## 3. The modelWe compute a numerical model of a jet with a sinusoidal variable
ejection velocity of period yr,
half-amplitude
km s We choose an initial jet density of
cm We assume that both the jet and the environment are initially neutral, and that both have an initial temperature of K (the precise value for this temperature not being important, since the Mach number of the flow is very high and all of the shocks are strong). The numerical simulation is carried out with the 3D adaptive grid code "yguazú-a", which is described in detail by Raga et al. (2000). In the configuration which has been used for the calculation, the code integrates the 3D gasdynamic equations, an advection equation for a passive scalar (with which different regions of the flow can be labeled), and a rate equation for the ionization of hydrogen. The simple cooling function discussed by Raga et al. (1999) has been included in the energy equation. The calculation is done on a hierarchical, binary adaptive grid with a maximum resolution of cm (along the three axes). The maximum resolution is only allowed in the region of space occupied by the material originally coming from the jet, and the region filled by environmental material is resolved at most with a grid of cm spacing. The resulting grid structure is illustrated in Fig. 2.
The flow stratification obtained after a
yr time-integration is shown in
Fig. 2, where we display the column density (integrated along the
Fig. 3 and Fig. 4 show the 3.6 cm, free-free
continuum maps predicted from the numerical jet model for different
integration times. The maps have been computed assuming that the
It is clear that the knots show a complex time-evolution. In Fig. 4, we see that over a period of 10 years knot F becomes fainter (for yr). Knot D also fades away, but knot E becomes brighter with increasing time. Knot D, however, had shown a dramatic intensity increase for yr (see Fig. 3). These complex changes in radio continuum intensity are illustrated
in Fig. 5, where we show the 3.6 cm emission integrated
along the
Given the extreme complexity of the flow, it is hard to understand in detail the light curves obtained for the successive knots. As pointed out by Raga & Noriega-Crespo (1998), the knots produced by a time-dependent ejection velocity have an intensity that first increases, and then decreases as the knots travel away from the source. In the present simulation, this effect is combined with the precession, which becomes more important at larger distances from the source (Raga et al. 1993). As the knots diverge from each other at larger distances from the source (Raga & Biro 1993), the trailing knots eventually cross the bow shock wings of previously ejected knots. These interactions lead to brightenings of the knots. A more dramatic effect is seen when the trajectory of one of the knots intersects the leading bow shock of the outflow. This is seen for knot C in the yr map, and for knot D in the yr map (see Fig. 3). A qualitative comparison of these results with the 3.6 cm VLA maps of the Serpens jet is presented in the following section. © European Southern Observatory (ESO) 2000 Online publication: January 29, 2001 |