Characterisation of candidate members of (136108) Haumea's family

Context. Ragozzine & Brown presented a list of candidate members of the first collisional family to be found among the trans-Neptunian objects (TNOs), the one associated with (136108) Haumea (2003 EL61). Aims. We aim to identify which of the candidate members of the Haumea collisional family are true members, by searching for water ice on their surfaces. We also attempt to test the theory that the family members are made of almost pure water ice by using optical light-curves to constrain their densities. Methods. We use optical and near-infrared photometry to identify water ice, in particular using the (J-HS) colour as a sensitive measure of the absorption feature at 1.6μm. We use the CH4 filter of the new Hawk-I instrument at the VLT as a short H-band (HS) for this as it is more sensitive to the water ice feature than the usual H filter. Results. We report colours for 22 candidate family members, including NIR colours for 15. We confirm that 2003 SQ317 and 2005 CB79 are family members, bringing the total number of confirmed family members to 10. We reject 8 candidates as having no water ice absorption based on our Hawk-I measurements, and 5 more based on their optical colours. The combination of the large proportion of rejected candidates and time lost to weather prevent us from putting strong constraints on the density of the family members based on the light-curves obtained so far; we can still say that none of the family members (except Haumea) require a large density to explain their light-curve