TNOs are Cool: Analysis of Classical Kuiper Belt Objects from Herschel Space Observatory Data

We present results from the sample analysis of classical Kuiper belt objects (CKBO) observed within the open time key program "TNOs are Cool" [Muller et al. 2009, Earth, Moon, Planets 105] of Herschel Space Observatory. About half of this sample was published in Vilenius et al. (2012, A&A 541) and together with earlier Spitzer Space Telecope results [Stansberry et al. 2008, in "Solar System Beyound Neptune" and Brucker et al. 2009, Icarus 201] the number of CKBOs with radiometric diameter/albedo solutions is increased to almost 40. About one quarter of these targets are dynamically cold. For the binaries (∼10) the effective radiometric size of the system is acquired to infer bulk densities. Our 3-band (70, 100, 160 µm) photometric observations with PACS cover the wavelength range where thermal emission from trans-Neptunian objects has its maximum. Each target was observed twice in order to characterize and remove the sky background, which is required in doing photometry of these faint targets. In order to derive simultaneously diameters and albedos from thermal-IR observations a thermal model is used in combination with absolute visual magnitudes and an assumed form of the phase integral. We use complementary data from Spitzer when available. The thermal model requires assumptions on the temperature distribution on the surface. We use the Near-Earth Asteroid Thermal Model [Harris 1998, Icarus 131] and fit the beaming parameter whenever data quality permits. We analyse correlations between the measured diameters and geometric albedos as well as between physical properties and colors or orbital elements for both the dynamically cold and hot CKBOs. We also analyse separately the correlations of large objects (D > ∼500 km). The anti-correlation found between diameter and geometric albedo of dynamically hot CKBOs [Vilenius et al., 2012] will be re-investigated. We acknowledge support from German DLR, project 50 OR 1108.