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Register a new userAbsolute magnitudes and slope parameters for 250,000 asteroids observed by Pan-STARRS PS1 - preliminary results
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We present the results of a Monte Carlo technique to calculate the absolute magnitudes (H) and slope parameters (G) of about 240000 asteroids observed by the Pan-STARRS1 telescope during the first 15 months of its 3-year all-sky survey mission. The systems exquisite photometry with photometric errors < 0.04 mag, and well-defined filter and photometric system, allowed us to derive accurate H and G even with a limited number of observations and restricted range in phase angles. Our Monte Carlo method simulates each asteroids rotation period, amplitude and color to derive the most-likely H and G, but its major advantage is in estimating realistic statistical+systematic uncertainties and errors on each parameter. The method was tested by comparison with the well-established and accurate results for about 500 asteroids provided by Pravec et al. (2012) and then applied to determining H and G for the Pan-STARRS1 asteroids using both the Muinonen et al. (2010) and Bowell et al. (1989) phase functions. Our results confirm the bias in MPC photometry discovered by (Juric et al. 2002).
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Using the first 18 months of the Pan-STARRS 1 survey we have identified 33 candidate high-amplitude objects for follow-up observations and carried out observations of 22 asteroids. 4 of the observed objects were found to have observed amplitude $A_{obs}geq 1.0$ mag. We find that these high amplitude objects are most simply explained by single rubble pile objects with some density-dependent internal strength, allowing them to resist mass shedding even at their highly elongated shapes. 3 further objects although below the cut-off for high-amplitude had a combination of elongation and rotation period which also may require internal cohesive strength, depending on the density of the body. We find that none of the high-amplitude asteroids identified here require any unusual cohesive strengths to resist rotational fission. 3 asteroids were sufficiently observed to allow for shape and spin pole models to be determined through light curve inversion. 45864 was determined to have retrograde rotation with spin pole axes $lambda=218pm 10^{circ}, beta=-82pm 5^{circ}$ and asteroid 206167 was found to have best fit spin pole axes $lambda= 57 pm 5^{circ}$, $beta=-67 pm 5^{circ}$. An additional object not initially measured with $A_{obs}>1.0$ mag, 49257, was determined to have a shape model which does suggest a high-amplitude object. Its spin pole axes were best fit for values $lambda=112pm 6^{circ}, beta=6pm 5^{circ}$. In the course of this project to date no large super-fast rotators ($P_{rot} < 2.2$ h) have been identified.
The NEOWISE dataset offers the opportunity to study the variations in albedo for asteroid classification schemes based on visible and near-infrared observations for a large sample of minor planets. We have determined the albedos for nearly 1900 asteroids classified by the Tholen, Bus and Bus-DeMeo taxonomic classification schemes. We find that the S-complex spans a broad range of bright albedos, partially overlapping the low albedo C-complex at small sizes. As expected, the X-complex covers a wide range of albedos. The multi-wavelength infrared coverage provided by NEOWISE allows determination of the reflectivity at 3.4 and 4.6 $mu$m relative to the visible albedo. The direct computation of the reflectivity at 3.4 and 4.6 $mu$m enables a new means of comparing the various taxonomic classes. Although C, B, D and T asteroids all have similarly low visible albedos, the D and T types can be distinguished from the C and B types by examining their relative reflectance at 3.4 and 4.6 $mu$m. All of the albedo distributions are strongly affected by selection biases against small, low albedo objects, as all objects selected for taxonomic classification were chosen according to their visible light brightness. Due to these strong selection biases, we are unable to determine whether or not there are correlations between size, albedo and space weathering. We argue that the current set of classified asteroids makes any such correlations difficult to verify. A sample of taxonomically classified asteroids drawn without significant albedo bias is needed in order to perform such an analysis.
The rotational state of asteroids is controlled by various physical mechanisms including collisions, internal damping and the Yarkovsky-OKeefe-Radzievskii-Paddack (YORP) effect. We have analysed the changes in magnitude between consecutive detections of approximately 60,000 asteroids measured by the PanSTARRS 1 survey during its first 18 months of operations. We have attempted to explain the derived brightness changes physically and through the application of a simple model. We have found a tendency toward smaller magnitude variations with decreasing diameter for objects of 1 < D < 8 km. Assuming the shape distribution of objects in this size range to be independent of size and composition our model suggests a population with average axial ratios 1 : 0.85 pm 0.13 : 0.71 pm 0.13, with larger objects more likely to have spin axes perpendicular to the orbital plane.
Context: Accurate measurements of diameters of trans-Neptunian objects are extremely complicated to obtain. Thermal modeling can provide good results, but accurate absolute magnitudes are needed to constrain the thermal models and derive diameters and geometric albedos. The absolute magnitude, Hv, is defined as the magnitude of the object reduced to unit helio- and geocentric distances and a zero solar phase angle and is determined using phase curves. Phase coefficients can also be obtained from phase curves. These are related to surface properties, yet not many are known. Aims: Our objective is to measure accurate V band absolute magnitudes and phase coefficients for a sample of trans-Neptunian objects, many of which have been observed, and modeled, within the TNOs are cool program, one of Herschel Space Observatory key projects. Methods: We observed 56 objects using the V and R filters. These data, along with those available in the literature, were used to obtain phase curves and measure V band absolute magnitudes and phase coefficients by assuming a linear trend of the phase curves and considering magnitude variability due to rotational light-curve. Results: We obtained 237 new magnitudes for the 56 objects, six of them with no reported previous measurements. Including the data from the literature we report a total of 110 absolute magnitudes with their respective phase coefficients. The average value of Hv is 6.39, bracketed by a minimum of 14.60 and a maximum of -1.12. In the case of the phase coefficients we report 0.10 mag per degree as the median value and a very large dispersion, ranging from -0.88 up tp 1.35 mag per degree.
We present the details of the photometric and astrometric calibration of the Pan-STARRS1 $3pi$ Survey. The photometric goals were to reduce the systematic effects introduced by the camera and detectors, and to place all of the observations onto a photometric system with consistent zero points over the entire area surveyed, the ~30,000 square degrees north of $delta$ = -30 degrees. The astrometric calibration compensates for similar systematic effects so that positions, proper motions, and parallaxes are reliable as well. The Pan-STARRS Data Release 2 (DR2) astrometry is tied to the Gaia DR1 release.
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