Mutual events of Haumea and Namaka

Haumea (formerly 2003 EL61, formerly Santa), the third brightest known Kuiper belt object, is perhaps the most unusual body in the outer solar system. Its extremely rapid rotation and subsequent elongation and its density of approximately 2.6 g cm^-3 immediately led to the hypothesis that Haumea suffered a giant impact which removed much of the water ice and left the body with the rapid spin (Rabinowitz et al. 2006). The subsequent discovery of two small satellites in orbit around the body (Brown et al. 2005, Brown et al. 2006), the confirmation that the largest satellite has the spectral signature expected from a collisional fragment (Barkume et al. 2006a), and the eventual realization that Haumea is the parent of an entire icy collisional family (Brown et al. 2007) confirms that the giant impact which gave Haumea its fast spin also shattered the icy mantle and ejected multiple fragments into orbit and beyond.

While satellite forming collisions have been modeled extensively for the earth and for Pluto, the best analogy to the Haumean collision is likely instead Mercury. Mercury suffered a mantle-shattering collision early in its history and was left with an anomalously large iron core surrounded by a thin rocky mantle, just as Haumea is now left with a large rocky core surrounded by a thin ice mantle. Little is known of the details of these mantle-shattering collisions, as none of the fragments from the Mercury event survived owing to their proximity to the sun. Haumea and its family thus provide the best opportunity known within the solar system for studying the effects and aftermath of this important process in both the inner and outer solar system.

One of the most important properties to determine is the complex shape of Haumea. Under the assumption that a body of that size should behave like an homogenous fluid, we assume that the rapid rotation of Haumea turns it into a triaxial ellipsoid with axis rations approximately 1:1.5:2. Deviations from this idealized shape will allow us to probe the interior density structure, test the assumptions of pure fluid behavior, and examine asymmetries caused by the giant impact. Haumea is, unfortunately, just below the resolution limit of HST. We thus have no way to direct examine the size and shape of this object.

Fortunately, a unique series of events occurring in the Haumean system over the next two years will allow us to probe the shape and size of Haumea and its satellites with unprecedented accuracy. After an intensive HST and Gemini LGS AO campaign last spring to determine the orbits of the interacting satellites, it has been determined that the orbit of the inner satellite is almost precisely edge-on. Like extra-solar planetary systems which have transits, edge-on satellites undergoing mutual events provide the opportunity for a host of studies that would not be present otherwise. Unlike extra-solar planets, however, the mutual event season on satellite systems is a short-lived phenomenon. On Haumea, we calculate that these events will last only a few years before the orbital motion of Haumea makes the satellite no longer appear edge-on. After these events are finished, the next events will not occur again for 140 years.

The mutual events t occurring include transits of the shadow of the satellite across the primary, transits of the satellite itself, eclipses of the satellite by the primary, and occultations of the satellite by the primary. As with extra-solar planetary systems, timing of these events places exquisite geometrical constraints on the satellite system, while measurement of the depth of these events gives unique information on sizes and albedos of the primary and the satellite.

Timing and analysis of mutual events provide the opportunity for a myriad of precise measurements of the system, including:

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  Precise size and shape. While we currently can only assume a shape for Haumea to an accuracy of perhaps 500 km, the timing of the eclipses and occultations of the satellite will provide single-chord sizes across Haumea that should be accurate to ~20 km!. Indeed, the most important observations might be the almost-grazing non-events which put extremely hard constraints on the otherwise poorly known short axis of Haumea. These observations will not only determine the true axis sizes of Haumea, but they will also be able to map small deviations from a perfect Jacobi ellipse.
  
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  Precise dynamics. Both satellite eclipses and transits will allow exceedingly precise dynamical timing. Over the next two years, the changing timing will give a much better measurement of the dynamical interaction between the two satellites and the primary than we had ever anticipated. These precise measurements should allow detection of the precessional effect caused by the rapidly spinning primary, which will, in turn, allow us to measure the non-spherical potential and thus interior structure of Haumea.
  
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  Size, albedo, and density of the satellite. A collisional fragment of the icy mantle should have a density of 1 g cm^-3 or less, almost three times lower than that of the primary. With the masses determined from the orbits, the size (relative to the primary) can be determined by precise measurement of the depth of a transit shadow, and the albedo (again relative to the primary) can be determined from the overall brightness variation as the satellite travels across the disk. Coupling these measurements with the now-precise size of the primary allows the first test of the hypothesis that the satellites and family members of Haumea are indeed low density chunks of the icy mantle.
  

Accurate measurement of these mutual events will not be trivial. Haumea has a mean V magnitude of 17.3 and an imperfectly characterized 0.25 magnitude variation over a 2 hour period. The shadow of the inner satellite will only cause a ~1.5% dip in the brightness of Haumea. The transit will cause another similar (and sometimes overlapping) dip of ~1.5%. Each of these dips lasts up to 4 hours, depending on the transit chord, but each individual ingress or egress takes a full ~15 minutes. A precise measurement of the total brightness during the same rotational phase without the transit will be required for careful comparison to ascertain when the events occurred. We obtained test observations last spring from the 48-inch Whipple telescope and found that this size telescope is barely sufficient for secure detection of the event. Precise timing or photometric information clearly requires 2-5 meter telescopes.

Analysis of these events correctly taking into account the size, shape, spin, shadowing, eclipsing, and transiting of the system will be complicated, but most of the parameters depend simply on geometry, which the transits themselves will help to constrain exceedingly well. But for any of the observations to be of use, many different chords across Haumea will be needed. Our collaboration hopes to get good measurements on all good events that will be visible in the next few years.