K. B. Clancy

The Deep Ecliptic Survey: A Search for Kuiper Belt Objects and Centaurs. II. Dynamical Classification, the Kuiper Belt Plane, and the Core Population

The Deep Ecliptic Survey (DES)?a search optimized for the discovery of Kuiper belt objects (KBOs) with the Blanco and Mayall 4 m telescopes at the Cerro Tololo Inter-American Observatory and Kitt Peak National Observatory?has covered 550 deg2 from its inception in 1998 through the end of 2003. This survey has a mean 50% sensitivity at VR magnitude 22.5. We report here the discoveries of 320 designated KBOs and Centaurs for the period 2000 March through 2003 December and describe improvements to our discovery and recovery procedures. Our data and the data products needed to reproduce our analyses in this paper are available through the NOAO survey database. Here we present a dynamical classification scheme, based on the behavior of orbital integrations over 10 Myr. The dynamical classes, in order of testing, are Resonant, Centaur, Scattered-Near, Scattered-Extended, and Classical. (These terms are capitalized when referring to our rigorous definitions.) Of the 382 total designated KBOs discovered by the DES, a subset of 196 objects have sufficiently accurate orbits for dynamical classification. Summary information is given for an additional 240 undesignated objects also discovered by the DES from its inception through the end of 2003. The number of classified DES objects (uncorrected for observational bias) are Classical, 96; Resonant, 54; Scattered-Near, 24; Scattered-Extended, 9; and Centaur, 13. We use subsets of the DES objects (which can have observational biases removed) and larger samples to perform dynamical analyses on the Kuiper belt. The first of these is a determination of the Kuiper belt plane (KBP), for which the Classical objects with inclinations less than 5? from the mean orbit pole yield a pole at R.A. = 27392 ? 062 and decl. = 6670 ? 020 (J2000), consistent with the invariable plane of the solar system. A general method for removing observational biases from the DES data set is presented and used to find a provisional magnitude distribution and the distribution of orbital inclinations relative to the KBP. A power-law model fit to the cumulative magnitude distribution of all KBOs discovered by the DES in the VR filter yields an index of 0.86 ? 0.10 (with the efficiency parameters for the DES fitted simultaneously with the population power law). With the DES sensitivity parameters fixed, we derive power-law indices of 0.74 ? 0.05, 0.52 ? 0.08, and 0.74 ? 0.15, respectively, for the Classical, Resonant, and Scattered classes. Plans for calibration of the DES detection efficiency function and DES magnitudes are discussed. The inclination distribution confirms the presence of hot and cold populations; when the geometric sin i factor is removed from the inclination distribution function, the cold population shows a concentrated core with a full width at half-maximum of approximately 46, while the hot population appears as a halo, extending beyond 30?. The inclination distribution is used to infer the KBO distribution in the sky, as a function of latitude relative to the KBP. This inferred latitude distribution is reasonably consistent with the latitude distribution derived from direct observation, but the agreement is not perfect. We find no clear boundary between the Classical and Scattered classes either in their orbital inclinations with respect to the KBP or in their power-law indices in their respective magnitude distributions. This leaves open the possibility that common processes have shaped the distribution of orbital parameters for the two classes.

The recent expansion of Pluto's atmosphere.

Stellar occultations—the passing of a relatively nearby body in front of a background star—can be used to probe the atmosphere of the closer body with a spatial resolution of a few kilometres (ref. 1). Such observations can yield the scale height, temperature profile, and other information about the structure of the occulting atmosphere. Occultation data acquired for Plutos atmosphere in 1988 revealed a nearly isothermal atmosphere above a radius of ∼1,215 km. Below this level, the data could be interpreted as indicating either an extinction layer or the onset of a large thermal gradient, calling into question the fundamental structure of this atmosphere. Another question is to what extent Plutos atmosphere might be collapsing as it recedes from the Sun (passing perihelion in 1989 in its 248-year orbital period), owing to the extreme sensitivity of the equilibrium surface pressure to the surface temperature. Here we report observations at a variety of visible and infrared wavelengths of an occultation of a star by Pluto in August 2002. These data reveal evidence for extinction in Plutos atmosphere and show that it has indeed changed, having expanded rather than collapsed, since 1988.

PROCEDURES, RESOURCES AND SELECTED RESULTS OF THE DEEP ECLIPTIC SURVEY

The Deep Ecliptic Survey is a project whose goal is to survey a largearea of the near-ecliptic region to a faint limiting magnitude (R ∼24) in search of objects in the outer solar system. We are collectinga large homogeneous data sample from the Kitt Peak Mayall 4-m and CerroTololo Blanco 4-m telescopes with the Mosaic prime-focus CCD cameras.Our goal is to collect a sample of 500 objects with good orbits to furtherour understanding of the dynamical structure of the outer solar system.This survey has been in progress since 1998 and is responsible for 272designated discoveries as of March 2003. We summarize our techniques,highlight recent results, and describe publically available resources.