3 Discussion

The H₂O spectra of the source G43.8-0.1 are a superposition of emission of many components, which frequently overlap in velocity or are blended with weaker features. In our single-dish observations, it was in many cases impossible to separate such spectra into components. For this reason, we analysed only variability of the emission of the most intense features and of the integral flux.

3.1 The flare at the radial velocity of 38.2 km s^-1

The emission at 38.2 km s^-1 was present in the H₂O spectrum of the source G43.8-0.1 from 1977 to 1979. Subsequent observations did not detect this emission. The emission at this velocity reappeared only at the end of 1996 in the form of a strong flare, which lasted for slightly longer than one year. In time, it happened near the activity minimum of the maser as a whole. Most probably, this flare was not local, because an increase of the emission at other radial velocities followed.

The flux density and linewidth of the flare at 38.2 km s^-1 are connected by the relationship $\ln F\propto\Delta V^{-2}$ (Fig. 5). This testifies that the maser, associated with this condensation, is unsaturated or partly unsaturated.

3.2 The maser condensation at 42.2 km s^-1

In this work, we have used the flux variability curve and the dependence between flux and linewidth variations, obtained in 1981-1993 (Lekht 1994). Figures 7 and 8 show these curves supplemented with the data obtained in 1994-1998.

We compared the two stages of evolution of the H₂O maser emission at 42.2 km s^-1: from the beginning of 1982 to the end of 1984 and from the beginning of 1994 to the end of 1996. Table 1 lists the following parameters of the emission for the two stages: duration of the stage in years, radial velocity, flux and linewidth at the beginning and end of each stage. We observe similarities in the qualitative and quantitative variations of all the parameters, excluding the linewidth, which was in both cases decreasing.

 

Table 1: The comparation of two evolutionary periods of H₂O maser emission for component with 42.2 km s^-1

Duration	$V_{\rm LSR}$	F	$\Delta V$
(Year)	(km s-1)	(Jy)	(km s-1)
1982 - 1984	42.30  42.15	470  1100	0.70  0.57
1994 - 1996	42.20  42.30	1300  500	0.55  0.48

In what followed, throughout 1997, the emission continued to fade, but the line was broadening, and the drift direction reversed. At the end of 1997, the intensity fell to such a level that we could reliably measure only the flux density. In 1998, the feature was very faint, and the accuracy of its parameters was insufficient to use in the corresponding plots. From July 1998, this component again started growing.

Figure 8 shows the complete evolution of the flux and linewidth of the 42.2 km s^-1 feature. Arrows show the evolutionary track in time. For the same maser condensation there are two branches of the dependence between F and $\Delta V$, relating to two different temporal stages of the evolution. It is obvious that the transition from one branch to the other took place between mid-1991 and early 1994. Precisely during this time interval, the linewidth was fluctuating. There was a considerable decrease of $\Delta V$ in the middle of this interval, i.e., at the beginning of 1993 (in two-month's time, $\Delta V$ decreased from 0.66 to 0.55 km s^-1).

We can suppose that some changes happened in the region hosting the maser emission at 42.2 km s^-1. This entailed oscillations of the linewidth and subsequent evolution of the emission along a different branch, which was described by functions $\ln F\propto\Delta V^{-2}$ and $\ln F\propto-\Delta V^{-2}$. The cause of this could be a transition from the saturated to unsaturated mode, a change in the maser geometry (probably due to passage of a shock) and/or some parameter of the medium, e.g. temperature. Lekht (1995) considered another possibility, namely action of the accreting matter on the given maser condensation.

In connection with our case, the latter mechanism seems less probable, because the variability of the 42.2 km s^-1 emission correlates with that of the integral flux (with a phase delay of one year). The maser condensation responsible for this emission is located in the long-living emission region. This testifies to the stability of the internal geometry of this region. Most likely, its emission is stimulated by a shock wave arising from interaction of the stellar wind with the surrounding material. Such regions are most frequently associated with directional molecular outflow and radio jets inside the molecular outflow (Koo 1989; Margulis & Snell 1989).

3.3 Activity of the central star in G43.8-0.1

Since 1981 we have recorded 10 flares of the emission. One of them was double-peaked, i.e. two consecutive flares took place at a radial velocity of 37.7 km s^-1 in 1987 and 1989. All the flares before April 1987 were short-living and most probably local. This assertion is substantiated by the fact that during the flares we observed no significant flux and radial-velocity variations of other features.

During the maximum activity of the entire maser (1987-1991), in addition to several strong flares, we observed a growth of the fluxes of all the features in the H₂O spectrum. In this case, the flares were global, i.e. associated with an increase of activity of the maser at whole. Earlier we noted that the variability of the integral H₂O flux did not bear a character of flares (Lekht 1995).

Thus, we may suppose that in G43.8-0.1 there exists some kind of equilibrium between the rate of accretion of matter onto the star, stellar luminosity variations and stellar radiation field, i.e. the process of formation of the star is more or less stationary. Nevertheless, there may exist oscillation processes with a period of the order of 10 years (Yorke & Krügel 1977; Garlick 1978; Tutukov & Shustov 1978), and the star formed may have maxima and minima of its activity. With regard to the earliest observations of the maser G43.8-0.1 in 1976 (Genzel & Downes 1977) and 1977 (Genzel et al. 1979), the activity minimum of the maser was in 1978-1980. In this case, the time interval between the activity minima of the H₂O maser is 18 $\pm$ 1 yr. This figure can be taken as the period of the central-star activity in G43.8-0.1 in the process of its formation.

There can be other reasons of the observed character of the maser emission variability, for instance, shock excitation. However, the lack of direct evidence of the existence of a molecular outflow in G43.8-0.1 (Shepherd & Churchwell 1996) and of VLA maps, obtained during our monitoring (1981-1998) does not allow an unambiguous interpretation of our data.