1 Introduction

Two of the outstanding problems concerning M-type Miras are the basic mechanism for dust formation and the mechanism for the generation of high mass loss rates of the order of 10^-6$M_{\odot}$yr^-1 (Habing 1996). Theoretical works of Wood (1979), Bowen (1988), Fleischer et al. (1992), Feuchtinger et al. (1993), Höfner & Dorfi (1997) and Winters et al. (2000) show that the high mass loss rates of Miras can be explained by a combination of pulsation of the star and radiation pressure acting on dust. Pulsation generates shock waves which move out through the atmosphere thereby increasing the mass density in the outer parts of the atmosphere. The passage of a shock leads to a sudden increase of the pressure, density and temperature of the gas. Behind the shock the gas cools via the emission of radiation, which leads to conditions that are favourable for dust formation. The shocks in the outer parts of the atmosphere thereby trigger the formation of dust. Radiation pressure acting on this dust leads to the high mass loss rate. One way to reveal the thermo- and hydro-dynamical conditions in a Mira atmospheres is to study the various emission lines which are emitted behind the shock front and which can be observed over a substantial portion of the pulsation period. Analysing a time-resolved series of these emission lines offers the possibility to determine the hydrodynamical conditions in different layers of the atmosphere influenced by the passing shock wave. In particular, the hydrodynamical conditions of the outer, dust-producing layers of the atmosphere can be studied with the help of metal emission lines which appear late in the pulsation cycle when the shock wave has reached these layers. M-type Miras show a variety of metal emission lines in all regions of wavelength. The most substantial observations in the optical wavelength region are the long-term observation of o Ceti by Joy (1954) and the observation of several Miras by Merrill (1940, 1945, 1946a, 1946b, 1947a, 1947b). In the near-IR wavelength region ( $\lambda < 1.01 \mu$m), Gillet et al. 1985a, 1985b, 1985c) observed various metal emission lines of SCar and o Ceti and interpreted the variation of the lines using a shock model. Using ISO, Aoki et al. (1998) found a number of high intensity emission lines of ionized metals in the mid-IR spectral region, although the stars examined were semi-regular and irregular variables rather than Miras. Finally, in the UV spectral region, Wood & Karovska (2000) observed emission lines in M-type Miras and suggested shock waves as an explanation for their variations.

Around maximum light, the hydrogen Balmer emission lines are prominent in the optical spectra. Deutsch & Merrill (1959) were the first to interpret these lines as due to shock waves and the theoretical work of Gorbatski (1961) verified their interpretation. A number of observational studies of Balmer emission lines have been made over the years, many concentrating on the mean radial velocity (Merrill 1940, 1945,1946a,1946b,1947a,1947b; Joy 1947, 1954; Gillet et al. 1985a,1985b,1985c). Quantitive data on Balmer line shapes, widths and line fluxes, as well as theoretical shock model estimates for the shock speed and temperature and line fluxes, are presented in Fox et al. (1984, 1985) and Gillet et al. (1983, 1985c). The emphasis of our work is on various metal emission lines of FeI, FeII, MgI, MnI and SiI, especially the lines which appear at late phases and which therefore should originate from the outer parts of the atmosphere. For two of the stars in our sample (RCar and RLeo) we observed forbidden lines [FeII] around minimum light. These lines were observed by Joy (1954) in o Ceti as well as by Merrill (1940, 1947b) in the M-type Miras UOri, RLeo, RHya and the S-type Mira $\chi$ Cyg. These lines must occur only very briefly in each cycle or perhaps only in occasional cycles as we did not see the lines in R Leo during the minimum of March 1999 whereas the lines were strong during the minimum of December 1999. Similarly, we did not see the lines during the minima of o Ceti (which we also observed, but do not present in this paper since no metal emission lines were detected) or R Hya although the lines have been reported in the past in these stars. We also note that although Joy (1954) reports the lines in o Ceti, Merrill (1940) explicitly notes that he did not see them.

Existing spectral observations have mostly concentrated on the mean radial velocities of the metal emission lines. There is very little published quantitive data on the shapes, widths and fluxes of metal emission lines. Here we present these quantities for a sample of six M-type Miras, namely RAql, RRSco, RCar, RLeo, SScl and RHya, which range in period from 281 days to 389 days. These Miras have been observed at multiple phases of the pulsation cycle. Because of this phase coverage, the data shows the history of the shock as it emerges through the deep photosphere (before maximum light) and then moves out through the atmosphere.

As well as the metal line observations, we also present similar data for the hydrogen emission lines H$\gamma$, H$\delta$, H$\zeta$, H$\eta$. These lines are most prominent around maximum light and they provide information about early history of the specific shock wave that generated the metal lines we observed.