Long-period variable stars (LPVs) are cool giant stars of low and intermediate masses, and the light variations associated with the pulsation of their envelope have long periods (typically 300 d) and large amplitudes (up to 9 magnitudes peak-to-peak in the visual). LPVs are subdivided into Mira Ceti-type variables (Mira stars or Miras) and semi-regular variables (of the SRa or SRb subtypes), depending on the light curve amplitude, shape and regularity.
Apart from the brightness fluctuations, LPVs are characterized by striking spectral changes:
Merrill (1955) was the first to suggest that the spectral changes observed in LPVs may be explained by the presence of a shock wave moving outward. This idea has been extensively investigated since then by a number of authors: e.g. Gorbatskii (1957, 1961), Willson (1972, 1976), Hinkle (1978), Willson & Hill (1979), Hill & Willson (1979), Wood (1979), Willson et al. (1982), Gillet & Lafon (1983, 1984, 1990), Fox et al. (1984), Fox & Wood (1985), Bessell et al. (1996), Fadeyev & Gillet (2000, 2001).
Nevertheless, due to the lack of a complete and self-consistent model describing the pulsation and its effect on the spectrum, most of the questions raised by the spectral peculiarities of LPVs remain unanswered so far. For instance, the origin of the emission lines is still debated: although most authors believe they are formed in the hot wake of the shock (Gillet 1988a; de la Reza 1986 and references therein), others reject the shock wave scenario and invoke instead purely non-LTE radiative processes (Magnan & de Laverny 1997).
The velocity of the shock front is also a matter of debate: large shock-wave velocity discontinuities (of the order of 60 km s-1) are indeed required to photodissociate H2 molecules and photoionize hydrogen atoms in order to subsequently produce by recombination the observed Balmer emission lines (Gillet et al. 1989). Velocity discontinuities of that order are indeed observed for fluorescent lines (Willson 1976), although double absorption lines yield much lower velocity discontinuities (of the order of 20-30 km s-1; e.g. Adams 1941; Merrill & Greenstein 1958; Maehara 1968; Tsuji 1971; Hinkle 1978; Hill & Willson 1979; Hinkle et al. 1997 and references therein). The lower velocity discontinuities derived from absorption lines are not necessarily incompatible with the theoretical requirement, since the velocities of the blue and red components suffer from a strong averaging effect due to the large extension of the region where they form.
The line-doubling phenomenon is source of conflicting theories. Some important progress towards its understanding has however been made recently thanks to a dedicated monitoring of Mira variables (Alvarez et al. 2000a, hereafter 1), which confirmed that the line-doubling phenomenon is caused by a shock wave propagating in the photosphere. It was shown that the temporal evolution of the red and blue peaks of the double absorption lines of the Mira variable RTCyg is consistent with the so-called "Schwarzschild scenario''. This scenario, originally presented by Schwarzschild (1952) for WVir Cepheids, relates the evolution of the line profile to the progression of a shock wave in the atmosphere. Alternative models accounting for line doubling without resorting to differential atmospheric motions (Karp 1975; Gillet et al. 1985a) can thus be definitively rejected.
It was shown in 1 that some LPVs (e.g., XOph) do not (or, at least, not clearly) exhibit line doubling around maximum light. This paper further investigates the questions raised by this result: what is the frequency of Mira variables exhibiting line-doubling around maximum light (Sect. 4.3.1) and what are their distinctive properties (Sect. 4)? To address these questions, a large sample of LPVs (mostly Mira variables) of various spectral types was monitored during 2 years (Sect. 2).
Copyright ESO 2001