are the envelopes of the following set of ellipses "classical" curves of mathematics, that appear naturally as the solution of a mathematical/physical problem;

$$\mathcal{E}:=\Biggr\{\frac{(x-e)^2}{a^2} +\frac{y^2}{a^2-e^2}-1\ =\ 0\Biggr\},\quad e\in[0,a)$$

the foci of these ellipses have coordinates $(0,0)$ and $(2e,0)$