Jeffrey A. Larsen

NEOWISE Observations of Near-Earth Objects: Preliminary Results

With the NEOWISE portion of the Wide-field Infrared Survey Explorer (WISE) project, we have carried out a highly uniform survey of the near-Earth object (NEO) population at thermal infrared wavelengths ranging from 3 to 22 μm, allowing us to refine estimates of their numbers, sizes, and albedos. The NEOWISE survey detected NEOs the same way whether they were previously known or not, subject to the availability of ground-based follow-up observations, resulting in the discovery of more than 130 new NEOs. The surveys uniform sensitivity, observing cadence, and image quality have permitted extrapolation of the 428 near-Earth asteroids (NEAs) detected by NEOWISE during the fully cryogenic portion of the WISE mission to the larger population. We find that there are 981 ± 19 NEAs larger than 1 km and 20,500 ± 3000 NEAs larger than 100 m. We show that the Spaceguard goal of detecting 90% of all 1 km NEAs has been met, and that the cumulative size distribution is best represented by a broken power law with a slope of 1.32 ± 0.14 below 1.5 km. This power-law slope produces ~13,200 ± 1900 NEAs with D > 140 m. Although previous studies predict another break in the cumulative size distribution below D ~ 50-100 m, resulting in an increase in the number of NEOs in this size range and smaller, we did not detect enough objects to comment on this increase. The overall number for the NEA population between 100 and 1000 m is lower than previous estimates. The numbers of near-Earth comets and potentially hazardous NEOs will be the subject of future work.

Fitting a Galactic Model to an All-Sky Survey

We use star counts from the APS Catalog of the POSS I to develop a Galactic model optimized for large, statistically significant data sets uniformly distributed over the sky. The power of the APS Catalog for Galactic structure studies comes from its large sky coverage, individual photometric calibrations, adequate scanning resolution, statistically significant sample sizes, color information, and, most importantly, multidirectional sampling. The APS Catalog is an extremely useful exploratory data set but requires new methodologies to maximize its usefulness. We have selected an 88-field subset, 16 deg2 each, from the catalog for a program of magnitude-limited star counts (12 ? O [blue] ? 20) within the completeness limit of the survey and in a realm where star-galaxy classification has minimal effects on the results. We have developed a simple three-component (disk, halo, thick disk) model optimized for efficiently and objectively analyzing star-count information. Our model not only produces model counts for our multidirectional data, but also returns a goodness of fit statistic. We use a genetic algorithm, a robust optimization technique well suited for this large multidirectional and multiple-parameter study, to optimize the fit and derive a self-consistent set of global parameters to model the Galaxy. With this global fit, we can identify significant deviations from symmetry in the Galaxys large-scale distribution of stars. The results from 12 independent executions or trials yielded consistent results. All of the model fits produced a flattened inner halo with c/a ? 0.5?0.6. The radial scale length of the disk, ?3.5 kpc, is higher than found in recent infrared surveys but agrees with older optical studies. The density normalizations to the plane for the thick disk and halo are consistent with previous work. Our model results all yielded a scale height for the thick disk of ?900 pc. The most surprising results from these global fits are relatively high values (>4 kpc) for the de Vaucouleurs radius for the halo and the radial scale length of the thick disk. The radial scale length for the thick disk is significantly larger than that for the old/thin disk and, if confirmed with additional work, may imply an independent origin for the thick disk. We also present evidence that the scale height and normalization of the thick disk may be variable with direction.

The Automated Plate Scanner Catalog of the Palomar Observatory Sky Survey. II. The Archived Database

The Minnesota Automated Plate Scanner Catalog of the POSS I has been available on-line since 1994. We are now archiving it for distribution to the national and international data centers. In this brief paper, we describe the calibration of the digitized data and the characteristics of the archived database.

Matched Filter Processing for Asteroid Detection

Matched filter (MF) processing has been shown to provide significant performance gains when processing stellar imagery used for asteroid detection, recovery, and tracking. This includes extending detection ranges to fainter magnitudes at the noise limit of the imagery and operating in dense cluttered star fields as encountered at low Galactic latitudes. The MF software has been shown to detect 40% more asteroids in high-quality Spacewatch imagery relative to the currently implemented approaches, which are based on moving target indicator (MTI) algorithms. In addition, MF detections were made in dense star fields and in situations in which the asteroid was collocated with a star in an image frame, cases in which the MTI algorithms failed. Thus, using legacy sensors and optics, improved detection sensitivity is achievable by simply upgrading the image-processing stream. This in turn permits surveys of the near-Earth asteroid (NEA) population farther from opposition, for smaller sizes, and in directions previously inaccessible to current NEA search programs. A software package has been developed and made available on the NASA data services Web site that can be used for asteroid detection and recovery operations utilizing the enhanced performance capabilities of MF processing.

The Search for Distant Objects in the Solar System Using Spacewatch

We have completed a low-inclination ecliptic survey for distant and slow-moving bright objects in the outer solar system. This survey used data taken over 34 months by the University of Arizonas Spacewatch Project based at Steward Observatory, Kitt Peak. Spacewatch revisits the same sky area every three to seven nights in order to track cohorts of main-belt asteroids. This survey used a multiple-night detection scheme to extend our rate sensitivity to as low as 0.012 arcsec hr-1. When combined with our plate scale and flux sensitivity (V ≈ 21), this survey was sensitive to Mars-sized objects out to 300 AU and Jupiter-sized planets out to 1200 AU. The survey covered approximately 8000 deg2 of raw sky, mostly within 10° of the ecliptic but away from the Galactic center. An automated motion-detection program was modified for this multinight search and processed approximately 2 terabytes of imagery into motion candidates. This survey discovered 2003 MW12, currently the tenth largest classical Kuiper Belt object. In addition, several known large Kuiper Belt objects and Centaurs were detected, and the detections were used with a model of our observational biases to make population estimates as a check on our survey efficiency. We found no large objects at low inclinations despite having sufficient sensitivity in both flux and rate to see them out as far as 1200 AU. For low inclinations, we can rule out more than one to two Pluto-sized objects out to 100 AU and one to two Mars-sized objects to 200 AU.

THE ASYMMETRIC THICK DISK: A STAR-COUNT AND KINEMATIC ANALYSIS. I. THE STAR COUNTS

We report a statistically significant asymmetry in the distribution of thick-disk and possibly inner halo stars interior to the solar orbit 1–2.5 kpc from the Sun. We have compared the star counts in the 120 POSS I fields, 40 each above and below the Galactic plane in quadrant I with the 40 complementary fields above the plane in quadrant IV. We find a spatially extended region in quadrant I with a significant excess, 20% to 25%, in the numbers of blue- and intermediate-colored stars. While the region of the asymmetric distribution is somewhat irregular in shape, it is also fairly uniform, stretching over several hundred square degrees on the sky. It is therefore a major substructure in the Galaxy due to more than small-scale clumpiness in the thick disk or inner halo. In this first paper, we describe the observations and our star-count and statistical analysis. We also discuss three possible explanations for the asymmetry: the fossil remnant of a merger, a triaxial thick disk or halo, and interaction of the thick-disk/inner halo stars with the bar in the disk.

Mapping the Asymmetric Thick Disk. III. The Kinematics and Interaction with the Galactic Bar

In the first two papers of this series, Larsen et?al. describe our faint CCD survey in the inner Galaxy and map the overdensity of thick disk stars in Quadrant 1 (Q1) to 5?kpc or more along the line of sight. The regions showing the strongest excess are above the density contours of the bar in the Galactic disk. In this third paper on the asymmetric thick disk, we report on radial velocities and derived metallicity parameters for over 4000 stars in Q1, above and below the plane, and in Quadrant 4 (Q4) above the plane. We confirm the corresponding kinematic asymmetry first reported by Parker et?al., extended to greater distances and with more spatial coverage. The thick disk stars in Q1 have a rotational lag of 60-70?km?s?1 relative to circular rotation, and the metal-weak thick disk stars have an even greater lag of 100?km?s?1. Both lag their corresponding populations in Q4 by 30?km?s?1. Interestingly, the disk stars in Q1 also appear to participate in the rotational lag by about 30?km?s?1. The enhanced rotational lag for the thick disk in Q1 extends to 4?kpc or more from the Sun. At 3-4?kpc, our sight lines extend above the density contours on the near side of the bar, and as our lines of sight pass directly over the bar the rotational lag appears to decrease. This is consistent with a gravitational wake induced by the rotating bar in the disk which would trap and pile up stars behind it. We conclude that a dynamical interaction with the stellar bar is the most probable explanation for the observed kinematic and spatial asymmetries.

Mapping the Asymmetric Thick Disk: The Hercules Thick-Disk Cloud

The stellar asymmetry of faint thick-disk/inner-halo stars in the first quadrant (l = 20°-45°) that was first reported by Larsen & Humphreys and investigated further by Parker et al. has recently been confirmed by the Sloan Digital Sky Survey (SDSS). Their interpretation of the excess in the star counts as a ringlike structure, however, is not supported by critical complementary data in the fourth quadrant, which is not covered by the SDSS. We present stellar density maps from the Minnesota Automated Plate Scanner Catalog of the POSS I showing that the overdensity does not extend into the fourth quadrant. The overdensity is most probably not a ring. It could be due to interaction with the disk bar, or it could be evidence of a triaxial thick disk or a merger remnant/stream. We call this feature the Hercules Thick-Disk Cloud.

Mapping the Asymmetric Thick Disk. I. A Search for Triaxiality

A significant asymmetry in the distribution of faint blue stars in the inner Galaxy, Quadrant 1 (l = 20°-45°) compared to Quadrant 4 was first reported by Larsen & Humphreys in 1996. Parker et al. greatly expanded the survey to determine its spatial extent and shape and the kinematics of the affected stars. This excess in the star counts was subsequently confirmed by Juric et al. using Sloan Digital Sky Survey data. Possible explanations for the asymmetry include a merger remnant, a triaxial thick disk, and a possible interaction with the bar in the disk. In this paper, we describe our program of wide field photometry to map the asymmetry to fainter magnitudes and therefore larger distances. To search for the signature of triaxiality, we extended our survey to higher Galactic longitudes. We find no evidence for an excess of faint blue stars at l ≥55° including the faintest magnitude interval. The asymmetry and star count excess in Quadrant 1 is thus not due to a triaxial thick disk.

Spacewatch Astrometry and Photometry of Near-Earth Objects

The Spacewatch Project uses four telescopes of apertures 0.9-m, 1.8-m, 2.3-m, and 4-m on Kitt Peak mountain in Arizona for followup astrometry of priority NEOs. Objects as faint as V=23 on the MPC’s NEO Confirmation Page, targets of radar, potential impactors, targets of spacecraft observations or visits, and PHAs with future close approaches to Earth receive priority for astrometry.