arXiv subjects

Imke de Pater

Publications and source records attributed to Imke de Pater.

At least 109 records · Page 6Linked to original sources

arXiv2015

Post-equinox dynamics and polar cloud structure on Uranus

Post equinox imaging of Uranus by HST, Keck, and Gemini telescopes has enabled new measurements of winds over previously sampled latitudes as well as measurements at high northern latitudes that have recently come into better view. These new observations also used techniques to greatly improve signal to noise ratios, making possible the detection and tracking of more subtle cloud features. The 250 m/s prograde jet peaking near 60 N was confirmed and more accurately characterized. Several long-lived cloud features have also been tracked. The winds pole-ward of 60 N are consistent with solid body rotation at a westward (prograde) rate of 4.3 deg/h with respect to Uranus' interior. When combined with 2007 and other recent measurements, it is clear that a small but well-resolved asymmetry exists in the zonal profile at middle latitudes, peaking at 35 deg, where southern winds are 20 m/s more westward than corresponding northern winds. High S/N Keck II imaging of the north polar region of Uranus reveals a transition from streaky bands below 60 N to a region from 60 deg to nearly the north pole, where widely distributed small bright spots, resembling cumulus cloud fields, with several isolated dark spots, are the dominant style of cloud features. This presents a stark contrast to 2003 detailed views of the south polar region of Uranus when no discrete cloud features could be detected in comparable Keck II near-IR images. The pressure levels of discrete clouds estimated from spatial modulations in H and Hcont images indicate that the polar cloud features are generally in the 1.3 to 2-3 bar range, as are equatorial and several mid-latitude features. Several of the brighter mid latitude features are found above the 1.2-bar level of methane condensation.

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arXiv2015

A nearby M star with three transiting super-Earths discovered by K2

Small, cool planets represent the typical end-products of planetary formation. Studying the archi- tectures of these systems, measuring planet masses and radii, and observing these planets' atmospheres during transit directly informs theories of planet assembly, migration, and evolution. Here we report the discovery of three small planets orbiting a bright (Ks = 8.6 mag) M0 dwarf using data collected as part of K2, the new transit survey using the re-purposed Kepler spacecraft. Stellar spectroscopy and K2 photometry indicate that the system hosts three transiting planets with radii 1.5-2.1 R_Earth, straddling the transition region between rocky and increasingly volatile-dominated compositions. With orbital periods of 10-45 days the planets receive just 1.5-10x the flux incident on Earth, making these some of the coolest small planets known orbiting a nearby star; planet d is located near the inner edge of the system's habitable zone. The bright, low-mass star makes this system an excellent laboratory to determine the planets' masses via Doppler spectroscopy and to constrain their atmospheric compositions via transit spectroscopy. This discovery demonstrates the ability of K2 and future space-based transit searches to find many fascinating objects of interest.

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arXiv2015

Methane depletion in both polar regions of Uranus inferred from HST/STIS and Keck/NIRC2 observations

From STIS observations of Uranus in 2012, we found that the methane volume mixing ratio declined from about 4% at low latitudes to about 2% at 60 deg N and beyond. This is similar to that found in the south polar regions in 2002, in spite of what appears to be strikingly different convective activity in the two regions. Keck and HST imaging observations close to equinox imply that the depletions were simultaneously present in 2007, suggesting they are persistent features. The depletions appear to be mainly restricted to the upper troposphere, with depth increasing poleward from about 30 deg N, reaching ~4 bars at 45 deg N and perhaps much deeper at 70 deg N. The latitudinal variations in degree and depth of the depletions are important constraints on models of meridional circulation. Our observations are qualitatively consistent with previously suggested circulation cells in which rising methane-rich gas at low latitudes is dried out by condensation and sedimentation of methane ice particles as the gas ascends to altitudes above the methane condensation level, then is transported to high latitudes, where it descends and brings down methane depleted gas. Since this cell would seem to inhibit formation of condensation clouds in regions where clouds are actually inferred from spectral modelling, it suggests that sparse localized convective events may be important in cloud formation. The small-scale latitudinal variations we found in the effective methane mixing ratio between 55 deg N and 82 deg N have significant inverse correlations with zonal mean latitudinal variations in cloud reflectivity in near-IR Keck images taken before and after the HST observations. If the CH4/H2 absorption ratio variations are interpreted as local variations in para fraction instead of methane mixing ratio, we find that downwelling correlates with reduced cloud reflectivity.

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arXiv2015

Record-breaking Storm Activity on Uranus in 2014

In spite of an expected decline in convective activity following the 2007 equinox of Uranus, eight sizable storms were detected on the planet with the near-infrared camera NIRC2, coupled to the adaptive optics system, on the 10-m W. M. Keck telescope on UT 5 and 6 August 2014. All storms were on Uranus's northern hemisphere, including the brightest storm ever seen in this planet at 2.2 $μ$m, reflecting 30% as much light as the rest of the planet at this wavelength. The storm was at a planetocentric latitude of $\sim$15$^{\circ}$N and reached altitudes of $\sim$330 mbar, well above the regular uppermost cloud layer (methane-ice) in the atmosphere. A cloud feature at a latitude of 32$^{\circ}$N, that was deeper in the atmosphere (near $\sim$2 bar), was later seen by amateur astronomers. We also present images returned from our HST ToO program, that shows both of these cloud features. We further report the first detection of a long-awaited haze over the north polar region.

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arXiv2013

Dispersion in Neptune's Zonal Wind Velocities from NIR Keck AO Observations in July 2009

We report observations of Neptune made in H-(1.4-1.8 μm) and K'-(2.0-2.4 μm) bands on 14 and 16 July 2009 from the 10-m W.M. Keck II Telescope using the near-infrared camera NIRC2 coupled to the Adaptive Optics (AO) system. We track the positions of 54 bright atmospheric features over a few hours to derive their zonal and latitudinal velocities, and perform radiative transfer modeling to measure the cloud-top pressures of 50 features seen simultaneously in both bands. We observe one South Polar Feature (SPF) on 14 July and three SPFs on 16 July at ~65 deg S. The SPFs observed on both nights are different features, consistent with the high variability of Neptune's storms. There is significant dispersion in Neptune's zonal wind velocities about the smooth Voyager wind profile fit of Sromovsky et al., Icarus 105, 140 (1993), much greater than the upper limit we expect from vertical wind shear, with the largest dispersion seen at equatorial and southern mid-latitudes. Comparison of feature pressures vs. residuals in zonal velocity from the smooth Voyager wind profile also directly reveals the dominance of mechanisms over vertical wind shear in causing dispersion in the zonal winds. Vertical wind shear is not the primary cause of the difference in dispersion and deviation in zonal velocities between features tracked in H-band on 14 July and those tracked in K'-band on 16 July. Dispersion in the zonal velocities of features tracked over these short time periods is dominated by one or more mechanisms, other than vertical wind shear, that can cause changes in the dispersion and deviation in the zonal velocities on timescales of hours to days.

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arXiv2013

Neptune at Summer Solstice: Zonal Mean Temperatures from Ground-Based Observations 2003-2007

Imaging and spectroscopy of Neptune's thermal infrared emission is used to assess seasonal changes in Neptune's zonal mean temperatures between Voyager-2 observations (1989, heliocentric longitude Ls=236) and southern summer solstice (2005, Ls=270). Our aim was to analyse imaging and spectroscopy from multiple different sources using a single self-consistent radiative-transfer model to assess the magnitude of seasonal variability. Globally-averaged stratospheric temperatures measured from methane emission tend towards a quasi-isothermal structure (158-164 K) above the 0.1-mbar level, and are found to be consistent with spacecraft observations of AKARI. This remarkable consistency, despite very different observing conditions, suggests that stratospheric temporal variability, if present, is $\pm$5 K at 1 mbar and $\pm$3 K at 0.1 mbar during this solstice period. Conversely, ethane emission is highly variable, with abundance determinations varying by more than a factor of two. The retrieved C2H6 abundances are extremely sensitive to the details of the T(p) derivation. Stratospheric temperatures and ethane are found to be latitudinally uniform away from the south pole (assuming a latitudinally-uniform distribution of stratospheric methane). At low and midlatitudes, comparisons of synthetic Voyager-era images with solstice-era observations suggest that tropospheric zonal temperatures are unchanged since the Voyager 2 encounter, with cool mid-latitudes and a warm equator and pole. A re-analysis of Voyager/IRIS 25-50 μm mapping of tropospheric temperatures and para-hydrogen disequilibrium suggests a symmetric meridional circulation with cold air rising at mid-latitudes (sub-equilibrium para-H2 conditions) and warm air sinking at the equator and poles (super-equilibrium para-H2 conditions). The most significant atmospheric changes are associated with the polar vortex (absent in 1989).

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arXiv2013

Constraining the Origins of Neptune's Carbon Monoxide Abundance with CARMA Millimeter-wave Observations

We present observations of Neptune's 1- and 3-mm spectrum from the Combined Array for Research in Millimeter-wave Astronomy (CARMA). Radiative transfer analysis of the CO (2-1) and (1-0) rotation lines was performed to constrain the CO vertical abundance profile. We find that the data are well matched by a CO mole fraction of 0.1^+0.2_-0.1 parts per million (ppm) in the troposphere, and 1.1^+0.2_-0.3 ppm in the stratosphere. A flux of 0.5-20 times 10^8 CO molecules cm-2 s-1 to the upper stratosphere is implied. Using the Zahnle et al. (2003) estimate for cometary impact rates at Neptune, we calculate the CO flux that could be formed from (sub)kilometer-sized comets; we find that if the diffusion rate near the tropopause is small (200 cm2 s-1), these impacts could produce a flux as high as 0.5^+0.8_-0.4 times 10^8 CO molecules cm-2 s-1. We also revisit the calculation of Neptune's internal CO contribution using revised calculations for the CO ->CH4 conversion timescale in the deep atmosphere (Visscher et al. 2011). We find that an upwelled CO mole fraction of 0.1 ppm implies a global O/H enrichment of at least 400, and likely more than 650, times the protosolar value.

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arXiv2012

The changing rotational excitation of C_3 in comet 9P/Tempel 1 during Deep Impact

The 4050Å band of C$_3$ was observed with Keck/HIRES echelle spectrometer during the {\em Deep Impact} encounter. We perform a 2-dimensional analysis of the exposures in order to study the spatial, spectral, and temporal changes in the emission spectrum of C$_3$. The rotational population distribution changes after impact, beginning with an excitation temperature of $\sim$45\,K at impact and increasing for 2\,hr up to a maximum of 61$\pm$5\,K. From 2 to 4 hours after impact, the excitation temperature decreases to the pre-impact value. We measured the quiescent production rate of C$_3$ before the encounter to be 1.0 $\times 10^{23}$ s$^{-1}$, while 2 hours after impact we recorded a peak production rate of 1.7 $\times 10^{23}$ s$^{-1}$. Whereas the excitation temperature returned to the pre-impact value during the observations, the production rate remained elevated, decreasing slowly, until the end of the 4\,hr observations. These results are interpreted in terms of changing gas densities in the coma and short-term changes in the primary chemical production mechanism for C$_{3}$.

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arXiv2011

The Allen Telescope Array Search for Electrostatic Discharges on Mars

The Allen Telescope Array was used to monitor Mars between 9 March and 2 June 2010, over a total of approximately 30 hours, for radio emission indicative of electrostatic discharge. The search was motivated by the report from Ruf et al. (2009) of the detection of non-thermal microwave radiation from Mars characterized by peaks in the power spectrum of the kurtosis, or kurtstrum, at 10 Hz, coinciding with a large dust storm event on 8 June 2006. For these observations, we developed a wideband signal processor at the Center for Astronomy Signal Processing and Electronics Research (CASPER). This 1024-channel spectrometer calculates the accumulated power and power-squared, from which the spectral kurtosis is calculated post-observation. Variations in the kurtosis are indicative of non-Gaussianity in the signal, which can be used to detect variable cosmic signals as well as radio frequency interference (RFI). During the three month period of observations, dust activity occurred on Mars in the form of small-scale dust storms, however no signals indicating lightning discharge were detected. Frequent signals in the kurtstrum that contain spectral peaks with an approximate 10 Hz fundamental were seen at both 3.2 and 8.0 GHz, but were the result of narrowband RFI with harmonics spread over a broad frequency range.

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arXiv2010

Observations of a Stationary Mid-Latitude Cloud System on Titan

We report the observation of a cloud system on Titan that remained localized near 40S latitude and 60W longitude for at least 34 hours. Ground-based observations obtained with the SINFONI imaging spectrograph at the Very Large Telescope over 4 consecutive nights recorded the lifetime and altitude of the unresolved cloud system. Concomitant measurements made by Cassini/VIMS over 3 hours resolved changes in the altitude and opacity of individual regions within the system during this time. Clouds are measured from 13 to 37 km altitude with optical depths per pixel ranging from $τ$=0.13 to 7. Short timescale rise times are consistent with previous measurements of the evolution of mid-latitude clouds; however the long timescale localization of the cloud structure is unexplained. We speculate about the role of meso-scale circulation in relation to cloud formation.

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arXiv2009

Component-resolved Near-infrared Spectra of the (22) Kalliope System

We observed (22) Kalliope and its companion Linus with the integral-field spectrograph OSIRIS, which is coupled to the adaptive optics system at the W.M. Keck II telescope on March 25 2008. We present, for the first time, component-resolved spectra acquired simultaneously in each of the Zbb (1-1.18 um), Jbb (1.18-1.42 um), Hbb (1.47-1.80 um), and Kbb (1.97-2.38 um) bands. The spectra of the two bodies are remarkably similar and imply that both bodies were formed at the same time from the same material; such as via incomplete re-accretion after a major impact on the precursor body.

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arXiv2008

A shift in Jupiter's equatorial haze distribution imaged with the Multi-Conjugate Adaptive Optics Demonstrator at the VLT

Jupiter was imaged during the Science Demonstration of the MCAO Demonstrator (MAD) at the European Southern Observatory's UT3 Very Large Telescope unit. Io and Europa were used as natural guide stars on either side of Jupiter, separated from each other by about 1.6 arcmin from 23:41 to 01:32 UT (2008 Aug 16/17). The corrected angular resolution was 0.090 arcsec across the entire field of view, as measured on background stars. The observations at 2.02, 2.14, and 2.16 micrometers were sensitive to portions of the Jovian spectrum with strong methane absorption. The data probe the upper troposphere, which is populated with a fine (~0.5 micrometer) haze. Two haze sources have been proposed: lofting of fine cloud particles into the stable upper troposphere, and condensation of hydrazine produced via ammonia photochemistry. The upper tropospheric haze is enhanced over Jupiter's equatorial region. Dramatic changes in the underlying cloud cover--part of the 2006/2007 "global upheaval"--may be associated with changes in the equatorial haze distribution now evident in the 2008 MAD images. Haze reflectivity peaked at 5 degrees N in HST/NICMOS data from 2005, but it now peaks at the equator. The observations suggest that haze variation is controlled by particle size, cloud source variation, diffusion, and horizontal transport.

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arXiv2008

The Distribution, Excitation and Formation of Cometary Molecules: Methanol, Methyl Cyanide and Ethylene Glycol

We present an interferometric and single dish study of small organic species toward Comets C/1995 O1 (Hale-Bopp) and C/2002 T7 (LINEAR) using the BIMA interferometer at 3 mm and the ARO 12m telescope at 2 mm. For Comet Hale-Bopp, both the single-dish and interferometer observations of CH3OH indicate an excitation temperature of 105+/-5 K and an average production rate ratio Q(CH3OH)/Q(H2O)~1.3% at ~1 AU. Additionally, the aperture synthesis observations of CH3OH suggest a distribution well described by a spherical outflow and no evidence of significant extended emission. Single-dish observations of CH3CN in Comet Hale-Bopp indicate an excitation temperature of 200+/-10 K and a production rate ratio of Q(CH3CN)/Q(H2O)~0.017% at ~1 AU. The non-detection of a previously claimed transition of cometary (CH2OH)2 toward Comet Hale-Bopp with the 12m telescope indicates a compact distribution of emission, D<9'' (<8500 km). For the single-dish observations of Comet T7 LINEAR, we find an excitation temperature of CH3OH of 35+/-5 K and a CH3OH production rate ratio of Q(CH3OH)/Q(H2O)~1.5% at ~0.3 AU. Our data support current chemical models that CH3OH, CH3CN and (CH2OH)2 are parent nuclear species distributed into the coma via direct sublimation off cometary ices from the nucleus with no evidence of significant production in the outer coma.

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arXiv2007

Temporal behavior of the SO 1.707 micron ro-vibronic emission band in Io's atmosphere

We report observations of the ro-vibronic transition of SO at 1.707 microns on Io. These data were taken while Io was eclipsed by Jupiter, on four nights between July 2000 and March 2003. We analyze these results in conjunction with a previously published night to investigate the temporal behavior of these emissions. The observations were all conducted using the near-infrared spectrometer NIRSPEC on the W.M. Keck II telescope. The integrated emitted intensity for this band varies from 0.8 x 10^27 to 2.4 x 10^27 photons/sec, with a possible link to variations in Loki's infrared brightness. The band-shapes imply rotational temperatures of 550-1000K for the emitting gas, lending further evidence to a volcanic origin for sulfur monoxide. An attempt to detect the ro-vibronic transition of SO at 0.97 microns was unsuccessful; simultaneous detection with the 1.707 micron band would permit determination of the SO column abundance.

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arXiv2006

Longitude-resolved imaging of Jupiter at lambda = 2 cm

We present a technique for creating a longitude-resolved image of Jupiter's thermal radio emission. The technique has been applied to VLA data taken on 25 January 1996 at a wavelength of 2 cm. A comparison with infrared data shows a good correlation between radio hot spots and the 5 micron hot spots seen on IRTF images. The brightest spot on the radio image is most likely the hot spot through which the Galileo probe entered Jupiter's atmosphere. We derived the ammonia abundance (= volume mixing ratio) in the hot spot, which is ~ 3 x 10^{-5}, about half that seen in longitude-averaged images of the NEB, or less than 1/3 of the longitude-averaged ammonia abundance in the EZ. This low ammonia abundance probably extends down to at least the 4 bar level.

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arXiv2006

A low density of 0.8 g/cc for the Trojan binary asteroid 617 Patroclus

The Trojan population consists of two swarms of asteroids following the same orbit as Jupiter and located at the L4 and L5 Lagrange points of the Jupiter-Sun system (leading and following Jupiter by 60 degrees). The asteroid 617 Patroclus is the only known binary Trojan (Merline et al. 2001). The orbit of this double system was hitherto unknown. Here we report that the components, separated by 680 km, move around the system centre of mass, describing roughly a circular orbit. Using the orbital parameters, combined with thermal measurements to estimate the size of the components, we derive a very low density of 0.8 g/cc. The components of Patroclus are therefore very porous or composed mostly of water ice, suggesting that they could have been formed in the outer part of the solar system.

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arXiv2006

A Bima Array Survey of Molecules in Comets Linear (C/2002 T7) and Neat (C/2001 Q4)

We present an interferometric search for large molecules, including methanol, methyl cyanide, ethyl cyanide, ethanol, and methyl formate in comets LINEAR (C/2002 T7) and NEAT (C/2001 Q4) with the Berkeley-Illinois-Maryland Association (BIMA) array. In addition, we also searched for transitions of the simpler molecules CS, SiO, HNC, HN13C and 13CO . We detected transitions of methanol and CS around Comet LINEAR and one transition of methanol around Comet NEAT within a synthesized beam of ~20''. We calculated the total column density and production rate of each molecular species using the variable temperature and outflow velocity (VTOV) model described by Friedel et al.(2005).Considering the molecular production rate ratios with respect to water, Comet T7 LINEAR is more similar to Comet Hale-Bopp while Comet Q4 NEAT is more similar to Comet Hyakutake. It is unclear, however, due to such a small sample size, whether there is a clear distinction between a Hale-Bopp and Hyakutake class of comet or whether comets have a continuous range of molecular production rate ratios.

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arXiv2005

BIMA Array Detections of HCN in Comets LINEAR (C/2002 T7) and NEAT (C/2001 Q4)

We present interferometric detections of HCN in comets LINEAR (C/2002 T7) and NEAT (C/2001 Q4) with the Berkeley-Illinois-Maryland Association (BIMA) Array. With a 25.4"x20.3" synthesized beam around Comet LINEAR and using a variable temperature and outflow velocity (VTOV) model, we found an HCN column density of N_T=6.4+-2.1x10^12 cm^-2, and a production rate of Q(HCN)=6.5+-2.2x10^26 s^-1, giving a production rate ratio of HCN relative to H_2O of ~3.3+-1.1x10^-3 and relative to CN of ~4.6+-1.5. With a 21.3"x17.5" synthesized beam around Comet NEAT and using a VTOV model, we found an HCN column density of N_T=8.5+-4.5x10^11 cm^-2, and a production rate of Q(HCN)=8.9+-4.7x10^25 s^-1, giving a production rate ratio of HCN relative to H_2O of ~7.4+-3.9x10^-4 and relative to CN of ~0.3+-0.2. For both comets, the production rates relative to H_2O are similar to those found in previous comet observations. For Comet LINEAR the production rate relative to CN is consistent with HCN being the primary parent species of CN, while for Comet NEAT it is too low for this to be the case.

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