Why is the librational water-ice band at 11.5 μm so difficult to detect?

Robinson, G.; Maldoni, M. M.

doi:10.1111/j.1365-2966.2010.17259.x

Cited by 8 publications

(22 citation statements)

References 43 publications

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“…e.g. Koike et al 1995;Begemann et al 1997;Posch et al 1999;Egan & Sloan 2001;Woitke et al 2006;Ireland & Scholz 2006;and Robinson & Maldoni 2010), and it can survive at high temperatures.…”

Section: Discussion and Interpretationmentioning

confidence: 99%

The mid-infrared diameter of W Hydrae

Zhao-Geisler

Quirrenbach

Köhler

et al. 2011

A&A

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Aims. Asymptotic giant branch (AGB) stars are among the largest distributors of dust into the interstellar medium, and it is therefore important to understand the dust formation process and sequence in their strongly pulsating extended atmosphere. By monitoring the AGB star W Hya interferometrically over a few pulsations cycles, the upper atmospheric layers can be studied to obtain information on their chemical gas and dust composition and their intracycle and cycle-to-cycle behavior. Methods. Mid-infrared (8-13 μm) interferometric data of W Hya were obtained with MIDI/VLTI between April 2007 and September 2009, covering nearly three pulsation cycles. The spectrally dispersed visibility data of all 75 observations were analyzed by fitting a circular fully limb-darkened disk (FDD) model to all data and individual pulsation phases. Asymmetries were studied with an elliptical FDD. Results. Modeling results in an apparent angular FDD diameter of W Hya of about (80 ± 1.2) mas (7.8 AU) between 8 and 10 μm, which corresponds to an about 1.9 times larger diameter than the photospheric one. The diameter gradually increases up to (105 ± 1.2) mas (10.3 AU) at 12 μm. In contrast, the FDD relative flux fraction decreases from (0.85 ± 0.02) to (0.77 ± 0.02), reflecting the increased flux contribution from a fully resolved surrounding silicate dust shell. The asymmetric character of the extended structure could be confirmed. An elliptical FDD yields a position angle of (11 ± 20) • and an axis ratio of (0.87 ± 0.07). A weak pulsation dependency is revealed with a diameter increase of (5.4 ± 1.8) mas between visual minimum and maximum, while detected cycle-to-cycle variations are smaller. Conclusions. W Hya's diameter shows a behavior that is very similar to the Mira stars RR Sco and S Ori and can be described by an analogous model. The constant diameter part results from a partially resolved stellar disk, including a close molecular layer of H 2 O, while the increase beyond 10 μm can most likely be attributed to the contribution of a spatially resolved nearby Al 2 O 3 dust shell. Probably due to the low mass-loss rate, close Fe-free silicate dust could not be detected. The results suggest that the formation of amorphous Al 2 O 3 occurs mainly at visual minimum. A possible close Al 2 O 3 dust shell has now been revealed in a few objects calling for self-consistent dynamic atmospheric models including dust formation close to the star. The asymmetry might be explained by an enhanced dust concentration along an N-S axis.

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“…e.g. Koike et al 1995;Begemann et al 1997;Posch et al 1999;Egan & Sloan 2001;Woitke et al 2006;Ireland & Scholz 2006;and Robinson & Maldoni 2010), and it can survive at high temperatures.…”

Section: Discussion and Interpretationmentioning

confidence: 99%

The mid-infrared diameter of W Hydrae

Zhao-Geisler

Quirrenbach

Köhler

et al. 2011

A&A

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“…Posch et al 1999;Fabian et al 2001). In particular, amorphous Al 2 O 3 provides significant opacity for wavelengths longwards of 10 μm (Koike et al 1995;Begemann et al 1997;Posch et al 1999;Woitke 2006a;Ireland & Scholz 2006;Robinson & Maldoni 2010). In addition, features of crystalline aluminum oxide (corundum, α-Al 2 O 3 ) at 12.7 μm and several modifications of titanium oxide (e.g.…”

Section: Spectral Features and Detectabilitymentioning

confidence: 99%

Dust and molecular shells in asymptotic giant branch stars

Zhao-Geisler

Quirrenbach

Köhler

et al. 2012

A&A

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Context. Asymptotic giant branch (AGB) stars are one of the largest distributors of dust into the interstellar medium. However, the wind formation mechanism and dust condensation sequence leading to the observed high mass-loss rates have not yet been constrained well observationally, in particular for oxygen-rich AGB stars. Aims. The immediate objective in this work is to identify molecules and dust species which are present in the layers above the photosphere, and which have emission and absorption features in the mid-infrared (IR), causing the diameter to vary across the N-band, and are potentially relevant for the wind formation. Methods. Mid-IR (8-13 μm) interferometric data of four oxygen-rich AGB stars (R Aql, R Aqr, R Hya, and W Hya) and one carbonrich AGB star (V Hya) were obtained with MIDI/VLTI between April 2007 and September 2009. The spectrally dispersed visibility data are analyzed by fitting a circular fully limb-darkened disk (FDD). Results. The FDD diameter as function of wavelength is similar for all oxygen-rich stars. The apparent size is almost constant between 8 and 10 μm and gradually increases at wavelengths longer than 10 μm. The apparent FDD diameter in the carbon-rich star V Hya essentially decreases from 8 to 12 μm. The FDD diameters are about 2.2 times larger than the photospheric diameters estimated from K-band observations found in the literature. The silicate dust shells of R Aql, R Hya and W Hya are located fairly far away from the star, while the silicate dust shell of R Aqr and the amorphous carbon (AMC) and SiC dust shell of V Hya are found to be closer to the star at around 8 photospheric radii. Phase-to-phase variations of the diameters of the oxygen-rich stars could be measured and are on the order of 15% but with large uncertainties. Conclusions. From a comparison of the diameter trend with the trends in RR Sco and S Ori it can be concluded that in oxygen-rich stars the overall larger diameter originates from a warm molecular layer of H 2 O, and the gradual increase longward of 10 μm can be most likely attributed to the contribution of a close Al 2 O 3 dust shell. The chromatic trend of the Gaussian FWHM in V Hya can be explained with the presence of AMC and SiC dust. The observations suggest that the formation of amorphous Al 2 O 3 in oxygenrich stars occurs mainly around or after visual minimum. However, no firm conclusions can be drawn concerning the mass-loss mechanism. Future modeling with hydrostatic and self-consistent dynamical stellar atmospheric models will be required for a more certain understanding.

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“…In this study we have primarily used the laboratory‐determined optical constants (the real and imaginary parts of the complex refractive index) for amorphous olivine (MgFeSiO 4 ) from Dorschner et al (1995), as available from the dusty (Ivezić & Elitzur 1995, 1997) optical constants library 1 . These optical constants were used by Robinson & Maldoni (2010) in their study of the water‐ice librational band and by Maldoni, Ireland & Robinson (2008) to represent the spectrum of the post‐AGB star IRAS 22036+5306. Hereafter, unless otherwise stated, the term ‘silicate’ means these specific laboratory silicates, as opposed to ‘astronomical silicates’, which will also be employed in the modelling of AFGL 961E in .…”

Section: The Opacity Data and Grain Propertiesmentioning

confidence: 99%

“…For this study we have used a radiative transfer model based on the quasi‐diffusion method of Leung (Leung 1975, 1976; Egan, Leung & Spagna 1988) as implemented by Mitchell & Robinson (1978, 1980). The most recent discussion of this implementation is presented by Robinson & Maldoni (2010). Briefly the model calculates the output spectrum from a star surrounded by a spherically symmetric dust shell, without significant approximation, for an arbitrary opacity law, an arbitrary density distribution and an arbitrary mixture of grain species, and includes the effects of (linearly) anisotropic scattering.…”

Section: The Radiative Transfer Modelmentioning

confidence: 99%

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The water-ice librational band: radiative transfer model for AFGL 961

Robinson

Smith

Maldoni

2012

Monthly Notices of the Royal Astronomical Society

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On the basis of radiative transfer models we have investigated the librational band of amorphous water‐ice (H2O‐ice), which might be expected to occur in the spectra of young stellar objects with dust shells, producing an absorption feature at about 13 μm. Models have been run with only bare silicate grains present, together with models including water‐ice mantles. We find that the 13‐μm feature may be substantially explained without the use of water‐ice if laboratory silicates are employed and the 10‐μm silicate feature is in self‐absorption, a phenomenon which may arise for shells of intermediate optical depth (). The self‐absorption process may lead to a variation in the shape of the local minimum between the 10‐ and 20‐μm silicate features, the local minimum occurring at about 13 μm, close to the position of the water‐ice librational band. This local minimum could easily be mistaken for the librational water‐ice band itself. Nevertheless, the librational band undoubtedly does contribute to the finer details of the shape of the 13‐μm absorption feature. We find that for shells of small optical depth the water‐ice feature is not distinguishable from the local minimum between the 10‐ and 20‐μm silicate features, while for shells of large optical depth it is washed out completely, so in neither of these two extreme cases should the feature be readily observable. The fact that relatively few objects exist in the intermediate state where the silicate feature is in self‐absorption may account for the extremely small number of cases in which the librational band apparently appears as a distinct feature. Radiative transfer models have been produced for the massive pre‐main‐sequence object AFGL 961E, with the aim of investigating whether the 13‐μm feature in its spectrum is due to the librational band of water‐ice. Clearly there is water‐ice somewhere in the line of sight to AFGL 961, as evidenced by the strong 3.1‐μm water‐ice band, and the case for the 13‐μm feature being due to the librational band is uniquely strong amongst all astronomical objects. However, we present evidence that the grains may have properties similar to laboratory silicates rather than astronomical silicates and that the feature may largely be a manifestation of the 10‐μm silicate feature being in self‐absorption, with amorphous water‐ice being a minor contributor. Neither bare astronomical silicates nor water‐ice mantled astronomical silicates will reproduce the 13‐μm feature.

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Why is the librational water-ice band at 11.5 μm so difficult to detect?

Cited by 8 publications

References 43 publications

The mid-infrared diameter of W Hydrae

The mid-infrared diameter of W Hydrae

Dust and molecular shells in asymptotic giant branch stars

The water-ice librational band: radiative transfer model for AFGL 961

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