Search

Introduction The near-Earth asteroid (153201) 2000 WO107 was selected as an M-asteroid candidate based on its known albedo value pv = 0.129 ± 0.058 [1] and the taxonomic X-type determined from spectral observations [2]. The ambiguous spectral X-type suggests one of the three E-, P-, or M-types, but in this case, given the known albedo, the asteroid is most consistent with an M-type. It has a significantly elongated orbit with e = 0.781, which crosses the orbits of all major inner planets, enters the orbit of Mercury, and approaches the Sun at a distance of q = 0.20 AU. The asteroid has a small size of about 0.5 km and is available for observation only during close approaches to the Earth. We initiated photometric observations of 2000 WO107 in the 2020 opposition aimed to obtain rotation parameters and estimate its shape and bulk density. Observations We carried out photometric observations of (153201) 2000 WO107 in November-December 2020, when the asteroid passed at a minimum distance of 0.029 AU from the Earth. Our observations were made in a coordinated manner at five observatories: the 1-m telescope of the Crimean Astrophysical Observatory, the 70-cm telescope of Abastumani Observatory, the 61-cm telescope of Skalnaté Pleso Observatory, the 36-cm telescope of Kitab Observatory, and the 0.25-m BART telescope of the Roque de los Muchachos Observatory. The observations covered the time interval from Nov. 28 to Dec. 9, 2020, and were done using CCD cameras in R-filter and, moreover, in Kitab without a filter. In addition, the asteroid was also observed in BVI filters on some nights. The measured lightcurves showed a very high amplitude (up to 1.24 mag at the phase angles of 65-70 deg) and made it possible to determine the rotation period of 5.03 hrs (Fig.1). Such a high amplitude characterizes the body of the asteroid as very elongated and possibly binary. Note the unusual flat minima are seen in the light curve. They are a characteristic manifestation of incomplete illumination of an asteroid at large phase angles for synchronous, as well as very elongated bodies of a nonconvex shape. It means that the asteroid is either a binary or a contact binary. Our assumptions are confirmed by radar observations carried out using the 70-m antenna at the Goldstone Observatory in the same opposition with our observations, which showed that the asteroid is a contact binary with a larger elongated primary component and a smaller secondary close to the sphere [3]. Moreover, the smaller body is attached to the end of the longest axis of the larger body. Figure 1. The composite light curve obtained from observational data on the night of November 28–29 during the passage of an asteroid at a minimum distance from the Earth. Modelling We simulated the scattering of light and radio waves from a bi-lobal asteroid in the assumption of two contacting ellipsoids. The axes of the two ellipsoids, the distance between their centers, the orientation of the rotation axis, the initial rotation phase and the period were used as fitting parameters in a Markov chain Monte Carlo (MCMC) algorithm. The resulted shape model fits well both the optical lightcurves and delay–Doppler images of 2000 WO107 obtained by Goldstone on Nov. 28, 29 and 30, 2020. We estimate the density of 1.5 g/cm3 from the requirement that the resulting shape of the asteroid is mechanically stable. An independent estimate of the asteroid density can be obtained from its Yarkovsky effect. Greenberg et al. (2020) analyzed measured positions for 2000 WO107 and found the Yarkovsky drift rate to be −24.37 ± 12.7 AU/My [4]. We simulated the Yarkovsky effect of a bi-lobal asteroid using the shape and size of the asteroid obtained from our modelling. Thus, we calculated the theoretical value of the Yarkovsky force acting on this asteroid. By comparing the theoretical Yarkovsky force with the observed Yarkovsky drift rate, we estimated the density of 2000 WO107. Within the error margin, the obtained value agrees with the density determined from the shape modeling. Conclusions Combination of photometric and radar data allows us to determine the rotation period and the position of the asteroid's pole, to predict its shape and size, and to measure the asteroid's density. The resulted density of the near-Earth asteroid (153201) 2000 WO107 is rather low, which implies the asteroid has a predominantly non-metallic composition and most probably belongs to primitive asteroids. Acknowledgment Ukrainian team was supported by the National Research Foundation of Ukraine (grant No. 2020.02/0371 “Metallic asteroids: search for parent bodies of iron meteorites, sources of extraterrestrial resources”). The authors are thankful to A. Novichonok and A. Zhornichenko for their help with observations. References [1] Mainzer, A., Grav, T., Bauer, J. et al. 2011. Astron. J. 743, Art. 156 [2] Binzel, R. P., Rivkin, A. S., Stuart, J. S., et al. 2004. Icarus 170, 259–294 [3] Benner, L. A. M. Goldstone Radar Observations Planning: (7753) 1988 XB, 2017 WJ16, and 2000 WO107. NASA JPL database “Asteroid Radar Research”. https://echo.jpl.nasa.gov/asteroids/1988XB/1988xb.2020.goldstone.planning.html [4] Greenberg, A.H., Margot, J.-L., Verma, A.K., Taylor, P.A., Hodge, S.E. 2020. Astron. J. 159, 92

The tangential YORP effect (or TYORP) is a radiation pressure torque, which acts on small irregularities of the asteroid surface due to their non-uniform heating. This effect causes asteroids to change their rotation rates, and in many cases, it can be larger than other non-gravitational torques. Other works have already considered TYORP produced by smooth boulders of different shapes on an asteroid's surface. Here, we present the new results on the contribution to TYORP due to the small-scale roughness on the surface of boulders or regolith. We carry out numeric simulations of the 2D heat conduction problem on a sinusoidal surface, average the radiation pressure force in time and space, and thus numerically find the TYORP experienced by the asteroid surface. We also create an approximate analytic theory of heat conduction on a slightly non-flat sinusoidal surface and use it to compute TYORP. We study the small-scale roughness of asteroid Ryugu published in other works and use it to evaluate its TYORP. As a result, we find that the numeric and analytic estimates of the tangential YORP produced by a rough surface are in good qualitative agreement with each other. The contribution of different sinusoidal harmonics to the tangential YORP is additive to good accuracy. It allows us to derive an approximate analytic formula, which expresses TYORP in terms of the Fourier spectrum of the shape of the asteroid surface. The TYORP contribution of the small-scale roughness can be comparable to or even greater than TYORP produced by boulders. Introduction The tangential Yarkovsky–O'Keefe–Radzievskii–Paddack effect (also tangential YORP, or TYORP) is caused by the recoil light pressure and drags the asteroid’s surface in the tangential direction [1]. It has previously been simulated for different geometries of smooth boulders [2, 3, 4]. Here, we study the contribution to TYORP arising from the roughness of the asteroid surface on a centimeter-scale, including both non-smoothness of the rocks and the non-flatness of the regolith. We present the recent analytic results from [5] and as yet unpublished results of the numeric simulation of the effect. Analytic model We use the perturbation theory to solve the heat conduction equation for a surface whose shape is described by a sinusoidal wave of small amplitude. The resulting temperature is used to compute the light pressure recoil force acting on the surface and thus to evaluate TYORP. The expression for TYORP is second-order in terms of the surface slopes, thus it gets vanishingly small if the surface is nearly flat. TYORP of surface roughness has other features previously observed for TYORP of boulders: it vanishes at very small and very big thermal parameters, it vanishes for very short and very long wavelengths of the sinusoidal perturbation of the surface, it is maximal if the thermal parameter is of the order of unity and the wavelength of the sinusoidal perturbation is of the order of the thermal wavelength. As a qualitative result of this approximate analytic theory, we also obtain that sinusoidal waves with non-commensurable wavelengths do not interfere with each other, and their TYORP is additive. Numeric model We create a computer program that numerically solves the 2D heat conduction equation under the curved surface, and uses the resulting temperature field to compute TYORP. The results of the program qualitatively agree with the analytic model, but differ in exact numeric values due to simplifying assumptions incorporated into the analytic model. We fit the numeric results with an analytic expression and thus get an equation for TYORP as simple as our analytic theory, but more precise than it. Numeric simulations of TYORP for a surface composed of two sinusoidal harmonics show that different harmonics are almost independent, with the TYORP of a surface being almost equal to the TYORP of individual harmonics. Results To apply the theory, we take the high-resolution shape model of asteroid (162173) Ryugu from [6] and decompose its shape into the Fourier harmonics. The additivity of TYORP for different Fourier harmonics tested both analytically and numerically allows us to derive a simple mathematical expression for TYORP as an integral over the Fourier power spectrum of the asteroid surface roughness. Application of this expression to Ryugu results in a TYORP value that is greater than the YORP effect due to the large-scale asymmetry of Ryugu computed from its global shape model [7]. Overall, TYORP produced by the asteroid surface roughness has the same order of magnitude as TYORP produced by boulders. This new component of the YORP effect needs to be computed to correctly predict the dynamics of asteroids, and here we show how this computation can be done. Acknowledgments This work was supported by the National Research Foundation of Ukraine, project N2020.02/0371 “Metallic asteroids: search for parent bodies of iron meteorites, sources of extraterrestrial resources”. References [1] Golubov O., Krugly Y. N., 2012, ApJL 752, L11[2] Golubov O., Scheeres D. J., Krugly Y. N., 2014, AJ 794, 22[3] Sevecek P., Broz M., Capek D., Durech J., 2015, MNRAS 450, 2104[4] Sevecek P., Golubov O., Scheeres D. J., Krugly Y. N., 2016, A&A 592, A115[5] Golubov O., Lipatova V., A&A, accepted[6] Otto K. A., Matz K. D., Schröder S. E., et al., 2021, MNRAS 500, 3178[7] Kanamaru M., Sasaki S., Morota T., et al., 2021, Journal of Geophysical Research: Planets 126, e2021JE006863.

An approach to distinguish p-wave from s-wave superconducting pairing symmetry and thus to select potential platforms for Majorana fermions is proposed in terms of electronic transport differences in a four terminal junction consisting of superconducting (S) and normal (N) terminals in the diffusive regime. The Keldysh Green’s function equations are derived in the θ-parametrisation, incorporating terms previously neglected in the literature. A stable procedure to solve these equations is presented. The supercurrent and differential conductance between two superconducting electrodes were calculated in the Keldysh–Usadel approximation. The N-terminals can be used to manipulate the energy distribution functions of electrons in the junction in order to control the overall charge transport. Our results provide a new experimental test to detect potential p-wave superconductivity. In fact, we show that the differential conductance of junctions containing p-wave superconductors is distinctly different from the differential conductance in junctions with s-wave superconductors, whereas the supercurrent through the junction is qualitatively similar. This is of importance for the search for Majorana fermions since it may help to design experiments to detect signatures of p-wave symmetry, which may lead to potential platforms for Majorana fermions.

Thin films of diluted magnetic alloys are widely used in superconducting spintronics devices. Most studies rely on transport measurements and assume homogeneous magnetic layers. Here we examine on a local scale the electronic properties of the well-known two-layer superconductor/ferromagnet structure Nb/CuNi. Scanning tunneling spectroscopy experiments demonstrated significant spatial variations of the tunneling conductance on nanoscale, with characteristic gapped, nongapped, and strongly zero-bias peaked spectra. The microscopic theory successfully reproduced the observed spectra and relied them to spatial variations of CuNi film thickness and composition, leading to strong variations of the effective exchange energy. The observed inhomogeneities put constraints on the use of diluted magnetic alloys in nanoscale devices.

In this work, we study magnetization dynamics in superconductor-ferromagnet-superconductor thin-film structures. Results of the broad-band ferromagnetic resonance spectroscopy are reported for a large set of samples with varied thickness of both superconducting and ferromagnetic layers in a wide frequency, field, and temperature ranges. Experimentally the one-dimensional anisotropic action of superconducting torque on magnetization dynamics is established; its dependence on thickness of layers is revealed. It is demonstrated that experimental findings support the recently-proposed mechanism of the superconducting torque formation via the interplay between the superconducting kinetic inductance and magnetization precession at superconductor-ferromagnet interfaces., 7 pages, 3 figures, 50 references