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 content inside the dust gaps of transition disks is important to establish their origin. Aims: We seek to constrain the surface density of warm gas in the disk of HD 139614, a Herbig Ae star with a transition disk exhibiting a dust gap from 2.3 +/- 0.1 to 5.3 +/- 0.3 AU. Methods: We have obtained ESO/VLT CRIRES high-resolution (R similar to 90 000) spectra of CO ro-vibrational emission at 4.7 mu m. We derived constraints on the disk's structure by modeling the line-profiles, the spectroastrometric signal, and the rotational diagrams using flat Keplerian disk models. Results: We detected v = 1 -> 0 (CO)-C-12, 2 -> 1 (CO)-C-12, 1 -> 0 (CO)-C-13, 1 -> 0 (CO)-O-18, and 1 -> 0 (CO)-O-17 ro-vibrational lines. 12CO v = 1 -> 0 lines have an average width of 14 km/s, Tgas of 450 K and an emitting region from 1 to 15 AU. (CO)-C-13 and (CO)-O-18 lines are on average 70 and 100 K colder, 1 and 4 km/s narrower, and are dominated by emission at R>6 AU. The (CO)-C-12 v = 1 -> 0 line-profile indicates that if there is a gap in the gas it must be narrower than 2 AU. We find that a drop in the gas surface density (delta_gas) at R < 5-6 AU is required to simultaneously reproduce the line-profiles and rotational diagrams of the three CO isotopologs. Delta_gas can range from 10(-2) to 10(-4) depending on the gas-to-dust ratio of the outer disk. We find that at 1 < R < 6 AU the gas surface density profile is flat or increases with radius. We derive a gas column density at 1 < R < 6 AU of N-H = 3 x 10(19) - 10(21) cm(-2). We find a 5sigma upper limit on NCO at R < 1 AU of 5 x 10(15) cm(-2) (N-H < 5 x 10(19) cm(-2)). Conclusions: The dust gap in the disk of HD 139614 has gas. The gas surface density in the disk at R<6 AU is significantly lower than the surface density expected from HD 139614's (10(-8) M-circle dot yr(-1)) rate assuming a viscous alpha-disk model. The gas density drop, the non-negative density gradient of 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.