Gas temperature structure across transition disk cavities

Abstract

Context.Most disks observed at high angular resolution show signs of substructures like rings, gaps, arcs, and cavities in both the gas and the dust. To understand the physical mechanisms responsible for these structures, knowledge about the gas surface density is essential. This, in turn, requires information on the gas temperature. Aims. The aim of this work is to constrain the gas temperature as well as the gas surface densities inside and outside the mm-dust cavities of two transition disks: LkCa15 and HD 169142, with dust cavities of 68 AU and 25 AU, respectively. Methods. We use some of the few existing ALMA observations of the J = 6 − 5 transition of 13 CO together with archival J = 2 − 1 data of 12 CO, 13 CO and C 18 O. The ratio of the 13 CO J = 6 − 5 to the J = 2 − 1 transition is used to constrain the temperature and is compared with that found from peak brightness temperature of optically thick lines. The spectra are used to resolve the innermost disk regions to a spatial resolution better than the beam of the observations. Furthermore, we use the thermochemical code DALI to model the temperature and density structure of a typical transition disk as well as the emitting regions of the CO isotopologues. Results. The 13 CO J = 6−5 and J = 2−1 transitions peak inside the dust cavity in both disks, indicating that gas is present in the dust cavities. The kinematically derived radial profiles show that the gas is detected down to 10 and 5 − 10 AU, much further in than the dust cavities in the LkCa15 and HD 169142 disks, respectively. For LkCa15, the steep increase towards the star in the 13 CO J = 6 − 5 transition, in contrast to the J = 2 − 1 line, shows that the gas is too warm to be traced by the J = 2 − 1 line and that molecular excitation is important for analysing the line emission. Quantitatively, the 6 − 5/2 − 1 line ratio constrains the gas temperature in the emitting layers inside the dust cavity to be up to 65 K, warmer than in the outer disk at 20−30 K. For the HD 169142, the lines are optically thick, complicating a line ratio analysis. In this case, the peak brightness temperature constrains the gas in the dust cavity of HD 169142 to be 170 K, whereas that in the outer disk is only 100 K. The data indicate a vertical structure in which the 13 CO 6 − 5 line emits from a higher layer than the 2 − 1 line in both disks, consistent with exploratory thermo-chemical DALI models. Such models also show that a more luminous central star, a lower abundance of PAHs and the absence of a dusty inner disk increase the temperature of the emitting layers and hence the line ratio in the gas cavity. The gas column density in the LkCa15 dust cavity drops by a factor > 2 compared to the outer disk, with an additional drop of an order of magnitude inside the gas cavity at 10 AU. In the case of HD 169142, the gas column density drops by a factor of 200−500 inside the gas cavity. Conclusions. The gas temperatures inside the dust cavities steeply increase towards the star and reach temperatures up to 65 K (LkCa15) an...

ALMA Detection of Dust Trapping around Lagrangian Points in the LkCa 15 Disk

“…The presence of material along the horseshoe orbit from the ALMA data presented here would prefer the lower planet mass scenario. As a higher mass planet would more effectively prevent gas flowing through the cavity, this low mass scenario is also more consistent with the presence of substantial amounts of molecular gas in the inner LkCa 15 disk (Leemker et al 2022). Nevertheless, this B42 planet may not be sufficient to fully account for both the wide inner cavity and the B101 ring further out, where additional planets and/or other mechanisms might act in concert to shape the LkCa 15 disk.…”

Disentangling the protoplanetary disk gas mass and carbon depletion through combined atomic and molecular tracers

“…Most of the observed CO isotopologue lines are centrally peaked, without resolving the inner cavity. The inner cavity is resolved in the 13 CO J=2-1 and C 18 O J=2-1 ALMA data, lately confirmed by Leemker et al (2022) using 13 CO J=2-1 data with higher spatial resolution and using the high velocity line wings that trace regions closer in to the star than the spatial resolution of the data (Bosman et al 2021c). The CN emission is centrally peaked and extends out to 1000 AU, similar to the 12 CO data.…”

“…There has been speculation about the existence of multiple planets inside the cavity based on asymmetric emission in scattered light and Hα emission (Kraus & Ireland 2012;Sallum et al 2015), but these were later shown to be likely due to disk features (Thalmann et al 2016;Currie et al 2019). The gas disk extends out to ∼1000 AU (Jin et al 2019) and has a significantly smaller cavity than the one found in the dust at 15-40 AU (van der Marel et al 2015;Jin et al 2019;Leemker et al 2022).…”

“…Additionally, this project makes use of data from various archival ALMA programs, which are summarized in Table 2. We include high-resolution CO J=2-1, 13 CO J=2-1 and C 18 O J=2-1 lines to constrain the radial profile of the CO emission, 13 CO J=6-5 to further constrain the temperature profile (Leemker et al 2022), and C 2 H to constrain the C/O ratio in the disk. CO isotopologue ALMA data were self-calibrated in CASA and CLEANed using a Keplerian mask.…”

“…CO isotopologue ALMA data were self-calibrated in CASA and CLEANed using a Keplerian mask. Further details of the data and initial calibration are described in Leemker et al (2022). C 2 H ALMA observations and reduction are described in more detail in Bergner et al (2019).…”