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 υ = 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 υ = 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 υ = 1 → 0, and are dominated by emission at R ≥ 6 AU. The ¹²CO υ = 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.

Based on CRIRES observations collected at the VLTI and VLT (European Southern Observatory, Paranal, Chile) with program 091.C-0671(B).