Alpha Pictoris

Alpha Pictoris (α Pic, α Pictoris) is the brightest star in the southern constellation of Pictor. It has an apparent visual magnitude of 3.27,^[2] which is bright enough to be viewed from urban areas in the southern hemisphere. This is actually a binary star system, whose components complete an orbit every three years and seven months.^[6] It is close enough for its distance to be measured using parallax shifts, which yields a value of roughly 97 light-years (30 parsecs) from the Sun, with a 5% margin of error.^[1] Alpha Pictoris has the distinction of being the south pole star of the planet Mercury.^[14]

α Pictoris

Location of α Pictoris (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Pictor
Right ascension	06h 48m 11.45512s[1]
Declination	−61° 56′ 29.0008″[1]
Apparent magnitude (V)	3.27[2]
Characteristics
Spectral type	A8 Vn kA6[3]
U−B color index	+0.13[2]
B−V color index	+0.21[2]
Astrometry
Radial velocity (Rv)	+20.6[4] km/s
Proper motion (μ)	RA: −66.07[1] mas/yr Dec.: +242.97[1] mas/yr
Parallax (π)	33.78±1.78 mas[1]
Distance	97 ± 5 ly (30 ± 2 pc)
Absolute magnitude (MV)	0.86[5]
Orbit[6]
Period (P)	1,316±2 days
Semi-major axis (a)	3.25 AU[a]
Eccentricity (e)	0.29±0.02
Inclination (i)	121±2°
Longitude of the node (Ω)	219±4°
Periastron epoch (T)	2456390±13 BJD
Argument of periastron (ω) (secondary)	353±4°
Details
A
Mass	1.6±0.1[6] M☉
Radius	3.55[7] R☉
Luminosity	38.7+6.0 −5.2[8] L☉
Surface gravity (log g)	3.48[9] cgs
Temperature	7,550±35[8] K
Metallicity [Fe/H]	−0.11[3] dex
Rotational velocity (v sin i)	206[10] km/s
Age	660[11][12] Myr
B
Mass	1.05±0.05[6] M☉
Temperature	5,068±91[6] K
Other designations
CD−61°1478, Gl 248, HD 50241, HIP 32607, HR 2550, LTT 2656, SAO 249647.[13]
Database references
SIMBAD	data

Properties

With an estimated age of 660 million years,^[11][12] the primary component is a relatively young Lambda Boötis star.^[15] The stellar classification of A8 Vn kA6^[3] shows this peculiarity, with the kA6 notation indicating weaker than normal calcium K-lines in the spectrum. The 'n' following the main sequence luminosity class of V indicates the absorption lines in the spectrum are broad and nebulous. This is caused by the rapid spin of the star, which has a high projected rotational velocity of 206 km/s.^[10] Spectroscopy shows narrow, time-varying absorption features being caused by circumstellar gas moving toward the star. This is not the result of interstellar matter, but a shell of gas along the orbital plane. Alpha Pictoris is categorized as a rapidly rotating shell star that may have recently ejected mass from its outer atmosphere.^[9][16]

Alpha Pictoris A is larger than the Sun, with a 60% greater mass^[6] and a 355% greater radius.^[7] It is radiating 40 times as much luminosity as the Sun from its outer atmosphere at an effective temperature of 7,550 K.^[8] At this heat, the star glows with the white hue of an A-type star.^[17] It is also one of the brightest known Delta Scuti variables.^[18] The space velocity components of this star in the galactic coordinate system are U = -22, V = -20 and W = -9 km/s.^[19]

Data from the Hipparcos mission uncovered the presence of a binary companion^[15] with a companion orbiting at a semimajor axis of around 3.25 AU, or three times the distance from Earth to the Sun.^[6] Alpha Pictoris is an X-ray source, which is unusual for an A-type star since stellar models don't predict them to have magnetic dynamos. This emission may instead be originating from the companion.^[9][20]

Notes

1. Calculated using a³ = (M₁ + M₂)  • P²,
   where a is the semi-major axis in astronomical units, M₁ and M₂ are the primary and secondary's mass respectively, in M_☉, and P is the period in years.