According to Pravec et al. [1], 15% of the population of the main belt asteroids are binary asteroids. These systems are essential to assess our knowledge about the origin and dynamical evolution of our solar system [2]. However, due to our current observational techniques, we estimate that the actual known samples identify only 2% of this population. Moreover, traditional observational techniques like direct imaging (revealing large object with small satellite) or optical and radar photometry (revealing close and icy objects) limit the variety of the current available sample, leading to a biased database. To enhance the current sample and fill this gap, the GAIAMOONS project has been launched in 2023. Its scope is to identify binary candidates using Gaia astrometric data and validating asteroid companions through stellar occultation observations. Gaia’s astrometric measurements are in fact that of the photocenter moving around the system’s center of mass [3].  Through data analysis, a list of 358 binary candidates has been established [4,5] and observational campaigns are underway to confirm the binary nature of these systems using the stellar occultation observation method [5].

Stellar occultation is a ground-based method that led to very exciting results. Indeed, this method allows to have access to its physical parameter such as its volume with a kilometric precision [6] as well as topographic details [7]. In fact, thanks to dynamic information about the asteroid pair, provided by the occultation observations, it is possible to trace the mass of the primary precisely and thus derive its density [8]. This way, coupled with infrared and spectrometric data, physical and dynamical conditions of the pair can be precisely determined.

To date, we predicted and observed 51 different stellar occultation events within the framework of the GAIAMOONS project. Each event, even with a single positive station, is of particular importance because it enables us to track the position of the occulting asteroid continuously and accurately refine its orbit. So, with continuous tracking, it is possible to get indirect clues about the existence of a companion with the deviation from the prediction. This way, some objects have already been followed through several new stellar occultation events and publicly available data from previous occultation observations.

A successful observation involving 30 stations—both amateur and professional astronomers—in Portugal, Spain, France, the Netherlands, Belgium, and Germany on October 23, 2024, revealed a detailed shape of (5044) Shestaka (Figure 1), showing the asteroid is shifted by 5.3 ± 0.047 km from its predicted ephemeris. Predictions indicated the satellite would lie outside the central zone, making double drops unlikely. Due to observation conditions, a large area wasn't covered, and no satellite was detected. This constrains its environment and eliminates probable satellite positions. Some topographic features on the primary were also revealed. The next occultation window is October 28, 2025, in southern Europe.

Figure 1: Post-occultation map showing the location of each station that participated in the
October 2024 occultation campaign by (5044) Shestaka

Figure 2: Green lines show observations from different stations. Solid parts indicate when the star was visible; hollow parts mark the occultation. Red segments represent timing uncertainties for immersion and emersion.

Distinct results were obtained for objects of varying sizes. In particular, asteroid (35420) 1998 AG6 exhibited signatures suggestive of the potential presence of a satellite or an unusual shape. Additionally, (1127) Mimi, which was the subject of four separate observations, showed a clear deviation, pointing to a possible interaction with its surrounding environment. Similarly, to name the most recurrent ones, (550) Senta, (247) Eukrate, and (1237) Genevieve underwent comparable observational approach, enabling improved constraints on their shape and positional parameters for subsequent studies. Overall, this presentation will highlight the main results obtained to date from past campaigns, including refined astrometric measurements, derived shape models and surface topographies, the identification of potential satellite candidates, and a critical evaluation of existing 3D models for selected objects.

Acknowledgments
The organisation of the observation campaigns was supported by Occultation Portal - Kilic et al. (2022). Occultation Portal: A web-based platform for data collection and analysis of stellar occultations. MNRAS, Volume 515, Issue 1, pp. 1346-1357. This work made use of the SORA package - Gomes-Júnior et al. (2022). SORA: Stellar occultation reduction and analysis. MNRAS, Volume 511, Issue 1, March 2022, Pages 1167–1181. This work made use of the PRAIA package - Assafin M., 2023a, Differential aperture photometry with PRAIA, Planetary and Space Science Planetary and Space Science, Volume 239, article id. 105816. This work made use of the Pymovie package - Anderson, B. (2019) PyMovie – A Stellar-Occultation Aperture-Photometry Program, Journal for Occultation Astronomy (ISSN 0737-6766), Vol. 9, No. 4, p. 9-13. This work is funded by the French national agency Agence National de la Recherche (ANR-22-CE49-0002). This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement.

References
[1] Pravec et al., 2007 Icarus, Binary asteroid population. 1. Angular momentum content
[2] Nesvorny et al., 2021 The Planetary Science Journal, Binary Planetesimal Formation from Gravitationally Collapsing Pebble Clouds
[3] Braga-Ribas et al. 2013, The Astrophysical Journal, THE SIZE, SHAPE, ALBEDO, DENSITY, AND ATMOSPHERIC LIMIT OF TRANSNEPTUNIAN OBJECT (50000) QUAOAR FROM MULTI-CHORD STELLAR OCCULTATIONS
[4] Tanga et al., 2023 Astronomy and Astrophysics, Gaia Data Release 3 The Solar System survey
[5] Liberato et al., 2024 Astronomy and Astrophysics, Binary asteroid candidates in Gaia DR3 astrometry
[6] Lallemand et al. 2024, SF2A Proceedings 2024, GAIAMOONS: Study of binary asteroids with stellar occultation and GAIA astrometry
[7] Rommel et al. 2023, Astronomy and Astrophysics, A large topographic feature on the surface of the trans-Neptunian object (307261) 2002 MS4 measured from stellar occultations
[8] Liu et al. 2024, Astronomy and Astrophysics, Asteroid (4437) Arecibo: Two ice-rich bodies forming a binary-Based on Gaia astrometric data

How to cite: Lallemand, R., Desmars, J., Sicardy, B., Tanga, P., Liu, Z., Liberato, L., Mary, D., Hestroffer, D., Langin, G., Dauvergne, J.-L., Saillenfest, M., Stachowicz, A., Leroy, A., and Kiliç, Y. and the co-authors: Stellar Occultations as a Tool to Detect and Characterize Binary Asteroids , EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-340, https://doi.org/10.5194/epsc-dps2025-340, 2025.

Gaia provides an exceptional opportunity to explore high-precision astrometric data for a large number of solar system objects. Thanks to its unprecedented precision, Gaia data can reveal the astrometric signatures of binary asteroids. This is the case of the binary system (4337) Arecibo, discovered by occultation. Using Gaia DR3 data [1], an astrometric wobble was clearly detected over several transits over a two days window.

In this study, we investigated the Arecibo system, combining all Gaia FPR observations. We first fit the heliocentric orbit, where the residuals contain the binary signal. This signal is proportional to the relative orbit, and a scaling factor related to the flux ratio and mass ratio of the components (see [2], [3] for the analytical formula). We then adjust the relative orbit to derive the key parameters, including full orbital parameters, flux and mass ratio. Based on estimated volumes, we obtain overall densities of around ρ₁ ≈ 1.2 and ρ₂ ≈ 1.6 for the primary and secondary, respectively. These results indicate that this system is composed of ice-rich bodies in the outer main belt [4].

Moreover, we are applying this method to other asteroid systems whose orbits are poorly known. Using Gaia observations combined with ground-based data, we have been able to refine their orbits. Furthermore, for trans-Neptunian objects, where the separation between the components is large enough for only the primary body to be observed by Gaia, the astrometric wobble can indicate the mass ratio. In such cases, Gaia data can be crucial in determining individual masses and, hence, density.

For binary candidates identified in previous studies [5], it is possible to solely use Gaia data or adding it with primary-only occultation data to perform mutual orbit determination. This can help estimate the position of a potential satellite, guiding future occultation observations for detection and confirmation.

Gaia's high-precision astrometric measurements enable us to estimate the individual masses and densities of solar system bodies. This provides valuable insights into the formation and evolution of the solar system. Additionally, by looking at orbital parameters, we can predict future mutual events and stellar occultations, which will provide further constraints on density. We aim to present preliminary results on main belt asteroid systems, TNBs and a few potential binaries, demonstrating how Gaia data contribute to our understanding of these objects and to future observation.

[1] Tanga, P., Pauwels, T., Mignard, F., et al. 2023, A&A, 674, A12

[2] Pravec, P. & Scheirich, P. 2012, Planet. Space Sci., 73, 56

[3] Lindegren, L. 2022

[4] Liu, Z., Hestroffer, D., Desmars, J., and David, P. 2024, A&A, 688, L23. 

[5] Liberato, L., Tanga, P., Mary, D., et al. 2024, A&A, 688, A50

How to cite: Liu, Z., Hestroffer, D., Desmars, J., Lallemand, R., David, P., and Liberato, L.: Binary system dynamics and physical property analysis using Gaia astrometry, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1274, https://doi.org/10.5194/epsc-dps2025-1274, 2025.

The spin and shape properties of asteroids carry valuable information about their collisional and dynamical history. In this study, we present an extensive new dataset of asteroid spin states and convex shape models derived from all available sparse photometric data, including Gaia DR3, ATLAS, ZTF, and other major optical surveys. This represents the most complete set of asteroid physical properties derived through lightcurve inversion to date, with significantly improved coverage and statistical reliability compared to previous works.

Combining Gaia DR3 with ground-based surveys not only increases the number of reliably modeled asteroids, but also improves the robustness of individual solutions thanks to better sampling and geometry coverage. This synergy highlights the importance of integrating space- and ground-based photometry, and paves the way for even larger datasets following future data releases—most notably Gaia DR4. The same methodology applied here can be directly extended to Gaia DR4, where longer time baselines and additional measurements will further enhance model quality and enable detailed studies of smaller or more slowly rotating bodies.

Using a unified inversion pipeline, we obtained unique spin and shape solutions for more than 25,000 asteroids. The model reliability benefits from cross-validation of photometry from multiple independent sources and optimized inversion techniques tuned for sparse time sampling. Our enhanced dataset enables us to analyze correlations between rotation period, size, elongation, and other physical parameters with unprecedented statistical power.

We focus particularly on asteroid families, using dynamical membership data to examine the evolution of spin properties within collisional groups. Young families, such as Veritas, show spin states that appear largely unaltered by dynamical or thermal processes. Their members exhibit a narrow distribution of rotation periods and broad distribution of elongations consistent with outcomes of catastrophic disruption and reaccumulation, reflecting their relatively pristine post-formation state.

In contrast, older families display a more evolved distribution of spin periods, with a noticeable excess of both fast and slow rotators. These features are consistent with long-term evolution driven by the YORP effect and collisional damping. The differences between young and old families reinforce the role of thermal torques in shaping asteroid spin distributions over Gyr timescales.

A central result of our study is the period–size diagram, which reveals a clear bimodality in the spin rate distribution: one group of fast rotators and another of slow rotators. This separation, statistically robust in our expanded sample, aligns well with the scenario proposed by Zhou et al. (2025), who interpret it as the combined outcome of YORP evolution and a population of tumbling or non-principal axis rotators. Our dataset confirms that this bimodality is not an artifact of selection effects or incomplete modeling, but a genuine structural feature of the asteroid spin distribution.

This work provides new constraints on asteroid rotational evolution, the age-dependent effects of YORP, and the initial conditions following family-forming collisions. Our results are essential for future efforts in thermophysical modeling, dynamical simulations of asteroid families, and constraining the long-term evolution of small bodies in the Solar System.

Figure 1: Distribution of spin axis directions (pole latitudes and longitudes) for asteroid members of the Veritas family. Each point represents one object with a derived shape and spin-state solution from combined Gaia DR3 and ground-based photometry. The color scale indicates the rotation period in hours. The nearly isotropic distribution of pole orientations suggests that the Veritas family is dynamically young and has not undergone significant YORP-induced spin alignment or collisional evolution.

How to cite: Hanuš, J. and Ďurech, J.: Spin and Shape Properties in Young and Evolved Asteroid Families: Insights from All-Sky Sparse Photometry, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-581, https://doi.org/10.5194/epsc-dps2025-581, 2025.

• Excess of slow rotators – A significant overabundance of asteroids with extremely slow rotation periods, first discovered in the 1980s (e.g. Pravec & Harris 2000 and references therein).
• Distribution of tumbling asteroids – A subset of asteroids that do not rotate about their principal axis (Harris 1994), yet their size-dependent distribution in the slow rotation regime follows a power law.

How to cite: Zhou, W.-H., Michel, P., Delbo, M., Wang, W., Wang, B. Y., Durech, J., and Hanuš, J.: Understanding the Long-term Rotational Evolution of Asteroids with Gaia, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-893, https://doi.org/10.5194/epsc-dps2025-893, 2025.

The first clustering analysis of asteroids following the shape of their reflectance photo-spectra was carried out by Chapman et al. (1975) using 24 narrow filters between 0.3 to 1 µm over 110 asteroids. They obtained two groups, the C- and S-types, as they were “similar to Carbonaceous and Stony-metallic meteorites”, respectively. Ten years later, David Tholen, a PhD student at the University of Arizona (USA), increased the sample up to 600 asteroids using albedo and 8 narrow filters, ranging from 0.3 to 1.1 µm, later called the Eight Color Asteroid Survey (ECAS, Zellner et al., 1985). In Tholen’s taxonomy, Chapman’s S-group was further subdivided into five classes: S-, Q-, A-, R-, and V-types. The C-group was divided into nine classes, all having low albedo (< 0.15) visible spectra ranging from blue, neutral, to flat-red: B, C, F, G, D, T, and P classes. The NUV (below 0.45 µm) became critical to distinguish between some of those taxonomies, such as B and F types. In the last decades of the 20th century, the Charge Coupled Devices (CCDs) appeared in planetary sciences, with their higher sensitivity in visible and near infrared wavelengths, but lower sensitivity in the blue region. Bus & Binzel (2002b, a) doubled Tholen’s sample with their second phase of the Small Main-Belt Asteroid Spectroscopic Survey (SMASSII), a set of low-resolution spectral observations of 1,341 asteroids in wavelengths between 0.43 to 0.92 µm. Although they increased the sample of asteroids, they lost albedo, critical to disentangle classes such as E-, M-, P-type, as the information below 0.43 µm, needed to identify Tholen’s classes among the C-complex. More recently, machine learning and probabilistic methods have been dealing with classifying small bodies based on their reflectance spectra: Mahlke et al. (2022) developed a classification method that allows to use partial spectra over visible and near-infrared (NIR) and obtain the different probabilities of a given spectrum to belong to a certain taxonomy. Interestingly, they incorporate the albedo as Tholen did in the 80s. 

We used the reflectance spectra of asteroids provided by Gaia DR3 (Gaia Collaboration 2022). The reflectance spectra were obtained by dividing each epoch spectrum by the mean of the solar analog stars selected and then averaging over the set of epochs. The resulting spectrum consists of 16 reflectance values covering a wavelength range from 0.374 to 1.034 µm, with a resolution of 0.044 µm, and normalized to unity at 0.550 ± 0.025 µm. However, since its publication in June 2022, no taxonomy has been published yet. In this abstract, we present a summary of our recent effort to classify a large number (>10,000) of asteroids in the Gaia DR3.

The first step to obtain this classification was to study, understand, try to correct, and/or clean the effects of some artifacts present in the spectra. Summarizing, Tinaut-Ruano et al (2023) found and corrected an artificial reddening on NUV wavelengths due to the use of Solar Analogs (SAs) that were valid in the visible but not in the NUV. Gallinier et al (2023) found a systematic reddening in wavelengths between 0.7 to 0.9 µm. Tinaut-Ruano et al (2024) have found a correlation between the NUV slope with the signal-to-noise ratio (SNR), and in this work, we found the same effect in the NIR. We have corrected the effect of the SA and cleaned the sample using thresholds in the SNR, limiting the sample with complete spectra. We use albedo to combine both Tholen (the last one with NUV information) and Mahlke (the one with the larger number of objects) taxonomies. After the cleaning and crossmatch with albedo databases, we ended with a sample larger than 15,000 asteroids. 

Following an iterative clustering process, we were able to classify more than 11,000 asteroids in 13 different taxons: S, V, A, K, E, M, P, D, C, G (Ch), B, F, and L-types. This is three times larger than any previous spectral taxonomic classification. We will show in the presentation how those taxons are distributed among different families and dynamic populations, as well as the typical features and physical properties of each taxon. Our taxonomic classification has the following advantages: i) the use of the NUV and albedo; ii) a large number of objects classified systematically in different dynamical populations and families; iii) some taxonomies with a limited number of known objects have increased their members substantially. However, this taxonomy can not be used to compare spectra from different sources, as some of the artifacts remain in the Gaia spectra. See the consistent reddening in the median Gaia spectra between 0.7 and 0.9 µm in the following figure. There, we compare the median spectra of the objects classified from Gaia (blue lines) and the references from Mahlke 2022 (red dotted line). In the title of each subplot is the number of spectra classified for each taxonomy.

Some examples of achievements of this taxonomy are: i) we found differences in the NUV between F-types and B-types as expected. Their classification is supported by their presence in different families, F are located mainly in Nysa-Polana, B in Themis, Iduna and Pallas; ii) we have classified almost 700 objects from Eos family as K type with a high confidence, this is 70 times more than were previously classified from spectra; iii) We have classified almost 200 objects as L types increasing the previously classified spectra by a factor 10 of this taxon. Furthermore, we will show some preliminary results derived from this taxonomic classification, such as the composition of the families and the link to the parent bodies.

All in all, this classification provides a unique opportunity for the community to explore the composition of different families, dynamic populations, or, in the other way around, to explore how different taxons distribute in the solar system.

How to cite: Tinaut-Ruano, F., Carry, B., and Sergeyev, A.: First taxonomic classification of Gaia DR3 asteroid reflectance spectra, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-965, https://doi.org/10.5194/epsc-dps2025-965, 2025.

Introduction :

Spectrally red and featureless asteroids, taxonomically classified as D-, P-, T-, and Z-type in the Bus-DeMeo and Mahlke’s taxonomies [1, 2], have traditionally been found in Jupiter’s Trojans and are considered to have an organic-rich surface. They are thought to have formed in the outer Solar System [3]. Very few have been so far discovered in the main belt, including the inner part [4, 5, 6]. Using Gaia’s spectral catalog, we have found and classified, both using Bus-DeMeo and Mahlke’s taxonomies, more than 900 primordial asteroids and conducted a statistical study on them.

Methods :

We have used the DR3 spectral catalog of solar system objects of the Gaia mission, referencing 60,518 spectra of asteroids in the visible [7]. Each spectrum is the mean of several observations at different epochs of a given asteroid, and it consists of 16 spectrophotometric points covering the 0.37-1.03 micron wavelength range.

First, we computed the signal-to-noise ratio (SNR) for all the asteroids of the DR3 catalog, we selected the main belt asteroids and discarded all the objects for which the SNR was < 30. We applied an automatic taxonomy classification tool between each asteroid and the mean spectra of the two taxonomies, spotting the two « best fits » for the classes of interest (in Bus-DeMeo taxonomy D and T-types and in Mahlke P, D, and Z). Then, to better constrain the taxonomic classification and to solve ambiguities with X-complex asteroids, we applied selection criteria based on the albedo and the slope.

We considered only low albedo asteroids (geometric albedo < 12% ). Secondly, we looked for objects having a spectral slope (in the 0.550-0.814 micron) in the 2-7 %/1000 Å range for the P-types, and higher than 7 %/1000 Å for the D and Z-types.

After the preliminary classification and selection criteria, we visually inspected each potential primordial asteroid in order to remove potential known problematic points [7] and do a final classification in both Bus-DeMeo and Malhke’s taxonomies.

We added to the final dataset 13 asteroids previously classified as D-types in the literature [6].

Results :

Fig1: Position of the dataset in the main belt. Blue dots represent the D and Z types, the red triangle the P-types, and the green stars the PD-types. The three vertical lines delimit the inner, middle, and outer main belt.

Using the Malhke taxonomy, we have found 446 D-types, 270 P-types, 193 Z-types, and nine PD-types (T-types for Bus-DeMeo taxonomy). This last class represents extremely red asteroids. D and Z-types represent 1,8% of Gaia spectral catalog considering asteroids with SNR > 30 as similarly done in this analysis, P-types represent 0,7%, and PD-types 0,02%. If we only consider Z-types, they represent 0,54% of the dataset, which is less than what was previously estimated: 1.1% [2].

Primordial asteroids are observed mainly in the outer belt (207 D-types, 91 Z-types, and 146 T-types), as expected (Fig. 1), but they are also present in the inner belt, even if in lower amounts (65 D-types, 14 Z-types, and 32 T-types). This indicates important migration processes in the Solar System with implantation of primordial asteroids from the outer solar system up to the inner main belt.

We computed the correlations between the spectral slope, diameter, albedo and orbital elements for D/ Z and P-types. For the D/Z types, we found a weak anti-correlation between the albedo and the semi-major axis, the farther away from the Sun they are, the darker they are this could attributed to space weathering effects [8]. Furthermore, there are more smaller P, D and Z-types in the inner main belt. They could have been driven to their current position by the Yarkovski force as proposed by [4].

We also compared the slope distribution of D-types in the main belt with different dynamical classes of TNOs and comets and found similarities with the less red population of centaurs, SDO, and detached objects. Yet, if those D-types were in fact implanted from the TNOs population, it does not explain why we don’t find redder objects in the D/Z-types asteroids.

Acknowledgement : This work has received support from France 2030 through the project named Académie Spatiale d'Île-de-France (https://academiespatiale.fr/) managed by the National Research Agency under bearing the reference ANR-23-CMAS-0041, as well as the Centre National d’Etude Spatial (CNES).

References :

[1] DeMeo et al. (2009), Icarus, 202, 160-180

[2] Mahlke et al. (2022) , A&A, 665, A26

[3] Levison et al. (2009), Nature, 460, 7253, 364-366.

[4] DeMeo et al. (2014), Icarus, 229, 392-399.

[5] Gartrelle et al. (2021), Icarus, 363.

[6] Humes et al. (2024), PSJ, 5,3, 80.

[7] Gaia Collaboration (2023), A&A, 674, A35.

[8] Lantz et al. (2017), Icarus, 285, 43-57.

How to cite: El-Bez-Sébastien, N. and Fornasier, S.: Research of primordial asteroids in the main belt using the Gaia spectral catalog, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-582, https://doi.org/10.5194/epsc-dps2025-582, 2025.

Asteroid families are groups of asteroids sharing similar orbits and surface properties resulting from the disruption of a parent body. Asteroid families are usually identified using hierarchical clustering algorithms (HCM) in the proper element phase space. The HCM has several limitations, such as being unable to separate overlapping families, identify interlopers, or detect old families. Spectroscopic data can overcome these issues, although they are less abundant than proper element measurements. The Gaia Data Release 3 (DR3) significantly improved spectroscopic analysis by providing over 60,000 asteroid reflectance spectra.
We have already proven that asteroid families can be identified using Gaia DR3 spectra alone by detecting the Tirela/Klumpkea and Watsonia families [1], two L-type families known for their connection to Barbarian asteroids. Barbarians, characterized by unique polarimetric properties [2], might represent an old population of asteroids preserving some of the properties of the early protoplanetary disk.

In this study, we focus on a low-inclination family in the middle belt, which, due to its large spread, challenges pure HCM-based identification. This region is also known to host Barbarians. By classifying Gaia spectra with the color taxonomy from [1], we identified a new L-type family with (460) Scania as its largest member. The distribution of the family in the (a, 1/D) plane, the so-called V-shape, is reported in Figure 1. This distribution was analyzed with the V-shape detection method [3], which confirmed that the observed structure is not a statistical artifact but a real family. Its age, estimated with the V-shape fitting method [4], is around 1.0 Gyr. The second-largest remnant, (1007) Pawlowia, is unusual because its size is comparable to the largest remnant. Further observations are needed to clarify its nature.

The V-shape was numerically reproduced by simulating a fragmentation event using the N-body integrator REBOUND [5]. The distribution of the synthetic fragments closely matches the observed one, as reported in Figure 2.

Barbarian asteroids exhibit negative polarization at large phase angles, unlike normal asteroids, which show a positive transition at the same phase angles. Among the family members, polarimetric data are available only for (460) Scania, (2085) Henan, and (2354) Lavrov. The latter has limited data, while the first two are likely Barbarians.
The V-shape reflects the Yarkovsky-driven drift in semi-major axis, which depends on spin orientation: prograde rotators drift outward, retrograde inward. A strong correlation was observed between spin obliquity and position within the family’s V-shape.

To verify that the L-type family was not a result of a misclassification of Gaia spectra, we compared it with existing taxonomic data. The memberships are in good agreement for large asteroids, but discrepancies emerged at smaller sizes due to missing or ambiguous classifications. Figure 3 compares the size distributions from Gaia, literature, and their combination, along with the geometric model [6], whose slope agrees with the observed distributions, but does not resolve the issue of the two similarly sized largest remnants.

In conclusion, our analysis likely identifies an L-type family in the middle Main Belt, potentially linked to Barbarian asteroids. We stress here that although our spectroscopy method addresses some HCM limitations, it still presents some biases, as it assumes compositional homogeneity within the family, which is the commonly accepted paradigm. In addition, the Gaia color taxonomy has some limitations, since it tends to overclassify objects into the C and S classes and may misclassify faint objects with low SNR spectra. Future Gaia releases, combined with data from other surveys, will offer a larger and improved spectroscopic sample, helping to refine our understanding of this family and the whole asteroid population.

Figure 1: Figure 1: The family members in the (a, 1/D) plane fitted by V-shapes corresponding to different ages.

Figure 2: Comparison between the proper elements of the family members observed by Gaia (grey circles) and the proper elements of the synthetic family members integrated in REBOUND (blue stars).

Figure 3: Size distributions corresponding to family memberships and the geometric model.

References

[1] Balossi, R., Tanga, P., Sergeyev, A., Cellino, A., Spoto, F. (2024), Gaia DR3 asteroid reflectance spectra: L-type families, memberships, and ages, A&A, 688, A221.
[2] Cellino, A., Belskaya, I. N., Bendjoya, Ph., et al. (2006), The strange polarimetric behavior of Asteroid (234) Barbara, Icarus, 180, 565–567.
[3] Bolin, B. T., Delbo, M., Morbidelli, A., Walsh, K. J. (2017), Yarkovsky V-shape identification of asteroid families, Icarus, 282, 290–312.
[4] Spoto, F., Milani, A., Knežević, Z. (2015), Asteroid family ages, Icarus, 257, 275–289.
[5] Rein, H., Liu, S.-F. (2012), REBOUND: an open-source multi-purpose N-body code for collisional dynamics, A&A, 537, A128.
[6] Tanga, P., Cellino, A., Michel, P., et al. (1999), On the size distribution of asteroid families: The role of geometry, Icarus, 141, 65–78.

How to cite: Balossi, R., Tanga, P., Delbo, M., and Cellino, A.: An ancient L- type family associated to (460) Scania in the Middle Main Belt as revealed by Gaia DR3 spectra, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-315, https://doi.org/10.5194/epsc-dps2025-315, 2025.

The most massive asteroids in the main belt perturb the trajectories of planets and other asteroids. High-precision astrometric measurements of the positions of the perturbed asteroids can show this deviation, especially with long observation arcs before and after the encounters. The large, perturbing asteroid masses can be included in the orbital fitting process to obtain new asteroid mass estimates [1]. We use all available astrometric observations of perturbed asteroids (referred as test asteroids) to estimate the mass of as many large asteroids (referred as perturbers) as possible. In particular, the Gaia Focused Product Release (FPR) [2] of high-precision astrometry allows more precise estimates of asteroid masses [3], which we attempt in general for large asteroids in the main belt.

Close Approach Search: We select an initial perturber list of N_L=1783 main belt, trojan, centaur and TNO asteroids with an inferred GM > 0.003 km³/s², based on a size estimate from the absolute magnitude and density from taxonomic type [4]. All asteroids in JPL’s Small-Body Database^[1] with well-determined orbits are considered as candidate test asteroids (1.07 million asteroids with MPC condition code = 0 out of the 1.4 million discovered asteroids). Then, we integrate the test asteroid orbits within their data arcs considering the perturbations of all planets (ephemeris model DE441 [5]) and perturber asteroids. We search for encounters with any asteroid in the perturber list. We find that 0.97 million objects have at least one encounter closer than 0.05 au with any of the objects in the perturber list within their respective data arcs.

Mass Estimation Methodology:

Test asteroid orbit determination: We correct astrometry for star catalog biases [6] and set station specific data weights [7], with specific treatment for Gaia FPR astrometry [8]. We fit the orbits of all test asteroids while estimating N_p perturber masses that the test asteroid encountered within its data arc, a subset of N_L. We set an apriori GM uncertainty to a large value, which prevents orbit determination divergence. We identify cases when the post-fit mass uncertainty is significantly smaller, which indicates there is real signal of the mass in the astrometry. We iterate the fitting process from updating the masses and perturber trajectories to the test asteroid orbit determination. After the orbital fit, we have the full (6+ N_p)x(6+ N_p) orbit covariance for each test asteroid, from which we sub-select the (N_pxN_p) perturber mass covariance.

Perturber masses: We compute the final mass estimates as a N-dimensional weighted mean of the individual estimates alongside the corresponding (N_LxN_L) covariance. The weighted mean only includes the test asteroids that had at least 1 perturber estimate with reduced uncertainty, extracted as the square root of the diagonal terms of the covariance. This full-size covariance allows us to identify cases with coupled mass estimates [1], typically due to encounters between those perturbers or encounters occurring shortly after one another.

Results: Out of all the test asteroids, we find ~10% of them are considered for the final perturber weighted mass estimates. The combination yields a total number of 77 mass estimates with SNR>10 (error <10%) and 231 estimates with SNR>3, which represent a significant addition to previous estimates from spacecraft position measurements [9] or other estimates in the literature [10]. This improvement is mostly enabled by the precision of Gaia astrometry [2,3], which limits the final uncertainties typically to be > 0.01 km³/s². However, on rare occasions very close encounters can be very well observed and lead to mass estimates with tiny uncertainties such as 0.039 0.001 km³/s² (445 Edna) or the smallest we found, 0.0032 0.0006 km³/s² (1952 Hesburgh). We did not find signal of mass estimates of Trojans, Centaurs or Trans-Neptunian Objects.

Main Belt Total Mass: The total mass of the main belt (sum of all a<4.6 au) is estimated to be 12.90  10⁻¹⁰ M_⊙, which is similar to previous estimates in the literature and consistent with the estimated errors.

Asteroid Densities: From the estimated masses, we derive updated density estimates for asteroids with well determined diameters and find consistency with their taxonomic types.

Conclusions: The latest Gaia data release allows us to estimate the masses of large main belt asteroids with higher precision and for more objects than previously attempted. We describe the details of the methodology and results in a journal publication. These mass estimates will improve the dynamical models of motion in the Solar System as well as our understanding of the densities of asteroids of given taxonomic classes.

Acknowledgments: This research was supported by an appointment to the NASA Postdoctoral Program at the Jet Propulsion Laboratory, administered by Oak Ridge Associated Universities under contract with NASA. D.F, J.G. and R.P. conducted this research at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA (80NM0018D0004).

References:

[1] Baer, J. and Chesley, S. R., (2017) AJ, 154:76,
[2] Gaia Collaboration et al, (2023) A&A, 680, A37
[3] Farnocchia, D., et al (2024) AJ, 168:21
[4] Carry, B., (2012) PSS, 73(1), 98-118.
[5] Park, R.S., et al (2021) AJ, 161:105
[6] Eggl, S., et al (2020) Icarus, 339, 113596
[7] Vereš, P., et al (2017) Icarus, 139-149
[8] Fuentes-Muñoz, O., et al (2024) AJ, 167:290.
[9] Farnocchia, D., et al (2021) Icarus, 369, 114594
[10] Fienga, A., et al (2020) MNRAS, 492, 589-602

How to cite: Fuentes-Munoz, O., Farnocchia, D., Giorgini, J. D., and Park, R. S.: Main Belt Asteroid Mass Estimation from High-Precision Astrometry, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1278, https://doi.org/10.5194/epsc-dps2025-1278, 2025.

ESA’s Gaia mission provides astrometry of asteroids at an unprecedented accuracy measured, at best, in milli- or even microarcseconds. However, the accuracy decreases for larger objects due to shape and size effects, such as the offset of the apparent photocenter and the true barycenter of the object. The effect has been studied with a highly accurate shape model for one target, (21) Lutetia [1], and in a statistical sense for the over 156,000 asteroids in Gaia Focused Product Release (FPR) [2] using a spherical shape approximation [3]. Both studies show that the residuals in the along-scan direction improve when the photocenter-barycenter offset is accounted for in the orbital fitting.

To take a step further, we first derive photocenter-barycenter offsets for 35 large main-belt asteroids using shape models reconstructed with the ADAM method from VLT/SPHERE/ZIMPOL images and available lightcurve data [4]. In addition, we apply a correction for the offset to Gaia Data Release 3 (DR3) astrometry and inspect the effect on orbit computation for asteroids (21) Lutetia and (511) Davida by fitting their orbits in a least-squares sense using the open-source orbit-computation software OpenOrb [5]. The offsets are computed following e.g., [6] for each transit epoch of a given target asteroid in Gaia DR3. The correction is performed by subtracting the offsets from the corresponding observations: each transit contains up to 9 observations, and it is assumed that the offset remains constant throughout a transit.

We find that the photocenter-barycenter offset is on average 4–7% of the diameter of the asteroid, which in many cases translates to an angular offset of some milliarcseconds. Considering Gaia’s astrometric precision, such an offset is significant. We also find a nearly linear dependence between the average magnitude of the offset and the size of the asteroid. In Fig. 1 the magnitude of the offset is computed as an average over all the DR3 transits of a given target. As expected, the magnitude of the offset increases as the diameter of the asteroid increases. The linear nature of the dependence suggests that there is a possibility to derive a useful empirical model for the offset based on its size. However, it is likely that such a model would have to take into account the shape of an asteroid in addition to its size as well as possible albedo variations. Additionally, we show that the orbital fits for (21) Lutetia and (511) Davida improve in terms of the number of rejected (outlier) observations and along-scan residuals.

Figure 1: Magnitude of the photocenter-barycenter offset averaged over DR3 transits as a function of the diameter of the asteroid for 35 large main-belt asteroids.

Finally, we will report on asteroid mass estimation based on asteroid-asteroid close encounters. Our goal of achieving mass estimates with improved and realistic uncertainties should be possible given the highly accurate Gaia astrometry and the addition of a photocenter-barycenter correction, which is relevant for the target asteroids for mass estimation, as they are often relatively large. The new mass estimates and the accurate shape models, that are used for the photocenter-barycenter offset correction, allow us to derive more realistic bulk density estimates.

References:

How to cite: Tuominen, E., Granvik, M., Muinonen, K., Tanga, P., and Carry, B.: Photocenter-barycenter offsets for Gaia observations with high-precision asteroid shape models, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1351, https://doi.org/10.5194/epsc-dps2025-1351, 2025.