An ISO/SWS study of the dust composition around S stars

Hony, S.; Heras, A. M.; Molster, Frank; Smolders, K.

doi:10.1051/0004-6361/200912017

Cited by 27 publications

(49 citation statements)

References 23 publications

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“…Optical constants have been taken from Jaeger et al (1994). Gehlenite We know from Hony et al (2009) that the peak of the silicate features in S stars is not at 9.8 µm as in M-type stars, but shifted towards redder wavelengths, up to 10.6 µm.…”

Section: Dust Decomposition Toolmentioning

confidence: 99%

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TheSpitzerspectroscopic survey of S-type stars

Smolders

Neyskens

Blommaert

et al. 2012

A&A

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Context. S-type AGB stars are thought to be in the transitional phase between M-type and C-type AGB stars. Because the composition of the circumstellar environment reflects the photospheric abundances, one may expect a strong influence of the stellar C/O ratio on the molecular chemistry and the mineralogy of the circumstellar dust. Aims. In this paper, we present a large sample of 87 intrinsic galactic S-type AGB stars, observed at infrared wavelengths with the Spitzer Space Telescope, and supplemented with ground-based optical data. Methods. On the one hand, we derive the stellar parameters from the optical spectroscopy and photometry, using a grid of model atmospheres. On the other, we decompose the infrared spectra to quantify the flux-contributions from the different dust species. Finally, we compare the independently determined stellar parameters and dust properties. Results. For the stars without significant dust emission features, we detect a strict relation between the presence of SiS absorption in the Spitzer spectra and the C/O ratio of the stellar atmosphere. These absorption bands can thus be used as an additional diagnostic for the C/O ratio. For stars with significant dust emission, we define three distinct groups, based on the relative contribution of certain dust species to the infrared flux. We find a strong link between group-membership and C/O ratio. Furthermore, we show that these groups can be explained by assuming that the dust-condensation can be cut short before silicates are produced, while the remaining free atoms and molecules can then be used to form the observed magnesium sulfides or the carriers of the unidentified 13 µm and 20 µm features. Finally, we present the detection of emission features attributed to molecules and dust characteristic to C-type stars, such as molecular SiS, hydrocarbons and magnesium sulfide grains. We show that we often detect magnesium sulfides together with molecular SiS and we propose that it is formed by a reaction of SiS molecules with Mg.

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Section: Dust Decomposition Toolmentioning

confidence: 99%

“…Furthermore, the figure shows that the sample of S-type AGB presented in Hony et al (2009) is biased towards higher massloss rates, because the limited sensitivity of the ISO/SWS spectrograph favors stars with a strong infrared excess. We have to keep both biases in mind when we compare our results with literature.…”

Section: Bias Towards Low Mass-loss Ratesmentioning

confidence: 99%

TheSpitzerspectroscopic survey of S-type stars

Smolders

Neyskens

Blommaert

et al. 2012

A&A

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“…For example, S-type stars, which are undergoing the transition from oxygen-rich to carbon-rich and thus have C/O ∼ 1, do not yet form carbon-rich dust. However, due to non-equilibrium chemistry effects carbon-rich molecules, such as PAHs, HCN, and C 2 H 2 can be seen (Hony et al 2009;Smolders et al 2010). In the carbon-rich case, the excess carbon is used to form, for example, CN, C 2 , C 2 H 2 , CH 4 , polycyclic aromatic hydrocarbons (PAHs), and SiC.…”

Section: Introductionmentioning

confidence: 99%

Carbonaceous molecules in the oxygen-rich circumstellar environment of binary post-AGB stars

Gielen

Cami

Bouwman

et al. 2011

A&A

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Context. The circumstellar environment of evolved stars is generally rich in molecular gas and dust. Typically, the entire environment is either oxygen-rich or carbon-rich, depending on the evolution of the central star. Aims. In this paper we discuss three evolved disc sources with evidence of atypical emission lines in their infrared spectra. The stars were taken from a larger sample of post-AGB binaries for which we have Spitzer infrared spectra, characterised by the presence of a stable oxygen-rich circumbinary disc. Our previous studies have shown that the infrared spectra of post-AGB disc sources are dominated by silicate dust emission, often with an extremely high crystallinity fraction. However, the three sources described here are selected because they show a peculiar molecular chemistry. Methods. Using Spitzer infrared spectroscopy, we study in detail the peculiar mineralogy of the three sample stars. Using the observed emission features, we identify the different observed dust, molecular and gas species. Results. The infrared spectra show emission features due to various oxygen-rich dust components, as well as CO 2 gas. All three sources show the strong infrared bands generally ascribed to polycyclic aromatic hydrocarbons. Furthermore, two sample sources show C 60 fullerene bands. Conclusions. Even though the majority of post-AGB disc sources are dominated by silicate dust in their circumstellar environment, we do find evidence that, for some sources at least, additional processing must occur to explain the presence of large carbonaceous molecules. There is evidence that some of these sources are still oxygen-rich, which makes the detection of these molecules even more surprising.

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“…The infrared spectra of S stars are diverse and show a mix of dust species typical for either oxygen-rich or carbon-rich circumstellar environments, molecular emission bands, or no excess emission at all (Chen & Kwok 1993;Hony et al 2009;Smolders et al 2010Smolders et al , 2012. This heterogeneity of the infrared appearance within one spectral class is caused by the peculiar chemical composition of the stellar atmosphere of S stars.…”

Section: Introductionmentioning

confidence: 99%

Discovery of a TiO emission band in the infrared spectrum of the S star NP Aurigae

Smolders

Verhoelst

Neyskens

et al. 2012

A&A

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We report on the discovery of an infrared emission band in the Spitzer spectrum of the S-type AGB star NP Aurigae that is caused by TiO molecules in the circumstellar environment. We modeled the observed emission to derive the temperature of the TiO molecules (≈600 K), an upper limit on the column density (≈10 17.25 cm −2 ) and a lower limit on the spatial extent of the layer that contains these molecules. (≈4.6 R ). This is the first time that this TiO emission band is observed. A search for similar emission features in the sample of S-type stars yielded two additional candidates. However, owing to the additional dust emission, the identification is less stringent. By comparing the stellar characteristics of NP Aur to those of the other stars in our sample, we find that all stars with TiO emission show large-amplitude pulsations, s-process enrichment, and a low C/O ratio. These characteristics might be necessary requirements for a star to show TiO in emission, but they are not sufficient.

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An ISO/SWS study of the dust composition around S stars

Cited by 27 publications

References 23 publications

TheSpitzerspectroscopic survey of S-type stars

TheSpitzerspectroscopic survey of S-type stars

Carbonaceous molecules in the oxygen-rich circumstellar environment of binary post-AGB stars

Discovery of a TiO emission band in the infrared spectrum of the S star NP Aurigae

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