A. Kryszczyńska

Shapes and rotational properties of thirty asteroids from photometric data

We have analyzed photometric lightcurves of 30 asteroids, and present here the obtained shapes, rotational periods and pole directions. We also present new photometric observations of five asteroids. The shape models indicate the existence of many features of varying degrees of irregularity. Even large main-belt asteroids display such features, so the resulting poles and periods are more consistent than those obtained by simple ellipsoid-like models. In some cases the new rotational parameters are rather different from those obtained previously, and in a few cases there were no proper previous estimates at all.

Retrograde spins of near-Earth asteroids from the Yarkovsky effect

Dynamical resonances in the asteroid belt are the gateway for the production of near-Earth asteroids (NEAs). To generate the observed number of NEAs, however, requires the injection of many asteroids into those resonant regions. Collisional processes have long been claimed as a possible source, but difficulties with that idea have led to the suggestion that orbital drift arising from the Yarkovsky effect dominates the injection process. (The Yarkovsky effect is a force arising from differential heating—the ‘afternoon’ side of an asteroid is warmer than the ‘morning’ side.) The two models predict different rotational properties of NEAs: the usual collisional theories are consistent with a nearly isotropic distribution of rotation vectors, whereas the ‘Yarkovsky model’ predicts an excess of retrograde rotations. Here we report that the spin vectors of NEAs show a strong and statistically significant excess of retrograde rotations, quantitatively consistent with the theoretical expectations of the Yarkovsky model.

New determination of the size and bulk density of the binary Asteroid 22 Kalliope from observations of mutual eclipses

Abstract In 2007, the M-type binary Asteroid 22 Kalliope reached one of its annual equinoxes. As a consequence, the orbit plane of its small moon, Linus, was aligned closely to the Suns line of sight, giving rise to a mutual eclipse season. A dedicated international campaign of photometric observations, based on amateur–professional collaboration, was organized and coordinated by the IMCCE in order to catch several of these events. The set of the compiled observations is released in this work. We developed a relevant model of these events, including a topographic shape model of Kalliope refined in the present work, the orbit solution of Linus as well as the photometric effect of the shadow of one component falling on the other. By fitting this model to the only two full recorded events, we derived a new estimation of the equivalent diameter of Kalliope of 166.2 ± 2.8 km , 8% smaller than its IRAS diameter. As to the diameter of Linus, considered as purely spherical, it is estimated to 28 ± 2 km . This substantial “shortening” of Kalliope, gives a bulk density of 3.35 ± 0.33 g / cm 3 , significantly higher than past determinations but more consistent with its taxonomic type. Some constraints can be inferred on the composition.

Photometry of 2006 RH120: an asteroid temporary captured into a geocentric orbit

Aims. From July 2006 to July 2007 a very small asteroid orbited the Earth within its Hill sphere. We used this opportunity to study its rotation and estimate its diameter and shape. Methods. Due to its faintness, 2006 RH120 was observed photometrically with the new 10-m SALT telescope at the SAAO (South Africa). We obtained data on four nights: 11, 15, 16, and 17 March 2007 when the solar phase angle remained almost constant at 74 ◦ . The observations lasted about an hour each night and the object was exposed for 7−10 s through the “clear” filter. Results. From the lightcurves obtained on three nights we derived two solutions for a synodical period of rotation: P1 = 1.375 ± 0.001 min and P2 = 2.750 ± 0.002 min. The available data are not sufficient to choose between them. The absolute magnitude of the object was found to be H = 29.9 ± 0.3 mag (with the assumed slope parameter G = 0.25) and its effective diameter D = 2− 7m , depending on the geometric albedo pV (with the most typical near-Earth asteroids albedo pV = 0.18 its diameter would be D = 3.3 ± 0.4 m). The body has an elongated shape with the a/b ratio greater than 1.4. It probably originates in low-eccentricity Amor or Apollo orbits. There is still a possibility, which needs further investigation, that it is a typical near-Earth asteroid that survived the aerobraking in the Earth’s atmosphere and returned to a heliocentric orbit similar to that of the Earth.

Photometry and models of selected main belt asteroids I. 52 Europa, 115 Thyra, and 382 Dodona

Photometric observations for 52 Europa (1995, 1997, 1999, 2000), 115 Thyra (1995, 1996, 1998, 2000), and 382 Dodona (1996, 1998, 1999, 2001) carried out at seven observatories are presented. Using all available lightcurves, the spin vectors, senses of rotation, and shape models of these three asteroids have been determined or refined.

Near infra-red spectroscopy of the asteroid 21 Lutetia. II. Rotationally resolved spectroscopy of the surface

Aims. In the framework of the ground-based science campaign dedicated to the encounter with the Rosetta spacecraft, the mineralogy of the asteroid (21) Lutetia was investigated. Methods. Near-infrared (NIR) spectra of the asteroid in the 0.8−2.5 μm spectral range were obtained with SpeX/IRTF in remote observing mode from Meudon, France in March and April 2006. We analysed these data together with previously acquired spectra - March 2003, August 2004. I-band relative photometric data obtained on 20 January 2006 using the 105 cm telescope from Pic du Midi, France has been used to build the ephemeris for physical observations. A χ2 test using meteorite spectra from the RELAB database was performed in order to find the best fit of complete visible + infrared (VNIR) spectra of Lutetia. Results. The new spectra reveal no absorption features. We find a clear spectral variation (slope), and a good correspondence between spectral variations and rotational phase. Two of the most different spectra correspond to two opposite sides of the asteroid (sub-Earth longitude difference around 180◦). For the neutral spectra a carbonaceous chondrite spectrum yields the best fit, while for those with a slightly positive slope the enstatitic chondrite spectra are the best analog. Based on the chosen subset of the meteorite samples, our analysis suggests a primitive, chondritic nature for (21) Lutetia. Differences in spectra are interpreted in terms of the coexistence of several lithologies on the surface where the aqueous alteration played an important role.

Asteroid 2867 Steins II. Multi-telescope visible observations, shape reconstruction, and rotational state

Context. Asteroid 2867 Steins is the first target of the Rosetta space mission with a flyby scheduled in September 2008. Aims. An early physical characterization is needed to optimize the flyby parameters and the science operations, and to maximize the scientific return. The aim of this article is to characterize the shape and rotational state of this asteroid. Methods. We compile a set of 26 visible light curves whose phase angle coverage extends from 7.5 ◦ to 41.7 ◦ , and perform their simultaneous inversion relying on convex modeling. Results. The full three-dimensional solution for asteroid 2867 Steins is rather spherical with axial ratios a/b = 1.17 and a/c = 1.25. The rotational state is characterized by a sidereal period of 6.04681 ± 0.00002 h, and the pole direction defined by its ecliptic coordinates λ ≈ 250 ◦ and β ≈− 89 ◦ has an uncertainty of about 5 ◦ . It is therefore almost exactly perpendicular to the ecliptic plane, and the viewing geometries are thus restricted to only ±20 ◦ about Steins’ equator. Consequently, the shape model is not strongly constrained, and the polar flattening has an uncertainty of about 10%. Inversion is basically scale-free, and absolute scaling comes from a measurement of its thermal emission with the Spitzer Space Telescope (Lamy et al. 2008, A&A, 487, 1187), yielding overall dimensions of 5.73 ± 0.52, 4.95 ± 0.45, and 4.58 ± 0.41 km.

Photometry and models of selected main belt asteroids. III. 283 Emma, 665 Sabine, and 690 Wratislavia

Photometric observations of 283 Emma (1998, 2000, 2001, 2004), 665 Sabine (1998, 1999, 2001, 2004, 2005), and 690 Wratislavia (1998, 2000, 2004, 2005-2006) carried out on 44 nights at two observatories are presented. Using all available lightcurves, the spin vectors, senses of rotation, and shape models for these three asteroids have been determined.

Eclipsing events in the binary system of the asteroid 90 Antiope

CCD observations of the binary asteroid 90 Antiope were carried out at four observatories (Borowiec, Pic du Midi, Kharkiv, and Chateau Renard) on 26 nights from October 2001 through February 2002. The results show a two-component lightcurve with each showing the same period of 16.505 ± 0.002 hours. The first component (with the amplitude of 0.10 mag) is associated with the rotation of two non-spherical bodies of the system; the second one, showing two sharp minima (with the amplitude in the range 0.05-0.12 mag, depending linearly on the phase angle) is due to eclipses in the binary system. The lightcurve suggests a synchronous rotation. The orientation of the systems orbit has been determined from the analysis of both the amplitudes and the duration of the eclipses. Some predictions of the possibility of observations of the eclipsing events during future oppositions have also been made.

Photometry and models of selected main belt asteroids IV. 184 Dejopeja, 276 Adelheid, 556 Phyllis

We report photometric lightcurve observations of 184 Dejopeja (apparition years: 2000, 2002, 2005, 2006), 276 Adelheid (2000, 2001, 2004, 2005, 2006), and 556 Phyllis (1998, 2000, 2002, 2004, 2005, 2006) carried out on 48 nights at four observatories. Using all of the available lightcurves, the spin vectors, senses of rotation, and shape models of these three asteroids have been determined.