Denise Catherine Stephens

Reflected Spectra and Albedos of Extrasolar Giant Planets. I. Clear and Cloudy Atmospheres

The reflected spectra of extrasolar giant planets are primarily influenced by Rayleigh scattering, molecular absorption, and atmospheric condensates. We present model geometric albedo and phase-integral spectra and Bond albedos for planets and brown dwarfs with masses between 0.8 and 70 Jupiter masses. Rayleigh scattering predominates in the blue while molecular absorption removes most red and infrared photons. Thus cloud-free atmospheres, found on giant planets with effective temperatures exceeding about 400 K, are quite dark in reflected light beyond 0.6 μm. In cooler atmospheres, first water clouds and then other condensates provide a bright reflecting layer. Only planets with cloudy atmospheres will be detectable in reflected light beyond 1 μm. Thermal emission dominates the near-infrared for warm objects with clear atmospheres. However the presence of other condensates, not considered here, may brighten some planets in reflected near-infrared light and darken them in the blue and UV. Bond albedos, the ratio of the total reflected to incident power, are sensitive to the spectral type of the primary. Most incident photons from early-type stars will be Rayleigh scattered, while most incident photons from late-type stars will be absorbed. The Bond albedo of a given planet thus may range from 0.4 to 0.05, depending on the primary type. Condensation of a water cloud may increase the Bond albedo of a planet by up to a factor of 2. The spectra of cloudy planets are strongly influenced by poorly constrained cloud microphysical properties, particularly particle size and supersaturation. Both Bond and geometric albedos are comparatively less sensitive to variations in planet mass and effective temperature.

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.

Physical Parameters of Two Very Cool T Dwarfs

We present new infrared spectra of the T8 brown dwarf 2MASS J04151954-0935066: 2.9-4.1 μm spectra obtained with the Infrared Camera and Spectrograph on the Subaru Telescope, and 5.2-14.5 μm spectra obtained with the Infrared Spectrograph on the Spitzer Space Telescope. We use these data and models to determine an accurate bolometric luminosity of log Lbol/L☉ = -5.67 and to constrain the effective temperature, gravity, mass, and age to 725-775 K, log g = 5.00-5.37, M = 33-58 MJup, and age = 3-10 Gyr. We perform the same analysis using published 0.6-15 μm spectra for the T7.5 dwarf 2MASS J12171110-0311131, for which we find a metal-rich composition ([Fe/H] ~ 0.3), and log Lbol/L☉ = -5.31, Teff = 850-950 K, log g = 4.80-5.42, M = 25-66 MJup, and age = 1-10 Gyr. These luminosities and effective temperatures straddle those determined with the same method and models for Gl 570D by Saumon et al. and make 2MASS J04151954-0935066 the coolest and least luminous T dwarf with well-determined properties. We find that synthetic spectra generated by the models reproduce the observed red through mid-infrared spectra of 2MASS J04151954-0935066 and 2MASS J12171110-0311131 very well, except for known discrepancies that are most likely due to the incomplete CH4 opacities. Both objects show evidence of departures from strict chemical equilibrium, and we discuss this result in the context of other late T dwarfs in which disequilibrium phenomena have been observed.

JHK Magnitudes for L and T Dwarfs and Infrared Photometric Systems

In the last few years a significant population of ultracool L and T dwarfs has been discovered. With effective temperatures ranging from ~2200 to 700 K, these objects emit most of their radiation in the near-IR, and their spectral energy distributions are dominated by strong molecular absorption bands. These highly structured energy distributions lead to JHK magnitudes that are extremely sensitive to the exact filter bandpass used. In the case of the T dwarfs, the differences between commonly used photometric systems can be as large as 0.4 mag at J and 0.5 mag at J–K. Near-IR magnitudes have been published for L and T dwarfs using a variety of photometric systems. Currently, the data obtained with these systems cannot be accurately compared or combined, as transformations based on the colors of hotter stars are not valid for L and T dwarfs. To address this problem, we have synthesized J, H, and K magnitudes for some of the common photometric systems and present transformation equations with respect to the most atmospheric-independent system, the Mauna Kea Observatory filter set. If the spectral type of the dwarf is known, our transformations allow data to be converted between systems to 0.01 mag, which is better than the typical measurement uncertainty. Transforming on the basis of color alone is more difficult because of the degeneracy and intrinsic scatter in the near-IR colors of L and T dwarfs; in this case J magnitudes can only be transformed to 0.05 mag and H and K to 0.02 mag.

Physical and Spectral Characteristics of the T8 and Later Type Dwarfs

We use newly observed and published near-infrared spectra, together with synthetic spectra obtained from model atmospheres, to derive physical properties of three of the latest type T dwarfs. A new R ≈ 1700 spectrum of the T7.5 dwarf HD 3651B, together with existing data, allows a detailed comparison to the well-studied and very similar dwarf Gl 570D. We find that HD 3651B has both higher gravity and higher metallicity than Gl 570D, with best-fit atmospheric parameters of Teff = 820-830 K, log g = 5.4-5.5, [m/H] = +0.2, and Kzz = 104 cm2 s-1. Its age is 8-12 Gyr, and its implied mass is 60-70 MJ. We perform a similar analysis of the T8 and T7.5 dwarfs 2MASS J09393548-2448279 and 2MASS J11145133-2618235 using published data, comparing them to the well-studied T8, 2MASS J04151954-0935066. We find that these two dwarfs have effectively the same Teff as the reference dwarf, and similar or slightly higher gravities, but lower metallicities. The derived parameters are Teff = 725-775 K and [m/H] = -0.3; log g = 5.3 - 5.45 for 2MASS J09393548-2448279 and log g = 5.0 - 5.3 for 2MASS J11145133-261823. The age and mass are ~10 Gyr and 60 MJ for 2MASS J09393548-2448279, and ~5 Gyr and 40 MJ for 2MASS J11145133-261823. A serious limitation to such analyses is the incompleteness of the line lists for transitions of CH4 and NH3 at λ ≤ 1.7 μm, which are also needed for synthesizing the spectrum of the later, cooler, Y type. Spectra of Saturn and Jupiter, and of laboratory CH4 and NH3 gas, suggest that NH3 features in the Y and J bands may be useful as indicators of the next spectral type, and not features in the H and K bands, as previously thought. However, until cooler objects are found, or the line lists improve, large uncertainties remain, as the abundance of NH3 is likely to be significantly below the chemical equilibrium value. Moreover, inclusion of laboratory NH3 opacities in our models predicts band shapes that are discrepant with existing data. It is possible that the T spectral class will have to be extended to temperatures around 400 K, when water clouds condense in the atmosphere and dramatically change the spectral energy distribution of the brown dwarf.

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.

Five New and Three Improved Mutual Orbits of Transneptunian Binaries

We present three improved and five new mutual orbits of transneptunian binary systems (58534) LogosZoe, (66652) Borasisi-Pabu, (88611) Teharonhiawako-Sawiskera, (123509) 2000 WK183, (149780) Altjira, 2001 QY297, 2003 QW111, and 2003 QY90 based on Hubble Space Telescope and Keck II laser guide star adaptive optics observations. Combining the five new orbit solutions with 17 previously known orbits yields a sample of 22 mutual orbits for which the period P, semimajor axis a, and eccentricity e have been determined. These orbits have mutual periods ranging from 5 to over 800 days, semimajor axes ranging from 1600 to 37,000 km, eccentricities ranging from 0 to 0.8, and system masses ranging from 2 � 10 17 to 2 � 10 22 kg. Based on the relative brightnesses of primaries and secondaries, most of these systems consist of near equal-sized pairs, although a few of the most massive systems are more lopsided. The observed distribution of orbital properties suggests that the most loosely-bound transneptunian binary systems are only found on dynamically cold heliocentric orbits. Of the 22 known binary mutual orbits, orientation ambiguities are now resolved for 9, of which 7 are prograde and 2 are retrograde, consistent with a random distribution of orbital orientations, but not with models predicting a strong preference for retrograde orbits. To the extent that other perturbations are not dominant, the binary systems undergo Kozai oscillations of their eccentricities and inclinations with periods of the order of tens of thousands to millions of years, some with strikingly high amplitudes.

The orbit, mass, size, albedo, and density of (65489) Ceto/Phorcys: A tidally-evolved binary Centaur

Abstract Hubble Space Telescope observations of Uranus- and Neptune-crossing object (65489) Ceto/Phorcys (provisionally designated 2003 FX128) reveal it to be a close binary system. The mutual orbit has a period of 9.554 ± 0.011 days and a semimajor axis of 1840 ± 48 km . These values enable computation of a system mass of ( 5.41 ± 0.42 ) × 10 18 kg . Spitzer Space Telescope observations of thermal emission at 24 and 70 μm are combined with visible photometry to constrain the systems effective radius ( 109 −11 +10 km ) and geometric albedo ( 0.084 −0.014 +0.021 ) . We estimate the average bulk density to be 1.37 −0.32 +0.66 g cm −3 , consistent with ice plus rocky and/or carbonaceous materials. This density contrasts with lower densities recently measured with the same technique for three other comparably-sized outer Solar System binaries (617) Patroclus, (26308) 1998 SM165, and (47171) 1999 TC36, and is closer to the density of the saturnian irregular satellite Phoebe. The mutual orbit of Ceto and Phorcys is nearly circular, with an eccentricity ⩽0.015. This observation is consistent with calculations suggesting that the system should tidally evolve on a timescale shorter than the age of the Solar System.

Mutual orbits and masses of six transneptunian binaries

Abstract We present Hubble Space Telescope observations of six binary transneptunian systems: 2000 QL 251 , 2003 TJ 58 , 2001 XR 254 , 1999 OJ 4 , (134860) 2000 OJ 67 , and 2004 PB 108 . The mutual orbits of these systems are found to have periods ranging from 22 to 137 days, semimajor axes ranging from 2360 to 10500 km, and eccentricities ranging from 0.09 to 0.55. These orbital parameters enable estimation of system masses ranging from 0.2 to 9.7 × 10 18 kg . For reasonable assumptions of bulk density (0.5 to 2.0 g cm −3 ), the masses can be combined with visible photometry to constrain sizes and albedos. The resulting albedos are consistent with an emerging picture of the dynamically “Cold” Classical sub-population having relatively high albedos, compared with comparably-sized objects on more dynamically excited orbits.