  We prove the existence of a one-parameter family of self-similar solutions
with time dependent tails for Smoluchowski's coagulation equation, for a class
of kernels $K(x,y)$ which are homogeneous of degree one and satisfy $K(x,1)\to
k_0>0$ as $x\to 0$. In particular, we establish the existence of a critical
$\rho_*>0$ with the property that for all $\rho\in(0,\rho_*)$ there is a
positive and differentiable self-similar solution with finite mass $M$ and
decay $A(t)x^{-(2+\rho)}$ as $x\to\infty$, with $A(t)=e^{M(1+\rho)t}$.
Furthermore, we show that (weak) self-similar solutions in the class of
positive measures cannot exist for large values of the parameter $\rho$.
