Let $\overline{ \mathcal{ M}}_{ g,A[n]}$ be the Hassett moduli stack of weighted stable curves, and let $\overline{ M}_{ g,A[n]}$ be its coarse moduli space. These are compactifications of $\mathcal{ M}_{ g,n}$ and $M_{ g,n}$ respectively, obtained by assigning rational weights $A = (a_{ 1},...,a_{ n})$, $0< a_{ i} \leq 1$ to the markings; they are defined over $\mathbb{ Z}$, and therefore over any field. We study the first order infinitesimal deformations of $\overline{ \mathcal{ M}}_{ g,A[n]}$ and $\overline{ M}_{ g,A[n]}$. In particular, we show that $\overline{ M}_{ 0,A[n]}$ is rigid over any field, if $g\geq 1$ then $\overline{ \mathcal{ M}}_{ g,A[n]}$ is rigid over any field of characteristic zero, and if $g+n > 4$ then the coarse moduli space $\overline{ M}_{ g,A[n]}$ is rigid over an algebraically closed field of characteristic zero. Finally, we take into account a degeneration of Hassett spaces parametrizing rational curves obtained by allowing the weights to have sum equal to two. In particular, we consider such a Hassett $3$-fold which is isomorphic to the Segre cubic hypersurface in $\mathbb{ P}^4$, and we prove that its family of first order infinitesimal deformations is non-singular of dimension ten, and the general deformation is smooth.
Categories: math.AG