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Toward a unified view of star formation in galaxies: the origin of protoSTAR clusters, FIlamentary molecular clouds, and prestellar Cores seen with Herschel – STARFICH

Toward a unified view of star formation in galaxies

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Studying the role of the filamentary structure of the interstellar medium in regulating the star formation process in galaxies

The star formation process plays a central role in astrophysics because it is closely linked to both the evolution of galaxies on large scales, and the formation of planetary systems on small scales. Studying the physical mechanisms responsible for star formation is thus of fundamental importance to better understand the origin and significance of our own Sun and solar system in the Universe. This is a complex problem, involving a wide range of scales and a subtle interplay between gravity, turbulence, magnetic fields, feedback mechanisms, heating and cooling effects in the interstellar medium. Despite this complexity, the net products of the star formation process on global scales, such as the distribution of stellar masses or the star formation rate in galaxies, appear to be governed by relatively simple, quasi-universal laws. One of the big questions today consists in understanding the origin of this universality. The general objective of the STARFICH project is to investigate the extent to which the quasi-universal filamentary structure revealed by the Herschel space observatory in the cold interstellar medium of our own galaxy may be responsible for regulating the efficiency of the star formation process in the dense molecular gas of galaxies.

The project combines observational and theoretical efforts. On the observational front, we have exploited, in a systematic manner, the wide-field submillimeter dust continuum images of Galactic clouds obtained with Herschel and have supplemented them by both higher-resolution studies with the APEX (« Atacama Pathfinder Experiment ») telescope in Chile and millimeter line observations of dense molecular gas tracers with the « Institut de Radioastronomie Millimétrique » (IRAM) telescopes, the Mopra telescope in Australia, and the Nobeyama 45m telescope in Japan. The properties observed toward nearby interstellar clouds have been compared to more global results obtained for local group galaxies. On the theoretical front, we have used the adaptive mesh refinement code RAMSES developed in the CEA/Irfu (AIM « Astrophysics – Instrumentation – Modelling ») Laboratory to perform magnetohydrodynamic simulations of the growth of dense structures within the cold molecular clouds inside which stars form. These numerical simulations have been confronted to the observational results.

We have shown that a dominant fraction of the dense molecular gas in Galactic interstellar clouds is structured in the form of filaments. We have identified a density threshold above which interstellar filaments fragment into prestellar condensations and form stars. We have also shown that the star formation efficiency in the dense molecular gas of galaxies is very similar to the efficiency with which the dense filaments of nearby galactic clouds convert their gas into stars (see illustration). Our results emphasize the quasi-universal character of the star formation process in galaxies and the key role played by the filamentary structure of interstellar clouds.

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The STARFICH project has led to 27 refereed publications involving both partners and more than 30 oral presentations in international conferences, including 12 invited talks. Several other publications are still in preparation. The project also stimulated the organisation of three international conferences, including a Symposium of the International Astronomical Union in 2015 (IAUS 315 : « From Interstellar Clouds to Star-Forming Galaxies : Universal Processes ? »).

Understanding star formation on both large and small scales is a major unsolved problem of modern astrophysics, fundamental in its own right and having a profound bearing on both galaxies and planet formation. Recent observational results with, e.g., Spitzer and Herschel suggest that it may be possible to understand both the global rate of star formation and the distribution of stellar masses (« IMF ») in galaxies by studying the physics of how dense structures (clouds, filaments, cores) form and grow in the interstellar medium (ISM) of our own Galaxy. Indeed, the star formation rate in both Galactic clouds and external galaxies is observed to be directly proportional to the mass of dense molecular gas above the surface density threshold beyond which interstellar filaments are gravitationally unstable. Furthermore, the prestellar core mass function (« CMF ») observed in nearby clouds closely resembles the IMF. Altogether, the early results from Herschel imaging surveys lead us to favor a scenario in which interstellar filaments and prestellar cores represent two fundamental steps in the star formation process: First, large-scale MHD turbulence generates filamentary clouds in the ISM; second, the densest, filaments fragment into prestellar cores (and ultimately protostars) through gravitational instability. The goals of the STARFICH project are 1) to confirm/refne this observationally-driven picture using additional spectroscopic constraints on the kinematics of a selected sample of cloud filaments imaged with Herschel, 2) to provide a solid theoretical framework based on detailed comparisons with numerical MHD simulations of molecular cloud formation and evolution, and 3) to investigate the extent to which the star formation rate is governed by a universal law from filament to galactic scales. Our approach is novel in that 1) we will combine theoretical and observational efforts and expertise within the same team, and 2) we will address the problem of star formation from large (GMC) to small (core) scale, allowing us to links the « macrophysics » to the « microphysics » of star formation. On the observational front, we plan extensive (sub-)millimeter continuum and spectral line studies with the Herschel, IRAM, APEX, and GBT telescopes. In particular, this project will greatly benefit from our large “Gould Belt" and HOBYS imaging surveys with Herschel.
On the theoretical front, special efforts will be devoted to relate the formation and evolution of prestellar cores within filaments to the larger-scale formation of molecular clouds from atomic gas in galaxies. We will extensively use the adaptive mesh refinement magnetohydrodynamic code RAMSES developed in our Astrophysics Division and large computational facilities available both at CEA and the French national level.

Project coordination

Philippe ANDRÉ (COMMISSARIAT A L'ENERGIE ATOMIQUE ET AUX ENERGIES ALTERNATIVES - CENTRE D'ETUDES NUCLEAIRES SACLAY) – pandre@cea.fr

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

LAB UNIVERSITE BORDEAUX 1
CEA/Irfu (AIM) COMMISSARIAT A L'ENERGIE ATOMIQUE ET AUX ENERGIES ALTERNATIVES - CENTRE D'ETUDES NUCLEAIRES SACLAY

Help of the ANR 420,000 euros
Beginning and duration of the scientific project: - 48 Months

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