Showing papers by "Joseph L. Hora published in 1998"

A Mid-Infrared Imaging Survey of Proto-Planetary Nebula Candidates

Abstract: We present the data from a mid-infrared (MIR) imaging survey of 66 proto-planetary nebula candidates using two MIR cameras (MIRAC2 and Berkcam) at the NASA Infrared Telescope Facility and the United Kingdom Infrared Telescope. The goal of this survey is to determine the size, flux, and morphology of the MIR emission regions, which sample the inner regions of the circumstellar dust shells of proto-planetary nebulae. We imaged these proto-planetary nebulae with narrowband filters(Δλ/λ~10%) at wavelengths of notable dust features. With our typical angular resolution of 1'', we resolve 17 sources, find 48 objects unresolved, and do not detect one source. For several sources we checked optical and infrared associations and positions of the sources. In table format, we list the size and flux measurements for all of the detected objects and show figures of all of the resolved sources. The proto-planetary nebula candidate sample includes, in addition to the predominant proto-planetary nebulae, extreme asymptotic giant branch stars, young planetary nebulae, a supergiant, and a luminous blue variable. We find that dust shells that are cooler (T~150 K) and brighter in the infrared are more easily resolved. Eleven of the seventeen resolved sources are extended and fall into one of two types of MIR morphological classes: core/elliptical or toroidal. Core/elliptical structures show unresolved cores with lower surface brightness elliptical nebulae. Toroidal structures show limb-brightened peaks suggesting equatorial density enhancements. We argue that core/ellipticals have denser dust shells than toroidals.

Characteristics of the Galileo Probe Entry Site From Earth-Based Remote Sensing Observations

Abstract: 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.

Imaging circumstellar environments with a nulling interferometer

Abstract: Extrasolar planets must be imaged directly if their nature is to be better understood. But this will be difficult, as the bright light from the parent star (or rather its diffracted halo in the imaging apparatus) can easily overwhelm nearby faint sources. Bracewell has proposed1 a way of selectively removing starlight before detection, by superposing the light from two telescopes so that the stellar wavefronts interfere destructively. Such a ‘nulling’ interferometer could be used in space to search for extrasolar Earth-like planets through their thermal emission and to determine through spectroscopic analysis if they possess the atmospheric signatures of life2,3,4. Here we report mid-infrared observations using two co-mounted telescopes of the Multiple Mirror Telescope that demonstrate the viability of this technique. Images of unresolved stars are seen to disappear almost completely, while light from a nearby source as close as 0.2 arcsec remains, as shown by images of Betelgeuse. With this star cancelled, there remains the thermal image of its surrounding, small dust nebula. In the future, larger ground-based interferometers that correct for atmospheric distortions (using adaptive optics) should achieve better cancellation, allowing direct detection of warm, Jupiter-size planets and faint zodiacal dust around other nearby stars5.

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

Abstract: 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.

Infrared array camera (IRAC) for the Space Infrared Telescope Facility (SIRTF)

Abstract: The Space Infrared Telescope Facility (SIRTF) contains three focal plane instruments, one of which is the Infrared Array Camera (IRAC). IRAC is a four-channel camera that provides simultaneous 5.12 × 5.12 arcmin images at 3.6, 4.5, 5.8, and 8 microns. The pixel size is 1.2 arcsec in all bands. Two adjacent fields of view in the SIRTF focal plane are viewed by the four channels in pairs (3.6 and 5.8 microns; 4.5 and 8.0 microns). All four detector arrays in the camera are 256 × 256 pixels in size, with the two short wavelength channels using InSb and the two longer wavelength channels using Si:As IBC detectors. The IRAC sensitivities (5 sigma, 200 sec) at 3.6, 4.5, 5.8, and 8.0 microns are 6, 7, 36, and 54 microJanskys, respectively. Two of the most important scientific objectives of IRAC will be to carry out surveys to study galaxy formation and evolution during the early stage of the Universe, and to search for brown dwarfs and superplanets.

Gemini near-infrared imager

Abstract: We discuss the main design features of the Gemini Near-IR Imager (NIRI) and its scientific capabilities NIRI is designed to fully exploit the excellent image quality and low telescope emissivity expected from the Gemini telescope on Mauna Kea It offers a range of pixel scales matched to different scientific objectives and has spectroscopic as well as polarimetric capabilities One of its main design features is the use of a near-IR 2 X 2 Shack-Hartmann wavefront sensor for tip-tilt and focus control

First results of nulling interferometry with the Multiple-Mirror Telescope

Abstract: We have successfully used nulling interferometry at 10 μm wavelength to interferometrically suppress a star's radiation. This technique was first proposed by Bracewell 20 years ago to image extra-solar planets and is now the basis for proposed space-borne instruments to search for Earth-like extra-solar planets and their spectroscopic signatures of habitability and life. In our experiment, the beams from two 1.8 m telescopes of the Multiple Mirror Telescope were brought into registration at a semi-transparent beamsplitter, and the images made coincident on an infrared array detector capable of taking rapid short exposure images. The atmospheric fluctuations caused the phase difference between the beams to fluctuate, changing the total flux of the star seen in the image plane. When the atmosphere caused the wavefronts to be exactly out of phase the entire stellar Airy pattern disappeared. For the unresolved star α Tauri the cancellation was such that a companion only 0.2 arcsec from the star and 25 times fainter would appear equal in intensity to the nulled star. The residual flux was spread into a wide halo suggesting the cause of this flux was imperfect cancellation of the aberrated wavefronts. To increase the precision of nulling beyond this first step several sources of error need to be addressed. We discuss the control of errors due to amplitude, polarization, chromatic differences, stellar leak, and sampling time. Improvements such as active phase tracking, adaptive optics, and cooled optics will increase the achievable gain of nulling interferometry and allow it to be used on fainter objects.

Precision slit design and fabrication for the Subaru IRCS instrument

Abstract: The IR Camera and Spectrograph for the Subaru telescope uses a series of reflective and transmissive slits. The width of the slits ranges from 48 micrometers to 440 micrometers . The requirements for both types of slits include sharp edge definition, good surface figure at cryogenic temperature and high reflectivity. Several different substrate materials and fabrication methods were investigated. The substrate materials considered include aluminum, copper, tungsten carbide, chromium carbide, and sapphire. The fabrication methods investigated include photo-etching micro machining using UV laser, electroforming, diamond turning, conventional polishing and electrical-discharge-machining. The pros and cons of each material and fabrication method will be described.

Mid-infrared (8è21 micron) imaging of protoèplanetary nebulae

Abstract: We present mid-infrared (8E21 km) images of thermal dust emission from two protoEplanetary nebulae (PPNs), IRAS 07134)1005 and IRAS 22272)5435, which show a strong 21 km 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 km and 21 km feature car- riers. In both sources, the dust temperatures range from D160E200 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%E5% of the carbon in these objects may be in the form of large PAH molecules. We construct optically thin, axially symmetric cylin- drical shell models to simulate the observed mid-IR morphologies and spectra, and calculate nebular masses of 0.26 for IRAS 07134 and 0.42 for IRAS 22272. Although the mid-IR emission pri- M _ M _ marily comes from warm (T B 190 K) dust, our models require a signi-cant cooler dust (T B 80 K) component to -t the observed mid- and far-IR spectral energy distributions. Subject headings: dust, extinction E infrared: ISM: continuum E ISM: abundances E planetary nebulae: general