“TNOs are Cool”: A survey of the trans-Neptunian region

Abstract

Context. Time series observations of the dwarf planet Haumea and the Plutinos 2003 VS 2 and 2003 AZ 84 with Herschel/PACS are presented in this work. Thermal emission of these trans-Neptunian objects (TNOs) were acquired as part of the "TNOs are Cool" Herschel Space Observatory key programme. Aims. We search for the thermal light curves at 100 and 160 µm of Haumea and 2003 AZ 84 , and at 70 and 160 µm for 2003 VS 2 by means of photometric analysis of the PACS data. The goal of this work is to use these thermal light curves to obtain physical and thermophysical properties of these icy Solar System bodies. Methods. When a thermal light curve is detected, it is possible to derive or constrain the object thermal inertia, phase integral and/or surface roughness with thermophysical modeling. Results. Haumea's thermal light curve is clearly detected at 100 and 160 µm. The effect of the reported dark spot is apparent at 100 µm. Different thermophysical models were applied to these light curves, varying the thermophysical properties of the surface within and outside the spot. Although no model gives a perfect fit to the thermal observations, results imply an extremely low thermal inertia (<0.5 J m −2 s −1/2 K −1 , hereafter MKS) and a high phase integral (>0.73) for Haumea's surface. We note that the dark spot region appears to be only weakly different from the rest of the object, with modest changes in thermal inertia and/or phase integral. The thermal light curve of 2003 VS 2 is not firmly detected at 70 µm and at 160 µm but a thermal inertia of (2 ± 0.5) MKS can be derived from these data. The thermal light curve of 2003 AZ 84 is not firmly detected at 100 µm. We apply a thermophysical model to the mean thermal fluxes and to all the Herschel/PACS and Spitzer/MIPS thermal data of 2003 AZ 84 , obtaining a close to pole-on orientation as the most likely for this TNO. Conclusions. For the three TNOs, the thermal inertias derived from light curve analyses or from the thermophysical analysis of the mean thermal fluxes confirm the generally small or very small surface thermal inertias of the TNO population, which is consistent with a statistical mean value Γ mean = 2.5 ± 0.5 MKS.

Haumea’s thermal emission revisited in the light of the occultation results

“…The lightcurves at 70 µm (MIPS) and 160 µm give a very similar picture, although the formal errors are bigger. Figure 3 explains nicely why previous studies Santos-Sanz et al 2017) resulted in much smaller thermal inertias for Haumea. In these projects, the light curve amplitudes were heavily overestimated due to the poorly characterised background.…”

“…The axis of their best-fit Jacobi triaxial ellipsoid had lengths of 1,920 × 1,540 × 990 km (2a × 2b × 2c) and a high density of about 2.6 g cm −3 . Santos-Sanz et al (2017) presented 100-and 160-µm thermal lightcurves of Haumea obtained by the Herschel-PACS instrument apparently also showing the 100 µm lightcurve asymmetry connected to the DRS. The radiometric studies implied a low thermal inertia (<0.5 J m −2 s −1/2 K −1 ) and a phase integral 3 larger than 0.73 for Haumea's surface.…”

“…Lellouch et al (2010) presented an analysis of the 2009 lightcurve data, and Fornasier et al (2013) of the 2010 data. Santos-Sanz et al (2017) focused on the two lightcurve data sets from 2009 and 2010.…”

“…The sky background in these two bands were taken from the measurements taken on the following day, when Haumea had moved about 30 arcsec. We performed aperture photometry on the background-subtracted images, and color-corrected the resulting fluxes (see also Santos-Sanz et al 2017). For the 100 and 160 µm lightcurve data we merged 3 and 6 repetitions, respectively.…”

Trans-Neptunian objects and Centaurs at thermal wavelengths

“…The next big step in thermal observations of Centaurs and TNOs came with Herschel's large Open Time Key Project on "TNOs are Cool: A survey of the trans-Neptunian region with Herschel" (Müller et al 2009) which produced more than 20 publications. The Herschel data (partly also Spitzer data) allowed to interpret the thermal emission of almost 170 TNOs and Centaurs: Müller et al 2010;Lellouch et al 2010;Lim et al 2010;Barucci et al 2012;Mommert et al 2012;Pál et al 2012;Santos-Sanz et al 2012;Vilenius et al 2012;Fornasier et al 2013;Kiss et al 2013;Lellouch et al 2013;Duffard et al 2014;Lacerda et al 2014;Vilenius et al 2014;Marton et al 2015;Pál et al 2015;Kiss et al 2016;Lellouch et al 2016;Pál et al 2016;Kovalenko et al 2017;Santos-Sanz et al 2017;Kiss et al 2018;Müller et al 2018;Vilenius et al 2018. In parallel, the WISE project performed an all-sky survey at MIR wavelengths and detected the brightest Centaurs (Bauer et al 2013).…”