Brill-Noether existence with secants

We completely characterize when a general curve of genus $g$ admits a non-degenerate degree $d$ map to projective space ${\mathbb{P}}^r$, that is $k$-secant along a linear space ${\mathbb{P}}^s\subset {\mathbb{P}}^r$ and deforms in a smooth family of expected dimension. This gives an optimal improvement of a theorem of Farkas, who gave necessary conditions, and proved sufficiency under additional numerical assumptions. Our main theorem shows that the natural necessary conditions are in fact sufficient, yielding a generalization of the classical Brill-Noether existence theorem. The proof relies on deformation theory of stable maps, and is valid in arbitrary characteristic.