Haumea family

The Haumea family is the only identified Trans-Neptunian collisional family; that is, the only group of TNOs with similar orbital parameters and physical characteristics that suggest they originated in the disruptive impact of a progenitor body.cite journal |last=Brown |first=Michael E. |authorlink= |coauthors=Barkume, Kristina M.; Ragozzine, Darin; Schaller, Emily L. |year=2007 |month= |title=A collisional family of icy objects in the Kuiper belt |journal=Nature |volume=446 |issue=7133 |pages=294–296 |doi=10.1038/nature05619 |url= |accessdate= |quote= ]

The dwarf planet dp|Haumea is the largest member of the family; other members include the moons of Haumea and the KBOs mpl|2002 TX|300, mpl|(24835) 1995 SM|55, mpl|(19308) 1996 TO|66, mpl|(120178) 2003 OP|32, and mpl|(145453) 2005 RR|43. These five KBOs are all estimated to be between 500 and 700 km in diameter, and so are likely to be dwarf planets themselves. (See List of plutoid candidates.) The dispersion of the proper orbital elements of the members is a few percent or less (5% for semi-major axis, 1.4° for the inclination and 0.08 for the eccentricity). The diagram illustrates the orbital elements of the members of the family in relation to other TNOs.

The objects' common physical characteristics include neutral colours and deep infrared absorption features (at 1.5 and 2.0 μm) typical of water ice.e.g. N. Pinilla-Alonso, J. Licandro, R. Gil-Hutton and R. Brunetto "The water ice rich surface of (145453) 2005 RR43: a case for a population of carbon-depleted TNOs?", A&A 468, L25-L28 (2007) [http://www.aanda.org/index.php?option=article&access=standard&Itemid=129&url=/articles/aa/full/2007/22/aa7294-07/aa7294-07.html] ]
*Collisional formation of the family requires a progenitor some 1660 km in diameter, with a density of ~2.0 g/cm³, similar to Pluto and Eris. During the formational collision, Haumea lost roughly 20% of its mass, mostly ice, and became denser.

The current orbits of the members of the family cannot be accounted for by the formational collision alone. To explain the spread of the orbital elements, an initial velocity dispersion of ~400 m/s is required, but such a velocity spread should have dispersed the fragments much further. This problem applies only to Haumea itself; the orbital elements of all the other objects in the family require an initial velocity dispersion of ~140 m/s. To explain this mis-match in the required velocity dispersion, Brown "et al." suggest that Haumea initially had orbital elements closer to those of the other members of the family and its orbit (especially the orbital eccentricity), changed "after" the collision. Unlike the other members of the family, Haumea is in a chaotic orbit, near the 7:12 resonance with Neptune, which would increase Haumea's eccentricity to its current value.

Haumea may not be the only elongated, rapidly rotating, large object in the Kuiper belt. In 2002, Jewitt and Sheppard suggested that dp|Varuna should be elongate, based on its rapid rotation. In the early history of the Solar System, the trans-Neptunian region would have contained many more objects than it does at present, increasing the likelihood of collisions between objects. Gravitational interaction with Neptune has since shifted many objects from the Kuiper belt to the more distant scattered disc.

The presence of the collisional family hints that Haumea and its "offspring" might have originated in the scattered disc. In today's sparsely populated Kuiper belt, the chance of such a collision occurring is less than 0.1 percent. The family could not have formed in the denser primordial Kuiper belt because such a close-knit group would have been disrupted by Neptune's subsequent migration into the belt, which is believed to have been the cause of its current low density. Therefore it appears likely that the dynamic scattered disc region, in which the possibility of such a collision is far higher, is the place of origin for the object which would become Haumea and its kin. [cite journal|title=On a Scattered Disc Origin for the mp|2003 EL|61 Collisional Family— an Example of the Importance of Collisions in the Dynamics of Small Sodies|author=Harold F. Levison, Alessandro Morbidelli, David Vokrouhlický and William F. Bottke|year=2008|journal= The Astronomical Journal|volume= 136|pages= 1079–1088| doi= 10.1088/0004-6256/136/3/1079|url=http://www.iop.org/EJ/abstract/1538-3881/136/3/1079|accessdate=2008-09-19]

Because it would have taken at least a billion years for the group to have diffused as far as it has, the collision which created the Haumea family is believed to have occurred very early in the Solar System's history. [cite journal|title=Candidate Members and Age Estimate of the Family of Kuiper Belt Object mp|2003 EL|61|author=D. Ragozzine; M. E. Brown|journal=The Astronomical Journal|volume=134|issue=6|pages= 2160–2167|year=2007|url=http://adsabs.harvard.edu/abs/2007arXiv0709.0328R|accessdate=2008-09-19|doi=10.1086/522334] This conflicts with the findings of Rabinowitz et. al. who found in their studies of the group that their surfaces were remarkably bright; their colour suggests that they have recently (i.e. within the last 100 million years) been resurfaced by fresh ice. Over a timescale as long as a billion years, energy from the Sun would have reddened and darkened their surfaces, and no plausible explanation has been found to account for their apparent youth. [cite journal|title=The Youthful Appearance of the 2003 EL61 Collisional Family|author=David L. Rabinowitz, Bradley E. Schaefer, Martha W. Schaefer, Suzanne W. Tourtellotte|year=2008|journal=ArXiv.org |url=http://arxiv.org/abs/0804.2864|accessdate=2008-09-20]

References