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Linking chemistry and physics in the planet-forming zones of disks

Periodic Reporting for period 1 - MOLDISK (Linking chemistry and physics in the planet-forming zones of disks)

Reporting period: 2022-01-01 to 2023-06-30

Stars like our Sun and planets like Earth and Jupiter form out of collapsing clouds of gas and dust between the stars. During the collapse, part of the material ends up in a rotating disk around the young star, which is the site where future planets form. Thanks to new ground-breaking observatories, especially the Atacama Large Millimeter/submillimeter Array (ALMA) and the recently launched James Webb Space Telescope (JWST), we can now zoom into these planet-construction sites with unprecedented sharpness and sensitivity for the first time. The main goal of this project is to follow the trail of molecules from the collapsing clouds to planet-forming disks and determine the chemical composition of the material that ultimately makes "us". Since chemistry and physics are intertwined, a related goal is to examine and quantify the key physical processes along this journey. The question of habitability of planets around other stars that may (or may not) be conducive to life has been of interest to society and humankind for many centuries. We are now in a position to start addressing this question scientifically. The results have been communicated not only in scientific publications but also in numerous public talks and outreach events and exhibitions to the general public.
The first 18 months of the program were focused on (i) performing and analyzing ALMA observations of gaseous complex organic molecules in protostellar systems, i.e. the earliest stages of star formation, for future comparison with JWST data of the same molecules in ices; and (ii) harvesting of the initial JWST results on gas and ices in protostellar systems and protoplanetary disks. Because the PI was heavily involved in the planning and building of the MidInfraRed Instrument (MIRI) for more than 25 years, the MOLDISK team has guaranteed access to early JWST data. More than 20 refereed papers have been published in this period.

The ALMA observations focused on methanol (CH3OH), a key molecule in interstellar chemistry, as well as on more complex O- and N-containing molecules, in programs co-led by van Gelder. A surprising finding is that abundance ratios of various complex molecules are remarkably constant among dozens of sources, in spite of their very different physical conditions. This points to a common formation site, likely on ices in the cold pre-stellar phase prior to collapse of the cloud. If correct, we should then see these same molecules in the ices with future JWST observations. Also, detailed modeling was performed to explain why some protostellar systems do not show complex molecules. In particular, the presence of a disk, lowering the temperature in the immediate environment, was found to be able to "hide" emission from gaseous complex molecules by freezing them out onto grains.

JWST was launch December 25 2021, followed by commissioning (leading to an overview paper on JWST-MIRI in orbit performance) and started routine observations by mid-July 2022. The first data from our guaranteed and open time programs on protostars and disks started to arrive soon after. One early highlight, led by Leiden colleague Melissa McClure, was the observation of the "darkest" ices to date in a dense cold cloud with nearly 100 mag of extinction. It demonstrated that the ice composition is remarkably robust as function of extinction and also hinted at the presence of more complex molecules in ices. The analysis of ices and gas in protostellar systems is onging, supported by Leiden Laboratory for Astrophysics measurements: in collaboration with colleague Harold Linnartz, a database of ice spectra for use by the entire worldwide community was published and made available online by postdoc Will Rocha. Additional spectra are being measured by PhD student Katie Slavicinska. Finally, the JWST spectra of protoplanetary disks turn out to be richer than expected, with even 13C isotopologs and benzene detected for the first time in a disk, leading to several "first results" papers.
Overall, MOLDISK is proceeding as planned, but thanks to the quality of the JWST data being even higher than expected, we are opening new avenues for research. Indeed, Indeed, the initial JWST spectra are full of surprises. In particular, the disks around very low mass stars (<0.2 LSun) turn out to be very rich in hydrocarbon molecules with surprisingly little water detected. This points to a very high C/O ratio in the terrestrial planet-forming zones of disks. New open time proposals have been submitted and approved on this intriguing class of sources.
JWST-MIRI spectrum of a very low mass star showing abundant hydrocarbon molecules in its disk