Periodic Reporting for period 1 - Filaments to stars (The origin of the IMF through multi-scale analysis of molecular clouds: unification of PDF and power spectrum analysis)
Reporting period: 2018-01-01 to 2019-12-31
The goal of the Filaments to Star project was to build a global and coherent picture of how cloud structures and magnetic fields have an impact on the formation of dense cores by applying state-of-the-art multi-scale analysis techniques on observations. This project has been realised at the Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) with the collaboration and under the supervision of Dr. Frédérique Motte. Along the two years of this project, most of the stated objectives have been realised, although some deviations exist from the initial project description. Notably, new links between the feature extraction technique that I developed and the statistical description of turbulent gas velocity and density fluctuations has open-up new interesting perspectives on the statistical analysis of astrophysical data associated with star-formation regions.
The first objective was to improve the multi-scale analysis of star-forming regions in order to bring a better understanding of the link between the filaments/cores and the turbulent environment. This objective has been fulfilled from several angles. The multi-scale segmentation method, based on the work initiated during my PhD, as been reviewed and substantially improved. This work as been published in Open Access in the Astronomy & Astrophysics journal (Robitaille et al. 2019) and made the cover of the August issue.
In this work, the now called Multi-scale non-Gaussian Segmentation (MnGSeg; open source code and software at : https://github.com/jfrob27/pywavan) technique was applied on the Herschel space observatory observations of the thermal dust emission at 250 m of the Polaris molecular complex and its derived gas density map. This particular region has been chosen by other recent studies, for its close location in our Galaxy (~150 pc) and as an early stage star-forming region, most probably dominated by turbulent processes. The major progress and result in this project, in addition to the improvement of the segmentation method itself, is that the study of the Polaris region allowed me to reveal new theoretical implications on the characterisation of the non-Gaussian processes in the interstellar medium (ISM).
The MnSeg technique allowed us to extract dense gaz structures in the ISM with specific statistical properties which corresponds well to the description of intermittent processes occurring in turbulence decay. I showed, with a wavelet formalism, that the non-Gaussian extraction perform with MnGSeg is in agreement with the anomalous scaling previously measured with more limited techniques like the structure function (Hily-Blant & Falgarone 2009; Federrath et al. 2010). In a submitted paper, we recently proved that the non-Gaussian part extracted using my multiscale analysis technique is associated with the multifractal topology of the gas. The upcoming effort of our research will be to link these local variations of density scaling with the IMF/CMF of star-forming regions.
Based on these new results, the following objectives have been explored, published or are part of an ongoing work:
- Constraining the origin of the IMF/CMF through multi-scale analysis techniques : new promising strategies are explored in order to join the concept of the multifractality of the ISM to the variation of core mass reservoirs in star forming regions
- Do massive star-formation regions have a distinct statistical signature? : The method has been applied on the nearby molecular complex Polaris Flare, which is the main subject of the publication by Robitaille et al. 2019. It is also currently applied on the massive star-formation region NGC2264, as well as on Cygnus X. Massive star-formation regions indeed show a different behaviour in terms of power distribution as a function of the spatial scales.
- Quantifying the influence of magnetic fields through the formation process of filaments and cold cores : The multi-scale aspect of the magnetic field fluctuations in the ISM is rarely investigated mainly because of the difficulty imposed by the vectorial values of polarisation maps. My previous works on the subject explored the multi-scale analysis of the amplitude of the gradient of the polarisation vector. During my Marie Curie project, I explored the linear property of wavelet transforms, where the continuous wavelet transform of a vector function is a vector whose components are the continuous wavelet transform of the different components.
In this work, the now called Multi-scale non-Gaussian Segmentation (MnGSeg; open source code and software at : https://github.com/jfrob27/pywavan) technique was applied on the Herschel space observatory observations of the thermal dust emission at 250 m of the Polaris molecular complex and its derived gas density map. This particular region has been chosen by other recent studies, for its close location in our Galaxy (~150 pc) and as an early stage star-forming region, most probably dominated by turbulent processes. The major progress and result in this project, in addition to the improvement of the segmentation method itself, is that the study of the Polaris region allowed me to reveal new theoretical implications on the characterisation of the non-Gaussian processes in the interstellar medium (ISM).
The MnSeg technique allowed us to extract dense gaz structures in the ISM with specific statistical properties which corresponds well to the description of intermittent processes occurring in turbulence decay. I showed, with a wavelet formalism, that the non-Gaussian extraction perform with MnGSeg is in agreement with the anomalous scaling previously measured with more limited techniques like the structure function (Hily-Blant & Falgarone 2009; Federrath et al. 2010). In a submitted paper, we recently proved that the non-Gaussian part extracted using my multiscale analysis technique is associated with the multifractal topology of the gas. The upcoming effort of our research will be to link these local variations of density scaling with the IMF/CMF of star-forming regions.
Based on these new results, the following objectives have been explored, published or are part of an ongoing work:
- Constraining the origin of the IMF/CMF through multi-scale analysis techniques : new promising strategies are explored in order to join the concept of the multifractality of the ISM to the variation of core mass reservoirs in star forming regions
- Do massive star-formation regions have a distinct statistical signature? : The method has been applied on the nearby molecular complex Polaris Flare, which is the main subject of the publication by Robitaille et al. 2019. It is also currently applied on the massive star-formation region NGC2264, as well as on Cygnus X. Massive star-formation regions indeed show a different behaviour in terms of power distribution as a function of the spatial scales.
- Quantifying the influence of magnetic fields through the formation process of filaments and cold cores : The multi-scale aspect of the magnetic field fluctuations in the ISM is rarely investigated mainly because of the difficulty imposed by the vectorial values of polarisation maps. My previous works on the subject explored the multi-scale analysis of the amplitude of the gradient of the polarisation vector. During my Marie Curie project, I explored the linear property of wavelet transforms, where the continuous wavelet transform of a vector function is a vector whose components are the continuous wavelet transform of the different components.
The results of the research presented above have been the subject of open access publications, international conference contributions and seminars. A paradigm shift is occurring in the astrophysical community studying the structuration of the ISM and the star-formation process. The community slowly realises that the star-formation process is not occurring in isolation from the ISM, but that this process is in constant interaction with its environment. Dense cores accrete mass from the parental structures and molecular cloud complexes are probably undergoing global hierarchical gravitational collapse. With this new paradigme, it becomes important to consider the ISM in its entirety when one wants to study the star-formation process in molecular clouds.
The research I have done during this Marie Sklodowska-Curie Individual Fellowship definitely use this approach in order to understand the structuration of the ISM and its impact on the star-formation process. This project will have an impact on future techniques analysing the density fluctuations in the ISM, its velocity field and its magnetic field.
This research is also opening on new projects in collaboration with people at IPAG, in France and also internationally.
The research I have done during this Marie Sklodowska-Curie Individual Fellowship definitely use this approach in order to understand the structuration of the ISM and its impact on the star-formation process. This project will have an impact on future techniques analysing the density fluctuations in the ISM, its velocity field and its magnetic field.
This research is also opening on new projects in collaboration with people at IPAG, in France and also internationally.