Periodic Reporting for period 2 - MOLDISK (Linking chemistry and physics in the planet-forming zones of disks)
Période du rapport: 2023-07-01 au 2024-12-31
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.
JWST was launched 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. The analysis of ices in protostellar systems has made significant progress, supported by Leiden Laboratory for Astrophysics measurements: a database of ice spectra for use by the entire worldwide community was published and made available online by postdoc Will Rocha. A highlight has been the first identification of individual complex organic molecules in ices, resulting in a NASA/ESA press release called "Cheers" that attracted worldwide attention; it was dedicated to Leiden colleague Harold Linnartz who led the laboratory efforts but tragically passed away on Dec 31 2023. Other results on ices include the analysis of 13CO2 ice to probe ice heating and processing, and the first detection of HDO ice in massive protostars.
Beautiful JWST images of disks, disk winds and jets have been published, allowing the linked accretion and ejection processes in the earliest stages of star formation to be probed. Many gaseous molecular lines probing the hot cores around protostars are also detected, including SO2 for the first time. In contrast with previous speculations, it does not appear to trace accretion shocks onto the disk.
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. A large diversity is found among the ~50 sources in our sample. Water is often found to be abundant in disks around Sun-like stars and has both warm and cold components, with some evidence for cold water being enhanced at the snow line due to drifting icy pebbles. Other disks show bright CO2 emission, perhaps due to small cavities in their inner disks. The role of dust structures in disks that could trap icy pebbles as imaged by ALMA is being assessed. Most intriguing are the spectra of disks around very low mass stars (about 20% of the mass of the Sun) which all show very prominent emission from hydrocarbon molecules including CH4, C2H2, C2H4, C2H6, C4H2, C3H4 and C6H6. This points to high C/O>1 ratios in their inner disks, perhaps due to destruction of hydrocarbon grains, alternatively due to drifting carbon-rich icy pebbles. Detailed modeling is ongoing to elucidate the origin of this surprising diversity.
Beautiful JWST images of disks, disk winds and jets have been published, allowing the linked accretion and ejection processes in the earliest stages of star formation to be probed. Many gaseous molecular lines probing the hot cores around protostars are also detected, including SO2 for the first time. In contrast with previous speculations, it does not appear to trace accretion shocks onto the disk.
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. A large diversity is found among the ~50 sources in our sample. Water is often found to be abundant in disks around Sun-like stars and has both warm and cold components, with some evidence for cold water being enhanced at the snow line due to drifting icy pebbles. Other disks show bright CO2 emission, perhaps due to small cavities in their inner disks. The role of dust structures in disks that could trap icy pebbles as imaged by ALMA is being assessed. Most intriguing are the spectra of disks around very low mass stars (about 20% of the mass of the Sun) which all show very prominent emission from hydrocarbon molecules including CH4, C2H2, C2H4, C2H6, C4H2, C3H4 and C6H6. This points to high C/O>1 ratios in their inner disks, perhaps due to destruction of hydrocarbon grains, alternatively due to drifting carbon-rich icy pebbles. Detailed modeling is ongoing to elucidate the origin of this surprising diversity.
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, the 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 little water detected. This points to a very high C/O ratio in the terrestrial planet-forming zones of disks, also affecting the kind of planets that can be built. New open time proposals have been submitted and approved for the second half of MOLDISK.