The long-period variables
(LPVs) located on the Asymptotic Giant Branch (AGB)
represent an important stage of stellar evolution in
which some are progenitors of planetary nebula while
others are directly evolving to the white dwarf stage
(see the reviews by Habing 1990, 1996,
and Wood 1990).
They play a major part in galactic evolution through
enrichment of the interstellar medium in heavy elements
via mass loss.
Several investigators (e.g. Whitelock et al. 1991) have studied correlations between observed quantities such as the period and amplitude of light variation, asymmetry of the light-curve shape (rise time expressed as a fraction of the period), the IR excess of the star and its JHKL colours, or parameters derived from the IRAS (Infrared Astronomical Satellite) Low Resolution Spectrograph and Point Source Catalog. The period seems to be a particularly important characteristic of LPVs. Campbell (1925) first showed that a statistical relationship exists between the period and the shape of the light curve. The period is also correlated to the infrared excess due to circumstellar dust (De Gioia-Eastwood et al. 1981; Jura 1986), and to the expansion velocity (Ukita 1982; Sivagnanam et al. 1989). These relations could be connected to the link between the period and the estimated mass loss rate (Wood 1990). Whitelock et al. (1986, 1991) have shown that larger mass loss is correlated to longer period and larger amplitude. Furthermore, it has been mentioned in many works that LPVs with more asymmetric light curves lose mass more rapidly than those with symmetric ones (see the review by Jura & Kleinmann 1992a).
As established previously by Feast (1963), and confirmed by many recent works (for example, Feast 1989; Kerschbaum & Hron 1992; Jura & Kleinmann 1992a, 1992b; Jura et al. 1993; Luri et al. 1996), LPVs exhibit distinctive kinematic properties and thus belong to different stellar populations. The period of the star could be a discriminant parameter.
These relationships are still not exactly understood, but it is clear that the period, amplitude, and asymmetry of the light curves of LPVs seem to be closely connected to their pulsation properties and therefore to their evolution. Hence these parameters might be used to define an automatic classification and to identify homogeneous groups of LPVs.
These evolved AGB-stars include three spectral types: M, S, and C, depending on the abundance ratio C/O in their surface layers and on their spectroscopic characteristics. S-stars are probably intermediate objects. A similar dichotomy also exists in their circumstellar material; the silicate and SiC dust features are observed respectively in the oxygen-rich (M-type and possibly S-type) and carbon-rich Miras. This can explain why IRAS colors alone are not sufficient to identify C LPVs. To distinguish carbon- from oxygen-rich stars, Epchtein et al. (1987) propose to use the K- and L-band photometric fluxes.
The purpose of this paper is to study the mean light curve parameters of LPVs, to classify these stars according to these parameters, and to use this classification together with IRAS colors to discriminate between carbon- and oxygen-rich LPVs (Sect. 6).
After a brief description of the data in Sects. 2 and 3, we present in Sect. 4 some statistical results on mean parameters of light curves obtained from the AAVSO visual data over 75 years for a sample of 355 LPVs. In Sect. 5 we present an automatic classification of our LPV sample based on the characteristics of their mean light curve, examine properties of stars belonging to each cluster and the link with previous classifications. Finally, we examine how the IRAS colors and our classification enable us to recognize carbon-rich LPVs.