Astrometry Projects

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This chapter describes two astrometry projects using remote observatory facilities. The first one was done by an accomplished citizen scientist. The second one is from the perspective of a professional astronomer.

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The Target Asteroids! citizen science program will support the NASA OSIRIS-REx asteroid sample return mission by characterizing asteroids that may be targets of future sample return missions and by analyzing asteroids which are analogs of the OSIRIS-REx target (101955) Bennu. Obtaining numerous low-precision photometric observations over a range of solar phase angles will provide direct measurements of the phase function, absolute magnitude, color and rotation period, and indirect measurements of the taxonomy, albedo and size.
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The opposition effect behavior is analyzed for 33 asteroids with well-measured magnitude-phase curves. It is shown that surface albedo is the main factor influencing the amplitude and width of the asteroid opposition effect. The amplitude of opposition effect depends on asteroid albedo in a non-monotonic way with the maximum for moderate albedo asteroids decreasing both for dark and for high albedo asteroids. This can be explained by the combined influence of both shadow hiding and coherent-backscatter mechanisms. The relative contribution of the coherent-backscatter effect to the asteroid magnitude-phase dependence close to opposition (α=0.3°) is estimated to be about 20-60% for low albedo asteroids and 80-90% for high and moderate albedo asteroids. At α>5° the shadow-hiding effect has the dominant influence. A tendency was found for asteroids with larger U-B colors to show larger values of the opposition effect. We also compare the opposition effects of asteroids with those of some atmosphereless satellites and meteorites.
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The fourth United States Naval Observatory (USNO) CCD Astrograph Catalog, UCAC4 was released in August 2012 (double-sided DVD and CDS data center Vizier catalog I/322). It is the final release in this series and contains over 113 million objects; over 105 million of them with proper motions. UCAC4 is an updated version of UCAC3 with about the same number of stars also covering all-sky. Bugs were fixed, Schmidt plate survey data were avoided, and precise 5-band photometry were added. Astrograph observations have been supplemented for bright stars by FK6, Hipparcos and Tycho-2 data to compile a UCAC4 star catalog complete to about magnitude R = 16. Epoch 1998 to 2004 positions are obtained from observations with the 20 cm aperture USNO Astrograph's red lens, equipped with a 4k by 4k CCD. Mean positions and proper motions are derived by combining these observations with over 140 ground- and space-based catalogs, including Hipparcos/Tycho and the AC2000.2, as well as unpublished measures of over 5000 plates from other astrographs. For most of the faint stars the first epoch plates from the Southern Proper Motion (SPM) and the Northern Proper Motion (NPM) programs form the basis for proper motions. These data are supplemented by 2MASS near-IR photometry for about 110 million stars and 5-band (B,V,g,r,i) APASS data for over 51 million stars. Thus the published UCAC4, as were UCAC3 and UCAC2, is a compiled catalog with the UCAC observational program being a major component. The positional accuracy of stars in UCAC4 at mean epoch is about 15 to 100 mas per coordinate, depending on magnitude, while the formal errors in proper motions range from about 1 to 10 mas/yr depending on magnitude and observing history. Systematic errors in proper motions are estimated to be about 1 to 4 mas/yr.
Astrometrica is an interactive software tool for scientific grade astrometric data reduction of CCD images. The current version of the software is for the Windows 32bit operating system family. Astrometrica reads FITS (8, 16 and 32 bit integer files) and SBIG image files. The size of the images is limited only by available memory. It also offers automatic image calibration (Dark Frame and Flat Field correction), automatic reference star identification, automatic moving object detection and identification, and access to new-generation star catalogs (PPMXL, UCAC 3 and CMC-14), in addition to online help and other features. Astrometrica is shareware, available for use for a limited period of time (100 days) for free; special arrangements can be made for educational projects.
The so-called Nysa-Polana complex of asteroids is a diverse and widespread group. It appears to be two overlapping families of different asteroid taxonomies: (44) Nysa is an E-type asteroid with the lowest number in the midst of a predominantly S-type cluster and (142) Polana is a B-type asteroid near the low-albedo B- and C-type cluster. Using the data from the Wide-field Infrared Survey Explorer (WISE) mission we have re-analyzed the region around the Nysa-Polana complex in the inner Main Belt, focusing on the low-albedo population. (142) Polana does not appear to be a member of the family of low-albedo asteroids in the Nysa-Polana complex. Rather, the largest is asteroid (495) Eulalia. This asteroid has never before been linked to this complex for an important dynamical reason: it currently has a proper eccentricity slightly below the range of most of the family members. However, its orbit is very close to the 3:1 mean motion resonance with Jupiter and is in a weak secular resonance. We show that its osculating eccentricity varies widely on short timescales and the averaged value diffuses over long timescales. The diffusive orbit, low-albedo, taxonomic similarity and semimajor axis strongly suggests that despite its current proper eccentricity, (495) Eulalia could have recently been at an orbit very central to the family. Hierarchical Clustering Method tests confirm that at an eccentricity of e=0.15, (495) Eulalia could be the parent of the family. The ``Eulalia family'' was formed between 900--1500 Myr ago, and likely resulted from the breakup of a 100--160 km parent body. There is also compelling evidence for an older and more widespread primitive family in the same region of the asteroid belt parented by asteroid (142) Polana.
We explore the correlation between an asteroid's taxonomy and photometric phase curve using the H, G12 photometric phase function, with the shape of the phase function described by the single parameter G12. We explore the usability of G12 in taxonomic classification for individual objects, asteroid families, and dynamical groups. We conclude that the mean values of G12 for the considered taxonomic complexes are statistically different, and also discuss the overall shape of the G12 distribution for each taxonomic complex. Based on the values of G12 for about half a million asteroids, we compute the probabilities of C, S, and X complex membership for each asteroid. For an individual asteroid, these probabilities are rather evenly distributed over all of the complexes, thus preventing meaningful classification. We then present and discuss the G12 distributions for asteroid families, and predict the taxonomic complex preponderance for asteroid families given the distribution of G12 in each family. For certain asteroid families, the probabilistic prediction of taxonomic complex preponderance can clearly be made. The Nysa-Polana family shows two distinct regions in the proper element space with different G12 values dominating in each region. We conclude that the G12-based probabilistic distribution of taxonomic complexes through the main belt agrees with the general view of C complex asteroid proportion increasing towards the outer belt. We conclude that the G12 photometric parameter cannot be used in determining taxonomic complex for individual asteroids, but it can be utilized in the statistical treatment of asteroid families and different regions of the main asteroid belt.
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  • B D Warner
  • R D Stephens
  • A W Harris
  • BD Warner