Context : Quantifying the gas surface density inside the dust cavities and gaps of transition disks is important to establish their origin .

Aims : We seek to constrain the surface density of warm gas in the inner disk of HD 139614 , an accreting 9 Myr Herbig Ae star with a ( pre- ) transition disk exhibiting a dust gap from 2.3 \pm 0.1 to 5.3 \pm 0.3 AU .

Methods : We observed HD 139614 with ESO/VLT CRIRES and obtained high-resolution ( R \sim 90 000 ) spectra of CO ro-vibrational emission at 4.7 \mu m. We derived constraints on the disk ’ s structure by modeling the CO isotopolog line-profiles , the spectroastrometric signal , and the rotational diagrams using grids of flat Keplerian disk models .

Results : We detected \upsilon = 1 \rightarrow 0 ~ { } ^ { 12 } CO , 2 \rightarrow 1 ^ { 12 } CO , 1 \rightarrow 0 ^ { 13 } CO , 1 \rightarrow 0 C ^ { 18 } O , and 1 \rightarrow 0 C ^ { 17 } O ro-vibrational lines .
Lines are consistent with disk emission and thermal excitation . ^ { 12 } CO \upsilon = 1 \rightarrow 0 lines have an average width of 14 km s ^ { -1 } , T _ { gas } of 450 K and an emitting region from 1 to 15 AU . ^ { 13 } CO and C ^ { 18 } O lines are on average 70 and 100 K colder , 1 and 4 km s ^ { -1 } narrower than ^ { 12 } CO \upsilon = 1 \rightarrow 0 , and are dominated by emission at R \geq 6 AU .
The ^ { 12 } CO \upsilon = 1 \rightarrow 0 composite line-profile indicates that if there is a gap devoid of gas it must have a width narrower than 2 AU .
We find that a drop in the gas surface density ( \delta _ { gas } ) at R < 5 - 6 AU is required to be able to simultaneously reproduce the line-profiles and rotational diagrams of the three CO isotopologs .
Models without a gas density drop generate ^ { 13 } CO and C ^ { 18 } O emission lines that are too broad and warm .
The value of \delta _ { gas } can range from 10 ^ { -2 } to 10 ^ { -4 } depending on the gas-to-dust ratio of the outer disk .
We find that the gas surface density profile at 1 < R < 6 AU is flat or increases with radius .
We derive a gas column density at 1 < R < 6 AU of N _ { H } = 3 \times 10 ^ { 19 } -10 ^ { 21 } cm ^ { -2 } ( 7 \times 10 ^ { -5 } -2.4 \times 10 ^ { -3 } g cm ^ { -2 } ) assuming N _ { CO } = 10 ^ { -4 } N _ { H } .
We find a 5 \sigma upper limit on the CO column density N _ { CO } at R \leq 1 AU of 5 \times 10 ^ { 15 } cm ^ { -2 } ( N _ { H } \leq 5 \times 10 ^ { 19 } cm ^ { -2 } ) .

Conclusions : The dust gap in the disk of HD 139614 has molecular gas .
The distribution and amount of gas at R \leq 6 AU in HD 139614 is very different from that of a primordial disk .
The gas surface density in the disk at R \leq 1 AU and at 1 < R < 6 AU is significantly lower than the surface density that would be expected from the accretion rate of HD 139614 ( 10 ^ { -8 } M _ { \odot } yr ^ { -1 } ) assuming a standard viscous \alpha -disk model .
The gas density drop , the non-negative density gradient in the gas inside 6 AU , and the absence of a wide ( > 2 AU ) gas gap , suggest the presence of an embedded < 2 M _ { J } planet at around 4 AU .