Suppose that $S$ is a non-singular complex projective surface that is defined over $\overline{\mathbb Q}$, namely $S\cong\text{Proj}\frac{\mathbb C[T\_1,T\_2,\ldots,T\_n]}{(f\_1,\ldots,f\_n)}$ where $f\_i$ has coefficients in $\overline{\mathbb Q}$ for every $i=1,\ldots,n$. If $S'$ is a minimal model of $S$, then can we conclude that also $S'$ is defined over $\overline{\mathbb Q}$? Does the blow-down preserve the field of definition (for algebraically closed fields)?



Minimal model of a non-singular complex projective surface defined over $\overline{\mathbb Q}$
I think so, yes. 


1) If $V\_{/\overline{\mathbb{Q}}}$ be a smooth projective variety. Then on Neron-Severi groups we have $NS(V) = NS(V\_{/\mathbb{C}})$. I believe this follows from the existence of the Picard scheme: the component group of a group scheme does not change under extension from one algebraically closed field to another.


2) Let $S\_{\overline{\mathbb{Q}}}$ be a smooth, projective surface. By Castelnuovo's Criterion, minimal models are obtained precisely by a finite sequence of contractions of $-1$-curves. By part 1), every $(-1)$-curve in the Neron-Severi group of $S\_{\mathbb{C}}$ is, up to algebraic equivalence, defined over $\overline{\mathbb{Q}}$, so the contractions can be done over $\mathbb{\overline{Q}}$.


**Added**: In the above there is an implicit assumption that the isomorphism class of the blowdown depends only on the Neron-Severi class of the $(-1)$-curve. I hope that's true; at the moment, I don't quite remember. Alternatively, the above argument certainly produces at least one $\mathbb{C}$-minimal model which is defined over $\overline{\mathbb{Q}}$. If the surface is irrational, minimal models are unique. If the surface is rational, then the minimal models are all known: see e.g. [here](http://www.encyclopediaofmath.org/index.php/Minimal_model). They can all be defined over $\overline{\mathbb{Q}}$.


