An original observatory
HERSCHEL is the first space observatory in the sub-millimetre wavelength range (55-672 microns). In this spectral range, the emissions' level from dust clouds (which pervade the galaxies) and cool cocoons is at its maximum. The cool cocoons are clouds composed of gas and dust which are collapsing because of gravity and giving birth to stars for the biggest. These objects (with temperatures down to 10K) are one of the reasons why HERSCHEL is also called the cold Universe's observatory.
HERSCHEL features a 3.5m mirror - a record in space - which powers three :
- Heterodyne Instrument for the Far Infrared (HIFI);
- Imaging instruments (PACS and SPIRE).
These imaging instruments produce images with an unprecedented resolution in this spectral range. This resolution is the basis of a key-discovery.
An unobservable interstellar medium on small scales...
Before the HERSCHEL's launch, the filamentary aspect of the matter which constitutes the interstellar medium was already known thanks to millimetre and radio observations from the ground (see next) and infrared space observatories such as IRAS, ISO and then Spitzer.
Turbulence phenomena resulting from the energy coming from the supernovas were thought to have triggered those structures' formation.
Even better, it has been possible to establish that the speeds within these big supersonic interstellar clouds (shock-wave sources) on large scales are decreasing in accordance with the measurement scale (Larson's scaling law). Impossible though to descent at a scale corresponding to the filaments' diameter taking into account the angular resolution available...
... and then came HERSCHEL
In 2009, within the frame of the Goult Belt Survey programme, leaded by the Astrophysics department of the CEA, the first images of HERSCHEL showed the finesse and richness of these structures in two near regions called Polaris and Aquila Rift. In Polaris there is no star formation whereas Aquila Rift is very fecund (see above). Hundreds of cool cocoons have been detected. The investigators noted that the very large majority of them were disposed along the densest filaments, just like a rosary. In addition, a very good correlation appeared between the mass distributions of stars and cool cocoons. This indicates that the filaments host the stellar formation.
But one question remained: how did they form, in such huge media where the average density is much lower than the highest vacuum the man could ever produce on Earth?
In 2011, the Goult Belt Survey programme obtained HERSCHEL's images of a third region: IGC5146. Like in the first two, a network of dust filaments clearly appeared. Thanks to the PACS and SPIRE's resolutions, it was finally possible to characterize the radial profile of the filaments' emissions. The team could thus deduce the density per length and width unit of each IGC 5146's filament and apply this method to the Aquila Rift and Polaris' images.
On this 90-filament sample of three different regions, covering three orders of magnitude in density per unit of length, some of which host no cool cocoon whereas others are studded, the filament's width is almost unchanging: 0.1 parsec +/- 0.03, i.e. approximately 0.3 light years. Surprised by this result, the investigators started to be particularly interested in this "universal" value and the Larson's law, especially on the still untapped small scales. This law predicts that the speed in the turbulences falls below the speed of sound (0.2 km/s in these clouds at approximately 10K) when under a specific scale... 0.1 pc, i.e. exactly the filaments' width!
Sonic booms at the origin of the stars!
These observations made a new scenario possible to establish: it is the transition between subsonic and supersonic regimes (by spreading, the shock-waves lose energy) which would create crowded zones in the range of 0.1 pc. This could be the reason of the presence of such uniformly broad filaments. A bit like the supersonic aircraft's boom... Then, depending on their density, these filaments may have different destiny: for the less dense, no evolution (like in Polaris); for the densest, accretion of supplementary matter by gravity; for the critical density questioning the filament's gravitational stability, fragmentation and fragments' collapse to give birth the pre-stellar objects (like in Aquila Rift and IGC5146).
The turbulent character on large scales was already known thanks to previous studies. On small scales, models predicted the filaments' stability conditions depending on their density. HERSCHEL brought the missing link: the high resolution ability on small scales.