Sometimes it can be worth observing from a different point of view! The team led  by Linda Podio, a researcher at the Italian National Institute for Astrophysics (INAF), observed a protoplanetary disk of less than 1 million years, which is almost edge-on. The edge-on geometry allowed observing the vertical structure of the disk and to resolve distinct chemical layers. The beautiful images obtained thanks to ALMA, the most powerful millimetre array on Earth located in the Atacama desert in Chile, revealed emission from several molecules. One of these is methanol, a key molecule for the formation of the so-called “complex organic molecules”. The results obtained in synergy with the team of astrochemistry of the Institut de Planétologie et d'Astrophysique in Grenoble (France) have been published on Astronomy & Astrophysics.

The research team observed the protoplanetary disk around the young forming star IRAS 04302+2247 located at 525 light year distance in the Taurus molecular cloud, a nursery of young stars. The observations revealed emission not only from CO, the most abundant molecule observable at mm wavelengths, but also from less abundant molecules, whose emission is fainter and needs extremely powerful and sensitive telescopes, such as formaldehyde (H2CO), carbon monosulfide (CS), cyanide (CN), and even methanol. Methanol is the mother of complex organic molecules and could be the starting point of prebiotic chemistry. However, to date methanol was observed only in a more evolved disk of about 10 million years.

«Our goal is to investigate what molecules are in disks and in what region of the disk they are located, to reveal key molecules for organic chemistry such as methanol, and to compare the disk chemical composition with that of comets, which are the leftover building blocks of giant planet cores and other planetary bodies. This will help us to reconstruct the formation and chemical origins of our Solar System. To this aim we started to collaborate with chemistry experts and with colleagues who study the Solar System and comets» says Linda Podio.

Antonio Garufi, a postdoc at INAF, explains: «The spectacular ALMA images allowed us to reveal emission due to the dust grains in the disk midplane as well as the emission from several molecules. The dust grains are the seeds for the formation of planets. The chemical composition of the forming planets clearly depends on what molecules are present in the disk midplane where the grains grow to form planetesimal. However, to see molecules in the midplane is difficult because in this cold region molecules freeze-out onto dust grains, covering them with an icy mantle, like water freezing on rocks in winter. Frozen molecules are invisible at the eyes of radio telescopes».

The protoplanetary disc IRAS 04302+2247 seen edge-on. The continuum emission (top left panel) comes from the dust grains sedimented in the disk plane (in orange) where the planets will form. The line emission of CO, CS and H2CO molecules comes mainly from the molecular layer (in green), while the disc plane appears darker because many molecules freeze on the dust grains. Credits: Podio et al. 2020. The protoplanetary disc IRAS 04302+2247 seen edge-on. The continuum emission (top left panel) comes from the dust grains sedimented in the disk plane (in orange) where the planets will form. The line emission of CO, CS and H2CO molecules comes mainly from the molecular layer (in green), while the disc plane appears darker because many molecules freeze on the dust grains. Credits: Podio et al. 2020.

This is the reason why the images obtained with ALMA show an X-like structure, where the emission from molecules mainly originates from an intermediate disk layer, whereas the disk midplane is darker given that molecules are frozen on icy grains. «However», Claudio Codella (INAF) annotates, «some organic molecules such as formaldehyde, although in smaller quantities, are still present in the cold midplane of the disk. This supports what suggested by theoretical models, that non-thermal mechanisms can inject part of the molecules trapped on the dust ices into the gas-phase».

The present work has been performed in the context of the project «ALMA-DOT: ALMA chemical survey of Disk-Outflow sources in Taurus», an ALMA survey led by Linda Podio, also the first author of the publication: «The project is focused on young disks in the Taurus star forming region, with ages of less than 1 million years, and likely associated with planet formation. The aim of the ALMA-DOT project is to chemically characterize such disks using the unprecedented combination of high-sensitivity and high-spatial resolution offered by ALMA. The ultimate goal is to shed light on the process leading to our Solar System».

Scientific paper: 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

Authors: Linda Podio, Antonio Garufi, Claudio Codella, Davide Fedele, Eleonora Bianchi, Francesca Bacciotti, Cecilia Ceccarelli, Cecile Favre, Seyma Mercimek, Kazi Rygl, Leonardo Testi