A gas density drop in the inner 6 AU of the transition disk around the Herbig Ae star HD 139614: Further evidence for a giant planet inside the disk?

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 ± 0.1 to 5.3 ± 0.3 AU. Methods. We observed HD 139614 with ESO/VLT CRIRES and obtained high-resolution (R ∼ 90 000) spectra of CO ro-vibrational emission at 4.7 μm.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 ν = 1 → 0 ¹²CO, 2 → 1 ¹²CO, 1→ 0 ¹³CO, 1→ 0 C¹⁸O, and 1→ 0 C¹⁷O ro-vibrational lines. Lines are consistent with disk emission and thermal excitation. ¹²CO ν = 1 → 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. ¹³CO and C¹⁸O lines are on average 70 and 100 K colder, 1 and 4 km s^-1 narrower than ¹²CO ν = 1 → 0, and are dominated by emission at R ≥ 6 AU. The ¹²CO ν = 1 → 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 (δ_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 ¹³CO and C¹⁸O emission lines that are too broad and warm. The value of δ_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 × 10¹⁹ -10²¹ cm^-2 (7 × 10^-5-2:4 × 10^-3 g cm^-2) assuming N_CO = 10^-4N_H. We find a 5σ upper limit on the CO column density N_CO at R ≤ 1 AU of 5 × 10¹⁵ cm^-2 (N_H ≤ 5 × 10¹⁹ cm^-2). Conclusions. The dust gap in the disk of HD 139614 has molecular gas. The distribution and amount of gas at R ≤ 6 AU in HD 139614 is very different from that of a primordial disk. The gas surface density in the disk at R ≤ 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_⊙ yr^-1) assuming a standard viscous α-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.