Epoch J2000.0 Equinox J2000.0
|Right ascension||12h 36m 01.03100s|
|Declination||−39° 52′ 10.2270″|
|Apparent magnitude (V)||5.80|
|Spectral type||A0 V / M2.5 V|
|B−V color index||+0.01|
|Radial velocity (Rv)||9.4 km/s|
|Proper motion (μ)||RA: –56.66 mas/yr
Dec.: –24.99 mas/yr
|Parallax (π)||13.74 ± 0.33 mas|
|Distance||237 ± 6 ly
(73 ± 2 pc)
|Mass||2.18 ± 0.10 M☉|
|Surface gravity (log g)||4.43 cgs|
|Metallicity [Fe/H]||–0.03 dex|
|Rotational velocity (v sin i)||152 km/s|
|Age||(8 ± 2) × 106 years|
HR 4796 is a binary star system in the southern constellation of Centaurus. Parallax measurements put it at a distance of 237 light-years (73 parsecs) from the Earth. The two components of this system have an angular separation of 7.7 arcseconds, which, at their estimated distance, is equivalent to a projected separation of about 560 Astronomical Units (AU), or 560 times the separation of the Earth from the Sun. The star and its ring resemble an eye, and it is sometimes known by the nickname "Sauron's Eye".
This is a young system with an estimated age of about 8 million years. The primary member A has a stellar classification of A0 V, while its smaller companion B is a red dwarf with a classification of M2.5 V. The luminosity class of 'V' indicates that both stars belong to the main sequence and are generating energy through the thermonuclear fusion of hydrogen at their cores. The primary is emitting this energy from its outer envelope at an effective temperature of about 9,378 K, which gives it the white hue characteristic of A-type stars. It has a radius about 168% of the radius of the Sun and 218% of the Sun's mass. By comparison, the secondary has only 30% of a solar mass. The abundance of elements other than hydrogen or helium, what astronomers term the star's metallicity, is similar to the proportion in the Sun.
In 1991, the primary was found to have an excess of infrared emission, implying that it has a circumstellar debris disk. Observations using the Near-Infrared Multi-Object Spectrometer aboard the Hubble Space Telescope in 2007 indicated that the dust had a reddish spectrum similar to that of tholins. The disk was resolved with the Hubble Space Telescope in 2009, confirming that it exists. Based on these images, the disk has a radius of 75 AU and a width of less than 18.5 AU. It may have some asymmetries and the center appears to be slightly offset from the star. The dust in the disk is likely the result of collisions between larger particles. In 2011, observations of the ring offset by the Subaru Telescope implies that one or more planets would likely exist within gaps tugging at its dust grains. With a new image in 2014, the configuration and alignment of the dust ring and HR4796A has been nicknamed "Eye of Sauron".
The space velocity of HR 4796 in the Galactic coordinate system is [U, V, W] = [−8.5, −18.3, −3.6] km s−1. This trajectory and the location of the system suggests that it may be a member of the TW Hydrae association of stars that share a common origin. A low-mass member of this association, identified as 2MASS J12354893−3950245, may be a tertiary component of the HR 4796 system. It has a proper motion matching HR 4796, suggesting it is gravitationally bound to the other two stars, and is separated from the pair by a distance of about 13,500 AU.
- van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv: , Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357
- Johnson, H. L.; et al. (1966), "UBVRIJKL photometry of the bright stars", Communications of the Lunar and Planetary Laboratory, 4 (99): 99, Bibcode:1966CoLPL...4...99J
- Schneider, G.; et al. (January 2009), "STIS Imaging of the HR 4796A Circumstellar Debris Ring", The Astronomical Journal, 137 (1): 53–61, arXiv: , Bibcode:2009AJ....137...53S, doi:10.1088/0004-6256/137/1/53
- Makarov, Valeri V. (March 2007), "Unraveling the Origins of Nearby Young Stars", The Astrophysical Journal Supplement Series, 169 (1): 105–119, Bibcode:2007ApJS..169..105M, doi:10.1086/509887
- Gerbaldi, M.; et al. (June 1999), "Search for reference A0 dwarf stars: Masses and luminosities revisited with HIPPARCOS parallaxes", Astronomy and Astrophysics Supplement, 137 (2): 273–292, Bibcode:1999A&AS..137..273G, doi:10.1051/aas:1999248
- Saffe, C.; et al. (October 2008), "Spectroscopic metallicities of Vega-like stars", Astronomy and Astrophysics, 490 (1): 297–305, arXiv: , Bibcode:2008A&A...490..297S, doi:10.1051/0004-6361:200810260
- Royer, F.; Zorec, J.; Gómez, A. E. (February 2007), "Rotational velocities of A-type stars. III. Velocity distributions", Astronomy and Astrophysics, 463 (2): 671–682, arXiv: , Bibcode:2007A&A...463..671R, doi:10.1051/0004-6361:20065224
- Huélamo, N.; et al. (December 2004), "Rotation periods of Post-T Tauri stars in Lindroos systems", Astronomy and Astrophysics, 428 (3): 953–967, arXiv: , Bibcode:2004A&A...428..953H, doi:10.1051/0004-6361:20034442
- "HD 109573 -- Star in double system", SIMBAD, Centre de Données astronomiques de Strasbourg, retrieved 2007-01-08
- Flora Graham (4 June 2014). "Eye of Sauron star spotted by planet-hunting camera". New Scientist.
- NewsCorp Australia (6 June 2014). "Sauron's Eye has been found. And it's watching us all, from above, in the form of ringed star HR 4796A". Courier Mail.
- "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, retrieved 2012-01-16
- Debes, J. H.; Weinberger, A. J.; Schneider, G. (2008). "Complex Organic Materials in the Circumstellar Disk of HR 4796A". Astrophysical Journal Letters. 673 (2): L191. arXiv: . Bibcode:2008ApJ...673L.191D. doi:10.1086/527546.
- "Subaru's sharp eye confirms signs of unseen planets in star's dust ring". Astronomy Magazine. 2012-01-04.
- Thalmann, C.; et al. (December 2011), "Images of the Extended Outer Regions of the Debris Ring around HR 4796 A", The Astrophysical Journal Letters, 743 (1): L6 (2011), arXiv: , Bibcode:2011ApJ...743L...6T, doi:10.1088/2041-8205/743/1/L6
- Kastner, J. H.; Zuckerman, B.; Bessell, M. (December 2008), "Identification of the TW Hydrae association member 2M1235-39: a tertiary component of the HR 4796 system", Astronomy and Astrophysics, 491 (3): 829–831, arXiv: , Bibcode:2008A&A...491..829K, doi:10.1051/0004-6361:200810580