Volume 642, October 2020
|Number of page(s)||7|
|Section||Letters to the Editor|
|Published online||09 October 2020|
Letter to the Editor
ALMA chemical survey of disk-outflow sources in Taurus (ALMA-DOT)
II. Vertical stratification of CO, CS, CN, H2CO, and CH3OH in a Class I disk⋆
INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
2 Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
3 INAF – Istituto di Radioastronomia & Italian ALMA Regional Centre, Via P. Gobetti 101, 40129 Bologna, Italy
4 Università degli Studi di Firenze, Dipartimento di Fisica e Astronomia, Via G. Sansone 1, 50019 Sesto Fiorentino, Italy
5 European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748, Garching bei München, Germany
6 Excellence Cluster Origins, Boltzmannstrasse 2, 85748 Garching bei München, Germany
Accepted: 17 August 2020
The chemical composition of planets is inherited from that of the natal protoplanetary disk at the time of planet formation. Increasing observational evidence suggests that planet formation occurs in less than 1−2 Myr. This motivates the need for spatially resolved spectral observations of young Class I disks, as carried out by the ALMA chemical survey of Disk-Outflow sources in Taurus (ALMA-DOT). In the context of ALMA-DOT, we observe the edge-on disk around the Class I source IRAS 04302+2247 (the butterfly star) in the 1.3 mm continuum and five molecular lines. We report the first tentative detection of methanol (CH3OH) in a Class I disk and resolve, for the first time, the vertical structure of a disk with multiple molecular tracers. The bulk of the emission in the CO 2−1, CS 5−4, and o–H2CO 31, 2 − 21, 1 lines originates from the warm molecular layer, with the line intensity peaking at increasing disk heights, z, for increasing radial distances, r. Molecular emission is vertically stratified, with CO observed at larger disk heights (aperture z/r ∼ 0.41−0.45) compared to both CS and H2CO, which are nearly cospatial (z/r ∼ 0.21−0.28). In the outer midplane, the line emission decreases due to molecular freeze-out onto dust grains (freeze-out layer) by a factor of > 100 (CO) and 15 (CS). The H2CO emission decreases by a factor of only about 2, which is possibly due to H2CO formation on icy grains, followed by a nonthermal release into the gas phase. The inferred [CH3OH]/[H2CO] abundance ratio is 0.5−0.6, which is 1−2 orders of magnitude lower than for Class 0 hot corinos, and a factor ∼2.5 lower than the only other value inferred for a protoplanetary disk (in TW Hya, 1.3−1.7). Additionally, it is at the lower edge but still consistent with the values in comets. This may indicate that some chemical reprocessing occurs in disks before the formation of planets and comets.
Key words: protoplanetary disks / astrochemistry / ISM: molecules / pulsars: individual: IRAS 04302 +2247
The reduced images and datacubes are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (188.8.131.52) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/642/L7
© ESO 2020
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