[Paper Review] "TNOs are Cool": A Survey of the Transneptunian Region IV. Size/albedo characterization of 15 scattered disk and detached objects observed with Herschel Space Observatory-PACS

P. Santos-Sanz, E. Lellouch+21 more|arXiv (Cornell University)|Feb 7, 2012

Astro and Planetary Science3 references59 citations

TL;DR

This study uses Herschel Space Observatory-PACS and Spitzer-MIPS infrared photometry to derive thermal models for 15 scattered disk and detached trans-Neptunian objects, determining their sizes, geometric albedos, and beaming factors. Key results include a significant correlation between albedo and diameter (brighter, larger objects are more reflective), and a positive correlation between albedo and perihelion distance, suggesting surface evolution due to sublimation or space weathering.

ABSTRACT

Physical characterization of Trans-Neptunian objects, a primitive population of the outer solar system, may provide constraints on their formation and evolution. The goal of this work is to characterize a set of 15 scattered disk (SDOs) and detached objects, in terms of their size, albedo, and thermal properties. Thermal flux measurements obtained with the Herschel-PACS instrument at 70, 100 and 160 μm, and whenever applicable, with Spitzer-MIPS at 24 and 70 μm, are modeled with radiometric techniques, in order to derive the objects' individual size, albedo and when possible beaming factor. Error bars are obtained from a Monte-Carlo approach. We look for correlations between these and other physical and orbital parameters. Diameters obtained for our sample range from 100 to 2400 km, and the geometric albedos (in V band) vary from 3.8 % to 84.5 %. The unweighted mean V geometric albedo for the whole sample is 11.2 % (excluding Eris); 6.9 % for the SDOs, and 17.0 % for the detached objects (excluding Eris). We obtain new bulk densities for three binary systems: Ceto/Phorcys, Typhon/Echidna and Eris/Dysnomia. Apart from correlations clearly due to observational bias, we find significant correlations between albedo and diameter (more reflective objects being bigger), and between albedo, diameter and perihelion distance (brighter and bigger objects having larger perihelia). We discuss possible explanations for these correlations.

Motivation & Objective

• To determine the physical properties—size, albedo, and thermal behavior—of 15 scattered disk and detached trans-Neptunian objects using thermal infrared observations.
• To assess the impact of orbital and physical parameters on surface albedo and thermal emission, particularly in relation to heliocentric distance and perihelion.
• To improve bulk density estimates for three binary TNO systems using newly derived sizes and published masses.
• To investigate correlations between albedo, diameter, and perihelion distance, testing for physical or observational biases.
• To validate and refine previous albedo and size estimates using a consistent radiometric modeling approach with error propagation via Monte Carlo simulations.

Proposed method

• Thermal flux measurements at 70, 100, and 160 μm from Herschel-PACS, supplemented by Spitzer-MIPS at 24 and 70 μm when available, were used as input data.
• Radiometric modeling was applied to fit the observed thermal emission, assuming a spherical body in thermal equilibrium with the Sun, using the standard thermal model (STM) with a beaming factor η.
• Monte Carlo simulations were used to propagate uncertainties in flux measurements and model parameters, yielding error bars on diameter, albedo, and beaming factor.
• The beaming factor η was derived from thermal fits for 7 objects, with values ranging from 1.02 to 1.48, indicating low thermal inertia and porous regoliths.
• Bulk densities were recalculated for three binary systems (Ceto/Phorcys, Typhon/Echidna, Eris/Dysnomia) using new size estimates and published mass measurements.
• Spearman rank correlation tests with error bars were used to assess statistical significance of observed correlations between albedo, diameter, and perihelion distance.

Experimental results

Research questions

• RQ1 What are the size and geometric albedo distributions of scattered disk and detached trans-Neptunian objects observed with Herschel-PACS?
• RQ2 How do the albedo and size of these objects correlate with their perihelion distance, and what physical mechanisms might explain such correlations?
• RQ3 What are the bulk densities of the Ceto/Phorcys, Typhon/Echidna, and Eris/Dysnomia binary systems based on updated size estimates?
• RQ4 How do the derived beaming factors compare with previous Spitzer results, and what do they imply about surface thermal properties?
• RQ5 To what extent are observed correlations between albedo, diameter, and perihelion distance driven by observational bias versus physical processes?

Key findings

• Diameters for the 15 objects range from 100 km to 2400 km, with a remarkably uniform distribution, though SDOs and detached objects may differ due to discovery bias.
• V-band geometric albedos range from 3.8% to 84.5%, with an unweighted mean of 11.2% for the full sample (excluding Eris), 6.9% for SDOs, and 17.0% for detached objects.
• The first size and albedo estimates for 2007 OR10 were derived: D = 1280 ± 210 km, pR = 18.5+7.6−5.2%, indicating a Quaoar-like surface composition.
• Bulk densities were re-estimated for three binaries: Ceto/Phorcys at 0.64+0.16−0.13 g cm⁻³, Typhon/Echidna at 0.36+0.08−0.07 g cm⁻³, and Eris/Dysnomia at 2.40+0.46−0.37 g cm⁻³.
• The beaming factor η had a weighted mean of 1.14 ± 0.15, consistent with low thermal inertia and porous, regolith-like surfaces, and compatible with Spitzer results.
• Significant positive correlations were found between albedo and diameter, and between albedo and perihelion distance, suggesting that larger and more distant objects retain brighter ices more effectively, possibly due to reduced sublimation or space weathering.