Photometric survey, modelling, and scaling of long-period and low-amplitude asteroids ^⋆

¹ Astronomical Observatory Institute, Faculty of Physics, A. Mickiewicz University, Słoneczna 36, 60-286 Poznań, Poland
e-mail: [email protected]
² Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
³ Instituto de Astrofísica de Canarias, C/vía Lactea s/n, 38205 La Laguna, Tenerife, Spain
⁴ Observatoire des Hauts Patys, 84410 Bédoin, France
⁵ Geneva Observatory, 1290 Sauverny, Switzerland
⁶ Les Engarouines Observatory, 84570 Mallemort-du-Comtat, France
⁷ Institute of Geology, A. Mickiewicz University, Krygowskiego 12, 61-606 Poznań, Poland
⁸ OAM – Mallorca, Camí de l’Observatori s/n, 07144 Costitx Mallorca, Illes Balears, Spain
⁹ Stazione Astronomica di Sozzago, 28060 Sozzago, Italy
¹⁰ Rose-Hulman Institute of Technology, CM 171 5500 Wabash Ave., Terre Haute, IN 47803, USA
¹¹ Departamento de Sistema Solar, Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
¹² Observatoíre du Bois de Bardon, 16110 Taponnat, France
¹³ Gran Telescopio Canarias (GRANTECAN), Cuesta de San José s/n, 38712 Breña Baja, La Palma, Spain
¹⁴ Astrophysics Division, Institute of Physics, Jan Kochanowski University, Świętokrzyska 15, 25-406 Kielce, Poland
¹⁵ Institute of Physics, Faculty of Natural Sciences, University of P. J. Šafárik, Park Angelinum 9, 040 01 Košice, Slovakia
¹⁶ Laboratory of Space Researches, Uzhhorod National University, Daleka st. 2a, 88000 Uzhhorod, Ukraine
¹⁷ NaXys, Department of Mathematics, University of Namur, 8 Rempart de la Vierge, 5000 Namur, Belgium
¹⁸ Mt. Suhora Observatory, Pedagogical University, Podchorążych 2, 30-084 Cracow, Poland
¹⁹ 4438 Organ Mesa Loop, Las Cruces, NM 88011, USA
²⁰ Command Module Observatory, 121 W. Alameda Dr., Tempe, AZ 85282, USA
²¹ Rue des Écoles 2, 34920 Le Crès, France
²² Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, 86001 Arizona, USA
²³ Kepler Institute of Astronomy, University of Zielona Góra, Lubuska 2, 65-265 Zielona Góra, Poland

Received: 30 June 2017
Accepted: 11 September 2017

Abstract

Context. The available set of spin and shape modelled asteroids is strongly biased against slowly rotating targets and those with low lightcurve amplitudes. This is due to the observing selection effects. As a consequence, the current picture of asteroid spin axis distribution, rotation rates, radiometric properties, or aspects related to the object’s internal structure might be affected too.

Aims. To counteract these selection effects, we are running a photometric campaign of a large sample of main belt asteroids omitted in most previous studies. Using least chi-squared fitting we determined synodic rotation periods and verified previous determinations. When a dataset for a given target was sufficiently large and varied, we performed spin and shape modelling with two different methods to compare their performance.

Methods. We used the convex inversion method and the non-convex SAGE algorithm, applied on the same datasets of dense lightcurves. Both methods search for the lowest deviations between observed and modelled lightcurves, though using different approaches. Unlike convex inversion, the SAGE method allows for the existence of valleys and indentations on the shapes based only on lightcurves.

Results. We obtain detailed spin and shape models for the first five targets of our sample: (159) Aemilia, (227) Philosophia, (329) Svea, (478) Tergeste, and (487) Venetia. When compared to stellar occultation chords, our models obtained an absolute size scale and major topographic features of the shape models were also confirmed. When applied to thermophysical modelling (TPM), they provided a very good fit to the infrared data and allowed their size, albedo, and thermal inertia to be determined.

Conclusions. Convex and non-convex shape models provide comparable fits to lightcurves. However, some non-convex models fit notably better to stellar occultation chords and to infrared data in sophisticated thermophysical modelling (TPM). In some cases TPM showed strong preference for one of the spin and shape solutions. Also, we confirmed that slowly rotating asteroids tend to have higher-than-average values of thermal inertia, which might be caused by properties of the surface layers underlying the skin depth.