Dimitrios Athanasopoulos

Context . The rotational properties of asteroids provide critical information about not only their internal structure, but also their collisional and thermal histories. Previous work has revealed a bimodal distribution of asteroid spin rates, dividing populations into fast and slow rotators; however, this separation remains poorly understood, for example, with regard to its dependence on composition. Aims . We investigated whether the valley that separates fast and slow rotators in rotational period–diameter space depends on asteroid composition. We approximated the composition using the asteroids’ spectral class. Methods . First, we extended the Minor Planet Physical Properties Catalogue (MP3C) to include the available spectral classes of asteroids. For each asteroid, we then selected the best diameter, rotational period, and spectral class. Building upon a semi-supervised machine-learning method, we quantified the valley between fast and slow rotators for S- and C-complex asteroids, which are linked to different types of meteorites: ordinary and carbonaceous chondrites, respectively. The method iteratively fits a linear boundary between the two populations in a rotational period–diameter space to maximise separation between them. Results . We find a clear compositional dependence of the valley: for C-complex asteroids, the transition occurs at longer periods than for S-complex, with P = 14.4 D _km ^0.739 (C-complex) and P = 11.6 D _{km 0.718} (S-complex), where the period and diameter are given in hours and kilometres, respectively. This corresponds to μQ ≃ 2 and 13 GPa, respectively, where μ is rigidity, which measures how strongly a body resists shear deformation under applied stress, and Q is the quality factor, which measures how efficiently a body dissipates mechanical energy when cyclically deformed. Conclusions . The dependence of the valley on spectral classes likely reflects compositional and structural differences: C-complex asteroids, being more porous and weaker, dissipate angular momentum more efficiently than stronger, more coherent S-complex asteroids. This represents quantitative evidence of class-dependent rotational valleys within asteroid populations.

Context. Based on the V-shape search method, two families, Athor and Zita, have been identified within the X-complex population of asteroids located in the inner main belt. The Athor family is ~3 Gyr old while the Zita family could be as old as the Solar System. Both families were found to be capable of delivering near-Earth asteroids (NEAs). Moreover, the Athor family was linked to the low-iron enstatite (EL) meteorites. Aims. The aim of our study is to characterise the spin states of the members of the Athor and Zita collisional families and test whether these members have a spin distribution consistent with a common origin from the break up of their respective family parent asteroids. Methods. To perform this test, our method is based on the well-established asteroid family evolution, which indicates that there should be a statistical predominance of retrograde-rotating asteroids on the inward side of family’s V-shape, and prograde-rotating asteroids on the outward side of family’s V-shape. To implement the method, we used photometric data from our campaign and the literature in order to reveal the spin states, and hence their rotation sense (prograde or retrograde), of the asteroids belonging to these families. We combined dense and sparse-in-time photometric data in order to construct asteroid rotational light curves; we performed the light curve inversion method to estimate the sidereal period and 3D convex shape along with the spin axis orientation in space of several family member asteroids. Results. We obtained 34 new asteroid models for Athor family members and 17 for Zita family members. Along with the literature and revised models, the Athor family contains 60% (72% considering only the family’s core) of retrograde asteroids on the inward side and, conversely, 76% (77% considering only the family’s core) of prograde asteroids on the outward side. We also found that the Zita family exhibits 80% of retrograde asteroids on the inward side. In addition, the Zita family presents an equal amount of prograde and retrograde rotators (50% each) on the outward side. However, when we applied Kernel density estimation (KDE), we also found a clear peak for prograde asteroids on the outward side, as expected from the theory. Conclusions. The spin states of these asteroids validate the existence of both families, with the Athor family exhibiting a stronger signature for the presence of retrograde-rotating and prograde-rotating asteroids on the inner and outer side of the family, respectively. Our work provides an independent confirmation and characterisation of these very old families, whose presence and characteristics offer constraints for theories and models of the Solar System’s evolution.

The aim of our study is to characterise the spin states of the members of the Athor and Zita collisional families and test whether these members have a spin distribution consistent with a common origin from the break up of their respective family parent asteroids. Our method is based on the asteroid family evolution, which indicates that there should be a statistical predominance of retrograde-rotating asteroids on the inward side of family's V-shape, and prograde-rotating asteroids on the outward side. We used photometric data from our campaign and the literature in order to reveal the spin states of the asteroids belonging to these families. We combined dense and sparse photometric data in order to construct lightcurves; we performed the lightcurve inversion method to estimate the sidereal period, spin axis and convex shape of several family members. We obtained 34 new asteroid models for Athor family members and 17 for Zita family members. Along with the literature and revised models, the Athor family contains 60% of retrograde asteroids on the inward side and, 76% of prograde asteroids on the outward side. We also found that the Zita family exhibits 80% of retrograde asteroids on the inward side and an equal amount of prograde and retrograde rotators on the outward side. However, when we applied Kernel density estimation, we also found a clear peak for prograde asteroids on the outward side, as expected from the theory. The spin states of these asteroids validate the existence of both families, with the Athor family exhibiting a stronger signature for the presence of retrograde-rotating and prograde-rotating asteroids on the inner and outer side of the family, respectively. Our work provides an independent confirmation and characterisation of these very old families, whose presence and characteristics offer constraints for theories and models of the Solar System's evolution.

Introduction:Numerous energetic collisions between asteroids in the inner Solar System have led to the creation of groups of fragments, known as asteroid collisional families. These families populate the Main Belt, each family typically originating from a single parent body that was shattered [1]. The fragments of the same collision form a characteristic V-shape in the plane defined by inverse diameter (1/D) and orbital semimajor axis (a), due to the Yarkovsky effect [2, 3]. It is a thermal force, which cause outward drift to prograde rotating asteroids and inward drift to retrograde. A method for the identification of very old and dispersed asteroid families has been developed [4,5] to search for families by their V-shapes. Such families may be invisible with the Hierarchical Clustering Method (HCM) [6, 7]. So far, six asteroid families have been discovered by using the V-shape method: the Eulalia and New Polana families [8], a low-albedo primordial family [5] with nominal age of about 4 Gyr, Athor family, ~3.0 Gyr-old, Zita family, ~4.5 Gyr-old [9] and an S-complex asteroid family, ~4.3 Gyr-old [10].Here, we study the spin states of asteroids that belong to the two X-complex asteroid families in the Inner Main Belt, Athor and Zita [9, 11]. Athor family is detectable also with HCM, conversely, the Zita family is not, as its members have dispersed in orbital elements (ghost family, according to [12]). Methodology:We used shape models that were retrieved from the literature or databases as well as multi-epoch observational datasets. Dense-in-time photometric data were retrieved from the literature and obtained from “Ancient Asteroids”1, our own observing campaign [13]. Specifically, we obtained 366 new lightcurves for 84 asteroids. Sparse-in-time photometric data were retrieved from sky surveys, namely: ASAS-SN, ATLAS, PTF, ZTF, and space missions, Gaia and TESS. Further data were collected from published asteroid models.We reduced and calibrated the photometric data following the same methodology of previous works [14, 15, 16] and using different weights for each dataset based on their accuracy [17]. Then we applied the Convex Inversion (CI) method, developed by Kaasalainen et al. [18, 19], for each asteroid from our sample. The CI reveals the spin axis orientation of asteroids, hence their sense of rotation. Results:Athor family:We obtained 31 new asteroid models for Athor family and 9 revised ones. Including the archival models, the spin state is known for 49 members.Figure 1 shows the spin states of Athor family members distributed in the family's V-shape. We found that the outward side of the family exhibits a statistical predominance of prograde asteroids of 76% and in the inward side retrograde asteroids of 60%. Considering only the family core, the outward side presents similar excess of prograde asteroids (77%) and inward side excess of retrograde asteroids (72%). The Kernel Density Estimation (KDE) visualizes qualitatively the underlying probability distribution for the retrograde and prograde rotators and presents two clear peaks: one for retrograde rotators in the inward side and another for prograde in the outward side.Zita family:Concerning the Zita family, we produced 17 new asteroid models and 7 revised. Including the archival models, the spin state is known for 32 members.Figure 2 shows the spin states of Zita family members distributed in the family's V-shape. We found that the 80% the Zita members in the inward side are retrograde rotators, while the outward side contains same number of prograde and retrograde asteroids. The KDE presents a clear peak for prograde in the outward side, while the maximum for retrograde rotators is around family’s V-shape center. Conclusions:The Athor family exhibits a statistical predominance of retrograde asteroids in the inward side and, conversely, predominance of prograde asteroids in the outward side. This result adds evidence that the members are fragments of the same parent body, an EL planetesimal, that was shattered about 3 Gy ago.The Zita family demonstrates an excess of retrograde asteroids in the inward side, but there is absence of asteroids due to orbital resonances. Conversely, the outward side does not present any predominance with an equal distribution of prograde and retrograde rotators. However, KDE exhibits a clear peak for prograde asteroids in the outward side.The spin states of these asteroids, derived from observations and modeling, validate the existence of the Athor and Zita families, with the Athor family exhibiting a stronger signature, which is expected as it is younger and more compact. This research provides independent confirmation and characterisation of these very old families, offering tight constraints for our Solar System's evolution models [10, 20]. References:[1] Zappalà et al. (1984). Icarus, 59(2), 261-285.[2] Bottke et al. (2006). Annu. Rev. Earth Planet. Sci., 34, 157-191.[3] Vokrouhlický et al. (2006). Icarus, 182(1), 118-142.[4] Bolin et al. (2017). Icarus, 282, 290-312.[5] Delbo et al. (2017). Science, 357(6355), 1026-1029.[6] Zappalà et al. (1990). Astron.J., 100, 2030-2046.[7] Zappalà et al. (1995). Icarus, 116(2), 291-314.[8] Walsh et al. (2013). Icarus, 225(1), 283-297.[9] Delbo et al. (2019). A&A, 624, A69.[10] Ferrone et al. (2023). A&A, 676, A5.[11] Avdellidou et al. (2022). A&A, 665, L9.[12] Dermott et al. (2021). MNRS, 505(2), 1917-1939.[13] Athanasopoulos et al. (2021). EPSC2021-355.[14] Hanuš et al. (2011) A&A, 530, A134.[15] Hanuš et al. (2013). A&A, 551, A67.[16] Athanasopoulos et al. (2022). A&A, 666, A116.[17] Hanuš et al. (2023). A&A, 679, A56.[18] Kaasalainen et al. (2001). Icarus, 153(1), 24-36.[19] Kaasalainen et al. (2001). Icarus, 153(1), 37-51.[20] Avdellidou et al. (2024). Science, eadg8092. Acknowledgments:D.A. acknowledges support from the Academy Complex Systems of the Universite Côte d’Azur under...

IntroductionWe invite observers to join an international observing campaign and obtain light curve of a selected number of the most ancient asteroids. Analysis of these data will be important to reconstruct the original state of the asteroid belt, which is a crucial problem of planetary science. This original state can be reconstructed through the identification of the oldest asteroid families. Traditional identification methods, like Hierarchical Clustering Methods (Zappala et al., 1990; HCM), have difficulties to recognise Gyr- and older asteroid families, whose members are very dispersed by the Yarkovsky effect. An innovative method, called V-shape search (Bolin et al., 2017), has been demonstrated effective (Deienno et al. 2021) to identify these aforementioned very old collisional families by searching for the signature of the size dependent dispersion of family members operated by the Yarkovsky effect.The method has already successfully identified two primordial families which likely formed before the giant planet orbital instability (Tsiganis et al., 2005) and could be as old as the Solar System and an ancient one that is ~3 Gyr-old (Delbo et al., 2017; 2019). There is evidence from observations and theoretical evolution models that there are more old families to be detected (Delbo et al., 2017; Dermott et al., 2018). However, the reliability of these V-shape families should be independently verified.A very important test of family membership is the anisotropic distribution of spin vectors of the asteroid family member, which is a fingerprint of the Yarkovsky effect evolution (Hanus et al. 2013). Namely, to test the working hypothesis that according to theories of asteroid orbital evolution under the Yarkovsky effect, members of the inward (outward) side of V-shape of a family have a statistical predominance of retrograde (prograde) objects (Fig. 1). This hypothesis has been tested (Hanus, et al., 2013) already for known families, and it is a reliable test for family membership. The observing campaign: Ancient AsteroidsFor this purpose, an international observing campaign called Ancient Asteroids has been put forward at different observatories worldwide: (University of Athens Observatory (UOAO), Greece, the BSA Observatory and Bigmuskie Observatory, Italy, the Observatoire de la Côte d’Azur (OCA), France, the Lowell Observatory in Arizona, United States, the Astronomical Institute of the Charles University in Prague, Czech Republic, the Aristotle University of Thessaloniki in Thessaloniki, Greece. The main goal of the campaign is to establish an international network of professional and amateur astronomers (Pro-Am collaboration), in order to perform photometric observations of a very specific sample of the most ancient asteroids. A special website (http://users.uoa.gr/~kgaze/research_asteroids_en.html) was developed for the purpose of the campaign, that includes guidelines for the participation, the observations and the image data evaluation and collection.In the frame of Ancient Asteroids campaign, a user-friendly web application was also developed for the target selection and observing plan preparation. The user can easily find the observable asteroids for the campaign based on the location, the time and the equipment limitations. The targets are displayed with a priority rate, in order to perform smart sampling. The user selects the favor target and an observing plan automatically is created. The contribution The obtained lightcurves from all the involved participants will be combined with data available in the literature, as well as with sparse data from space missions (Gaia, TESS, etc) and global sky surveys (PTF, LSST, ATLAS, etc). Thus, the spin state of the asteroids can be revealed. The results will be ingested to the Minor Planet Physical Properties Catalogue (MP3C) program (https://mp3c.oca.eu/catalogue/index.htm), the largest database of asteroid physical properties, which complies with the EU vision for open data. The observing campaign Ancient Asteroids has started to collect photometric observations with the contribution of amateur astronomers from Italy, France and Greece are submitted. The observations will reveal the spin state of the members, which are crucial for the testing the hypothesis of the family membership. This research will potentially lead to a better understanding of the first stages of the evolution of the Solar System, the mechanism at the origin of the formation of the asteroids and the planet formation processes. AcknowledgmentsThis work was also partially supported by the ANR ORIGINS (ANR-18-CE31-0014) and by the French National Program of Planetology (PNP). Here we make use of asteroid physical properties data from Minor Planet Physical Properties Catalog (https://mp3c.oca.eu/). ReferencesBolin, B. T. et al. (2017). Icarus, 282, 290-312.Deienno, R. et al. (2021). Icarus, 357, 114218.Delbo, M. et al. (2019). Astronomy & Astrophysics, 624, A69.Delbo, M. et al. (2017). Science, 357, 6355.Dermott, S. F. et al. (2018). Nature Astronomy, 2, 7.Hanus, J. et al. (2013). Astronomy & Astrophysics, 559.Hanus, J. et al. (2013). Astronomy & Astrophysics, 551.Tsiganis K. et al. (2005). Nature, 435, 7041.Zappala, V. et al. (1990). Astrophysical Journal, 100, 2030-2046.

The so-called “primordial family”, is a recently discovered collisional family [1] that could be as old as the Solar system. It contains low-albedo asteroids and is located in the inner Main Belt. This family was identified by the V-shape technique [2] and is estimated to be at least 4 Gyr old, meaning that occurred before the giant planet instability [3]. Asteroids 2768 Gorky (1972 RX3) and 9086 (1995 SA3) are members of the primordial family and have been observed in order to determine their rotational period, spin pole and shape, which will give insights about their membership and the family evolution. The photometric data are obtained mainly by the University of Athens Observatory (UOAO) [4]. Models of the asteroids are derived by the lightcurve inversion method, which is stored in the Database of Asteroid Models from Inversion Techniques (DAMIT) [5].1. IntroductionThe so-called primordial family formed by an impact event 4 Gyr ago or even earlier, i.e. before the giant planet instability [3]. The identification of the primordial family has been done by using the V-shape technique [2]. The V-shape in (α,1/D) space is occurred due to the thermal Yarkovsky effect [6, 7, 2], where prograde-rotating asteroids drifting outwards and retrograde ones inward. As only the inward side of the V-shape of the primordial family is well defined, the members should have retrograde spin. Otherwise, the membership of asteroids with prograde spin could be disputed or the contradicting spin sense could be due to YORP cycles or a collision or a close encounter with other asteroid [8, 9]. Thus, it is essential to observe thoroughly the asteroids belonging to the primordial family, in order to determine their rotational state and shape.2. ObservationsAsteroid 2768 Gorky (1972 RX3) is member of the primordial asteroid family of the inner main belt [1]. Its rotational period is 4.507 ± 0.010 h [10], while its spin pole and shape remain unknown. Observations from Sopot Astronomy Observatory (SAO) for 2 nights in September 2019, in phase angle of 32º, are available in Asteroid Lightcurve Photometry Database (ALCDEF)[11] and present a period of 4.445 ± 0.003 h [12]. On the Observatoire de Genève website of R. Behrend [13] rotational period of the asteroid was reported to be 4.5118 ± 0.0007 h. Asteroid 2768 Gorky (1972 RX3) has been observed from UOAO for 5 nights in December 2019, in phase angles between 10º to 14º.Asteroid 9086 (1995 SA3) is member of the primordial asteroid family in the inner main belt [1]. Its rotational period, spin pole and shape remain unknown. UOAO has been observed this asteroid for 22 nights from October to November 2019, in phase angle ranging between 4º to 22º.3. Methodology and Preliminary ResultsDifferential photometry of the asteroid has been obtained for each sky field. All images were dark and flat-field corrected and unguided. The measured values are in magnitudes and have been transformed properly in relative flux. The determination of the rotational period of the two asteroids and the estimation of the spin pole and shape of 2768 Gorky (1972) has been carried out through the lightcurve inversion method stored in DAMIT [5]. 3.1 Asteroid 2768 Gorky (1972 RX3) The lightcurve of asteroid 2768 Gorky (1972 RX3) present a low amplitude of ~0.1 mag (or ~0.1 intensity units), as Figure 1 shows. Generating 50 random spin poles as initial input for DAMIT software, the results have the distribution presented in Figure 2. The plot in this figure constrains the possible spin poles of the asteroid, while the χ2 value and the dark facet percentage should have the lowest value. 3.2 Asteroid 9086 (1995 SA3) The lightcurve of asteroid 9086 (1995 SA3) present also a low amplitude of ~ 0.2 mag (or ~0.2 intensity units). The low brightness of the object in combination with the low amplitude of its lightcurve render the detection of its periodicity difficult, due to the background noise. In order to address this problem, the exposures of each night were combined in packs of 3, in order to increase the S/N and make the detection of light variation more prominent. The resulted period spectrum from DAMIT is presenting in Figure 3. 4. Summary and Conclusions The rotational period of 2768 Gorky (1972 RX3) has been calculated as 4.510 ± 0.020 h, in agreement with the literature. The spin pole is not defined well enough. It is needed observations from more than one apparition for more robust results. Asteroid 9086 (1995 SA3), despite the extensive observations, does not present a clear lightcurve, as a result to have not unique solution in the period. 5. Further work The determination of asteroids’ rotational state and shape requires observations from more than one apparition. Alternatively, combing the already ground based with sparse data from sky surveys or space-borne could also drive to unique solutions. Acknowledgements We would to acknowledge Dr. Hanus J. for his valuable help and advices for the lightcurve analysis that performed. References [1] Delbo, M. et al. (2017). Science, 357, 1026-1029. [2] Bolin, B. T. et al. (2017). Icarus, 282, 290-312. [3] Tsiganis, K., et al. (2005). Nature, 435(7041), 459-461. [4] Gazeas, K. (2016). Revista Mexicana de Astronomía y Astrofísica, 48, 22-23. [5] Ďurech, J. et al. (2010). Astronomy & Astrophysics, 513, A46. (DAMIT’s website: astro.troja.mff.cuni.cz) [6] Milani, A. et al. (2014). Icarus, 239, 46-73. [7] Spoto, F. et al. (2015). Icarus, 257, 275-289. [8] Carruba, V. et al. (2013). Monthly Notices of the Royal Astronomical Society, 433(3), 2075-2096. [9] Delisle, J. B., & Laskar, J. (2012). Astronomy & Astrophysics, 540, A118. [10] Pray, D. P. et al. (2008). Minor Planet Bulletin, 35, 34-36. [11] Asteroid Lightcurve Photometry Database (ALCDEF)’s website: alcdef.org [12] Benishek, V. (2020). Minor Planet Bulletin, 47(1), 75-83. [13] Observatoire de...

Context. Families of asteroids generated by the collisional fragmentation of a common parent body have been identified using clustering methods of asteroids in their proper orbital element space. However, there is growing evidence that some of the real families are larger than the corresponding cluster of objects in orbital elements, and there are families that escaped identification by clustering methods. An alternative method has been developed in order to identify collisional families from the correlation between the asteroid fragment sizes and their proper semi-major axis distance from the family centre (V-shape). This method has been shown to be effective in the cases of the very diffuse families that formed billions of years ago. Aims. Here we use multiple techniques for observing asteroids to provide corroborating evidence that one of the groups of asteroids identified as a family from the correlation between size and proper semi-major axis of asteroids are real fragments of a common parent body, and thus form a collisional family. Methods. We obtained photometric observations of asteroids in order to construct their rotational light curves; we combine them with the literature light curves and sparse-in-time photometry; we input these data in the light curve inversion methods, which allow us to determine a convex approximation to the 3D shape of the asteroids and their orientation in space, from which we extract the latitude (or obliquity) of the spin pole in order to assess whether an object is prograde or retrograde. We included in the analysis spin pole solutions already published in the literature aiming to increase the statistical significance of our results. The ultimate goal is to assess whether we find an excess of retrograde asteroids on the inward side of the V-shape of a 4 Gyr asteroid family identified via the V-shape method. This excess of retrograde rotators is predicted by the theory of asteroid family evolution. Results. We obtained the latitude of the spin poles for 55 asteroids claimed to belong to a 4 Gyr collisional family of the inner main belt that consists of low-albedo asteroids. After re-evaluating the albedo and spectroscopic information, we found that nine of these asteroids are interlopers in the 4 Gyr family. Of the 46 remaining asteroids, 31 are found to be retrograde and 15 prograde. We also found that these retrograde rotators have a very low probability (1.29%) of being due to random sampling from an underlying uniform distribution of spin poles. Conclusions. Our results constitute corroborating evidence that the asteroids identified as members of a 4 Gyr collisional family have a common origin, thus strengthening their family membership.

Introduction Asteroid families generated by the collisional fragmentation of a common parent body have been identified using clustering methods of asteroids in their proper orbital element space (Broz & Morbidelli 2013; Tsirvoulis et al. 2018; Dermott et al. 2021). However, there is growing evidence that some of the real families are larger than the corresponding cluster of objects in orbital elements, as well as there are families that escaped identification from clustering methods (Milani et al. 2014). An alternative method has been developed by Bolin et al. (2017); Delbo et al. (2017), in order to identify collisional families from the correlation between the asteroid fragment sizes and their proper semimajor axis distance from the family center (V-shape). This method has been shown to be effective in the cases of the very diffused families that have formed Gyrs ago. Based on this method, a 4 Gyr-old (so-called primordial) collisional family of the inner main belt has been identified consisting of low-albedo asteroids (Delbo et al. 2017). The theory of asteroid family evolution predicts that there is an excess on retrograde asteroids in the inward side of the family’s V-shape. For this reason, photometric observations were performed in order to construct their rotational light curves and determine their shape and spin state.   Dataset and Observations We combined data of asteroid lightcurves that we collected from the databases, sparse photometric data obtained from different surveys and existing shape models. Aiming to enlarge our input dataset used for the shape modelling, which would potentially lead to new and improved shape solutions, we performed additional ground-based photometric observations. An international observing campaign has been initiated in the framework of our international initiative called Ancient Asteroids[1], aiming to collect dense photometric data for asteroids that belong to the oldest asteroid families (Athanasopoulos et al. 2021).   Method The photometric datasets include both dense photometric data from ground-based facilities, as well as sparse data from several sky surveys and space missions. Appropriate analysis techniques were used for each type of dataset to extract the asteroid’s rotational light curve and use the convex inversion (CI) method developed by Kaasalainen & Torppa (2021); Kaasalainen et al. (2021). So far, the CI has been used to derive asteroid models for more than 3,460 asteroids that are stored in the DAMIT database.     Results The spin state for 54 family members was determined by additionally using literature and sparse photometric data from ground and space observatories. Moreover, we measured rotation periods for 8 asteroids for the first time. Combining new and literature data, we determined shapes and spin states for 54 asteroids that belong to the nominal population of the primordial family. This corresponds to 50% of the population in the sliver between the left-wing border of the Polana and the primordial family (see Fig. 1). Specifically, we calculated 23 new complete asteroid models, 16 revised and 8 new partial models, where 32 asteroids have retrograde rotation and 22 prograde. Based on our analysis, we indicated 9 interlopers in the sample of 54 studied objects. From these 45 confirmed asteroid family members, 29 asteroids (65%) models have retrograde rotation and 16 prograde, including also the partial solutions.   Fig. 1: Panel A: The primordial family members are presented in proper semi-major axis vs. inverse diameter plane, along with the low albedo asteroids. Yellow diamond markers present members with known spin pole from the literature. Moreover, "plus", cross and square markers show the sources of dense lightcurves for these members. Panel B: The left side of the V-shape of the primordial family, where the red markers show the retrograde and blue markers the retrograde asteroids respectively.   Conclusion We carried out a campaign of photometric observations of those asteroids that have been claimed to be members of one of the oldest collisional (primordial) families in the Solar System and we extract the lightcurves, spin state and shape for 45 members. The statistical predominance of the retrograde spin poles is due to a physical process, as it was claimed by Delbo et al. 2017, namely formation as collisional fragments of a common parent body, a subsequent dynamical evolution driven by the Yarkovsky effect. The results of this research constitute corroborating evidence that the asteroids as members of a 4 Gry-old collisional family have a common origin, thus strengthening their family membership.   Acknowledgments MD and CA acknowledge support from ANR “ORIGINS” (ANR-18-CE31-0014). This work is based on data provided by the Minor Planet Physical Properties Catalogue (MP3C) of the Observatoire de la Côte d’Azur. The research of JH has been supported by the Czech Science Foundation through grant 20-08218S. The work of OP has been supported by INTER-EXCELLENCE grant LTAUSA18093 from the Ministry of Education, Youth, and Sports. Support for T.W.-S.H. was provided by NASA through the NASA Hubble Fellowship grant HST-HF2-51458.001-A awarded by the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. We thank the Las Cumbres Observatory and their staff for its continuing support of the ASAS-SN project, supported by the Gordon and Betty Moore Foundation through grant GBMF5490 to the Ohio State University and funded in part by the Alfred P. Sloan Foundation grant G-2021-14192 and NSF grant AST-1908570. Development of ASAS-SN has been supported by NSF grant AST-0908816, the Mt. Cuba Astronomical Foundation, the Center for Cosmology and AstroParticle Physics at the Ohio State University, the Chinese Academy of Sciences South America...

Ultra-short orbital period contact binaries (P_orb < 0.26 d) host some of the smallest and least massive stars. These systems are faint and rare, and it is believed that they have reached a contact configuration after several Gyrs of evolution via angular momentum loss, mass transfer, and mass loss through stellar wind processes. This study is conducted in the frame of the Contact Binaries Towards Merging (CoBiToM) Project and presents the results from light curve and orbital analysis of 30 ultra-short orbital period contact binaries, with the aim to investigate the possibility of them being red nova progenitors, eventually producing merger events. Approximately half of the systems exhibit orbital period modulations, as a result of mass transfer or mass loss processes. Although they are in contact, their fill-out factor is low (less than 30 per cent), while their mass ratio is larger than the one in longer period contact binaries. This study investigates the orbital stability of these systems and examines their physical and orbital parameters in comparison to those of the entire sample of known and well-studied contact binaries, based on combined spectroscopic and photometric analysis. It is found that ultra-short orbital period contact binaries have very stable orbits, while very often additional components are gravitationally bound in wide orbits around the central binary system. We confirmed that the evolution of such systems is very slow, which explains why the components of ultra-short orbital period systems are still Main Sequence stars after several Gyrs of evolution.