A dust and gas cavity in the disc around CQ Tau revealed by ALMA (1905.00909v1)

Abstract: The combination of high resolution and sensitivity offered by ALMA is revolutionizing our understanding of protoplanetary discs, as their bulk gas and dust distributions can be studied independently. In this paper we present resolved ALMA observations of the continuum emission ($\lambda=1.3$ mm) and CO isotopologues ($^{12}$CO, $^{13}$CO, C$^{18}$O $J=2-1$) integrated intensity from the disc around the nearby ($d = 162$ pc), intermediate mass ($M_{\star}=1.67\,M_{\odot}$) pre-main-sequence star CQ Tau. The data show an inner depression in continuum, and in both $^{13}$CO and C$^{18}$O emission. We employ a thermo-chemical model of the disc reproducing both continuum and gas radial intensity profiles, together with the disc SED. The models show that a gas inner cavity with size between 15 and 25 au is needed to reproduce the data with a density depletion factor between $\sim 10^{-1}$ and $\sim 10^{-3}$. The radial profile of the distinct cavity in the dust continuum is described by a Gaussian ring centered at $R_{\rm dust}=53\,$au and with a width of $\sigma=13\,$au. Three dimensional gas and dust numerical simulations of a disc with an embedded planet at a separation from the central star of $\sim20\,$au and with a mass of $\sim 6\textrm{-} 9\,M_{\rm Jup}$ reproduce qualitatively the gas and dust profiles of the CQ Tau disc. However, a one planet model appears not to be able to reproduce the dust Gaussian density profile predicted using the thermo-chemical modeling.

• The paper confirms a prominent dust and gas cavity with a gas depletion from 15 to 25 AU and a Gaussian dust ring centered at 53 AU.
• The study uses high-resolution ALMA data of continuum emission and CO isotopologues analyzed via the thermo-chemical DALI code to map disc structure.
• Three-dimensional simulations indicate that an embedded planet of 6–9 Jupiter masses may explain the observed disc features and elevated dust-to-gas ratio.

Analysis of Dust and Gas Cavities in the CQ Tau Disc Using ALMA Observations

This paper provides a comprehensive paper of the disc around CQ Tau, an intermediate-mass pre-main-sequence star, using ALMA observations. The authors explore the dust and gas distribution within the disc and ascertain the presence of a cavity, with particular emphasis on analyzing the continuum emission and various CO isotopologues.

Methodology

The paper employs high-resolution ALMA data, focusing on the continuum emission at 1.3 mm and the integrated intensity of CO isotopologues (^12CO, ^13CO, and C^18O J=2-1). The authors apply a thermo-chemical code, DALI (Dust And LInes), to self-consistently compute the dust and gas temperature, molecular abundances, and excitation state of molecules in the disc. This model effectively reproduces both the continuum and gas radial intensity profiles alongside the disc's spectral energy distribution (SED).

Key Findings

1. Cavity Detection: The ALMA observations confirm a prominent cavity in the CQ Tau disc. The authors model this cavity's gas and dust distribution, establishing a gas cavity radius between 15 and 25 AU, with a density depletion factor ranging from 10^-1 to 10^-3. The dust cavity is described by a Gaussian ring centered at 53 AU, with a standard deviation of 13 AU.
2. Gas and Dust Discrepancies: The spatial extent of the gas, traced by CO isotopologues, is found to be larger than that of the dust, indicating a discrepancy often observed in protoplanetary discs. The results suggest different mechanisms affecting gas and dust distributions, potentially driven by processes like radial drift or differential grain growth.
3. Simulations and Planetary Influence: Three-dimensional numerical simulations suggest that a massive embedded planet, approximately 6 to 9 Jupiter masses situated around 20 AU, could account for the observed features in the disc. These findings highlight the potential of a planetary body significantly influencing the disc's structure and dynamics.
4. Dust-to-Gas Ratio: The paper estimates a global dust-to-gas ratio of approximately 0.09, significantly higher than the canonical interstellar medium value of 0.01. This suggests depletion in volatile carbon, possibly due to sequestration in larger icy bodies or conversion into more complex molecules.

Implications

The insights drawn from this paper are multi-faceted. Understanding the cavity's dynamics elucidates the complex interplay between dust and gas in disc evolution and planet formation. The modeling and simulation approaches presented here provide valuable frameworks for future exploration of transitional discs and investigation of the initial conditions for planet formation. Observational benchmarks provided by CO isotopologues and continuum emissions can guide refinement in disc models and simulations.

Moreover, the capability to infer unseen planetary presence based on disc features broadens the observational strategy of exoplanet detection and characterization. The high dust-to-gas ratio findings motivate further inquiry into chemical processes within the disc, potentially influencing disc lifetime and planet formation efficiency.

Future Directions

The paper suggests that more refined observations and models are necessary to explore alternative clearing mechanisms, such as photoevaporative winds or dead zones. Higher resolution and sensitivity in future ALMA campaigns could resolve fine structures and asymmetries, further contributing to our understanding of disc-planet interactions. Additionally, refining dust grain size distributions and incorporating multi-grain simulations may provide a more comprehensive understanding of dust dynamics and its role in shaping disc morphology.

As advancements in both observational capabilities and modeling techniques continue, the intricate processes governing disc evolution and subsequent planet formation will become increasingly demystified, paving the way for significant theoretical developments in astrobiology and planetary science.