Keith S. Noll

Detection of ozone on Ganymede

An absorption band at 260 nanometers on the trailing hemisphere of Ganymede, identified as the Hartley band of ozone (O3), was measured with the Hubble Space Telescope. The column abundance of ozone, 4.5 × 1016 per square centimeter, can be produced by ion impacts or by photochemical equilibrium with previously detected molecular oxygen (O2). An estimated number density ratio of [O3]/[O2] ≈ 10−4 to 10−3 requires an atmospheric density orders of magnitude higher than upper limits from spacecraft occultation experiments. Apparently, this O2-O3 “atmosphere” is trapped in Ganymedes surface ice, an inference consistent with the shift and broadening of the band compared with the gas-phase O3 band.

Evidence for two populations of classical transneptunian objects: The strong inclination dependence of classical binaries

Abstract We have searched 101 Classical transneptunian objects for companions with the Hubble Space Telescope. Of these, at least 21 are binary. The heliocentric inclinations of the objects we observed range from 0.6°–34°. We find a very strong anticorrelation of binaries with inclination. Of the 58 targets that have inclinations of less than 5.5°, 17 are binary, a binary fraction of 29 ± 7 6 % . All 17 are similar-brightness systems. On the contrary, only 4 of the 42 objects with inclinations greater than 5.5° have satellites and only 1 of these is a similar-brightness binary. This striking dichotomy appears to agree with other indications that the low eccentricity, non-resonant Classical transneptunian objects include two overlapping populations with significantly different physical properties and dynamical histories.

The albedo spectrum of Europa from 2200 Å to 3300 Å

We obtained a spectrum of Europa from 2224 to 3302 A with high signal to noise and a resolution of 6.3 A with the Hubble space telescope faint object spectrograph. We detect a broad absorption feature centered at 2800 A, seen previously at lower signal to noise in International Ultraviolet Explorer spectra of Europa. No additional absorption features are detected. We compare the observed spectrum to laboratory spectra of sulfur ions in water ice and sulfur dioxide ice on water ice. The absorption in Europa is a closer match to sulfur dioxide ice. The poor fit of ice irradiated by sulfur ions may indicate that a direct source of SO2 is required to explain the observed absorption.

Detection of Abundant Carbon Monoxide in the Brown Dwarf Gliese 229B

We report the detection of carbon monoxide in the atmosphere of the brown dwarf Gliese 229B in spectra obtained at wavelengths near 4.7 μm. The spectrum is consistent with mole fractions of CO of more than 50 parts per million (ppm), much higher than expected for gas in thermochemical equilibrium at the temperature of the atmosphere we observe. As in Jupiter, mixing in the upper atmosphere of Gl 229B may prevent equilibrium between CO and CH4 at low temperatures. The average flux from 4.55 to 5.10 μm in Gl 229B is measured to be 3.6±0.2 mJy, lower than previous broadband measurements.

The correlated colors of transneptunian binaries

Abstract We report resolved photometry of the primary and secondary components of 23 transneptunian binaries obtained with the Hubble Space Telescope. V–I colors of the components range from 0.7 to 1.5 with a median uncertainty of 0.06 magnitudes. The colors of the primaries and secondaries are correlated with a Spearman rank correlation probability of 99.99991%, 5 sigma for a normal distribution. Fits to the primary vs. secondary colors are identical to within measurement uncertainties. The color range of binaries as a group is indistinguishable from that of the larger population of apparently single transneptunian objects. Whatever mechanism produced the colors of apparently single TNOs acted equally on binary systems. The most likely explanation is that the colors of transneptunian objects and binaries alike are primordial and indicative of their origin in a locally homogeneous, globally heterogeneous protoplanetary disk.

Detection of Six Trans-Neptunian Binaries with NICMOS: A High Fraction of Binaries in the Cold Classical Disk

We have analyzed a homogeneous set of observations of eighty-one transneptunian objects obtained with the NIC2 camera on the Hubble Space Telescope with the goal of identifying partially resolved binaries. Using PSF-fitting we have identified six likely binaries in addition to the three new binaries already found in this data set. We find that 11% of transneptunian objects are binaries at separation and brightness limits of the NIC2 camera. The identification of these new binaries significantly increases the known lower limit to the binary fraction among transneptunian objects. The origin of such a high fraction of binaries remains to be determined. Most interestingly, detectable binaries appear to be about four times more common among the cold classical disk than in the dynamically excited populations.We have analyzed a homogeneous set of observations of 81 trans-Neptunian objects obtained with NIC2 on the Hubble Space Telescope with the goal of identifying partially resolved binaries. Using PSF fitting we have identified six likely binaries in addition to the three new binaries already found in this data set. We find that 11% of trans-Neptunian objects are binaries at the separation and brightness limits of NIC2. The identification of these new binaries significantly increases the known lower limit to the binary fraction among trans-Neptunian objects. The origin of such a high fraction of binaries remains to be determined. Most interestingly, detectable binaries appear to be about 4 times more common among the cold classical disk than in the dynamically excited populations.

HD 98800: A unique stellar system of post-T tauri stars

HD 98800 is a system of four stars, and it has a large infrared excess that is thought to be due to a dust disk within the system. In this paper we present new astrometric observations made with Hipparcos, as well as photometry from Hubble Space Telescope WFPC2 images. Combining these observations and reanalyzing previous work allow us to estimate the age and masses of the stars in the system. Uncertainty in these ages and masses results from uncertainty in the temperatures of the stars and any reddening they may have. We find that HD 98800 is most probably about 10 Myr old, although it may be as young as 5 Myr or as old as 20 Myr old. The stars in HD 98800 appear to have metallicities that are about solar. An age of 10 Myr means that HD 98800 is a member of the post T Tauri class of objects, and we argue that the stars in HD 98800 can help us understand why post T Tauris have been so elusive, HD 98800 may have formed in the Centaurus star-forming region, but it is extraordinary in being so young and yet so far from where it was born.

Detection of SO2 on Callisto with the Hubble Space Telescope

We have detected SO2 in ultraviolet spectra of Callisto obtained with the Hubble Space Telescopes Faint Object Spectrograph. An absorption band centered at 280 nm appears in the spectrum of Callistos leading hemisphere, but is not apparent in the spectrum of the trailing hemisphere. The band is similar to the SO2 band on Europas trailing hemisphere. Callistos leading hemisphere spectrum can be well fit with models that include SO2 ice absorption with N(SO2) ≥ 6 × 1016 cm−2. Callistos leading hemisphere is modified by impacts with micrometeorites; this may directly or indirectly be a source of sulfur dioxide.

The Onset of Methane in L Dwarfs

We have detected weak absorption features produced by the strong ν3 methane band at 3.3 μm in two L dwarfs, 2MASSW J1507476-162738 and 2MASSI J0825196+211552, classified as spectral types L5 and L7.5, respectively. These absorptions occur in objects warmer than any in which methane previously has been detected and mark the first appearance of methane in the ultracool star-to-brown dwarf spectral sequence.

The origin and vertical distribution of carbon monoxide in Jupiter

Six clearly-resolved lines of the CO 1-0 vibration-rotation band near 4.7 microns have been observed in Jupiter at a resolution of 0.07/cm. CO is not found to be concentrated in the stratosphere, but is shown to be present in the troposphere at a mole fraction of 1.6 + or - 0.3 x 10 to the -9th, suggesting that rapid vertical mixing is the source of CO. Results indicate that the global oxygen abundance in Jupiters gaseous envelope below the cloud-forming regions must be near the solar value, and that intervening clouds with an optical depth of 0.5-4 are present above the line-forming region.