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Teaser, summary, work performed and final results

Periodic Reporting for period 2 - PROMISE (Origins of the Molecular Cloud Structure)

Teaser

Our cosmic home—the Milky Way galaxy—continuously gives birth to new stars: on average, a couple of stars such as our own Sun are formed every year. These new stars form from the interstellar gas that is an indivisible component of the Milky Way. If a large-enough amount...

Summary

Our cosmic home—the Milky Way galaxy—continuously gives birth to new stars: on average, a couple of stars such as our own Sun are formed every year. These new stars form from the interstellar gas that is an indivisible component of the Milky Way. If a large-enough amount of this gas is located in a small-enough space, the gravity of the gas causes a gravitational collapse, followed by a formation of a new star. Exactly how does this process proceed is not yet well known. Specifically, it remains unknown how the different physical processes—such as gravity and turbulence—set the rate and efficiency with which the new stars form. As a result, our comprehensive understanding of how galaxies like Milky Way build up their stellar content remains lacking.

Overall, the question of “How do the stars form?” is linked to the fundamental question of our own origins in the Universe. Exploring and understanding such a topic has its main societal value in enabling and promoting critical, fact-based thinking and deep understanding of our own natural environment. On a more practical level, the study of the topic requires methods and technologies highly relevant in the modern society, for example, in the fields of data science, computer science, and engineering. Thus, the question “How do the stars form?” interfaces with the society by being a motivating question by its own right, by providing a route to develop skills that are in high demand in today’s society, and by providing a unique, curiosity-driven perspective to complex problem-solving.

The PROMISE project addresses fundamental open questions related to how the star formation process proceeds in the interstellar gas. The foremost goal of the project is to accurately map how the gas is distributed in a large number of star-forming gas clouds in the Milky Way. The exact distribution of gas determines its energetics, and from therein, where exactly the new stars can form. The PROMISE project focuses in exploiting novel and innovative observational techniques to map the gas distribution in thousands of gas clouds in the Milky Way in a detail that has not been available before. This enables PROMISE to make a step forward in understanding how the interstellar gas is distributed and how it is transformed into new stars in the Milky Way.

Work performed

The PROMISE project has made significant progress in studying the formation of new stars in three fronts: in deriving detailed maps of giant interstellar gas clouds in the Milky Way, in finding out how these giant gas clouds break up into smaller and smaller pieces, down to the units that give birth to individual stars, and in determining how gravity and turbulence affect the internal structure of gas clouds.

The most substantial goal of the PROMISE project is to derive a novel map picturing thousands of gas clouds in the centre plane of the Milky Way. The map will be the most detailed and largest map of such clouds so far; obtaining it has been made possible by novel methodology we have recently developed. However, obtaining the map requires a substantial amount of algorithm development and computing work. During the first half of the PROMISE project, our group has worked actively on those tasks, and currently, the map is almost complete. We expect it to be finalised within the next few months.

Related to the above work, we have also developed a new methodology to study how exactly the gas is moving in the interstellar gas clouds. The movement of gas can be studied with the help of the molecular line emission that originates from the clouds; the emission lines contain information on the velocity structure of the gas. Our novel technique takes advantage of machine-learning to identify detailed velocity patterns from such data. This is done automatically, so that large amounts of data can be analyzed efficiently, without human intervention. This enables analyses of huge amounts of data—such a tool is highly useful not only for our project, but also for astronomers working on other questions with similar data.

The PROMISE project has made strong progress in studying how the internal structure of individual gas clouds is organized, i.e., how the gas clouds fragment to smaller and smaller units of gas. We have been extremely successful in the competition to use the most advanced technologies for this purpose, namely the Atacama Large Millimeter/submillimeter Array (ALMA). The PROMISE group is currently among the most active groups in the world studying this topic. Our first results have already shown that the existing understanding on cloud fragmentation need to be fundamentally revised—the models currently used to describe the fragmentation do not seem to be adequate. Our first results are published in Kainulainen et al. (2017, Astronomy & Astrophysics, 600, 141) and in Mattern et al. (2018, Astronomy & Astrophysics, 616, 78).

One necessary step in studying star formation in interstellar gas clouds is that we need to carefully map how many new stars the clouds are forming. In this topic, we have performed the first study that determines how many stars are formed in highly elongated, massive filamentary structures in the Milky Way. A significant fraction of the Milky Way’s star-forming gas is organised in such a spider-web-like gas network. Our study found that despite this curious morphology, the massive filamentary structures form stars just as much as the other gas clouds in the Milky Way (Zhang et al. 2019, accepted to Astronomy & Astrophysics, arXiv:1811.02197).

We have also worked on determining the roles of different physical processes—gravity, turbulence, and magnetic fields—in shaping the structure of gas clouds. The internal structure of clouds is expected to be affected by the relative strengths of these processes. We have performed a study to determine these relative strengths. The first, tentative results indicate that the commonly-used models of cloud structure are not very accurate. Specifically, the role of gravity may be stronger than the current models assume and the role of the environment in which the clouds reside may have a strong influence on how their internal structure evolves. The results are published in Kainulainen & Federrath (2017, Astronomy & Astrophysics, 608, L3) and Chira et al

Final results

The PROMISE project has so far been focused on deriving and obtaining observational data. During the second half of the project, these data will be in hand and the project is expected to concentrate on the analysis and exploitation of the obtained data. Consequently, it is expected that the main result papers will be published. Specifically, we foresee the following progress and results until the end of the project.

The PROMISE mapping of giant gas clouds in the Milky Way will be finished and the resulting data product will be released to the community. Once in hand, the map will enable us to perform a number of key studies. We will study the internal structure of a large number—thousands—of gas clouds, especially from the perspective of cloud fragmentation. This will be a major outcome of the PROMISE map: we will determine how exactly the cloud structures are built up, from large to small scales. We will also study how the fragmentation depends on the velocity structure of the gas as revealed by our novel methodology. We also expect that the availability of the PROMISE data will prompt several new studies, for example, detailed works of especially interesting gas clouds revealed by the data.

In the remainder of the PROMISE project, our success in cloud fragmentation studies with ALMA is expected to result in a more systematic picture of how the cloud substructure connects with the protostars that form in the clouds. We have so far only published the very first results of our ALMA projects—several projects are waiting to be analyzed and exploited. We expect that in the second half of the PROMISE project, these analyses will continue actively. The results allow us to take the first steps in building a new scenario for how the internal cloud structure evolves through fragmentation towards star formation. Such work is expected to continue beyond the limits of the PROMISE project; the PROMISE group’s work will function as the path maker for the future ALMA studies.

The information resulting from the analyses above is vital for developing accurate models for how rapidly and how efficiently stars form in the interstellar gas clouds. Bringing our observational results in contact with the existing models of interstellar medium physics and star formation is the key goal of the second half of the project. We expect that our results will immediately affect those models and help in developing new ones. Such improvements would represent a major step forward in understanding the star formation process and they would mark a highly successful end of the PROMISE project.