Time variation of SWS spectra of M-type Mira variables

Abstract

Abstract. 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 (2.38-45.2 µm) of Z Cyg shows prominent silicate emission bands at 10 µm and 18 µm 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 µm to 18 µm 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 fitted by optically thin dust shell models with one single silicate dust emissivity profile. 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 fitted most consistently with a model in which the inner dust shell temperature is 700 ± 100 K at maximum. From the model fits we derive a ratio of the 18 µm and 10 µm emission efficiencies of Q(18 µm)/Q(10 µm) = 0.51±0.08. The Z Cyg dust also has a broad feature in the 20-25 µm region in addition to the 18 µm 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 20 µm emissivity of circumstellar dust, possibly related to the presence of another dust component.

A Uniform Database of 2.4–45.4 Micron Spectra from the Infrared Space Observatory Short Wavelength Spectrometer

“…Vandenbussche et al (2002) and Onaka et al (2002) presented several spectra of stellar sources processed from OLP 10 pipeline results. To increase the sample of spectra for comparison, we also examine results from earlier versions of the pipeline from other authors.…”

“…On the other hand, our spectra are considerably less noisy from 3.0 to 3.7 lm, probably because we discard most of the out-of-band data from Bands 1D and 1E and they do not. Onaka et al (2002) observed Z Cyg, an oxygen-rich M5 star, as part of a study of infrared variability in circumstellar dust emission. Figure 10 compares our spectra with the same observations reduced by Onaka et al (2002) from OLP 10.1 pipeline output.…”

“…Onaka et al (2002) observed Z Cyg, an oxygen-rich M5 star, as part of a study of infrared variability in circumstellar dust emission. Figure 10 compares our spectra with the same observations reduced by Onaka et al (2002) from OLP 10.1 pipeline output. As with the Band 1 data of Vandenbussche et al (2002), the two reduction methods produce generally indistinguishable results.…”

Multiwavelength Study of Pulsation and Dust Production in Mira Variables Using Optical Interferometry for Constraints

“…Much of this work is based on multi-wavelength studies of various components or regions in these atmospheres including masers and radio photospheres (Reid & Menten, 1997Cotton et al 2008), molecular photospheres, especially with optical/infrared interferometry (Ragland et al 2008, Perrin et al 2004, and dust production, especially with respect to pulsational phase (Onaka et al 2002, Matsuura et al 2002, Marengo et al 2007). This study seeks to increase our information on the interaction between pulsation and dust production by obtaining simultaneous data from space (infrared spectroscopy) and the ground (infrared interferometry) on a chemically complete sample of mira variables.…”

Spitzer'IRAC Observations of AGB Stars

“…Just to mention a few, H 2 O, SiO, CO 2 , CO and silicate dust features have been detected in stars with atmospheric C/O ratio < 1, while C 2 H 2 , HCN, CS, C 3 and carbonaceous dust (SiC and amorphous carbon) have been found in carbon stars, having C/O > 1. The presence and strength of these spectral features depends on the chemistry of the stellar atmosphere [5,6] and the mass loss rate, and can change with time [7] because AGB stars are Long Period Variables generically classified as Mira, semiregular or irregular pulsators.…”