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Context. The JAXA Hayabusa2 mission will perform the first ever sample return from a primitive asteroid. The target near-Earth asteroid (162173) Ryugu will be reached in mid-2018 and its samples will be returned to the Earth by the end of 2020. Aims. We want to improve the current knowledge of the compositional and rotational properties of Ryugu, which are still presenting some uncertainties that might a ect the mission operations and scientific return. Methods. We acquired high-quality photometric time-series data with the FORS2 instrument at the Very Large Telescope of the European Southern Observatory (ESO-VLT, Chile). We also acquired four FORS2 visible spectra and three X-shooter spectra in the 0.35-2.15 μm range, at di erent rotational phases. Results. We obtained the currently highest-quality visual light-curve of Ryugu. A best solution of ∼7.63 h is found for the rotational period, while a short-period solution (i.e., P ≈ 3:8 h) is ruled out by the clearly non-symmetric light-curve. The obtained spectra are generally similar and featureless, but present a drop-o of the reflectance at <0.45 μm, suggesting the presence of aqueously altered minerals on Ryugu. The best meteorite analogs for Ryugu are represented by thermally altered CM carbonaceous chondrites. Conclusions. Our new photometric data help to refine the target reference model used by the Hayabusa2 team for the mission preparation and implementation, improving our knowledge of Ryugu's spin properties. Our new spectra constrain the compositional and geological context of the Ryugu's surface in order to prepare the planning of mission observations and support the working group for the selection of possible landing and sampling sites [ABSTRACT FROM AUTHOR]

Context. Quasi-periodic modulations of the stellar light curve may result from dark spots crossing the visible stellar disc. Owing to di erential rotation, spots at di erent latitudes generally have di erent rotation periods. Hence, by studying spot-induced modulations, it is possible to learn about stellar surface (di erential) rotation and magnetic activity. Recently, a method based on the Lomb-Scargle periodogram of light curves has been proposed to identify the sign of the di erential rotation at the stellar surface. Aims. Our goal is to understand how the modulation of the stellar light curve due to the presence of spots and the corresponding periodogram are a ected by both the stellar and spot properties. Methods. We generate synthetic light curves of stars with di erent properties (inclination angle, limb darkening, and rotation rate) and spot configurations (number of spots, latitude, intensity contrast, and size). By analysing their Lomb-Scargle periodograms, we compute the ratio between the heights of the second and first harmonics of the rotation period (peak-height ratio). Results. We find that the peak-height ratios are essentially a function of a single parameter, the fraction of time the spot is visible, which is related to the sinusoidality of the spot modulation. We identify the conditions under which the periodogram analysis can actually provide an estimate of the spot latitudes and/or the stellar inclination angle. We also identify possible sources of error in the identification of the sign of the di erential rotation. [ABSTRACT FROM AUTHOR]

Aims. Approach observations with the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) experiment onboard Rosetta are used to determine the rotation period, the direction of the spin axis, and the state of rotation of comet 67P's nucleus. Methods. Photometric time series of 67P have been acquired by OSIRIS since the post wake-up commissioning of the payload in March 2014. Fourier analysis and convex shape inversion methods have been applied to the Rosetta data as well to the available ground-based observations. Results. Evidence is found that the rotation rate of 67P has significantly changed near the time of its 2009 perihelion passage, probably due to sublimation-induced torque.We find that the sidereal rotation periods P1 = 12:76129 ± 0:00005 h and P2 = 12:4043 ± 0:0007 h for the apparitions before and after the 2009 perihelion, respectively, provide the best fit to the observations. No signs of multiple periodicity are found in the light curves down to the noise level, which implies that the comet is presently in a simple rotation state around its axis of largest moment of inertia. We derive a prograde rotation model with spin vector J2000 ecliptic coordinates λ = 65° ± 15°, β= +59° ± 15°, corresponding to equatorial coordinates RA = 22°, Dec = +76°. However, we find that the mirror solution, also prograde, at λ = 275° ± 15°, β= +50° ± 15° (or RA = 274°, Dec = +27°), is also possible at the same confidence level, due to the intrinsic ambiguity of the photometric problem for observations performed close to the ecliptic plane. [ABSTRACT FROM AUTHOR]

Context. We have developed a dual-band (140 and 220GHz) mm-wave imaging camera based on superconducting kinetic inductance detector (KID) arrays. Each array contains 132 superconducting resonators whose resonant frequencies are shifted by mm-wave photons absorption. The read out is achieved with a single electronics chain per band, taking advantage of the intrinsic KID frequencydomain multiplexability. The arrays are easily scalable and well adapted for future large format focal plane instruments. NIKA (formerly Néel IRAM KID Array, now New IRAM KID Array) has been specifically designed for the IRAM 30m telescope at Pico Veleta, and is one of the first instruments using KIDs to have made measurements of astronomical sources. Aims. In this Letter we describe the solutions adopted to improve the calibration accuracy and the sensitivity of the instrument, and we report on the outcome of the 3rd NIKA observing run of October, 2011. Methods. We use a fast electronic modulation of the readout tone for each KID pixel in order to linearize the instrument calibration, which we track with measurements of planets.We also adopt a new design of the KIDs, sensitive to both polarizations, to increase the amount of radiation absorbed and thus the optical efficiency of the system. Results. We measured an average sensitivity on the sky of 21 mJys0.5 per beam at 140 GHz and 140 mJys0.5 at 220 GHz in the best observing conditions (τ220 ≃ 0.2) after atmospheric noise decorrelation. The sensitivity at 220 GHz was limited by the atmospheric attenuation and loading as well as a reduction in the spectral bandwidth due to a misplaced filter. We found the repeatability in the photometry over the entire observing run to be better than 10% in both bands, thus demonstrating a significant improvement over the previous runs. We also find good agreement between NIKA measurements of faint astronomical sources and previous measurements of the same sources. [ABSTRACT FROM AUTHOR]

Context. Comet 67P/Churyumov-Gerasimenko is the target of the European Space Agency Rosetta spacecraft rendez-vous mission. Detailed physical characteristation of the comet before arrival is important for mission planning as well as providing a test bed for ground-based observing and data-analysis methods. Aims. To conduct a long-term observational programme to characterize the physical properties of the nucleus of the comet, via ground-based optical photometry, and to combine our new data with all available nucleus data from the literature. Methods. We applied aperture photometry techniques on our imaging data and combined the extracted rotational lightcurves with data from the literature. Optical lightcurve inversion techniques were applied to constrain the spin state of the nucleus and its broad shape. We performed a detailed surface thermal analysis with the shape model and optical photometry by incorporating both into the new Advanced Thermophysical Model (ATPM), along with all available Spitzer 8–24 μm thermal-IR flux measurements from the literature. Results. A convex triangular-facet shape model was determined with axial ratios b/a = 1.239 and c/a = 0.819. These values can vary by as much as 7% in each axis and still result in a statistically significant fit to the observational data. Our best spin state solution has Psid = 12.76137 ± 0.00006 h, and a rotational pole orientated at Ecliptic coordinates λ = 78°(±10°), β = + 58°(±10°). The nucleus phase darkening behaviour was measured and best characterized using the IAU HG system. Best fit parameters are: G = 0.11 ± 0.12 and HR(1,1,0) = 15.31 ± 0.07. Our shape model combined with the ATPM can satisfactorily reconcile all optical and thermal-IR data, with the fit to the Spitzer 24 μm data taken in February 2004 being exceptionally good. We derive a range of mutually-consistent physical parameters for each thermal-IR data set, including effective radius, geometric albedo, surface thermal inertia and roughness fraction. Conclusions. The overall nucleus dimensions are well constrained and strongly imply a broad nucleus shape more akin to comet 9P/Tempel 1, rather than the highly elongated or “bi-lobed” nuclei seen for comets 103P/Hartley 2 or 8P/Tuttle. The derived low thermal inertia of <15 J m-2 K-1 s-1/2 is comparable with that measured for other comets scaled to similar heliocentric distances, and implies a surface regolith finer than lunar surface material. [ABSTRACT FROM AUTHOR]