TPGK facing donk turn bet

[kmar1085]

vilian is 55/9, what do you do on the turn here? I cant really see him having AK-AJ here, being that he checked his option then just called my raise PF. The shove on the turn just feels like a weak ace, at best...

PokerStars No-Limit Hold'em, $0.50 BB (6 handed) Hand History Converter Tool from FlopTurnRiver.com (Format: 2+2 Forums)

MP ($15.15)
CO ($28.90)
Hero ($49.75)
SB ($7.45)
BB ($18.10)
UTG ($44.20)

Preflop: Hero is Button with T[img]/images/graemlins/spade.gif[/img], A[img]/images/graemlins/spade.gif[/img]. CO posts a blind of $0.50.
<font color="#666666">2 folds</font>, CO (poster) checks, <font color="#CC3333">Hero raises to $2.5</font>, <font color="#666666">2 folds</font>, CO calls $2.

Flop: ($5.75) 7[img]/images/graemlins/heart.gif[/img], A[img]/images/graemlins/diamond.gif[/img], 7[img]/images/graemlins/diamond.gif[/img] <font color="#0000FF">(2 players)</font>
CO checks, <font color="#CC3333">Hero bets $4</font>, <font color="#CC3333">CO raises to $8</font>, Hero calls $4.

Turn: ($21.75) 2[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(2 players)</font>
<font color="#CC3333">CO bets $18.4 (All-In)</font>, Hero?