Astron. Astrophys. 357, 515-519 (2000)

1. Introduction

Carbon stars are highly evolved stars which can be considered a direct evidence for evolution along the asymptotic giant branch (Chan & Kwok 1988, 1990; Willems 1988). Among their interesting features there is the characteristic ejection of large amounts of processed material into the interstellar medium (Knapp & Morris 1985) and for this reason they are often regarded as chief factories of interstellar dust grains. In particular the atmospheres of carbon stars are generally thought to be the place were carbon and silicon carbide (SiC) grains can easily condense (Tielens 1990).

To study the exact nature of the carbon grains (amorphous or crystalline) as well as of the SiC component (-SiC or -SiC; Borghesi et al. 1985), some years ago our group started a study of the observed infrared spectra of carbon stars in relation to the extinction properties, of both carbon and SiC submicron particles, measured in our laboratory (Borghesi et al. 1985, 1986; Bussoletti et al. 1987; Orofino et al. 1990, 1991; Blanco et al. 1994, 1998).

In particular, in the most recent paper (Blanco et al. 1998, hereinafter Paper I) the spectroscopic and photometric data relative to a sample of 55 carbon stars showing the 11.3 µm feature have been fitted in the wavelength range between 0.4 µm and 100 µm. This has been done by means of a radiative transfer model using the laboratory extinction spectra of amorphous carbon and silicon carbide grains. The transfer code allowed us to determine also, in a self-consistent way, the grain equilibrium temperature of the various species at different distances from the central star and all the relevant circumstellar parameters, which can be very important for the evolutionary study of carbon stars.

To get meaningful information on the nature and physical properties of the dust grains responsible for the 11.3 µm feature and the underlying continuum, the fitting procedure of the spectra has been applied individually to every single source. For this reason it has been possible to take into account not only any variation in position and shape of the SiC band, but also the general trend of the observed flux density of each individual object.

Our analysis has shown that all the sources, in addition to the amorphous carbon grains accounting for the continuum emission, need the presence of -SiC particles while some of them require also -SiC. Moreover, the presence of one or both types of SiC particles seems correlated neither with the total optical thickness nor with any other physical and geometrical parameter of the circumstellar envelope.

Our fitting procedure may be used, also, to derive the distances and the mass-loss rates which are in general very poorly known. The various estimates of the distances, taken from the general literature, show considerable discrepancies, while the evaluations of the mass-loss rates can be in error by more than an order of magnitude. The best-fit parameters found in Paper I have been already used to calculate the mass-loss rates from the central stars of our 55 carbon star sample (see Table 2 of Paper I). In this work we extend our analysis calculating the distances of the same sample of carbon stars by using the physical and geometrical parameters found in Paper I. In Sect. 2 we present the derived distances and describe the computational method which has the advantage of being, on the average, more accurate than those used for the same stars by other authors.

Using such distances and the mass-loss rates for all the 55 stars of our sample, we show that the correlation between the absolute band strength of the SiC feature and the mass loss-rates, already found by other authors with an independent method (Skinner & Whitmore 1988, hereinafter SW), still holds and it is statistically strengthened by the increased number of stars taken into consideration. This correlation, presented in Sect. 3, provides an easy method to derive the distance and/or the mass-loss rate of other carbon stars not included in our sample.

© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000
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