ALMA chemical survey of disk-outflow sources in Taurus (ALMA-DOT)

V. Sample, overview, and demography of disk molecular emission

¹ INAF, Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
e-mail: antonio.garufi@inaf.it
² Univ. Grenoble Alpes, CNRS, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), 38000 Grenoble, France
³ INAF, Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
⁴ Università degli Studi di Firenze, Dipartimento di Fisica e Astronomia, Via G. Sansone 1, 50019 Sesto Fiorentino, Italy
⁵ INAF−Istituto di Radioastronomia & Italian ALMA Regional Centre, Via P. Gobetti 101, 40129 Bologna, Italy
⁶ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁷ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany

Received: 21 September 2020
Accepted: 10 December 2020

Abstract

We present an overview of the ALMA chemical survey of disk-outflow sources in Taurus (ALMA-DOT), a campaign devoted to the characterization of the molecular emission from partly embedded young stars. The project is aimed at attaining a better understanding of the gaseous products delivered to planets by means of high-resolution maps of the assorted lines probing disks at the time of planet formation (≲1 Myr). Nine different molecules are surveyed through our observations of six Class I/flat-spectrum sources. As part of a series of articles analyzing specific targets and molecules, in this work we describe the sample and provide a general overview of the results, focusing specifically on the spatial distribution, column densities, and abundance ratios of H₂CO, CS, and CN. In these embedded sources, the ¹²CO emission is dominated by envelope and outflow emission while the CS and, especially, the H₂CO are good tracers of the gaseous disk structure. The spatial distribution and brightness of the o-H₂CO 3_1,2−2_1,1 and CS 5−4 lines are very similar to each other and across all targets. The CN 2−1 line emission is fainter and distributed over radii larger than the dust continuum. The H₂CO and CS emission is always dimmed in the inner ~50 au. While the suppression by the dusty disk and absorption by the line-of-sight material significantly contributes to this inner depression, an actual decrease in the column density is plausible in most cases, making the observed ring-like morphology realistic. We also found that the gaseous disk extent, when traced by H₂CO (150−390 au), is always 60% larger than the dust disk. This systematic discrepancy may, in principle, be explained by the different optical depth of continuum and line emission without invoking any dust radial drift. Finally, the o-H₂CS 7_1,6−6_1,5 and CH₃OH 5_0,5−4_0,4 line emission are detected in two disks and one disk, respectively, while the HDO is never detected. The H₂CO column densities are 12−50 times larger than those inferred for Class II sources while they are in line with those of other Class 0/I. The CS column densities are lower than those of H₂CO, which is an opposite trend with regard to Class II objects. We also inferred abundance ratios between the various molecular species finding, among others, a H₂CS/H₂CO ratio that is systematically lower than unity (0.4−0.7 in HL Tau, 0.1 − 0.2 in IRAS 04302+2247, and <0.4 in all other sources), as well as a CH₃OH/H₂CO ratio (<0.7 in HL Tau and 0.5−0.7 in IRAS 04302+2247) that is lower than the only available estimate in a protoplanetary disks (1.3 in TW Hya) and between one and two orders of magnitude lower than those of the hot corinos around Class 0 protostars. These results are a first step toward the characterization of the disk’s chemical evolution, which ought to be complemented by subsequent observations of less exceptional disks and customized thermo-chemical modeling.