HU Delphini

HU Delphini, also known as Gliese 791.2, is a star system in the constellation of Delphinus. Its apparent magnitude is 13.07.^[3] With a trigonometric parallax of 113.4 ± 0.2 mas,^[3] it is about 28.76 light-years (8.82 parsecs) away from the Solar System.

HU Delphini

A visual band light curve for HU Delphini, plotted from ASAS-SN data.[1] The main plot shows the long-term variability, and the inset plot shows the periodic variability.
Observation data Epoch J2000      Equinox J2000
Constellation	Delphinus
Right ascension	20h 29m 48.344s[2]
Declination	+09° 41′ 20.25″[2]
Apparent magnitude (V)	13.07[3]
Characteristics
Spectral type	M4.5V[4]
U−B color index	+1.29[5]
B−V color index	+1.64[5]
Variable type	Flare star[6]
Astrometry
Radial velocity (Rv)	-34.24 ± 0.07[3] km/s
Proper motion (μ)	RA: 673.1[3] mas/yr Dec.: 122.0[3] mas/yr
Parallax (π)	113.4±0.2 mas[3]
Distance	28.76 ± 0.05 ly (8.82 ± 0.02 pc)
Absolute magnitude (MV)	13.46 / 16.73[3]
Orbit[3]
Period (P)	538.6 ± 0.1 d
Semi-major axis (a)	0.1037 ± 0.0005″
Eccentricity (e)	0.558 ± 0.005
Inclination (i)	145.4 ± 0.6°
Longitude of the node (Ω)	102.2 ± 1.1°
Periastron epoch (T)	JD 2451022.7 ± 0.9
Argument of periastron (ω) (secondary)	12.2 ± 1.5°
Semi-amplitude (K1) (primary)	4.09 ± 0.08 km/s
Semi-amplitude (K2) (secondary)	8.54 ± 0.13 km/s
Details[3]
HU Del A
Mass	0.237 ± 0.004 M☉
Metallicity [Fe/H]	0.07 dex
Rotation	0.3085±0.0005[7] days
Rotational velocity (v sin i)	35.1±0.2[7] km/s
HU Del B
Mass	0.114 ± 0.002 M☉
Other designations
HU Del, GJ 791.2, CCDM J20298+0941AB, LHS 3556, 2MASS J20294834+0941202, G 24-16[6]
Database references
SIMBAD	data
ARICNS	A
	B

HU Delphini is a binary star with a well-defined period of 538.6 days. The orbit has been derived from astrometry as well as through spectral observations, although that has been difficult because of the high projected rotational velocity. It is also fairly eccentric, at 0.558.^[3]

Both stars in the system are red dwarfs. The primary component of the system is only 23.7% as massive as the Sun,^[3] so it is fully convective.^[4] As a result, there are frequent starspots on its surface, especially near poles.^[7] While the normal surface temperature of the primary is 3000 K, the starspots themselves are cooler: only 2700 K. It is also a flare star,^[4] the first flare been detected in August 2000.^[8] The secondary star has a mass of 11.4% that of the Sun.^[3]

See also

• List of star systems within 25–30 light-years