Volume 537, January 2012
|Number of page(s)||15|
|Section||Galactic structure, stellar clusters and populations|
|Published online||20 December 2011|
Molecular absorption and/or emission and dust features affect the [3.6] − [4.5] and [3.6] − [8.0] colors. In order to understand the observed trend of increasing [3.6] − [4.5] and [3.6] − [8.0] colors of late-type stars with pulsation type, we analysed ISO-SWS spectra (see Fig. A.1). From the flux-calibrated ISO-SWS library of Sloan et al. (2003), we selected spectra of stars with known spectral types and variabilities: AGB-Miras, AGB-SRs, and OH/IR stars, and RSGs.
Water, SiO, CO2, and CO absorption may be present in the 3.0 to 5.0 μm spectral region of late-type stars (Sylvester et al. 1999; Matsuura et al. 2002; Cami 2002). ISO-SWS spectra of RSGs and AGB-SRs are remarkably different from those of AGB Miras (see Fig. A.1). Typically RSGs and AGB-SRs do not have strong molecular signatures in between 3 and 4 μm, and display a decreasing flux in this region (GLIMPSE 3.6 μm). AGB-Miras are dominated by water, SiO, CO2, and CO absorption, and the resulting spectral flux between 3 and 4 μm can have a positive slope. This is attributed to a different “MOLspheres”, layers of molecular material in an extended atmosphere (Miras), where water is the main source of opacity (Tsuji et al. 1997; Matsuura et al. 2002; Verhoelst et al. 2009). The strength of water absorption varies with stellar pulsation phase (Matsuura et al. 2002).
Verhoelst et al. (2009) explains the continuum shape of RSGs in the 2.5–3 μm regions with an extra dust component (metallic Fe, amorphous C, or O-rich micron-sized grains) or with chromospheric emission. The similarity of the continuum of AGB SRs and RSG stars in the 2.5 and 3 μm region suggests that the attribution of extra continuum emission to free-free
radiation from chromospheric activities is less likely. Regularly pulsating RSGs have larger mass-loss (as suggested by the strength of the silicate emission at 9.7 μm) than RSGs with irregular pulsations (Fig. A.1).
The ISO-SWS flux densities at the effective wavelength of the GLIMPSE filters were measured, and flux densities were converted into magnitudes using the GLIMPSE zero-points. The synthetic [3.6] − [4.5] versus [3.6] − [8.0] diagram is shown in Fig. A.2, and well reproduces the sequence of increasing [3.6] − [4.5] with [3.6] − [8.0] colors. A clear sequence of redder colors is seen in AGB stars, going from SRs, to Miras and OH/IR stars. An increase in the [3.6] − [8.0] color is mostly due to an increase of silicate emission at 9.7 μm, which for optically thin envelopes well correlates with mass-loss. Their [3.6] − [4.5] colors are redder than those of AGB SR stars (Fig. A.1), because in the spectra of Miras there is strong water absorption. RSGs and AGB SR stars have weaker water absorption than AGB Miras. Yang & Jiang (2011) suggest that RSGs have bluer [3.6] − [4.5] due to a continuum depression around 4.5 μm by CO bands. RSG-SR stars have redder [3.6] − [4.5] colors than RSGs with irregular pulsations. Only weak water absorption is seen in some variable RSG stars.
Left: a representative samples of ISO-SWS spectra of AGB stars. Over-plotted transmission curves of the four Spitzer/IRAC filters. Right: a representative samples of ISO-SWS spectra of RSG stars. Over-plotted transmission curves of the four Spitzer/IRAC filters.
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Synthetic GLIMPSE [3.6] − [8.0] versus [3.6] − [4.5] diagram of evolved late-type stars observed with ISO-SWS (Sloan et al. 2003). AGB-Miras are shown with black dots, AGB-SR with crosses, RSG-SR with diamonds, RSG irregular with plus signs, and OH/IR stars with starred symbols. The arrow indicates the direction of the reddening vector following the extinction ratios derived by Indebetouw et al. (2005).
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Right: synthetic [3.6] − [4.5] of RSG stars versus mass-loss rates. Magnitudes are obtained from ISO-SWS spectra (Sloan et al. 2003), and mass-loss rates are given by Verhoelst et al. (2009). Starred symbols are RSG stars with periodic pulsations (SR), while diamonds are irregular RSG stars. Left: synthetic [3.6] − [8.0] of RSG stars modeled by Verhoelst et al. (2009) versus their inferred mass-loss rates. Starred symbols are RSG stars with periodic pulsations (SR), while diamonds are irregular RSG stars.
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In Fig. A.3, we plot our synthetic GLIMPSE colors of RSG stars versus mass-loss rates, as inferred by Verhoelst et al. (2009). Both [3.6] − [4.5] and [3.6] − [8.0] colors correlate with mass-loss rates. The correlation of mass loss with the [3.6] − [8.0] is explained by the increasing silicate strength at 9.7 μm. The correlation of mass loss with [3.6] − [4.5] color can be due to CO at 4.3 μm, and/or to the existence of an extra source of opacity (dust component), as suggested by Verhoelst et al. (2009).
© ESO, 2012
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