η CARINAE'S DUSTY HOMUNCULUS NEBULA FROM NEAR-INFRARED TO SUBMILLIMETER WAVELENGTHS: MASS, COMPOSITION, AND EVIDENCE FOR FADING OPACITY.

Infrared observations of the dusty, massive Homunculus Nebula around the luminous blue variable η Carinae are crucial to characterize the mass-loss history and help constrain the mechanisms leading to the Great Eruption. We present the 2.4 - 670 µm spectral energy distribution, constructed from legacy ISO observations and new spectroscopy obtained with the Herschel Space Observatory. Using radiative transfer modeling, we find that the two best-fit dust models yield compositions which are consistent with CNO-processed material, with iron, pyroxene and other metal-rich silicates, corundum, and magnesium-iron sulfide in common. Spherical corundum grains are supported by the good match to a narrow 20.2 µm feature. Our preferred model contains nitrides AlN and Si3N4 in low abundances. Dust masses range from 0.25 to 0.44 M ʘ but M tot ≥ 45 M ʘ in both cases due to an expected high Fe gas-to-dust ratio. The bulk of dust is within a 5″ × 7″ central region. An additional compact feature is detected at 390 µm. We obtain L IR = 2.96 × 106 L ʘ, a 25% decline from an average of mid-IR photometric levels observed in 1971-1977. This indicates a reduction in circumstellar extinction in conjunction with an increase in visual brightness, allowing 25-40% of optical and UV radiation to escape from the central source. We also present an analysis of 12CO and 13CO J = 5 - 4 through 9 - 8 lines, showing that the abundances are consistent with expectations for CNO-processed material. The [12C II] line is detected in absorption, which we suspect originates in foreground material at very low excitation temperatures.

Beam profile for the Herschel-SPIRE Fourier transform spectrometer.

One of the instruments on board the Herschel Space Observatory is the Spectral and Photometric Imaging Receiver (SPIRE). SPIRE employs a Fourier transform spectrometer with feed-horn-coupled bolometers to provide imaging spectroscopy. To interpret the resultant spectral images requires knowledge of the wavelength-dependent beam, which in the case of SPIRE is complicated by the use of multimoded feed horns. In this paper we describe a series of observations and the analysis conducted to determine the wavelength dependence of the SPIRE spectrometer beam profile.

Demonstration of spectral and spatial interferometry at THz frequencies.

A laboratory prototype spectral-spatial interferometer has been constructed to demonstrate the feasibility of the double-Fourier technique at far infrared (FIR) wavelengths (0.15-1 THz). It is planned to use this demonstrator to investigate and validate important design features and data-processing methods for future astronomical FIR interferometer instruments. In building this prototype, we have had to address several key technologies to provide an end-end system demonstration of this double-Fourier interferometer. We report on the first results taken when viewing single-slit and double-slit sources at the focus of a large collimator used to simulate real sources at infinity. The performance of the prototype instrument for these specific field geometries is analyzed to compare with the observed interferometric fringes and to demonstrate image reconstruction capabilities.

Cascaded interferometric imaging spectrometer.

We present what we believe to be a novel method for order sorting a Fabry-Perot interferometer using a Fourier transform spectrometer (FTS) in tandem. We demonstrate how the order sorting is achieved using a model instrument response as an example of an instrument working in the 5-25 microm band, although the method is generally applicable at all wavelengths. We show that an instrument of this type can be realized with a large bandwidth, a large field of view, and good transmission efficiency. These attributes make this instrument concept a useful technique in applications where true imaging spectroscopy is required, such as mapping large astronomical sources. We compare the performance of the new instrument to grating and standard FTS instruments in circumstances where the measurement is background and detector noise limited. We use a figure of merit based on the field of view and speed of detection and find that the new system has a speed advantage over a FTS with the same field of view in all circumstances. The instrument will be faster than a grating instrument with the same spectral resolution once the field of view is >13 times larger under high background conditions and >50 times larger with detector performances that match the photon noise from Zodiacal light.