frankohara's Cannondale Capo

It's a special star nut

I thought it was a special steerer :-)

I know Cervelo used to (maybe still do) issue a metal sleeve which you were supposed to bond into the carbon steerer, and then use a horrible star nut which gripped the metal liner, not the carbon.

My personal solution is to bond in a custom nut about halfway down the section of steerer which is clamped by the stem, that provides some reinforcement against the stem clamp's radial compression which is the effect Cannondale are obviously aiming for. The disadvantages are that it's non-adjustable, irreversible and expensive (because every bike has its own custom one-off nut) but it's what I have on all my carbon-steerer forks.