T. de Jong

An oxygen-rich dust disk surrounding an evolved star in the Red Rectangle

The Red Rectangle is the prototype of a class of carbon-rich reflection nebulae surrounding low-mass stars in the final stages of evolution. The central star of this nebula has ejected most of its layers (during the red-giant phase), which now form the surrounding cloud, and is rapidly evolving to a white dwarf. This star is also a member of a wide binary system, which is surrounded by a thick, dusty disk of material,. Here we report infrared observations of the Red Rectangle that reveal the presence of oxygen-rich material: prominent emission bands from crystalline silicates, and absorption lines arising from carbon dioxide. The oxygen-rich material is located in the circumbinary disk, in contrast to the previously known carbon-rich dust, which is found mainly in the extended nebula,. The properties of the oxygen-rich dust are similar to those of dusty disks surrounding young stars, which are believed to be the sites of planet formation. Grain processing, and perhaps even planet formation, may therefore also be occurring in the circumbinary disk of this evolved star.

IRAS 09425-6040: A Carbon Star surrounded by highly crystalline Dust

We present infrared spectroscopy and millimeter photometry and spectroscopy of the peculiar carbon star IRAS 09425-6040. The 2-15 μ m spectrum, as well as the CO millimeter line observations are typical for a (J-type) carbon star with moderate mass-loss rate. The 15-45 μ m spectrum is dominated by strong emission bands from Mg-rich and Fe-poor crystalline silicates. IRAS 09425-6040 has the highest abundance of crystalline silicates (75 per cent) observed in any source so far. The ISO data, combined with IRAS and millimeter wavelength photometry indicate the presence of large cold grains. The observations indicate that the carbon star IRAS 09425-6040 is surrounded by a stationary, massive, highly crystalline oxygen-rich dust disk which is depleted of gas. These properties are very similar to those of the disk seen in the Red Rectangle. We propose that IRAS 09425-6040 is the evolutionary progenitor of the central binary of the Red Rectangle nebula.

W Hya : Molecular inventory by ISO-SWS

Infrared spectroscopy is a powerful tool to probe the inventory of solid state and molecular species in circumstellar ejecta. Here we analyse the infrared spectrum of the Asymptotic Giant Branch star W Hya, obtained by the Short and Long Wavelength Spectrometers on board of the Infrared Satellite Observatory. These spectra show evidence for the presence of amorphous silicates, aluminum oxide, and magnesium-iron oxide grains. We have modelled the spectral energy distribution using laboratory-measured optical properties of these compounds and derive a total dust mass-loss rate of 3× 10 −10 Myr −1 .W e find no satisfactory fit to the 13 µm dust emission feature and the identification of its carrier is still an open issue. We have also modelled the molecular absorption bands due to H2O, OH, CO, CO2 ,S iO, and SO 2 and estimated the excitation temperatures for different bands which range from 300 to 3000 K. It is clear that different molecules giving rise to these absorption bands originate from different gas layers. We present and analyse high-resolution Fabry-Perot spectra of the three CO2 bands in the 15 µm region. In these data, the bands are resolved into individual Q-lines in emission, which allows the direct determination of the excitation temperature and column density of the emitting gas. This reveals the presence of a warm (� 450 K) extended layer of CO2, somewhere between the photosphere and the dust formation zone. The gas in this layer is cooler than the 1000 K CO2 gas responsible for the low-resolution absorption bands at 4.25 and 15 µm. The rotational and vibrational excitation temperatures derived from the individual Q-branch lines of CO2 are different (∼450 K and 150 K, respectively) so that the CO2 level population is not in LTE.

Time variation of SWS spectra of M-type Mira variables I. Z Cyg and its dust optical properties

The M-type Mira variable star, Z Cyg, was observed with the Short-Wavelength Spectrometer (SWS) on board the Infrared Space Observatory (ISO) 7 times at roughly 60 day intervals over one and a half period. The infrared spectrum (2.38{45.2m) of Z Cyg shows prominent silicate emission bands at 10 ma nd 18m and displays quite large variations over the observed period. The variation in the infrared spectrum of Z Cyg is synchronized with the visual light curve. The circumstellar emission and the 10 mt o 18m silicate band ratio increases at maximum and decreases at minimum, indicating a variation in the dust temperature with phase. Apart from minor emission features which may be partly due to oxide dust, the observed spectra can be tted by optically thin dust shell models with one single silicate dust emissivity prole. Thus silicate is the dominant dust component in the circumstellar shell of Z Cyg. The variation in the integrated infrared flux and in the dust temperature derived from the observed spectra can be interpreted in terms of the variation in the luminosity of the central star if proper dust optical properties are adopted. Conversely, the dust emissivity can be estimated from the variations in the infrared spectrum. The derived optical properties are relatively insensitive to the assumptions made in the model analysis because of the optically thin nature of the dust shell. Possible evidence for dust formation near minimum is discussed. The observed variation of the dust shell spectrum of Z Cyg is tted most consistently with a model in which the inner dust shell temperature is 700 100 K at maximum. From the model ts we derive a ratio of the 18 ma nd 10m emission eciencies of Q(18 m)=Q(10 m) = 0:510:08. The Z Cyg dust also has a broad feature in the 20{25m region in addition to the 18m silicate band. The optical properties of dust grains around Z Cyg are compared with those in circumstellar shells of other oxygen-rich late-type stars and it is shown that there are variations in the 20m emissivity of circumstellar dust, possibly related to the presence of another dust component.

ISO-SWS spectra of OH/IR stars

We present the spectra of 5 OH/IR stars observed with the Short Wavelength Spectrometer (SWS) on board the Infrared Space Observatory (ISO). The spectra are dominated by dust features, both in the amorphous and crystalline state and illustrate the influence of the mass loss rate on the observed spectral features.

ISO-SWS spectra of AGB stars

In this contribution, we present a few highlights of the guaranteed time program to observe AGB stars with differing chemical compositions and mass loss rates using ISO1 SWS. We briefly discuss C2H2 absorption in C-stars and O-rich stars with the 13 μm dust emission.

Time variation of mid-infrared spectra on M-type variables

Time variation of mid-infrared (MIR) spectra of M-type Mira variables has been investigated by ISO/SWS observations of Z Cyg and T Cep. The variation in the Z Cyg spectrum can be accounted for by the variation in the stellar luminosity, if appropriate dust emissivity parameters are adopted. No apparent variation in the spectral shape despite of the intensity variation suggests that emission from high-temperature grains dominates in the MIR spectrum of T Cep

SPECTROSCOPIC STUDY OF CARBON STARS WITH THE ISO-SWS

We have observed ten carbon stars with different mass-loss rates using the Short Wavelength Spectrometer (SWS) on board ISO. We found that not only the spectral energy distribution and the dust features, but that also that the strength and/or shape of molecular absorption features in the infrared spectrum varies with the near-infrared color temperature, i.e. with the thickness of the circumstellar envelope.

ISO-SWS observations of the time variability of oxygen-rich Mira variables

We present the first observation of variation of the infrared spectrum of circumstellar dust emission around an oxygen-rich Mira variable star, Z Cyg, over an entire light variation cycle, based on the periodic SWS01 observations. The 10 µm and 20 µm ‘silicate features’ become stronger relative to the photospheric emission at maximum than at minimum. In addition, the relative intensity of the 10 µm to the 20 µm features increases at maximum, indicating an increase of the temperature of circumstellar dust grains. A simple model analysis suggests enhanced dust formation near the photosphere around maximum, leading to a scenario that dust nucleation may have occurred near minimum and the dust grains may have subsequently grown till maximum. The strong observed variation suggests that the variability must be taken into account in the interpretation of the infrared spectra of oxygen-rich Mira variables.

IRAS Observations of NGC 4696 — Cooling or Evaporation Flow?

We have analysed 60 and 100 μm IRAS observations of NGC 4696, the central elliptical galaxy of the Centaurus cluster. Ground-based optical photometry and spectroscopic data as well as ultraviolet (IUE) and X-ray (EINSTEIN) measurements constrain the interpretation of the infrared data. The dust responsible for the infrared emission has a temperature of 24 K, a total mass of 2.5 106 M⨀ and emits a total infrared luminosity of 1.5 109 L ⨀. It is heated by photons from the interstellar radiation field in the elliptical galaxy and by hot electrons from the X-ray gas. About 10 % of the radiating dust mass is located in a small ”horse shoe” shaped dust lane in the centre of NGC 4696. We argue that the other 90 % is distributed over a much larger region (R ≃ 10 kpc) in the galaxy.