Crystalline silicate dust around evolved stars: I. The sample stars

This is the first paper in a series of three where we present the first comprehensive inventory of solid state emission bands observed in a sample of 17 oxygen-rich circumstellar dust shells surrounding evolved stars. The data were taken with the Short and Long Wavelength Spectrographs on board of the Infrared Space Observatory (ISO) and cover the 2.4 to 195 μm wavelength range. The spectra show the presence of broad 10 and 18 μm bands that can be attributed to amorphous silicates. In addition, at least 49 narrow bands are found whose position and width indicate they can be attributed to crystalline silicates. Almost all of these bands were not known before ISO. The incredible richness of the crystalline silicate spectra observed by ISO allows detailed studies of the mineralogy of these dust shells, and is a telltale about the origin and evolution of the dust. We have measured the peak positions, widths and strengths of the individual, continuum subtracted bands. Based on these measurements, we were able to order the spectra in sequence of decreasing crystalline silicate band strength. We found that the strength of the emission bands correlates with the geometry of the circumstellar shell, as derived from direct imaging or inferred from the shape of the spectral energy distribution. This naturally divides the sample into objects that show a disk-like geometry (strong crystalline silicate bands), and objects whose dust shell is characteristic of an outflow (weak crystalline silicate bands). All stars with the 33.6 μm forsterite band stronger than 20 percent over continuum are disk sources. We define spectral regions (called complexes) where a concentration of emission bands is evident, at 10, 18, 23, 28, 33, 40 and 60 μm. We derive average shapes for these complexes and compare these to the individual band shapes of the programme stars. In an Appendix, we provide detailed comments on the measured band positions and strengths of individual sources

The circumstellar dust shell of the post-AGB star HD 161796

We have modeled the complete optical to millimeter spectrum of the Post-Asymptotic Giant Branch (Post-AGB) star HD 161796 and its circumstellar dust shell. A full 2–200 μm spectrum taken with the Infrared Space Observatory was used to constrain the dust properties. A good fit is achieved using only 4 dust components: amorphous silicates, the crystalline silicates forsterite and enstatite, and crystalline water ice, contributing respectively about 63, 4, 6 and 27% to the total dust mass. The different dust species were assumed to be co-spatial but distinct, resulting in different temperatures for the different grain populations. We find a temperature for the crystalline H2O ice of 70 K, which is higher than thermal equilibrium calculations of pure H2O ice would give. This implies that the ice must be formed as a mantle on top of an (amorphous) silicate core. In order to form H2O ice mantles the mass loss rate must exceed some yr-1. With a water-ice fraction of 27% a lower limit for the gas to dust mass ratio of 270 is found. At a distance of 1.2 kpc (Skinner et al. [CITE]) and adopting an outflow velocity of 15 km s-1 (Likkel et al. [CITE]) an AGB mass loss rate of ( yr-1) is found, which lasted 900 years and ended 430 years ago. During this phase a total of 0.46 was expelled. The mass loss rate was high enough to account for the presence of the H2O ice

The complete ISO spectrum of NGC 6302

We present the combined Infrared Space Observatory Short-Wavelength Spectrometer and Long-Wavelength Spectrometer 2.4-197 μm spectrum of the Planetary Nebula NGC 6302 which contains in addition to strong atomic lines, a series of emission features due to solid state components. The broad wavelength coverage enables us to more accurately identify and determine the properties of both oxygen- and carbon-rich circumstellar dust. A simple model fit was made to determine the abundance and typical temperature of the amorphous silicates, enstatite and forsterite. Forsterite and enstatite do have roughly the same abundance and temperature. The origin and location of the dust in a toroidal disk around the central star are discussed

The 69-mu m forsterite band as a dust temperature indicator

A band of pure crystalline forsterite (100 per cent Mg2SiO4) occurs at 69.67 μm at room temperature (295 K); for olivines with ≳10 per cent Fe the corresponding feature is at ≳73 μm. The Mg-rich forsterite feature is observed in a variety of ISO LWS spectra, but the corresponding Fe-rich olivine feature is not. For the 10 astronomical sources in our sample, the forsterite band peaks in the 68.9–69.3 μm range and narrows with decreasing peak wavelength. This is consistent with the shortwards shifting of the peak observed when laboratory samples are cooled to 77 K (69.07 μm) and 3.5 K (68.84 μm). The shifted peak is produced by lattice contraction and the sharpening is due to a decrease in phonon density at lower temperatures. However, the astronomical bands are narrower than those of the laboratory samples. By comparing the laboratory and astronomical peak wavelengths, we deduce characteristic forsterite 69-μm band temperatures that are in the 27–84 K range for the eight post-main-sequence objects in our sample. These values are shown to be consistent with the local continuum temperatures derived using a β=1.5 dust emissivity index, similar to derived interstellar values of the opacity index. For the pre-main sequence-objects HD 100546 and MWC 922, the characteristic 69-μm forsterite band temperatures (127±18 and 139±10 K, respectively) are significantly higher than those of the post-main-sequence objects and are more than twice as high as their local continuum temperatures deduced using β=1.5. The assumption of large grains (β=0) can produce agreement between the derived 69-μm and continuum temperatures for one of these objects but not for the other — a spatial separation between the forsterite and continuum-emitting grains may therefore be implied for it. We conclude that observations of the peak wavelength and FWHM of the 69-μm forsterite band show great promise as a new diagnostic of characteristic grain temperatures

Production of dust by massive stars at high redshift

The large amounts of dust detected in sub-millimeter galaxies and quasars at high redshift pose a challenge to galaxy formation models and theories of cosmic dust formation. At z > 6 only stars of relatively high mass (> 3 Msun) are sufficiently short-lived to be potential stellar sources of dust. This review is devoted to identifying and quantifying the most important stellar channels of rapid dust formation. We ascertain the dust production efficiency of stars in the mass range 3-40 Msun using both observed and theoretical dust yields of evolved massive stars and supernovae (SNe) and provide analytical expressions for the dust production efficiencies in various scenarios. We also address the strong sensitivity of the total dust productivity to the initial mass function. From simple considerations, we find that, in the early Universe, high-mass (> 3 Msun) asymptotic giant branch stars can only be dominant dust producers if SNe generate <~ 3 x 10^-3 Msun of dust whereas SNe prevail if they are more efficient. We address the challenges in inferring dust masses and star-formation rates from observations of high-redshift galaxies. We conclude that significant SN dust production at high redshift is likely required to reproduce current dust mass estimates, possibly coupled with rapid dust grain growth in the interstellar medium.Comment: 72 pages, 9 figures, 5 tables; to be published in The Astronomy and Astrophysics Revie

Circumstellar silicate mineralogy

This paper reviews spectra obtained with the SWS on board of ISO of dust shells around O-rich objects. These spectra reveal the presence of many new emission features between 10 and 45 mu m. These bands are generally much narrower than the well-known 10 and 20 mu m silicates features. The strength of these features relative to the underlying broad continuum varies from source to source (similar or equal to 5-50%). The 10 mu m region shows evidence for the presence of Al(2)O(3) grains. At longer wavelength, the spectra are dominated by features due to crystalline olivine and pyroxene. The exact peak position of these features shows that the emitting grains consist of the Mg-rich end-members of these minerals with an Fe-content o