TNOs are Cool: A survey of the trans-Neptunian region. VIII. Combined Herschel PACS and SPIRE observations of 9 bright targets at 70--500 micron (1305.0449v2)

Abstract: Transneptunian objects (TNOs) are bodies populating the Kuiper Belt and they are believed to retain the most pristine and least altered material of the solar system. The Herschel Open Time Key Program entitled "TNOs are Cool: A survey of the trans-Neptunian region" has been awarded 373 h to investigate the albedo, size distribution and thermal properties of TNOs and Centaurs. Here we focus on the brightest targets observed by both the PACS and SPIRE multiband photometers: the dwarf planet Haumea, six TNOs (Huya, Orcus, Quaoar, Salacia, 2002 UX25, and 2002 TC302), and two Centaurs (Chiron and Chariklo). Flux densities are derived from PACS and SPIRE instruments using optimised data reduction methods. The spectral energy distribution obtained with the Herschel PACS and SPIRE instruments over 6 bands (centred at 70, 100, 160, 250, 350, and 500 $\mu$m), and with Spitzer-MIPS at 23.7 and 71.4 $\mu$m has been modelled with the NEATM thermal model in order to derive the albedo, diameter, and beaming factor. For the Centaurs Chiron and Chariklo and for the 1000 km sized Orcus and Quaoar, a thermophysical model was also run to better constrain their thermal properties. We derive the size, albedo, and thermal properties, including thermal inertia and surface emissivity, for the 9 TNOs and Centaurs. Several targets show a significant decrease in their spectral emissivity longwards of $\sim$300 $\mu$m and especially at 500 $\mu$m. Using our size estimations and the mass values available in the literature, we also derive the bulk densities for the binaries Quaoar/Weywot (2.18$^{+0.43}_{-0.36}$ g/cm$^3$), Orcus/Vanth (1.53$^{+0.15}_{-0.13}$ g/cm$^3$), and Salacia/Actea (1.29$^{+0.29}_{-0.23}$ g/cm$^3$). Quaoar's density is similar to that of the other dwarf planets Pluto and Haumea, and its value implies high contents of refractory materials mixed with ices.

A Comprehensive Overview of the TNOs are Cool: Combined Herschel Observations of 9 Bright Targets

The paper presents a detailed paper of nine bright trans-Neptunian objects (TNOs) and Centaurs using observations from both the Herschel PACS and SPIRE at wavelengths of 70-500 $\mu$m. This paper is part of the "TNOs are Cool: A survey of the trans-Neptunian region" project, aimed at enhancing our understanding of TNOs by determining their sizes, albedos, and thermal properties using radiometric methods.

Observational Techniques and Data Analysis

The analysis involves high-sensitivity measurements from the Herschel Space Observatory, combined with Spitzer-MIPS and WISE data. The employed Herschel data reduction techniques and models, in particular the NEATM thermal model and thermophysical modeling, enable precise determination of the physical properties of these distant celestial bodies. A key aspect of this paper is its utilization of the comprehensive spectral energy distribution obtained across multiple bands to derived parameters like albedo, diameter, and thermal inertia.

Derived Physical and Thermal Properties

The major results from the NEATM and TPM analysis include:

• Size and Albedo: Sizes range from 218 km (Chiron) to 1239.5 km (Haumea), with albedo values spanning 3.7% to 80.4%. The derived albedos, especially the low values for large objects like Chariklo, suggest varied and complex surface compositions across these bodies.
• Thermal Properties: The paper finds beaming factors mostly around ~1, except for Quaoar. Thermal inertia differences, derived from TPM analysis, indicate diverse thermal histories and surface characteristics.
• Surface Emissivity: Emission at long wavelengths show a trend of decreasing emissivity, likely indicating cold and less-reflective subsurface layers or particular grain sizes present in the surface regolith. Chiron displays the most pronounced emissivity drop.

Discussion on Density and Composition

The paper also estimates bulk densities using available mass information and derived sizes for binary systems, including Quaoar/Weywot (2.18 g/cm³), Orcus/Vanth (1.53 g/cm³), and Salacia/Actea (1.29 g/cm³). These densities align with some dwarf planets, suggesting internal composition mixing ices with rocky materials.

Implications and Concluding Remarks

The paper offers significant enhancements to our understanding of TNO and Centaur physical characteristics, promoting better models of solar system formation and evolution. One implication is the need for theories accommodating the observed diversity in thermal properties and surface emissivity. The findings also add to a growing body of evidence that these bodies possess complex histories, possibly including resurfacing events and varying thermal regimes.

In summary, this comprehensive observation-based characterization provides a rich dataset for further modeling and comparative studies, especially in understanding the mechanisms driving surface modification in these remote solar system regions. Future studies could focus on more resolving power on binary system dynamics, ongoing cycles of cometary activity, or novel spectral features indicating surface chemical diversity.