ROASTM: A Hybrid Thermal Model for Infrared Asteroid Lightcurve Observations

The volume of thermal infrared observations of asteroids has significantly risen in the past decade with the WISE/NEOWISE survey [1]. This trend will continue when the planned NEOSurveyor [2] and NEOMIR [3] surveys begin observing thousands of near-Earth and main-belt asteroids. Simple thermal models that can be implemented on large datasets with low computational effort are an essential part of the analysis. The Near-Earth Thermal Model (NEATM; 4) has been the preferred choice because of its applicability and relatively high accuracy in diameter determinations [5].

With the WISE/NEOWISE survey, observations of asteroid thermal lightcurves are becoming more frequent, and the application of the NEATM model is restricted to spherical shapes. Moreover, the upcoming surveys mentioned herein will observe asteroids at high phase angles where a greater fraction of thermal emission originated from the nighttime hemisphere. Models like the fast-rotating model (FRM, a.k.a. iso-latitudinal model; 6) and the Nighttime Emission Simple Thermal Model (NESTM; 7) account for non-zero thermal emission from the unilluminated hemisphere yet still have only been applied to spherical shapes.

To overcome these limitations, I describe the Rotating Asteroid Simple Thermal Model (ROASTM). The model is essentially a hybrid blend of the NEATM and FRM models that represent endmember surface temperature distributions. In the case of NEATM, the surface temperatures mimic thermal equilibrium on the dayside hemisphere of an object where the hottest location is the sub-solar point. On the other hand, the FRM represents a case of infinite heat conduction and isothermal temperature at each latitude. ROASTM uses a weighted combination of both models in order to closely match a realistic temperature distribution (Figure 1) that is a result of finite heat conduction [8]. In this scheme two parameters are used: f_iso, which is the weighting factor of the isolatitudinal temperatures, and eta (i.e. the beaming parameter) which can only increase the daytime temperature.

Figure 1. Example ROASTM temperature distribution.

The ROASTM temperature distribution can be applied to any general shape to generate a thermal lightcurve for fitting to multiple observations. I present a python package that can be used to fit thermal infrared observations with these simple thermal models. A shape class is used to generate a triangular mesh for different simple shapes, including simple spheres, triaxial ellipsoids [9], and so-called Cellinoid ellipsoids [10] (Figure 2). I have also implemented the option for specifying the spin axis obliquity. In such cases, the sub-solar and sub-observer coordinates can be calculated. Subsequently, ROASTM can be used in a pole scanning procedure to provide initial constraints on the spin axis and thermal inertia for later refinement using a more sophisticated thermophysical model [11].

Figure 2. Cellinoid shape implementation.

 

References:

[1] Mainzer et al. (2014) The Astrophysical Journal 792, 1:30, 14 pp. [2] Mainzer et al. (2024) The Planetary Science Journal, 4, 12:224, 19 pp. [3] Conversi et al. (2024) Proceedings of the SPIE, 13092, 130922H 8 pp. [4] Harris (1998) Icarus 131, 291–301. [5] Mommert et al. (2018) The Astronomical Journal 155, 2:74, 10 pp. [6] Lebofsky & Spencer (1989) In Asteroids II, University of Arizona Press. [7] Wolters & Green (2009) MNRAS 400, 204–218. [8] Spencer et al. (1989) Icarus, 78, 337–354. [9] MacLennan & Emery (2019) The Astronomical Journal, 157, 1:2, 17 pp. [10] A. Cellino, et al. (1989) Icarus 78, 2, 298–310.