Circumstellar environments - III. M-supergiants

Abstract

The spectra of M-supergiants in the 7–100μm region are considered as part of a study of late-type stars based on the IRAS Low Resolution Spectrometer (LRS) Catalogue. The circumstellar silicate dust features at 9.7 and 18μm are observed to be of variable width. Most stars exhibit the classical circumstellar silicate dust features, typified by those of α Ori, modelled in a previous paper (Skinner & Whitmore). Some stars exhibit much broader features, VX Sgr being the brightest example. The latter stars can be fitted with a similar model, where the continuum is shown to be dominated by free–free emission. Dust condensing within outflows from various oxygen-rich stars generally produce similar emission characteristics, suggesting that the dust grains have a similar composition. This study shows that the strength of the spectral emission features produced by circumstellar dust in the infrared are correlated with surface temperature and luminosity, but not with pulsation characteristics of the stars. Radial pulsations are probably not linked directly to the mass-loss driving mechanism for the M-supergiants. We present mass-loss rates for 31 M-supergiants, which range from |$6.5\times10^{-7} \enspace \text {to}\enspace 4.3\times10^{-4} M_\odot\text {yr}^{-1}$|⁠. The stars AD Per and MZ Cas are M-supergiants which show the 11.5μm SiC feature, a characteristic of carbon stars.