycle has been observed as of 2015.
In a more recent study,[10] the authors used more than 2,000 high-resolution spectra collected over 16 years with the STELLA robotic telescopes in Tenerife, an unprecedented amount of homogeneous spectroscopic data from a star. From the data, 99 time-series Doppler images were reconstructed, showing the spot evolution on the stellar surface between 2006 and 2022. One of the main findings of the study is that the surface changes of XX Tri do not show Sun-like magnetic cycles, based on which the authors conclude that the star's dynamo is non-periodic in nature, most likely chaotic.
This study is the first to demonstrate how the huge starspots cause a tiny displacement of XX Tri in the sky, which appears virtually as a point source when observed from Earth. The reason for this is that while the photocenter of a homogeneous (=unspotted) stellar disk is the same as the geometric center of the star, huge starspots on the stellar disk repel the photocenter in the opposite direction to the spots. In the case of XX Tri, which is 630 light-years away, the photocenter of the stellar disk can shift by up to 10% of the star's radius relative to the geometric center, causing an apparent displacement of 24 micro-arcseconds in the celestial position of the star (the diameter of a hair at a distance of 1000 km). This is similar to the expected astrometric displacement caused by a Saturn-mass planet in a one year orbit around a Sun at about 300 lightyears distance. Therefore, separating the effects of spots and exoplanets seems very challenging, if not impossible, in particular in cases of similar periodicity, the study[10] concludes.
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References
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