Asteroids with Unusually High Albedo in Primitive Asteroid Families: Joint Analysis of TESS and Sparse ZTF Photometry

Primitive asteroids are of particular interest for studies of the origin and early evolution of the Solar System, since their composition is believed to preserve, to a large extent, the primordial material of the protoplanetary disk that has undergone only limited thermal and chemical processing. Such objects are usually assigned to carbonaceous taxonomic classes C, B, G, F and related subclasses, which are characterized by low geometric albedo (p_V < 0.1) and relatively neutral or slightly red-sloped reflectance spectra in the visible range [1].

Interest in primitive taxonomic asteroid types increased significantly following the successful sample-return missions from primitive near-Earth asteroids. The Hayabusa2 mission to the C-type asteroid Ryugu and the OSIRIS-REx mission to the B-type asteroid Bennu provided direct evidence of the mineralogical and chemical composition of primitive bodies and revealed complex processes of aqueous alteration and space weathering on their surfaces. These missions demonstrated that primitive asteroids preserve material from the early Solar System and may contain organic compounds and hydrated minerals [2-4]. The returned samples also improved the interpretation of remote photometric and spectroscopic observations of low-albedo asteroids.

According to the analysis of physical properties of asteroid families by Masiero et al. [5], combined with the dynamical family catalog compiled by Nesvorný et al. [6], the main asteroid belt contains 122 dynamically identified asteroid families. Approximately 50 out of these 122 families are dominated by low-albedo (p_V < 0.1) asteroids. Among the largest and most scientifically important low-albedo asteroid families are Themis, Hygiea and Pallas. These families are located primarily in the outer and middle main asteroid belt.

Data from infrared satellite surveys (IRAS, AKARI, WISE) and subsequent spectroscopic studies have shown that several primitive dynamical families contain asteroids with geometric albedos noticeably higher than the typical value (p_V ≈ 0.1) [7-12].

Photometric observations of asteroids at different phase angles allow the investigation of their phase-brightness relations, which provide an important tool for determining absolute magnitudes and refining photometric estimates of geometric albedo [13-15].

Our study aims to improve the phase curves of such asteroids with unusually high albedo in the largest primitive families by integrating high-cadence TESS photometry with sparse photometry from ZTF survey. Failure to properly account for the rotational modulation of brightness increases the dispersion of the phase-curve data points and introduces systematic biases in the derived phase-function parameters. Neglecting this correction can significantly affect the resulting values of the absolute magnitude H, the HG/HG₁₂ parameters, the linear phase coefficient β, and the amplitude of the opposition effect.

Systematic bias in the absolute magnitude H values propagates directly into errors in the derived visual geometric albedo [16].

To address this issue, we introduce a method that constructs rotational lightcurve using data from the Transiting Exoplanet Survey Satellite (TESS). We validate the rotation period by eliminating alias solutions, confirming the expected double-peaked light-curve morphology, and performing an independent robustness assessment using Phase Dispersion Minimization (PDM), alongside model selection based on the Bayesian Information Criterion (BIC). A strict geometric requirement of this approach is the alignment of the TESS-derived template with the precise opposition corresponding to the ZTF. We therefore partition the multi-filter survey data into distinct oppositional epochs. In addition, we require not only a broad temporal overlap between TESS and the ground-based surveys, but also that ZTF measurements occur strictly within the active TESS sector observing window. Ultimately, only single-opposition data are used.
Once a temporally consistent “TESS template - ZTF opposition” pair is established, we apply the rotational correction to the ZTF survey data. For all filters within a given opposition, we fit a universal phase shift tied to the object’s rotational epoch, whereas the amplitude scaling of the correction is determined separately for each specific photometric band. Finally, after outlier rejection, the data are binned with respect to α to produce robust, representative phase-curve points across the targeted phase-angle intervals.

To independently validate this framework, we acquired dedicated ground-based photometry. Spectrophotometric observations for the Themis family members (2560) Siegma and (7024) Impey were carried out with the 0.5-m D500WS telescope (Newtonian optical system). Images were obtained using a Moravian C4-16000 CCD camera in the SDSS photometric system with broadband filters g, r, i, z.

By combining the newly determined absolute magnitudes with diameters from WISE, we re-evaluated their geometric albedos. We also conducted a secondary consistency check utilizing the empirical correlation between the phase slope β and p_V [13,15]. The updated p_V estimates for (2560) Siegma settled into the 0.05-0.09 interval, which is completely standard for dark, primitive bodies. Similarly, the β-derived and photometric assessments for (7024) Impey point to a low albedo in the 0.04-0.06 range.

We used TESS light curves together with sparse ZTF photometry to study 78 members of the Themis, Hygiea, and Pallas families with anomalously high geometric albedos. Rotationally corrected phase curves and revised albedo estimates were successfully obtained for 22 targets.

The obtained results will be presented and discussed in detail at the conference.


References 

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