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Astron. Astrophys. 322, 924-932 (1997)

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2. Spectra of AGB circumstellar envelopes

The IRAS color-color diagram (van der Veen & Habing 1988) obtained by IR photometry at 12, 25 and 60 µm, provides a first-order discrimination of AGB sources, according to their infrared excess, variability and C abundance. However, as shown in Paper I, such IRAS color-color diagram can lead to misclassification of AGB CSEs due to the large width of IRAS filters (this is true in particular for the one at 12 µm, which cannot discriminate between silicates and carbonaceous features). Although narrow band filters allow a better discrimination (as in the case of TIRCAM photometric system), a detailed description of the dust emission/absorption features requires mid-IR spectra.

IRAS LRS (1986) provide spectral energy distributions in the wavelength range 7.7-22.6 µm for about 4,000 AGB sources, which are classified with a two-digit code. The first digit (main class) is related to the spectral index [FORMULA] of the continuum (assuming [FORMULA]), while the second indicates the strength of the main feature, in emission or absorption, present in the mid-IR window. In particular, spectra of O-rich sources are identified by the 9.7 µm silicate feature, that is found in emission or absorption in the LRS classes 2n and 3n, respectively. C-rich envelopes are mainly found in the 4n class, defined by the presence of the 11.3 µm SiC feature in emission. Post-AGB objects of both types are in the main classes 6n and 7n, characterized by very large infrared excess. Finally, both O-rich and C-rich sources may fall in classes 1n and 0n (featureless spectra), where a few LRS with self-absorbed SiC feature can be found (Omont et al. 1993).

Table 1 lists the spectral characteristics of the sources in Paper I with known LRS; the star AFGL 1822 was added to the TIRCAM sample in order to have an evolved O-rich envelope with the 9.7 µm silicate feature in absorption. Post-AGB sources have not been included because their modelling involves very different physical conditions, dominated by the hot radiation emitted by the central star (B or A spectral type) on its way to become a white dwarf.


[TABLE]

Table 1. List of AGB sources from Paper I with known LRS. The evolved O-rich envelope AFGL 1822 was added to have a 3n LRS class source. Last column: LML90 LRS subclass, when available.


Note that the LRS class 2n contains sources with different characteristics; a tentative classification is in the LML90 paper, where the silicate band profiles in optically thin O-rich envelopes are divided into seven subclasses, according to their shape and the presence of secondary peaks. The subclass for our O-rich sources, when given by LML90, is reported in Table 1; the S-star W And is tentatively attributed to the "S" feature subclass, characterized by a broad peak at 10.3 µm. The other sources fall in subclass "Sil" (with a strong feature peaked at 9.8 µm, typical of 2n LRS with [FORMULA]) and subclass "Sil+" (having a secondary "bump" at 11.3 µm). LML90, and Simpson (1991) suggest that these differences be related to a kind of "mineralogical diversity" in the oxidic CSE dust, although this explanation is still controversial (see e.g. Ivezi & Elitzur 1995, hereafter IE95). We tried to model the shape of the silicate feature in our sources belonging to different subclasses using several sets of opacities and dust mixtures.

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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