Trans-Neptunian objects and Centaurs at thermal wavelengths

Abstract

The thermal emission of transneptunian objects (TNO) and Centaurs has been observed at mid-and far-infrared wavelengths -with the biggest contributions coming from the Spitzer and Herschel space observatories-, and the brightest ones also at sub-millimeter and millimeter wavelengths. These measurements allowed to determine the sizes and albedos for almost 180 objects, and densities for about 25 multiple systems. The derived very low thermal inertias show evidence for a decrease at large heliocentric distances and for high-albedo objects, which indicates porous and low-conductivity surfaces. The radio emissivity was found to be low ( r =0.70±0.13) with possible spectral variations in a few cases. The general increase of density with object size points to different formation locations or times. The mean albedos increase from about 5-6% (Centaurs, Scattered-Disk Objects) to 15% for the Detached objects, with distinct cumulative albedo distributions for hot and cold classicals. The color-albedo separation in our sample is evidence for a compositional discontinuity in the young Solar System. The median albedo of the sample (excluding dwarf planets and the Haumea family) is 0.08, the albedo of Haumea family members is close to 0.5, best explained by the presence of water ice. The existing thermal measurements remain a treasure trove at times where the far-infrared regime is observationally not accessible. Radiometric techniques 3.1 Models to interpret thermal measurementsRadiometry is a powerful technique to derive size, albedo, and thermal properties from thermal infrared measurements. The technique consists in the exploitation and interpretation of thermal data (see Section 2) in combination with data in the visible. The visible data are mainly obtained from the ground and include the object's absolute magnitude and estimates for the phase integral q. The most accurate radiometric properties are determined when objects are observed close to their thermal emission peaks, preferentially shortwards and longwards of the peak. For Centaurs and TNOs these peaks are located in the FIR regime between ≈50 and 100 µm (accessible from space only).There are different thermal models to obtain size and albedo of atmosphereless small bodies: simple models like the Standard Thermal Model (STM), the Fast Rotating Thermal Model (FRM), or the near-Earth asteroid thermal model (NEATM) or more complex thermophysical models (TPM) are used (see Delbo' et al. 2015 for a detailed discussion). For Centaurs and TNOs, mostly the NEATM are applied. In cases where the object is lacking shape or spin properties, the NEATM is often used. The beaming factor η is then either fitted to multi-band thermal measurements or a default value is applied. The STM/NEATM concepts can also handle elongated shapes (Brown 1985;Lellouch et al. 2017;Vilenius et al. 2018), but have severe limitations when thermal measurements cover different aspect angles 14 .In cases where the rotation period is known, the TPM concepts have the advantage that in addition to the stan...

The Moon at thermal infrared wavelengths: a benchmark for asteroid thermal models

“…Thermophysical modeling techniques are widely used to derive radiometric properties of asteroids (e.g., Delbo et al 2015) and more distant bodies (e.g., Müller et al 2020). Most of the radiometric studies are based on infrared (IR) measurements taken close to the objects' thermal emission peak: IRAS (Tedesco et al 2002a) at 12, 25, 60, and 100 µm, MSX (Tedesco et al 2002b) at 4.29, 4.35, 8.28, 12.13, 14.65, and 21.3 µm, AKARI-IRC (Usui et al 2011(Usui et al , 2013 at 9 and 18 µm, or WISE/NEOWISE (Mainzer et al 2011(Mainzer et al , 2016 at 11 and 22 µm.…”

“…Most of the radiometric studies are based on infrared (IR) measurements taken close to the objects' thermal emission peak: IRAS (Tedesco et al 2002a) at 12, 25, 60, and 100 µm, MSX (Tedesco et al 2002b) at 4.29, 4.35, 8.28, 12.13, 14.65, and 21.3 µm, AKARI-IRC (Usui et al 2011(Usui et al , 2013 at 9 and 18 µm, or WISE/NEOWISE (Mainzer et al 2011(Mainzer et al , 2016 at 11 and 22 µm. For more distant, colder Centaurs or trans-Neptunian objects, key studies used data from Spitzer-MIPS at 24 and 70 µm (Stansberry et al 2008), or from Herschel-PACS at 70, 100, and 160 µm (Müller et al 2020).…”

Small Bodies: Near and Far Database for thermal infrared observations of small bodies in the Solar System

“…Beyond Jupiter and in the transneptunian region, the typical surface temperatures drop from ∼100 K to 30-50 K and the corresponding emission with a peak in the far-infrared (50-100 µm) is almost exclusively observed from space. Centaurs and TNOs were suitably observable with the far-infrared detectors of the Spitzer Space Telescope (mostly at 70 µm, Stansberry et al 2008) and Herschel Space Observatory (70, 100 and 160 µm, Müller et al 2009Müller et al , 2019. Recent reviews on the thermal emission of asteroids and more distant objects by space instruments can be found in Müller et al (2019), Mainzer et al (2015), and references therein.…”

A multi-chord stellar occultation by the large trans-Neptunian object (174567) Varda

“…The geometric albedo we derive for Varda (p = 0.099 ± 0.002, see Table 5) can be compared to those of other TNOs. The dynamically hot population has a median geometric albedo of 0.085 +0.084 −0.045 (Müller et al 2020), excluding the Haumea family 10 and dwarf, planets noting that the median geometric albedo for cold classicals is 0.14 +0.09 −0.07 (Ibid.). More recently, Müller et al (2020) provided an overview of TNO physical properties at thermal wavelengths, in particular, for a sample of 26 hot classical TNOs (excluding Makemake, Haumea, and its family), with geometric albedo values ranging between 0.032 and 0.310, a median at 0.084, and a mean value at 0.102.…”

“…The Haumea family is characterised by high geometric albedo values, with a median 0.48 +0.28 −0.18(Müller et al 2020). 11 This is calibrated with a nominal value of 60% as this gives an object with f m = 70% and a density of ≈750 kg m −3 .…”