Astronomical instrumentation

Abstract:

We present 10 and 20-μm spectroscopic observations of a sample of F- and G-type carbon-rich post-AGB objects, three of which were known to exhibit the unidentified emission feature at 21 μm. We also find the F3I post-AGB object SAO 163075 to exhibit a (weak) 21-μm emission feature. We additionally obtained a 10-μm spectrum of IRAS 05341+0852, which has been reported to have a possible 21-μm emission band in its IRAS LRS spectrum, and obtained new 10 and 20-μm spectra of the carbon-rich bipolar post-AGB sources GL 2688 and GL 618, the extreme carbon star GL 3068, and the planetary nebulae IRAS 21282+5050 and He 2-447, in order to study the evolution of C-rich dust from the early post-AGB through to the planetary nebula (PN) phases. The 7.5-13-μm spectra of the 21-μm band objects exhibit broad plateau emission, shortwards of 9μm and from 10-13μm, superposed upon which, in addition to the well-known UIR-band at 11.3 μm, are several new features, at 7.9, 8.2, 10.6, 11.5 and 12.2 μm, differing from those observed in standard UIR-band spectra. An excellent match is found between the wavelengths of these new features and those of bands in the spectrum of chrysene (C18H12), one of the simplest PAH molecules. The absence of the new features in the spectra of earlier spectral-type post-AGB objects and PN is consistent with the expected complete dehydrogenation of any PAH molecules having less than 20-25 carbon atoms when exposed to the ultraviolet radiation fields of stars with spectral types earlier than F. Chrysene is not responsible for the 21-μm emission bands observed in the spectra of the cool post-AGB objects. Possible identifications for the 21-μm band in terms of highly hydrogenated 2-dimensional PAH molecules or 3-dimensional fulleranes (hydrogenated fullerenes) are discussed. The mid-infrared spectrum of GL 2688 is largely featureless, apart from two broad weak emission features between 9.5-10.5 and 10.5-12.2 μm. The profiles of the broad 10-13-μm absorption features in the spectra of the extreme carbon star GL 3068 and the C-rich bipolar post-AGB object GL 618 are compared. For GL 3068 the profile shape and the wavelength of peak absorption, near 11 μm, are consistent with absorption by silicon carbide particles. However, the absorption observed towards GL 618 is considerably broader and peaks at 12 μm. Its profile is very similar to that of the 10-13-μm emission plateau observed in the spectra of the 21-μm band objects, suggesting that it arises from absorption by a large column of highly-hydrogenated PAH-type species.

Abstract:

We present linear spectropolarimetry in the wavelength range 4.5-4.8 μm of the embedded infrared source W33A. Our observations show for the first time the presence of polarization associated with the CO and XCN ice features, demonstrating that the absorbers reside in or on aligned grains. Both narrow and broad components of the solid CO feature near 4.67 μm are polarized. The detection of polarization associated with the narrow CO component is particularly significant, as the ices responsible are thought to exist only in dense, cold regions of molecular clouds, where gas and grain temperatures are expected to be closely coupled and traditional grain alignment mechanisms should become ineffective. We discuss the significance of this result with regard to current grain alignment theories. Mechanisms in which grain rotational energy is enhanced by interaction with cosmic rays merit further investigation. © 1996. The American Astronomical Society. All rights reserved.

Abstract:

3D, the new MPE NIR imaging spectrometer, provides us with a unique opportunity to probe in detail the structure of the stars, ionized gas, and hot molecular gas in the very centers of AGN. The instrument delivers data cubes with 16×160.5″ pixels which are 256 spectral channels deep. Thus, in a single observation we are able to obtain data on the entire K-Band over an 8″×8″ field of view, with a spectral resolution of R = λ/Δλ = 1000. In this paper we detail the working principles behind the instrument, and show first results from observations of the inner regions of the Seyfert 1 galaxy NGC 7469 made at the Calar Alto observatory. Copyright ©1996 Elsevier Science Ltd.

Abstract:

The new MPE near infrared imaging spectrometer 3D represents a new generation of astronomical instrumentation. It is based on a 2562 NICMOS-3 Rockwell array and can simultaneously obtain 256 H- or K-band spectra at R= 1100 or 2100 from a square 16×16 pixel field on the sky. Typical pixel scales are 0.3″/pixel or 0.5″/pixel. 3D is a combination of a novel image slicer and a liquid nitrogen cooled long slit spectrometer. It includes high definition on-axis lens optics, a high efficiency directly ruled KRS-5 grism as well as a cold closed-loop piezo-driven tilt mirror allowing full spectral sampling. The instrument efficiency including detector is 15%. Combining the advantages of imaging and spectroscopy increases the observing efficiency on key astronomical objects (e.g. galactic nuclei) by such a large factor over existing grating or Fabry-Perot spectrometers that subarcsecond near-IR spectroscopy of faint Seyferts, starbursts, quasars, or distant galaxy clusters becomes feasible for the first time with 4m-class telescopes. As a portable instrument 3D has already been successfully deployed on several 2 and 4m-class telescopes.