Final Report Summary - ORISTARS (Toward a Complete View of Star Formation: The Origin of Molecular Clouds, Prestellar Cores, and Star Clusters)
The overall objective of the ORISTARS project was to test, refine, and explore the implications of a new paradigm for star formation motivated by Herschel observations of nearby Galactic molecular clouds which emphasize the key role of interstellar filaments (see review chapter by André+2014 in Protostars and Planets VI – p. 27-51 – for a description of this filamentary paradigm). On the research front, the project aimed to study 1) the origin of interstellar filaments, 2) the fragmentation of filaments into prestellar cores and its possible link to the origin of the initial mass function (IMF), as well as 3) the rotational sub-fragmentation of prestellar cores into binary protostars and the birth of protostellar disks. 4) On the instrumental/technological front, the project aimed to equip the new generation millimeter continuum camera of the IRAM 30m telescope (“NIKA-2”) with a Polarization Channel, with the longer term goal of probing the role played by magnetic fields in the structure and evolution of star-forming filaments.
Key results were achieved on each of these four aspects over the duration of the project:
1) Concerning the formation of filaments, a strong observational constraint is the characteristic ~ 0.1 pc width measured with Herschel for molecular filaments (Arzoumanian+2018). A promising theoretical scenario is that filaments correspond to the dissipative structures of the interstellar turbulence cascade and that diffusive processes such as ambipolar diffusion set the filament width (Hennebelle 2013; Hennebelle & André 2013). High-resolution, 3D MHD simulations of both driven and decaying turbulence performed with the RAMSES code confirmed that non-ideal MHD effects such as ion-neutral friction tend to broaden dense filamentary structures and likely play a key role in setting their characteristic transverse scale (Ntormousi, Hennebelle+2016).
2) Systematic extraction of cores and filaments in the wide-field submm continuum images taken as part of the Herschel Gould Belt survey with the multi-scale, multi-wavelength algorithms getsources (Men’shchikov+2012) and getfilaments (Men’shchikov 2013) has provided a complete census of prestellar cores in nearby molecular clouds, made publicly available on the open-access database gouldbelt-herschel.cea.fr/archives. The Herschel results confirm the existence of a close link between the prestellar core mass function (CMF) and the IMF and suggest that the gravitational fragmentation of filaments is the dominant physical mechanism responsible for the peak of the IMF (e.g. André+2014 – Könyves, André+2015).
The ArTéMiS submm camera, successfully installed by our instrument group on the APEX 12m telescope (cf. www.eso.org/public/images/eso1341a/) and open to the ESO/OSO communities since 2014, has also allowed us to start probing the properties of filaments and the connection with dense cores beyond the Gould Belt with a factor 3.5 higher angular resolution than Herschel.
3) A detailed analysis of the results of the Large Program “CALYPSO” with the IRAM millimeter interferometer (cf. irfu.cea.fr/Projets/Calypso/Welcome.html) confirmed a low frequency of occurrence of large (radius > 50-100 AU) protostellar disks and multiple systems at the Class 0 stage, compared to more evolved Class I protostars (Maury+2015, 2018): At most 25% of Class 0 protostars appear to have large disks with radii > 60 AU. These findings support a magnetized accretion scenario, in which magnetic fields contribute to prevent the formation of large protostellar disks due to magnetic braking and jet/outflow launching.
4) Last but not least, thanks to the ORISTARS project/funding, a Polarization-Channel was added to the design of the New IRAM KID Array (NIKA-2) camera for the 30m telescope (cf. ipag.osug.fr/nika2/Welcome.html). Unpolarized science observations with NIKA2 started in October 2017 and first tests of the polarization channel, carried out at the 30m telescope in November 2017, are very promising, suggesting that NIKA2 will be more sensitive in polarization mode than in total power mode. A large program of 300 hours of guaranteed time observations with NIKA2-POL (“B-FUN” – 015-17 by André, Bracco+) was approved by IRAM (cf. www.iram.es/ IRAMES/mainWiki/Continuum/NIKA2/Main) and will normally start at the 30m telescope in 2019.
Key results were achieved on each of these four aspects over the duration of the project:
1) Concerning the formation of filaments, a strong observational constraint is the characteristic ~ 0.1 pc width measured with Herschel for molecular filaments (Arzoumanian+2018). A promising theoretical scenario is that filaments correspond to the dissipative structures of the interstellar turbulence cascade and that diffusive processes such as ambipolar diffusion set the filament width (Hennebelle 2013; Hennebelle & André 2013). High-resolution, 3D MHD simulations of both driven and decaying turbulence performed with the RAMSES code confirmed that non-ideal MHD effects such as ion-neutral friction tend to broaden dense filamentary structures and likely play a key role in setting their characteristic transverse scale (Ntormousi, Hennebelle+2016).
2) Systematic extraction of cores and filaments in the wide-field submm continuum images taken as part of the Herschel Gould Belt survey with the multi-scale, multi-wavelength algorithms getsources (Men’shchikov+2012) and getfilaments (Men’shchikov 2013) has provided a complete census of prestellar cores in nearby molecular clouds, made publicly available on the open-access database gouldbelt-herschel.cea.fr/archives. The Herschel results confirm the existence of a close link between the prestellar core mass function (CMF) and the IMF and suggest that the gravitational fragmentation of filaments is the dominant physical mechanism responsible for the peak of the IMF (e.g. André+2014 – Könyves, André+2015).
The ArTéMiS submm camera, successfully installed by our instrument group on the APEX 12m telescope (cf. www.eso.org/public/images/eso1341a/) and open to the ESO/OSO communities since 2014, has also allowed us to start probing the properties of filaments and the connection with dense cores beyond the Gould Belt with a factor 3.5 higher angular resolution than Herschel.
3) A detailed analysis of the results of the Large Program “CALYPSO” with the IRAM millimeter interferometer (cf. irfu.cea.fr/Projets/Calypso/Welcome.html) confirmed a low frequency of occurrence of large (radius > 50-100 AU) protostellar disks and multiple systems at the Class 0 stage, compared to more evolved Class I protostars (Maury+2015, 2018): At most 25% of Class 0 protostars appear to have large disks with radii > 60 AU. These findings support a magnetized accretion scenario, in which magnetic fields contribute to prevent the formation of large protostellar disks due to magnetic braking and jet/outflow launching.
4) Last but not least, thanks to the ORISTARS project/funding, a Polarization-Channel was added to the design of the New IRAM KID Array (NIKA-2) camera for the 30m telescope (cf. ipag.osug.fr/nika2/Welcome.html). Unpolarized science observations with NIKA2 started in October 2017 and first tests of the polarization channel, carried out at the 30m telescope in November 2017, are very promising, suggesting that NIKA2 will be more sensitive in polarization mode than in total power mode. A large program of 300 hours of guaranteed time observations with NIKA2-POL (“B-FUN” – 015-17 by André, Bracco+) was approved by IRAM (cf. www.iram.es/ IRAMES/mainWiki/Continuum/NIKA2/Main) and will normally start at the 30m telescope in 2019.