Mid-infrared observations of photodissociation regions and MIRSI: A Mid-infraRed Spectrometer and Imager developed for ground based observing.

摘要

The structure of high-mass, star-forming cloud complexes containing photodissociation regions (PDRs) viewed edge-on is examined through 8.7–20.6 μm images and 8–13 μm spectra. For a homogeneous, neutral cloud illuminated by a bright OB star, PDR theory predicts that the ultra-violet (UV) radiation is attenuated exponentially (e−1.8A_ν). The predicted UV attenuation is confirmed by observations of broad emission features found at 8.6, 11.2, and 12.7 μm, commonly attributed to emission from polycyclic aromatic hydrocarbons (PAHs). The PAH emission is located between the edges of HII regions and layers of [CI] emission, agreeing with PDR theory. By modeling the spatial variations in the emission from ionized gas and PAHs, the geometry of the PDR and the gas density are determined. With constraints placed on the geometry of the PDR, PAHs must be destroyed almost instantaneously or not at all inside HII regions, disagreeing with previous determinations. According to PAH theory, the transition from ionized to neutral PAHs at PDR surfaces is traceable by the relative strengths of the PAH features. On large and small size scales, the relative strengths of the 8.6, 11.2, and 12.7 μm PAH features at the bar of the Orion Nebula indicate that there is not a simple transition from ionized to neutral PAHs across the PDR.; Over half of the observations presented were acquired with a Mid-InfraRed Spectrometer and Imager, MIRSI, completed at Boston University and currently operating at the NASA InfraRed Telescope Facility (IRTF). MIRSI utilizes the largest available mid-infrared array (320 x 240) developed by Raytheon/Santa Barbara Research Corporation for ground-based infrared applications. MIRSI's advantage at the IRTF is a wide field of view (85&inches; x 64&inches;) with diffraction limited imaging (0.7&inches; at 10 μm with a 0.27&inches; pixel scale). MIRSI switches between spectroscopic and imaging modes in a minute, enabling quick association of the spatial and spectral emission components. Meeting the camera's design goals, a one sigma detection with MIRSI at 10 μm (47% bandpass) is four mJy for an on-source exposure time of one minute. At Boston University, I wrote software to interface the host computer with hardware components, designed and fabricated mechanical and electrical components, and optimized and characterized MIRSI's performance.