Thermal diameters versus occultation diameters of TNOs: a new tool to search for satellites?

Abstract

Here we report the possibility to identify the existence of satellites by finding anomalies when we compare thermal effective diameters with occultation-derived effective diameters. The comparison has to be done very carefully, taking into account many different aspects often not considered. Our sample of objects includes only those that have multichord stellar occultations with more than 3 chords. We find that the effective diameters from thermal models are, overall, in good agreement with the occultation-derived effective diameters, although they slightly underestimate the real sizes. In the case of 2002 TC₃₀₂ we found a clear excess in the thermal data, indicating that a satellite might be responsible for it, and this scenario seems compatible with time series photometry and astrometry of 2002 TC₃₀₂ taken along several years.

 

Introduction

Satellites around TNOs have mostly been discovered through high-spatial-resolution imaging, mainly using the HST. Unfortunately, this causes a bias against finding satellites close to their primaries. To avoid this bias one can try to look for satellites from periodic residuals in rotational lightcurves [1], and one can also try to identify satellites by the astrometric wobble caused in the system [2]. Also, contact binaries can be inferred through high-amplitude rotational lightcurves [3].

Binary TNOs and TNOs with satellites carry valuable information on the processes and the physics governing the formation of the TNOs from the initial protoplanetary disc. Therefore, efforts to search for satellites in close orbits are important. A potential new tool to address the topic is the comparison of occultation-derived effective diameters with thermal diameters. For non-spherical bodies (like most of TNOs except those that rotate slowly), the concept of diameter makes no sense. For non-spherical bodies we can use the concept “effective diameter” to mean the diameter of a spherical object with the same volume as the body. Then we talk about effective diameters “in equivalent volume sense”. We can use the term effective diameter to mean the diameter of a sphere with the same projected area as that of the TNO, but if the TNO is a triaxial body and is rotating, we have to specify also the rotation phase or correct to the mean projected area.  

There are many tricky aspects and details that must be addressed before comparing the results from thermal observations with occultation observations. First of all, we have to compare the effective diameter in the projected area sense, not in volume. Also, one has to account for the presence of known satellites, which in some of the Herschel "`TNOs are cool"' papers was unknown or not shown in the tables, so the reported diameters are often for the combination of the primary plus the satellite, whereas the occultations only recorded the primaries. Also, the rotational phase at the occultation time has to be considered in the case of triaxial bodies. In addition, the different aspect angle at the time of the Herschel-Spitzer observations compared to the occultation has to be considered (this is often very small, only a few degrees, but should be computed if possible). On the other hand, we have to compare occultation observations with thermal observations analyzed the same way with the same kind of thermal models (not with thermophysical models). So, careful analysis has to be done.

Observations from the literature

From a sample of ~10 TNOs for which we have accurate occultation diameters and were observed within the “TNOs are cool” Herschel Key project, we have taken into account all the aspects mentioned in the introduction and have generated a table with all the relevant parameters.

Results

We found that thermal models tend to slightly underestimate the true diameters. This is because all the analyzed TNOs are elongated, and as expected from the theoretical work by [4], highly elongated asteroids get their diameters underestimated when using thermal models, which are built for spherical bodies. We had already noticed this in the case of the occultation by Haumea [5], but we can now confirm the conclusion for other TNOs. Nevertheless, the general difference of the thermal diameters in comparison with the occultation ones is small, not as large as in the Haumea case. One notable exception to this is 2002 TC₃₀₂ whose thermal diameter is considerably larger than that from the occultation. We have reassessed the thermal fluxes to double check that this is the case. Our preliminary results indicate that the Herschel PACS fluxes used in [6] came from the combination of observations at two epochs, but a close look at the Herschel PACS images of the second epoch revealed contamination from a bright source. If only the uncontaminated epoch is used to derive the fluxes, the effective diameter is even larger than that reported in [6] and the fit improves considerably. On the other hand, time series photometry and astrometry of this body tend to support the existence of a large satellite orbiting at a distance of the order for 2000 km [7]. The approximate diameter of the satellite would be around 300 km.

References

[1] Fernández-Valenzuela et al. (2019) Astrophys. J. Lett. 883 L21 7 pp. [2] Ortiz et al. (2011) Astron. Astrophys 525, id.A31, 12 pp. [3] Thirouin et al. 2018 Astron. J. 155, id. 248, 16 pp. [4] Brown, R. H. 1985, Icarus, 64, 53 [5] Ortiz et al. 2017, Nature, 550, 219-222, [6] Fornasier, et al. (2013), Astron Astrophys, 555, A15 [7] Ortiz et al. (2020) Astron. Astrophys. In Press. arXiv e-prints,arXiv:2005.08881