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 star 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]
  | |
| 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) | 1618+1407 −325 days |
| Semi-major axis (a) | 36+15 −2 mas |
| Eccentricity (e) | 0.39+0.35 −0.17 |
| Inclination (i) | 118±3° |
| Longitude of the node (Ω) | 24±5° |
| Periastron epoch (T) | 953+707 −640 |
| Argument of periastron (ω) (secondary) | 92+22 −44° |
| Details | |
| Mass | 2.04[5] M☉ |
| Radius | 1.6[7] R☉ |
| Luminosity | 13[8] L☉ |
| Surface gravity (log g) | 3.48[9] cgs |
| Temperature | 7530[9] K |
| Metallicity [Fe/H] | –0.11[3] dex |
| Rotational velocity (v sin i) | 206[10] km/s |
| Age | 660[11][12] Myr |
| Other designations | |
| Database references | |
| SIMBAD | data |
Properties
With an estimated age of 660 million years,[11][12] this 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 is larger than the Sun, with twice[5] the mass and a 60% greater radius.[7] It is radiating 13[8] times as much luminosity as the Sun from its outer atmosphere at an effective temperature of 7530 K.[9] At this heat, the star glows with the white hue of an A-type star.[17] The space velocity components of this star in the galactic coordinate system are U = -22, V = -20 and W = -9 km/s.[18]
Data from the Hipparcos mission indicate this may be an unresolved binary system with a companion orbiting at a semimajor axis of around 1 AU, or the same distance that the Earth orbits from the Sun.[15] 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][19]