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Setting the Stage for Solar System Formation

Periodic Reporting for period 2 - S4F (Setting the Stage for Solar System Formation)

Reporting period: 2017-02-01 to 2018-07-31

Low-mass stars like our Sun are formed in the dense regions of dark clouds of dust and gas that obscure their visible light. Deep astronomical observations at infrared and submillimeter wavelengths are uniquely suited to probe the inner regions of these young stellar objects and unravel their structures, as well as the physical and chemical processes involved. These earliest stages are particularly interesting because the properties of the deeply embedded objects reflect the star formation process itself and how it relates to its environment. It is for example during this stage that the final mass of the star and the properties of its disk – and thus ability to form planets – are determined. It is also during these stages that the first seeds for the chemical evolution of the protoplanetary disk are planted and where some complex organic, possibly prebiotic, molecules may be formed. The aim of the program is to map the physics and chemistry of the early Solar System using new high resolution, high sensitivity observations from the Atacama Large Millimeter/submillimeter Array (ALMA) coupled with state-of-the-art radiative transfer tools and theoretical simulations to address some of the key questions concerning the physics and chemistry of the earliest stages of the Solar System: How is the chemistry of the earliest protostellar stages related to the physical structure and evolution of the young stellar object and its surrounding environment? Which complex organic molecules are present in the inner regions of low-mass protostars? What are the chances the rich chemistry of the earliest stages is incorporated into planetary systems such as our own?
The action has been highly active and made excellent progress on addressing the questions outlined above. We have (i) initiated and executed programs to measure the physical and chemical conditions of the gas surrounding embedded protostars, (ii) obtained, reduced and published data from ALMA on the complex organic chemistry of embedded protostars including first detections of a number of complex organic molecules and their isotopologues and (iii) explored the link between the physical structure of protostars and their chemical signatures through sophisticated numerical simulations and detailed observations.

The nexus of the program has so-far been a large survey using ALMA of the complex chemistry of a system of solar-type young stars. The data show a wealth of molecular lines from different species that we identify through spectroscopic data obtained in laboratories on Earth. These observations have so-far resulted in a number of discoveries of molecules – some that had never been seen in the interstellar medium before and some that had only been detected in more extreme environments, for example in the center of our Galaxy. One example, is the first discovery of methyl chloride that members of our team also discovered toward the Solar System Comet 67P as part of the ESA/Rosetta mission. The discovery demonstrates a link between the chemistry between the earliest protostellar stages and our own Solar System.

The high quality data also allow us to make significant progress on making detailed and accurate inventories of molecular species in these regions and address their origin. For example, the sensitivity of the data allow us to measure the abundances of rare isotopologues of these complex organics accurately. The results of these measurements support a picture where many of these species are formed during the densest cold phases in the evolution of young stars where most of the chemistry takes place on the surfaces of dust grains. The accurate measurements also pose new challenges with respect to mimicking these results in laboratory experiments as well as through detailed chemical models.

Finally, the results also pose new questions, e.g. whether this chemistry is ubiquitous - i.e. that all forming young stars are characterised by similar chemical processes and thereby have a chemical complexity that can be incorporated in their protoplanetary disks. We have carried out observations of a number of young protostars from different star forming regions and in different evolutionary stages and also explored the results from large-scale numerical simulations. These observations suggest that some chemical differences indeed are found between objects that may be tied to the exact mechanism by which these disks are formed.
The ALMA observations in themselves represent a revolution in terms of the achievable angular resolution and sensitivity critical for studying the physical and chemical processes taking place on the smallest scales toward newly formed protostars. We have been highly successful in taking advantage of this opportunity and thereby move the field beyond the previous state of the art. Likewise, the comparison between these detailed observations and the predictions from the numerical simulations are possible through the effort of developing the techniques for bridging the gap between these different fields, in particular through detailed continuum and line radiative transfer calculations.