FA50 CREW USED A PORTION OF NOTAMED CLOSED RWY FOR TKOF AT COS.

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TAXIING FOR TKOF; CLRED TO TAXI TO RWY 17R. (N 1600 FT OF RWY 17R NOTAMED CLOSED.) TAXIED N ON TXWY A TO INTXN OF TXWY A AND RWY 17R (ABOUT 750 FT FROM N END). WAITED APPROX 10 MINS FOR SEVERAL TOUCH AND GO ACFT AND ONE BARON DEP AHEAD OF US ALL OF WHICH USED AVAILABLE RWY ABEAM OUR POS (750 FT POINT). FINALLY CLRED 'INTO POS AND HOLD' WHICH WE DID (FORGETTING FIRST 1600 FT CLOSED NOTAM). A FEW SECONDS LATER WE WERE CLRED FOR TKOF; COPLT FLYING FROM L SEAT APPLIED TKOF PWR AND AT THE SAME TIME THAT I NOTED 'TKOF PWR SET' THE TWR ADVISED US THAT 'THE FIRST 1600 FT OF THAT RWY ARE CLOSED.' ABOUT THIS TIME WE WERE GOING THROUGH 80 KTS AT ABOUT THE 1200 FT POINT AND LOOKING DOWN THE RWY I COULD SEE 2 THIN YELLOW CHEVRONS PAINTED ON THE RWY DENOTING CLOSURE AT ABOUT THE 1600 FT POINT. THE TKOF CONTINUED NORMALLY WITH NO OTHER MENTION OF THE RWY CLOSURE BY THE TWR. THE RWY WAS NOTAMED CLOSED FOR THE FIRST 1600 FT. I DON'T RECALL IF THIS WAS ALSO SPELLED OUT ON THE ATIS; HOWEVER THERE WERE NO MARKINGS ON THE RWY OR TXWY AT THE POINT WE WERE CLRED FROM AND NO ADVISORIES FROM THE TWR UNTIL AFTER WE HAD BEEN CLRED FOR TKOF. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: CREW WAS FLYING A FALCON 50. THE CREW TRAINS IN BOTH THE L SEAT AND R SEAT. HE DID NOT FEEL THAT SEAT SWAPPING CONTRIBUTED TO THE INCIDENT. HE PRIMARILY FAULTS THE COMPLETE LACK OF ARPT RWY MARKINGS THAT INDICATE A CLOSED PART OF THE RWY. HE ALSO BELIEVES THAT THE TWR COULD HAVE GIVEN THEM A REMINDER.