Focus on slow rotators - first results from stellar occultations campaign on long-period asteroids

<p><strong>Most asteroids are slow rotators  </strong>                                                                                                                                                              <br />Results from space missions like Kepler and TESS (Molnar et al. 2018; Pal et al. 2020), but also ground-based surveys e.g. ATLAS and ZTF (Erasmus et al. 2021) reveal unexpectedly large numbers of slowly rotating asteroids in the main belt, with periods reaching up to a few thousand hours. Actually majority of asteroids rotate slowly, with periods impossible to be fully covered during one night. As a consequence, their spin and shape models are mostly missing, skewing the big picture of asteroids spins, shapes, sizes and other properties.    <br />                                                                                                                                                                                        <br />Our long-term project (see e.g. Marciniak et al. 2015; and 2021) targets a few tens of slow rotators with periods up to 60 hours, to obtain their full lightcurves in an efficient world-wide observing scheme. Dense lightcurves then serve for spin and shape reconstructions via lightcurve inversion. Recently we launched a special campaign among stellar occultation observers<sup>1,2</sup>, to put these models into scale, and also to verify the shape solutions, often allowing to break the mirror-pole ambiguity.              </p> <p>                                    <br /><strong>Sample result</strong>                                                                       <br />Our lightcurve survey resulted e.g. in a rich enough lightcurve dataset to create unique spin and shape model of asteroid (439) Ohio, a challenging slow rotator displaying 37-hours period. At the same time, two successful, multi-chord stellar occultations by this asteroid have been registered within the "slow rotators" campaign in March 2022. Together with one archival occultation from PDS database (Herald et al., 2019) they allowed us to precisely scale our shape model of (439) Ohio using the method by Durech et al. (2011). This has independently confirmed the size and location of shape features down to single kilometers on this 74-km body (see the figure below). </p> <p><img src="" alt="" width="1026" height="259" /></p> <p><strong>Fig. 1.</strong> Shape model of asteroid (439) Ohio (blue contours) fitted to chords from three stellar occultations (black lines). Grey segments denote occultation timing uncertainties. North is up and west is right. Volume equivalent diameter: 74 +/- 2 km.</p> <p><strong>Conclusions and future prospects </strong>                  <br />Presented scheme resulted recently in obtaining quality shape models for 15 slow rotators, most of them for the first time. Sizes determined from fitting these models to stellar occultations agree with sizes from infrared space missions WISE (Wright et al. 2010), IRAS (Neugebauer et al. 1984), and AKARI (Usui et al. 2011), yet providing better accuracy, or resolving previous inconsistencies in size determinations in some cases. That stems from creating robust spin and shape models instead of spherical shape approximation with unknown spin used in Simple Thermal Model (Harris & Lagerros 2002). For roughly half of the targets this fitting allowed to reject one of two mirror pole solutions, clearly pointing to the preferred one, thus removing the ambiguity inherent to lightcurve inversion. Such well determined and scaled asteroid shapes will e.g. constitute a solid basis for precise density determinations, once their masses are available (for some of our targets expected already from Gaia mission). Spin and shape models in general keep filling the gap caused by various biases. </p> <p><em>Acknowledgements:</em> This work was supported by the National Science Centre, Poland, through grant no. 2020/39/O/ST9/00713.</p> <p>                                                                                                   <br /><strong>References: </strong>                                                                                               <br />Durech, J., Kaasalainen, M., Herald, D. et al. 2011, Icarus 214, 652 <br />Erasmus, N., Kramer, D., McNeill, A. et al. 2021, MNRAS 505, 3872 <br />Harris, A. W., Lagerros, J. S. V. 2002, in Asteroids III, 205 <br />Herald, D., Frappa, E., Gault, D., et al. 2019, Asteroid Occultations V3.0, NASA Planetary Data System <br />Marciniak, A., Pilcher, F., Oszkiewicz, D., et al. 2015, Planet. Space Sci., 118, 256 <br />Marciniak, A., Durech, J., Ali-Lagoa, V., et al. 2021, A&A, 657, A87 <br />Molnar, L., Pal, A., Sarneczky, K., et al. 2018, ApJS, 234, 37 <br />Neugebauer, G., Habing, H. J., van Duinen, R., et al. 1984, ApJ, 278, L1 <br />Pal, A., Szakats, R., Kiss, C., et al. 2020, ApJS, 247, 26 <br />Usui, F., Kuroda, D., Mueller, T. G., et al. 2011, PASJ, 63, 1117 <br />Wright, E. L., Eisenhardt, P. R. M., Mainzer, A. K., et al. 2010, AJ, 140, 1868</p> <p><sup>1</sup>https://www.iota-es.de/neglected_asteroids.html <br /><sup>2</sup>https://cloud.occultwatcher.net/campaigns</p>