A gas density drop in the inner 6 AU of the transition disk around the Herbig Ae star HD 139614

Abstract

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 12 CO, 2→1 12 CO, 1→0 13 CO, 1→0 C 18 O, and 1→0 C 17 O ro-vibrational lines. Lines are consistent with disk emission and thermal excitation. 12 CO 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. 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 υ = 1 → 0, and are dominated by emission at R≥ 6 AU. The 12 CO 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 13 CO and C 18 O 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 × 10 19 − 10 21 cm −2 (7 × 10 −5 − 2.4 × 10 −3 g cm −2 ) assuming N CO = 10 −4 N H . We find a 5σ upper limit on the CO column density N CO at R≤1 AU of 5 × 10 15 cm −2 (N H ≤ 5 × 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≤ 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. A. Carmona et al.: CO ro-vibrational emission in the transition disk HD 139614. 12 CO 1-0 P(6) 12 CO 1-0 P(7) 12 CO 1-0 P(9) 2.0 2.2 2.4 2.6 12 CO 1-0 P(10) -40 -20 0 20 40 0.2 0.6 1.0 12 CO 1-0 P(11) 12 CO 1-0 P(12) 12 CO 1-0 P(13) 12 CO 1-0 P(15) 13 CO 1-0 R(6) 13 CO 1-0 R(5) 13 CO 1-0 R(4) 2.0 2.2 13 CO 1-0 R(2) -40 -20 0 20 40 0.2 0.6 1.0 13 CO 1-0 R(0) 13 CO 1-0 P(1) 13 CO 1-0 P(2) 13 CO1-0P4 12 CO2-1P9 C 18 O 1-0 R(7) C 18 O 1-0 R(6) C 18 O 1-0 R(5)

New Insights into the Nature of Transition Disks from a Complete Disk Survey of the Lupus Star-forming Region

“…All disks to date that have been sufficiently resolved show gas cavities that are smaller than the dust cavities, an indication of one or more undetected companions (Zhu et al 2011;Pinilla et al 2012;Fung et al 2014). Small gas rings were previously suggested by near-infrared observations of the rovibrational CO lines in some transition disks (Pontoppidan et al 2008;Brown et al 2012;Carmona et al 2017), but ALMA observations allowed the determination of the radial structure of the gas surface density profile through imaging of the rotational CO line, tracing the bulk of the gas, suggesting the clearing by a companion. A consequence of this clearing process is the existence of pressure bumps at the outer gap gas edge, where millimeter dust will be trapped due to gas-dust drag (e.g., Weidenschilling 1977;Zhu et al 2011;Pinilla et al 2012) and show a narrow dust ring or in certain cases asymmetric dust rings due to Rossby-wave instability (Barge & Sommeria 1995;Klahr & Henning 1997) or eccentric planet orbits (Ataiee et al 2013;Ragusa et al 2017).…”

Hints for icy pebble migration feeding an oxygen-rich chemistry in the inner planet-forming region of disks

“…Modeling work by Antonellini et al (2016) found that the infrared water spectrum should respond to the formation and size of an inner cavity with a specific spectral signature, where the depletion of hotter to colder gas by increasing the inner cavity size produces a decrease of higher-excitation lines at shorter wavelengths (3-17 µm) to lower-excitation lines at longer wavelengths (25-35 µm), a spectral signature that is observed in the data (Banzatti et al 2017). Thermo-chemical model fits to spectrally-resolved infrared CO emission in Herbig disks estimated the gas column density as a function of inner disk radius, and clarified that when not observed, the CO gas column density must be depleted by at least a few orders of magnitude (Bruderer 2013;Carmona et al 2017;Bosman et al 2019). In some cases, spatiallyresolved imaging has also shown depletion of CO gas inside dust cavities (Pontoppidan et al 2008;van der Marel et al 2016van der Marel et al , 2018.…”

Reading between the lines

“…In addition, narrow-line components (originating in more compact but relatively hot and dense regions, such as hot spots at the stellar surface or in the accretion shock region) would be stronger for the lines with high transition probability because low transition probability lines would be more susceptible to collisional deexcitation in these environments. Such departures from disklike profile related to optical depth are also observed in CO lines (e.g., see Panić et al 2008;Hein Bertelsen et al 2016;Carmona et al 2017), contributing to the box-like appearance of optically thick lines. As in such cases, Fe I lines with various optical depths can give information concerning the vertical temperature structure.…”