Physical Characterization of Asteroid (16583) Oersted Combining Stellar Occultation and Photometric Data

TL;DR

This study addresses the challenge of resolving the physical parameters of a mid-sized main-belt asteroid by integrating multi-chord stellar occultationdata with sparse photometry and WISE thermal infrared observations. The authors apply convex lightcurve inversion to obtain an initial shape and spin state, then refine this with the ADAM non-convex reconstruction constrained by occultation chords, yielding a silhouette that reveals surface concavities and a diameter near $D \approx 21$ km. Thermophysical analysis using TPM with WISE fluxes delivers a thermal inertia of $\Gamma \approx 37$ J m$^{-2}$ s$^{-1/2}$ K$^{-1}$ and a geometric albedo of $p_V \approx 0.046$, consistent with radiometric NEOWISE estimates. The results demonstrate the power of combining occultations, photometry, and thermal data to produce robust asteroid models and emphasize the important contribution of citizen scientists to small-body characterization.

Abstract

We report a successful observation of a stellar occultation by asteroid (16583) Oersted, enabling a detailed physical characterization of its shape, spin state, and surface properties. Our goal is to determine the physical parameters of Oersted by combining multi-chord occultation timing, sparse optical photometry, and thermal infrared observations. Such asteroids (size$\sim$20 km) are rarely modeled in this detail due to observational limitations, making Oersted a valuable case study. We applied convex lightcurve inversion to sparse photometric data to derive an initial shape and spin state. This model was then refined and scaled using non-convex shape modeling with the ADAM algorithm, incorporating constraints from the occultation chord profile. Thermophysical modeling based on WISE thermal infrared fluxes was used to determine the asteroid's effective diameter, geometric albedo, and thermal inertia. The non-convex shape model reveals localized surface concavities and provides a size estimate consistent with radiometric measurements. The derived thermal inertia is typical for asteroids of comparable size. This work demonstrates the effectiveness of combining stellar occultations, photometry, and thermal infrared data for asteroid modeling and highlights the valuable contributions of citizen scientists, who played a key role in capturing the occultation and constraining the asteroid's profile.

Figures (7)

• Figure 1: Observed occultation chords (black line segments) from the March 3, 2024 event, projected onto the sky plane, along with the best-fit non-convex ADAM shape model of asteroid Oersted (grey silhouette) at the corresponding rotational phase. Each chord represents the stellar disappearance and reappearance as seen from a specific observer location listed in Table \ref{['tab:observers']}. The model silhouette is scaled and oriented to minimize residuals between the chord endpoints and the projected shape boundary. The close match demonstrates the validity of the shape and spin solution.
• Figure 2: Periodogram of asteroid Oersted based on sparse photometric data. The global minimum, corresponding to a sidereal rotation period of $P_{\mathrm{sid}} = 4.956677$ hours, is indicated by the vertical line. The horizontal solid blue line marks the adopted threshold for the $\chi^2$ values used to evaluate the uniqueness of the period solutions.
• Figure 3: Non-convex shape model of asteroid Oersted reconstructed using the ADAM algorithm. The model was scaled to match the stellar occultation chords observed on March 3, 2024. The inclusion of occultation data enables the resolution of surface concavities and the overall elongation of the body. Three viewing perspectives are shown: the first two are equator-on views separated by $90^\circ$ in longitude, and the third is a pole-on view.
• Figure 4: Comparison of thermophysical model fits using the convex shape of asteroid Oersted without (left) and with (right) color correction. The plot shows the chi-square ($\chi^2$) as a function of thermal inertia for various levels of surface roughness, characterized by the mean surface slope angle $\bar{\theta}$. Each curve corresponds to a different roughness value, as indicated in the legend. The best-fit thermal inertia is $\Gamma = 37$ J m$^{-2}$ s$^{-1/2}$ K$^{-1}$ in both cases. Color correction leads to a marginal decrease in the fit quality, increasing the minimum $\chi^2$ from 1.66 to 1.71, but does not significantly affect the derived physical parameters.
• Figure 5: Thermophysical model fit for asteroid Oersted using the non-convex ADAM shape model. The plot shows the chi-square ($\chi^2$) as a function of thermal inertia for various levels of surface roughness, characterized by the mean surface slope angle $\bar{\theta}$. Each curve corresponds to a different roughness value, as indicated in the legend. The best-fit solution corresponds to $\Gamma = 37$ J m$^{-2}$ s$^{-1/2}$ K$^{-1}$ and $\bar{\theta} = 32.8^\circ$, with a minimum reduced $\chi^2$ of 2.00.