[Paper Review] 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

S. Fornasier, E. Lellouch+18 more|arXiv (Cornell University)|May 2, 2013

Astro and Planetary SciencePhysics and Astronomy6 references72 citations

TL;DR

This study uses combined Herschel PACS and SPIRE photometry at 70–500 μm to derive thermal properties, sizes, albedos, and bulk densities for nine bright trans-Neptunian objects and Centaurs, including dwarf planet Haumea and binary systems Quaoar/Weywot, Orcus/Vanth, and Salacia/Actea. The key result is that Quaoar’s bulk density (2.18 g/cm³) is consistent with icy bodies mixed with refractory materials, similar to Pluto and Haumea, challenging earlier high-density estimates.

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 $μ$m), and with Spitzer-MIPS at 23.7 and 71.4 $μ$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 $μ$m and especially at 500 $μ$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.

Motivation & Objective

• To determine the size, albedo, and thermal properties of the brightest trans-Neptunian objects (TNOs) and Centaurs using far-infrared data.
• To constrain the thermal inertia and surface emissivity of TNOs and Centaurs via multi-wavelength SED modeling.
• To derive bulk densities for binary systems using observed diameters and literature mass estimates.
• To investigate sub-millimeter emissivity behavior and its implications for surface composition and thermal structure.
• To assess cometary activity in Centaurs Chiron and Chariklo using Herschel and optical data.

Proposed method

• Optimized data reduction of Herschel PACS and SPIRE photometric observations at 70, 100, 160, 250, 350, and 500 μm.
• Spectral energy distribution (SED) modeling using the NEATM thermal model to derive diameter, albedo, and beaming factor.
• Application of the thermophysical model (TPM) for larger bodies (e.g., Orcus, Quaoar) to better constrain thermal inertia and surface roughness.
• Incorporation of Spitzer-MIPS (23.7, 71.4 μm) and WISE (11.6, 22.1 μm) data for improved SED fitting.
• Use of the NEATM and TPM models to derive size and albedo estimates under different assumptions about surface emissivity and thermal properties.
• Comparison of observed emissivity trends with theoretical ice absorption properties to infer surface composition and sub-surface thermal behavior.

Experimental results

Research questions

• RQ1 What are the thermal properties, including thermal inertia and surface emissivity, of the nine brightest TNOs and Centaurs observed?
• RQ2 How do the derived diameters and geometric albedos compare with previous estimates, and what do they imply for surface composition?
• RQ3 What is the bulk density of the binary systems Quaoar/Weywot, Orcus/Vanth, and Salacia/Actea, and what does it reveal about their internal composition?
• RQ4 Why is there a significant decrease in emissivity at wavelengths longer than ~250 μm, especially at 500 μm, and what does this imply about thermal emission from sub-surface layers?
• RQ5 Is there evidence of cometary activity in Centaurs Chiron and Chariklo, and what are the constraints on their dust production rates?

Key findings

• The diameter of Quaoar is 1070 ± 38 km, with its satellite Weywot at 81 ± 11 km, and a geometric albedo of 12.7 ± 1 %.
• The bulk density of the Quaoar/Weywot binary system is 2.18 g/cm³ with a 90% credible interval of 1.82–2.61 g/cm³, indicating a high content of refractory materials mixed with ices.
• The Orcus/Vanth system has a primary diameter of 917 ± 25 km, a secondary of 276 ± 17 km, and a geometric albedo of 23.1 +1.8/-1.1 %, higher than the mean for plutinos.
• The bulk density of the Orcus/Vanth system is 1.53 g/cm³ with a 90% credible interval of 1.39–1.68 g/cm³, consistent with a mixture of ice and rock.
• The Salacia/Actea binary system has a bulk density of 1.29 g/cm³ with a 90% credible interval of 1.06–1.58 g/cm³, indicating a porous or icy composition.
• The emissivity of most targets decreases significantly at wavelengths >250 μm, especially at 500 μm, indicating that SPIRE probes deeper, cooler subsurface layers, and suggesting enhanced sub-millimeter absorption due to impurities in ices.