4,161 research outputs found

    Automated Classification of 2000 Bright IRAS Sources

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    An Artificial Neural Network (ANN) has been employed using a supervised back-propagation scheme to classify 2000 bright sources from the Calgary database of IRAS (Infrared Astronomy Satellite) spectra in the wavelength region of 8-23 microns. The data base has been classified into 17 pre-determined classes based on spectral morphology. We have been able to classify more than 80 percent of the 2000 sources correctly at the first instance. The speed and robustness of the scheme will allow us to classify the whole of LRS database, containing more than 50,000 sources in the future.Comment: 26 pages, To appear in ApJS after July 200

    A self-consistent photoionization-dust continuum-molecular line transfer model of NGC 7027

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    A model to simulate the entire spectrum (1000 Å to 1 cm) of the high-excitation young planetary nebula NGC 7027 is presented. The ionized, dust, and molecular components of the object are modeled using geometric parameters obtained from visible, radio, infrared, and CO data. The physical processes considered include recombination lines of H and He, collisional excited lines of metals, bf and ff continuum radiations, two-photon radiation, dust continuum radiation, and molecular rotational and vibrational transitions. The dust component is assumed to be heated by a combination of direct starlight and the line and continuum radiation from the ionized nebula. The molecular component of the nebula is coupled to the dust component through the stimulated absorption of the dust continuum radiation. Specifically, we compare the predicted fluxes of the CO rotational lines and the 179.5 μm water rotational line to those observed by the Infrared Space Observatory satellite. From the observed ratio of these lines, we derived a gas phase water-to-H 2 abundance ratio of 3.2 × 10 -7 assuming that the water is uniformly mixed with the CO. © 1997. The American Astronomical Society. All rights reserved.published_or_final_versio

    Infrared space observatory spectroscopy of extreme carbon stars

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    We present Infrared Space Observatory (ISO) SWS01 (2-43 μm) and LWS01 (43-195 μm) observations of five highly evolved carbon stars with extremely thick circumstellar envelopes. The unidentified 30 μm emission feature is detected in the SWS01 spectra of all five sources. This feature seems to be resolved into two subfeatures at 25.5 and 28.3 μm in two sources. These same two sources show a weak emission feature near 20 μm which may be the same as the 21 μm feature seen in carbon-rich proto-planetary nebulae. Absorption features at 13.7 μm attributed to C 2H 2 are also detected in four sources. No dust feature is detected in the LWS spectra. The only line present is the 157.74 μm line of C +, which could be interstellar in origin. We present radiative transfer models for the spectra of these five stars assuming that the dust is amorphous carbon. The derived optical depths at 11.2 μm range from 1.4 to 4.5, corresponding to optical depths at V = 190-600, making these stars among the asymptotic giant branch (AGB) stars with the most optically thick dust envelopes. From the model fittings, we found the 30 μm features to be optically thick with 5%-8% of the total stellar luminosities being emitted in the feature. For four of the five stars, the models also suggest that their mass-loss rates have been increasing rapidly over the last few 10 4 yr periods, with current mass-loss rates as high as 4 × 10 -4 (D/kpc) M ⊙ yr -1. Such high mass-loss rates imply that these stars are in the final stages of AGB evolution. The observations of these extreme carbon stars therefore represent the best cases to study the transition from AGB to proto-planetary nebulae. infrared: stars - radiative transfer - stars: carbon - stars: evolution -stars: late-type - stars: mass loss 1 Based on observations made with ISO, an ESA project with instruments funded by the ESA member states (especially the PI countries: France, Germany, the Netherlands, and the United Kingdom) with the participation of ISAS and NASA.published_or_final_versio

    2-45 micron infrared spectroscopy of carbon-rich proto-planetary nebulae

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    Infrared Space Observatory (ISO) 2-45 μm observations of seven proto-planetary nebulae (PPNs) and two other carbon-rich objects are presented. The unidentified emission features at 21 and 30 μm are detected in six sources, including four new detections of the 30 μm feature. This previously unresolved 30 μm feature is now resolved and found to consist of a broad feature peaking at 27.2 μm (the "30 μm" feature) and a narrower feature at 25.5 μm (the "26 μm" feature). This new 26 μm feature is detected in eight sources and is particularly strong in IRAS Z02229 + 6208 and 16594-4656. The unidentified infrared (UIR) emission features at 3.3, 6.2, 7.7, and 11.3 μm, which are commonly observed in planetary nebulae and H II regions, are also seen in these PPNs. However, their strengths relative to the continuum plateaus at 8 and 12 μm are weaker than in planetary nebulae. The 6.9 μm feature, seen almost exclusively in PPNs, is strong. New millimeter CO and HCN observations were made; they support the carbon-rich nature of the objects and yield the expansion velocities of the gaseous envelopes. The spectral energy distributions of these PPNs were fitted with a radiative-transfer model, taking into account the emission features at 21, 26, and 30 μm. A significant fraction of the total energy output is emitted in these features: as high as 20% in the 30 μm feature and 8% in the 21 μm feature. The fact that so much energy is carried in these features suggests that the material responsible for these features must be made of abundant elements and most likely involves carbon. SiS 2 appears to be ruled out as the emitter of the 21 μm feature due to the absence of a predicted companion feature.published_or_final_versio