Irreducibility and components rigid in moduli of the Hilbert scheme of smooth curves

Abstract

Denote by $\mathcal {H}_{d,g,r}$ the Hilbert scheme of smooth curves, that is the union of components whose general point corresponds to a smooth irreducible and non-degenerate curve of degree d and genus g in $\mathbb {P}^r$. A component of $\mathcal {H}_{d,g,r}$ is rigid in moduli if its image under the natural map $\pi :\mathcal {H}_{d,g,r} \dashrightarrow \mathcal {M}_{g}$ is a one point set. In this note, we provide a proof of the fact that $\mathcal {H}_{d,g,r}$ has no components rigid in moduli for $g > 0$ and $r=3$, from which it follows that the only smooth projective curves embedded in $\mathbb {P}^3$ whose only deformations are given by projective transformations are the twisted cubic curves. In case $r \ge 4$, we also prove the non-existence of a component of $\mathcal {H}_{d,g,r}$ rigid in moduli in a certain restricted range of d, $g>0$ and r. In the course of the proofs, we establish the irreducibility of $\mathcal {H}_{d,g,3}$ beyond the range which has been known before.