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Using fine structure line emission to observe the life-cycle of molecular clouds

Periodic Reporting for period 1 - BEYONDCO (Using fine structure line emission to observe the life-cycle of molecular clouds)

Reporting period: 2016-10-06 to 2018-10-05

Stars are born in the densest regions of molecular clouds (MCs), but the process appears to be very inefficient, with MCs converting only a few percent of their gas budget into stars per dynamical time. The underlying physical processes that regulate the star formation rate in the interstellar medium (ISM) are still unknown and hotly debated, with candidates ranging from turbulence and magnetic fields to stellar feedback. Further debate stems from the observational evidence that while the “dense” regions in MCs in the solar neighbourhood appear to explain the observed galactic star formation relations, the same approach fails to explain the SFR towards the central molecular zone (CMZ) of the Milky Way. The primary reason for this debate is that we still do not understand how MCs are assembled and destroyed — the two processes that ultimately set the timescale over which a cloud can form stars. The problem is that carbon monoxide (CO), the main tracer of MC structure and dynamics, is only sensitive to the cold interiors of MCs and not their envelopes, and models show that CO may form relatively late in the assembly process. Therefore, alternative tracers that can probe gas in the absence of CO are needed to make further progress in understanding MC formation and destruction.

There is a growing consensus within the international community that fine structure line (FSL) tracers -- specifically ionised and atomic carbon ([CII] and [CI]) and atomic oxygen ([OI]) -- are the key probes of the earliest stages of cloud assembly, as they are chemically abundant in regions surrounding MCs where CO is not. As such, FSLs are able to trace the low-density transition from atomic to molecular gas that marks the boundary from the warm ISM to the cold reservoirs in which stars form. They also constitute the main coolants of ISM during this transition, thus providing a way to measure the energetics of the ISM. It is only within the last few years that we have gained the technological capabilities to map FSL emission in and around Galactic star formation sites. First with the Herschel Space Observatory and now with the Stratospheric Observatory for Infrared Astronomy (SOFIA), [CII] and [OI] mapping are possible. The Atacama Pathfinder Experiment (APEX) telescope can also map [CI] emission. With these facilities, it is now possible to measure the mass and dynamics of the “CO-dark” molecular and cold atomic gas that surround and permeate MCs, and combine this with our extensive knowledge of CO-traced cloud and the star formation inventory within.

The main objective of this project was to use FSL tracers alongside more traditional MC tracers to gain a broader understanding of the connection between MCs and their environment. We did this by examining the morphology and dynamics of a sample of clouds and comparing those results to numerical simulations, which helped us to identify the important physical mechanisms in the cloud formation.
We have obtained maps of four molecular clouds, including both quiescent and active star-formation regions. In addition to the standard CO mapping, we also observed atomic carbon maps and ionised carbon maps. We have conducted an analysis of the morphologies of the different tracers and found that while atomic carbon traces essentially the same material as CO, the ionised carbon line exhibits a very different morphology, probing the extended envelope of the cloud. In order to interpret these observations, we have undertaken a suite of numerical simulations of the cloud formation process, following the time-dependent chemistry of these carbon species. This enables us to characterise the physical conditions of the gas in which these tracers are excited. We find that ionised carbon is a potential tracer of cloud formation mechanism, but note the predicted line brightness is at the limit of the current observational capabilities. The ionised carbon also shows elevated line widths, signifying that the gas is more turbulent. Our further studies will verify if any of our regions are part of a large scale atomic flow, a commonly invoked mechanism for cloud assembly. If confirmed, our ionised carbon measurements will provide powerful constraints on the turbulent state of large scale gas flows.
The assembly of molecular clouds is a complex dynamical process. To model such a process, one must simulate a broad range of physical scenarios. Since there are countless permutations of the starting conditions, the models must be informed by observations. Until recently, both the modelling techniques and the observational tracers needed to test the models were inaccessible. This project is a manifestation of this progress, where new observations have fed into the improvement of state-of-the-art numerical simulations, and this feedback process will continue as our work together progresses and as such accelerate scientific progress.