MINDS: The DR Tau disk

I. Combining JWST-MIRI data with high-resolution CO spectra to characterise the hot gas

¹ Leiden Observatory, Leiden University, 2333 CC Leiden, The Netherlands
e-mail: temmink@strw.leidenuniv.nl
² Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching, Germany
³ Université Paris-Saclay, CNRS, Institut d'Astrophysique Spatiale, 91405 Orsay, France
⁴ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁵ STAR Institute, Université de Liège, Allée du Six Août 19c, 4000 Liège, Belgium
⁶ Max-Planck-Institut für Astronomie (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
⁷ Dept. of Astrophysics, University of Vienna, Türkenschanzstr. 17, 1180 Vienna, Austria
⁸ ETH Zürich, Institute for Particle Physics and Astrophysics, Wolfgang-Pauli-Str. 27, 8093 Zürich, Switzerland
⁹ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
¹⁰ Centro de Astrobiología (CAB), CSIC-INTA, ESAC Campus, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
¹¹ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
¹² Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin D02 XF86, Ireland
¹³ Kapteyn Astronomical Institute, Rijksuniversiteit Groningen, Post-bus 800, 9700AV Groningen, The Netherlands
¹⁴ SRON Netherlands Institute for Space Research, Niels Bohrweg 4, NL-2333 CA Leiden, The Netherlands
¹⁵ Department of Astronomy, Stockholm University, AlbaNova University Center, 10691 Stockholm, Sweden
¹⁶ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
¹⁷ Space Research Institute, Austrian Academy of Sciences, Schmiedlstr. 6, 8042, Graz, Austria
¹⁸ TU Graz, Fakultät für Mathematik, Physik und Geodäsie, Petersgasse 16 8010 Graz, Austria
¹⁹ Centro de Astrobiología (CAB, CSIC-INTA), Carretera de Ajalvir, 28850 Torrejón de Ardoz, Madrid, Spain

Received: 11 December 2023
Accepted: 20 March 2024

Abstract

Context. The MRS mode of the JWST-MIRI instrument has been shown to be a powerful tool to characterise the molecular gas emission of the inner region of planet-forming disks. Investigating their spectra allows us to infer the composition of the gas in these regions and, subsequently, the potential atmospheric composition of the forming planets. We present the JWST-MIRI observations of the compact T-Tauri disk, DR Tau, which are complemented by ground-based, high spectral resolution (R ~ 60 000–90 000) CO ro-vibrational observations.

Aims. The aim of this work is to investigate the power of extending the JWST-MIRI CO observations with complementary, high-resolution, ground-based observations acquired through the SpExoDisks database, as JWST-MIRI’s spectral resolution (R ~ 1500– 3500) is not sufficient to resolve complex CO line profiles. In addition, we aim to infer the excitation conditions of other molecular features present in the JWST-MIRI spectrum of DR Tau and link those with CO.

Methods. The archival complementary, high-resolution CO ro-vibrational observations were analysed with rotational diagrams. We extended these diagrams to the JWST-MIRI observations by binning and convolution with JWST-MIRI’s pseudo-Voigt line profile. In parallel, local thermal equilibrium (LTE) 0D slab models were used to infer the excitation conditions of the detected molecular species.

Results. Various molecular species, including CO, CO₂, HCN, and C₂H₂, are detected in the JWST-MIRI spectrum of DR Tau, with H₂O being discussed in a subsequent paper. The high-resolution observations show evidence for two ¹²CO components: a broad component (full width at half maximum of FWHM ~33.5 km s⁻¹) tracing the Keplerian disk and a narrow component (FWHM ~ 11.6 km s⁻¹) tracing a slow disk wind. The rotational diagrams yield CO excitation temperatures of T ≥ 725 K. Consistently lower excitation temperatures are found for the narrow component, suggesting that the slow disk wind is launched from a larger radial distance. In contrast to the ground-based observations, much higher excitation temperatures are found if only the high-J transitions probed by JWST-MIRI are considered in the rotational diagrams. Additional analysis of the ¹²CO line wings suggests a larger emitting area than inferred from the slab models, hinting at a misalignment between the inner (i ~ 20°) and the outer disk (i ~ 5°). Compared to CO, we retrieved lower excitation temperatures of T ~ 325-900 K for ¹²CO₂, HCN, and C₂H₂.

Conclusions. We show that complementary, high-resolution CO ro-vibrational observations are necessary to properly investigate the excitation conditions of the gas in the inner disk and they are required to interpret the spectrally unresolved JWST-MIRI CO observations. These additional observations, covering the lower-J transitions, are needed to put better constraints on the gas physical conditions and they allow for a proper treatment of the complex line profiles. A comparison with JWST-MIRI requires the use of pseudo-Voigt line profiles in the convolution rather than simple binning. The combined high-resolution CO and JWST-MIRI observations can then be used to characterise the emission, in addition to the physical and chemical conditions of the other molecules with respect to CO. The inferred excitation temperatures suggest that CO originates from the highest atmospheric layers close to the host star, followed by HCN and C₂H₂ which emit, together with ¹³CO, from slightly deeper layers, whereas the CO₂ emission originates from even deeper inside or further out of the disk.