arXiv:1104.4114v1 [astro-ph.EP] 20 Apr 2011 Astronomy & Astrophysics manuscript no. aa˙2009 c  ESO 2018 August 22, 2018 A study of asteroid pole-latitude distribution based on an extended set of shape models derived by the lightcurve inversion method J. Hanuˇ s 1∗ , J. ˇ Durech 1 , M. Broˇ z 1 , B. D. Warner 2 , F. Pilcher 3 , R. Stephens 4 , J. Oey 5 , L. Bernasconi 6 , S. Casulli 7 , R. Behrend 8 , D. Polishook 9 , T. Henych 10 , M. Lehk ´ y 11 , F. Yoshida 12 , and T. Ito 12 1 Astronomical Institute, Faculty of Mathematics and Physics, Charles University in Prague, V Holeˇ soviˇ ck´ ach 2, 18000 Prague, Czech Republic ∗ e-mail: hanus.home@gmail.com 2 Palmer Divide Observatory, 17995 Bakers Farm Rd., Colorado Springs, CO 80908, USA 3 4438 Organ Mesa Loop, Las Cruces, NM 88011, USA 4 Goat Mountain Astronomical Research Station, 11355 Mount Johnson Court, Rancho Cucamonga, CA 91737, USA 5 Kingsgrove, NSW, Australia 6 Observatoire des Engarouines, 84570 Mallemort-du-Comtat, France 7 Via M. Rosa, 1, 00012 Colleverde di Guidonia, Rome, Italy 8 Geneva Observatory, CH-1290 Sauverny, Switzerland 9 Benoziyo Center for Astrophysics, The Weizmann Institute of Science, Rehovot 76100, Israel 10 Astronomical Institute, Academy of Sciences of the Czech Republic, Friova 1, CZ-25165 Ondejov, Czech Republic 11 Severni 765, CZ-50003 Hradec Kralove, Czech republic 12 National Astronomical Observatory, Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan Received 17-02-2011 / Accepted 13-04-2011 ABSTRACT Context. In the past decade, more than one hundred asteroid models were derived using the lightcurve inversion method. Measured by the number of derived models, lightcurve inversion has become the leading method for asteroid shape determination. Aims. Tens of thousands of sparse-in-time lightcurves from astrometric projects are publicly available. We investigate these data and use them in the lightcurve inversion method to derive new asteroid models. By having a greater number of models with known physical properties, we can gain a better insight into the nature of individual objects and into the whole asteroid population. Methods. We use sparse photometry from selected observatories from the AstDyS database (Asteroids – Dynamic Site), either alone or in combination with dense lightcurves, to determine new asteroid models by the lightcurve inversion method. We investigate various correlations between several asteroid parameters and characteristics such as the rotational state and diameter or family membership. We focus on the distribution of ecliptic latitudes of pole directions. We create a synthetic uniform distribution of latitudes, compute the method bias, and compare the results with the distribution of known models. We also construct a model for the long-term evolution of spins. Results. We present 80 new asteroid models derived from combined data sets where sparse photometry is taken from the AstDyS database and dense lightcurves are from the Uppsala Asteroid Photometric Catalogue (UAPC) and from several individual observers. For 18 asteroids, we present updated shape solutions based on new photometric data. For another 30 asteroids we present their partial models, i.e., an accurate period value and an estimate of the ecliptic latitude of the pole. The addition of new models increases the total number of models derived by the lightcurve inversion method to ∼200. We also present a simple statistical analysis of physical properties of asteroids where we look for possible correlations between various physical parameters with an emphasis on the spin vector. We present the observed and de-biased distributions of ecliptic latitudes with respect to different size ranges of asteroids as well as a simple theoretical model of the latitude distribution and then compare its predictions with the observed distributions. From this analysis we find that the latitude distribution of small asteroids (D < 30 km) is clustered towards ecliptic poles and can be explained by the YORP thermal effect while the latitude distribution of larger asteroids (D > 60 km) exhibits an evident excess of prograde rotators, probably of primordial origin. Key words. minor planets, asteroids - photometry - models 1. Introduction The lightcurve inversion method (LI) is a powerful tool that al- lows us to derive basic physical properties of asteroids (the ro- tational state and the shape) from their disk-integrated photom- etry (see Kaasalainen & Torppa 2001; Kaasalainen et al. 2001, 2002). This photometry can be dense-in-time, which typically consists of tens to a few hundreds of individual data points ob- served during one revolution. This is in contrast to sparse-in- time, where the typical separation of individual measurements is large compared to the rotation period. For sparse data, we usu- ally have a few measurements per night, such as in the case of astrometric sky surveys. In the following text, we use the terms “dense lightcurves” and “sparse lightcurves”. To obtain a unique spin and shape solution, we need a set of at least a few tens of dense lightcurves observed during at least three apparitions. Based on simulated photometric data and the survey cadence of the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS), Kaasalainen (2004) showed that we can also use only sparse data for the inversion technique. 1