Aditya Dayal

Characteristics of the Galileo probe entry site from Earth‐based remote sensing observations

A reassessment of ground-based observations confirms to better than a 98% confidence level that the Galileo probe entered a 5-μm hot spot, a region of unusual clarity and dryness, some 900±300 km north of its southern boundary. Cloud conditions at that point were similar to those in the center of this region, some 600 km further north. At the time of the probe entry, the region was evolving to a slightly larger size and even thinner cloud conditions, as evidenced by its rapidly brightening appearance at 4.78 μm. The low reflectivity of the region in red light is highly anticorrelated with 4.78-μm thermal emission, but this correlation breaks down in the blue. In general, the reflectivity of most hot spots is remarkably uniform, although the 4.78-μm thermal emission is highly variable. A cloud structure most consistent with both the observed reflected sunlight and thermal emission properties consists of two layers: (1) a cloud layer above the 450-mbar level extending up to the 150-mbar level that probably consists of submicron sized particles and (2) a tropospheric cloud that is probably below the 1-bar level, possibly ammonia hydrosulfide, with low optical thickness in the infrared. A population of particles larger than ∼3 μm, clearly present at the NH3 ice cloud level outside hot spots, is absent inside them. The NH3 gas abundance near 300–400 mbar pressure does not appear to be unusually depleted in hot spots. Zonal structures in the tropospheric temperature field near the probe entry site were not correlated with the location of 5-μm hot spots but moved at speeds closer to the internal rotation rate of the planet. The properties of the tropospheric thermal waves at the probe entry latitude show little correlation to the properties of the 5-μm hot spot waves. Temperatures at the probe entry site derived from remote sensing are warmer than the Atmospheric Structure Instrument (ASI) experiment results near the tropopause, probably because the low-temperature ASI features are confined to regions smaller than the ∼6000-km resolution characteristic of the remote sensing.

Revealing the Photodissociation Region: HST/NICMOS Imaging of NGC 7027

We report results from a Hubble Space Telescope (HST) and Near-Infrared Camera and Multiobject Spectrometer (NICMOS) program to study the distribution of hot neutral (molecular hydrogen) and ionized circumstellar material in the young planetary nebulae NGC 7027. HST/NICMOS provided very high spatial resolution imaging in line and continuum emission, and the stability and large dynamic range needed for investigating detailed structures in the circumstellar material. We present dramatic new images of NGC 7027 that have led to a new understanding of the structure in this important planetary nebula. The central star is clearly revealed, providing near-infrared fluxes that are used to directly determine the stellar temperature very accurately (T = 198,000 K). It is found that the photodissociation layer as revealed by near-infrared molecular hydrogen emission is very thin (ΔR ~ 6 × 1015 cm) and is biconical in shape. The interface region is structured and filamentary, suggesting the existence of hydrodynamic instabilities. We discuss evidence for the presence of one or more highly collimated, off-axis jets that might be present in NGC 7027. The NICMOS data are combined with earlier Hubble Space Telescope data to provide a complete picture of NGC 7027 using the highest spatial resolution data to date. The evolutionary future of NGC 7027 is discussed.

Earth-Based Observations of the Galileo Probe Entry Site

Earth-based observations of Jupiter indicate that the Galileo probe probably entered Jupiters atmosphere just inside a region that has less cloud cover and drier conditions than more than 99 percent of the rest of the planet. The visual appearance of the clouds at the site was generally dark at longer wavelengths. The tropospheric and stratospheric temperature fields have a strong longitudinal wave structure that is expected to manifest itself in the vertical temperature profile.

Mid-Infrared (8-21 micron) Imaging of Proto-Planetary Nebulae

We present mid-infrared (8-21 μm) images of thermal dust emission from two proto-planetary nebulae (PPNs), IRAS 07134+1005 and IRAS 22272+5435, which show a strong 21 μm emission feature. Both of the sources are well resolved and show evidence for axial symmetry. From our images, we calculate temperature and optical depth maps and estimate the abundance of the 11 μm and 21 μm feature carriers. In both sources, the dust temperatures range from ~160-200 K. The optical depths in IRAS 07134 are about a factor of 3 lower than those in IRAS 22272, but the emission is optically thin in both sources. Our analyses of the feature-to-continuum ratios suggests that 0.5%-5% of the carbon in these objects may be in the form of large PAH molecules. We construct optically thin, axially symmetric cylindrical shell models to simulate the observed mid-IR morphologies and spectra, and calculate nebular masses of 0.26 M☉ for IRAS 07134 and 0.42 M☉ for IRAS 22272. Although the mid-IR emission primarily comes from warm (T ≈ 190 K) dust, our models require a significant cooler dust (T ≈ 80 K) component to fit the observed mid- and far-IR spectral energy distributions.

The Etched Hourglass Nebula MyCn 18. I. HUBBLE SPACE TELESCOPE Observations

We have obtained emission-line and continuum images of the young planetary nebula MyCn 18 with the Wide Field Planetary Camera 2 on the Hubble Space Telescope (HST). Although from the ground MyCn 18 appeared to have a triple-ring structure similar to SN 1987A, the HST images show that MyCn 18 has an overall hourglass shape. A series of arcs appear to be etched on the walls of the hourglass near its rims. In the complex central region of the nebula we find a small, inner hourglass structure and two rings. Ring 1 is a bright elliptical ring, and ring 2 a smaller, higher excitation ring. The outer and inner hourglass, and ring 1 and ring 2, all have different centers, and none are coincident with the central star. The hourglass shape of the main nebula is consistent with the predictions of the generalized interacting-winds hypothesis for planetary nebula formation. However, the complex inner nebular structure of MyCn 18 and the offset of the central star from the center of the nebula remain a mystery. We discuss several mechanisms for producing the offset of the central star. Although none are found to be completely satisfactory, those involving a binary central star probably offer the best hope of successful explanation.

Subarcsecond Mid-Infrared Structure of the Dust Shell around IRAS 22272+5435*

We report sub-arcsecond imaging of extended mid-infrared emission from a proto-planetary nebula (PPN), \iras 22272+5435, performed at the MMT observatory with its newly upgraded 6.5 m aperture telescope and at the Keck observatory. The mid-infrared emission structure is resolved into two emission peaks separated by

INFRARED OBSERVATIONS OF DUST EMISSION FROM COMET HALE-BOPP

0\arcsec.5 - 0\arcsec.6

MIRAC2: a mid-infrared array camera for astronomy

in the MMT 11.7

The abundance distribution of C4H in IRC + 10216

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The Etched Hourglass Nebula MyCn 18. II. A Spatio-kinematic Model

image and in the Keck 7.9, 9.7, and 12.5