Though elongated in shape, 2003 AZ84 displays a small lightcurve amplitude due to its rotation axis being oriented nearly pole-on; the variability is mainly caused by albedo features on its surface.[11][10]
It is considered a very likely dwarf planet by astronomers Gonzalo Tancredi and Michael Brown.[15][16] However, Will M. Grundy et al. conclude that objects such as this, in the size range of 400–1,000 km, with albedos less than ≈0.2 and densities of ≈1.2 g/cm3 or less, have likely never compressed into fully solid bodies, let alone differentiated or collapsed into hydrostatic equilibrium, and so are highly unlikely to be dwarf planets.[17]
Physical characteristics
The Spitzer Space Telescope has estimated its size at 686±96 km,[18] while an analysis of a combination of Spitzer and Herschel data yielded a slightly higher estimate of 727.0+61.9 −66.5 km.[19] These results are in agreement with each other.[a]
The large size of 2003 AZ84 makes it a possible dwarf planet. However, if one assumes it to be in hydrostatic equilibrium, the density that results is too low for it to be solid, and hence it may not be a dwarf planet. Its mass is unknown since the satellite has not been recovered.[9]
A stellar occultation in 2010 measured a single chord of 573±21 km.[21] But this is only a lower limit for the diameter of 2003 AZ84 because the chord may not have passed through the center of the body.[22]
In 2017, stellar occultations and data from its rotational lightcurve suggested that 2003 AZ84 had an elongated shape, presumably due to its rapid rotation rate of 6.71 hours, similar to Haumea and Varuna.[10] That would give 2003 AZ84 approximate dimensions of 940×766×490 km, with its longest axis nearly twice as long as its polar axis.
The spectra and colors of 2003 AZ84 are very similar to those of Orcus, another large object in 2:3 resonance with Neptune. Both bodies have a flat featureless spectrum in the visible and moderately strong water ice absorption bands in the near-infrared, although 2003 AZ84 has a lower albedo. Both bodies also have a weak absorption band near 2.3 μm, which may be caused by ammonia hydrate or methane ice.[23]
Orbit and rotation
2003 AZ84 orbits the Sun at an average distance of 39.4 astronomical units (AU) and completes a full orbit in 247 years.[1] It is in a 2:3 orbital resonance with Neptune; 2003 AZ84 completes two orbits around the Sun for every three orbits completed by Neptune.[14] Since it is in a 2:3 resonance with Neptune, 2003 AZ84 is classified as a plutino.[14] Its orbit is inclined to the ecliptic by 13.6 degrees.[1] The orbit of 2003 AZ84 is moderately eccentric, with an orbital eccentricity of 0.183.[1] As of July 2019[update], 2003 AZ84 is currently located 44.43 AU (6.647×109 km) from the Sun.[12] It had approached its aphelion (furthest distance from the Sun) in 1982[24] and will come to its perihelion (closest distance to the Sun) in 2107.[1] Simulations by the Deep Ecliptic Survey show that over the next 10 million years 2003 AZ84 will not come closer (qmin) than 31.6 AU from the Sun (it will stay farther away than Neptune).[3]
The rotation period of this minor planet was first measured by Scott Sheppard in 2003. Light curves obtained by Sheppard at the University of Hawaiʻi's 2.2-meter telescope gave an ambiguous rotation period of either 6.71 or 13.42 hours, with a brightness variation of 0.14 magnitudes (U=2).[25] The shorter rotation period refers to the single-peaked solution, expected if the brightness variations resulted from albedo spots. The longer rotation period is for a double-peaked solution, more consistent with an elongated shape that is rotating edge-on.[26]
Polar view of 2003 AZ84's orbit (yellow) along with other plutinos.
Orbit of 2003 AZ84 (blue) compared to the orbits of Pluto and Neptune.
Using observations with the Hubble Space Telescope, the discovery of a satellite of 2003 AZ84 was reported in IAUC 8812 on 22 February 2007.[8][6][27] The object was measured with a separation of 0.22 arcsec and an apparent magnitude difference of 5.0.[6] As of 2012[update], attempts to recover the satellite have failed.[9] The unrecovered satellite is estimated to be about 72±12 km in diameter.[7][8]
Notes
^The results of the previous observation of (208996) 2003 AZ84 by the Herschel were reported in 2010. They were obtained using chop/nod technique yielding 910±60 km.[20] The difference can be explained by the large light-curve amplitude of (208996) 2003 AZ84 and the fact that in 2010 the radiation from it was measured at one particular time, while 2012 determination was based on the time-averaged data.[19]
^ abMommert, M.; Harris, A. W.; Kiss, C.; Pál, A.; Santos-Sanz, P.; Stansberry, J.; et al. (4 May 2012). "TNOs are cool: A survey of the trans-Neptunian region. V. Physical characterization of 18 Plutinos using Herschel-PACS observations". Astronomy & Astrophysics. 541 (A93): 17. arXiv:1202.3657. Bibcode:2012A&A...541A..93M. doi:10.1051/0004-6361/201118562. S2CID119253817.
^Müller, T. G.; Lellouch, E.; Stansberry, J.; Kiss, C.; Santos-Sanz, P.; Vilenius, E. (16 July 2010). ""TNOs are Cool": A survey of the trans-Neptunian region I. Results from the Herschel science demonstration phase (SDP)". Astronomy & Astrophysics. 518 (L146): L146. arXiv:1005.2923. Bibcode:2010A&A...518L.146M. doi:10.1051/0004-6361/201014683. S2CID118635387.
^Delsanti, A.; Merlin, F.; Guilbert-Lepoutre, A.; Bauer, J.; Yang, B.; Meech, K. J. (28 September 2010). "Methane, ammonia, and their irradiation products at the surface of an intermediate-size KBO? A portrait of Plutino (90482) Orcus". Astronomy & Astrophysics. 520 (A40): 15. arXiv:1006.4962. Bibcode:2010A&A...520A..40D. doi:10.1051/0004-6361/201014296. S2CID118745903.
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