XX Trianguli

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.^[8] 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]

A visual band light curve for XX Trianguli, adapted from Strassmeier (1999)^[9]

XX Trianguli

Doppler images of a giant starspot on XX Trianguli.
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Triangulum
Right ascension	02h 03m 47.11380s[1]
Declination	+35° 35′ 28.6692″[1]
Apparent magnitude (V)	8.1 – 8.7[2]
Characteristics
Spectral type	K0 III[3]
U−B color index	+0.78[2]
Variable type	RS CVn[2]
Astrometry
Radial velocity (Rv)	−26.35±0.18[4] km/s
Proper motion (μ)	RA: −53.222[1] mas/yr Dec.: −14.160[1] mas/yr
Parallax (π)	5.0820±0.0497 mas[1]
Distance	642 ± 6 ly (197 ± 2 pc)
Orbit
Period (P)	23.9674±0.0005[5] days
Inclination (i)	60±10[6]°
Details[5]
Mass	1.1+0.2 −0.3 M☉
Radius	8.95±0.23 R☉
Luminosity	337±2 L☉
Surface gravity (log g)	2.82±0.05 cgs
Temperature	4,627±46 K
Metallicity [Fe/H]	−0.13±0.04 dex
Rotation	23.470–24.716 days
Rotational velocity (v sin i)	20.0±0.6 km/s
Age	7.4+2.3 −3.2 Gyr
Other designations
XX Tri, BD+34°363, GJ 3130, HD 12545, HIP 9630, SAO 55233[7]
Database references
SIMBAD	data

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.^[6]

The star is "covered with large high-latitude and even polar spots and with occasional small equatorial spots".^[6] XX Tri is notable for having a huge starspot larger than the diameter of the Sun, discovered using Doppler imaging.^[10] 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.