Physical Observations of 2005 UD: A Mini-Phaethon

Abstract

Planet-crossing object 2005 UD is dynamically similar to purported Geminid meteor-stream parent 3200 Phaethon, suggesting an association between the two. We present new physical observations taken to characterize 2005 UD when at 1.6 AU from the Sun. Assuming equal albedos, 2005 UD is about 4 times smaller than 3200 Phaethon, with a diameter of $1:3 AE 0:1 km. The body shows periodic brightness variations of 0.4 mag that are compatible with an aspherical nucleus in rotation with a period near 5.2 hr. At optical wavelengths, 2005 UD is similar to or slightly bluer than the Sun. This property is relatively uncommon among near-Earth objects but is shared by Phaethon, strengthening the association between the two. No evidence for ongoing mass loss at rates as small as 0.01 kg s À1 is found in the surface brightness profile, and we conclude that the fraction of the surface that is losing mass is not greater than 10 À4. Overall, we conclude that the dynamical similarities between 2005 UD and 3200 Phaethon are matched by physical similarities between these bodies, except that the former contains only about 2% of the mass of the latter. Phaethon, 2005 UD, and the Geminid meteoroids may be fragments produced by the breakup of a precursor object.

Surface heterogeneity of 2005 UD from photometric observations

“…This lower limit implies an asteroidal picture for the bulk properties of 2005 UD, rather than the cometary picture. The amplitude of the lightcurve, axis ratio, and critical density agrees well with values reported by Jewitt & Hsieh (2006).…”

“…The periodogram exhibits the maximum at a period of 2.6155 h. Considering the lightcurve is due to the rotation of the irregularly shaped body, double the obtained periodicity 5.2310 h is the actual rotational period. Figure 2 is the phase curve assuming the periodicity of 5.2310 h. Jewitt & Hsieh (2006) utilized the phase dispersion minimization (PDM) method to obtain the rotational period of 5.2492 h, which is quite close to ours.…”

“…Very recently, Jewitt & Hsieh (2006) have reported their analysis of the photometric observation of 2005 UD. The results of their broadband multi-color photometry support the possible dynamical association between 2005 UD and (3200) Phaethon.…”

Activity in Geminid Parent (3200) Phaethon

“…With τ d τ c , we can assume that heat deposited on the surface of Phaethon has conducted to the deep interior. The core temperature calculated as in Jewitt & Hsieh (2006), T c = 300 K, is too high for water ice to survive in the core. Accordingly, we consider it unlikely that ongoing activity in Phaethon could be driven by the sublimation of deeply buried water ice; Phaethon is simply too hot, even in its core.…”

“…The ∼1.3 km diameter asteroid 2005 UD shares both a dynamical link to Phaethon, and a physical link in the sense that both it and Phaethon are optically blue. Since blue asteroids constitute only about 4% of the general asteroid population, the finding that 2005 UD and Phaethon have related orbits and are both blue is highly suggestive of a connection or common origin (Jewitt & Hsieh 2006; Kinoshita et al 2007). Asteroid 1999 YC may also be dynamically related to Phaethon but its surface appears more nearly neutral with respect to the color of the Sun (Kasuga & Jewitt 2008).…”