A NEW MID-INFRARED CAMERA FOR GROUND-BASED ASTRONOMY AND AN INFRARED STUDY OF PLANETARY NEBULAE

Abstract

ABSTRACT The dissertation is composed of two parts. The first part is a description of the Mid-Infrared Array Camera (MIRAC), a new camera for ground-based astronomy. The second part of the dissertation is an infrared study of planetary nebulae utilizing observations with the new camera. MIRAC is a collaborative effort between the University of Arizona, Smithsonian Astrophysical Observatory, and Naval Research Laboratory. It currently utilizes a Hughes 20 X 64 Si:As impurity-band-conduction (IBC) detector array with a capacitive transimpedance amplifier (CTIA) readout, which is sensitive to IR radiation from 2 to 26 microns. The camera is equipped with 10% bandiwidth filters at 2.2, 3.8, 4.6, 8.8, 9.8, 11.7, and 12.5 microns, and a wide-band 8.0-12.8 micron (N-band) filter. There is also a 20% filter at 20.5 microns and an 8-14 micron CVF with a resolution of 1.8%. An adjustable slide with several pupil stops allows the camera to be configured for telescopes with focal ratios ranging from 15 to 45. Two externally selectable magnifications are provided by the fully reflective optics, giving plate scales of 0.35" and 0.7" pixel-1 on the NASA IRTF. The MIRAC electronics provides timing signals and coadds successive frames at a maximum rate of 10 kHz for the full array, and higher rates for a partial array readout. the data are transferred via a serial interface to a PC for real-time display, storage, and further processing. MIRAC recently achieved a NEFD of 0.010 Jy arcsec-2 at 8.8, 11.7, and 12.5 microns for a 900-s on-source integration on the Steward Observatory 1.5-m telescope. Planetary nebulae (PNe) are formed from circumstellar material ejected during the late stages of stellar evolution. These nebulae are an important source of enrichment for the interstellar medium. In many PNe, there is an excess of emission in the IR, indicating the presence of dust. There are several different components seen in the IR emission, including a family of unidentified IR (UIR) emission features at 3.3, 6.2, 7.7, and 11.3 microns. The source morphology provides information on the structure of the nebula as well as the carriers of the emission features. Images in the near- and/or mid-IR are presented for the following PNe: IC 418, BD+30degree3639, J900, NGC 2392, NGC 6543, AFGL 2688, and M 2-9. The mid-IR images were obtained using MIRAC, and the images at J, H, and K were obtained using the Steward Observatory 64 X 64 Hg:Cd:Te array camera. In BD+30degree3639, the UIR emission is seen to be spatially distinct from the IR continuum and the ionized gas as traced by the 12.8 micron [Ne II] line emission. The average temperature across the nebula of the dust responsible for the mid-IR continuum emission is 172 K. In IC 418, the size of the nebula increases as a function of wavelength in the near-IR images at J, H, and K. Emission from the halo is detected in the H and K images. In NGC 2392 and NGC 6543, evidence for excess emission is seen in the distribution of the near-IR flux. The distribution of IR emission differs significantly from the emission observed at optical wavelengths. In the bipolar nebulae AFGL 2688 and M 2-9, structures in the IR emission are seen that could be related to the equatorial density enhancements that have caused the bipolar morphology.